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In situ neutron diffraction experiments have been performed to investigate the deformation mechanisms on CoCrFeNi high entropy alloys (HEAs) with various amounts of doped Cu. Lattice strain evolution and diffraction peak analysis were used to derive the stacking fault probability, stacking fault energy, and dislocation densities. Such diffraction analyses indirectly uncovered that a lower degree of Cu doping retained the twinning behavior in undoped CoCrFeNi HEAs, while increasing the Cu content increased the Cu clusterings which suppressed twinning and exhibited prominent dislocation strengthening. These results agree with direct observations by transmission electron microscopy. [ABSTRACT FROM AUTHOR]

The uniaxial mechanical properties of the high entropy alloy (HEA) Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 doped with 1.1 at.% C have been determined for both the as-cast and recrystallized states at temperatures from 77 to 973 K. Very high yield strengths were obtained at 77 K of 928 MPa for the as-cast HEA and 1037 MPa for the recrystallized HEA. The yield strength decreased sharply to 293 K and more slowly to 973 K, where it was ~200 MPa [11] in both conditions. The lattice friction stress, which results from the increase of the Peierls barrier, was the major contributor to the large yield strength at 77 K. On the other hand, the grain boundaries lose their strengthening ability at a high temperature, which largely accounts for the sharp drop in yield strength of the fine-grained (5 μm) recrystallized HEA at 973 K. The elongation to failure was highest at 293 K and 673 K with the lowest values at 77 K. Transmission electron microscopy was used to study the effects of temperature on the deformation mechanisms of the HEAs, especially the temperature dependence of the microband-induced plasticity effect. At temperatures ranging from 77 K to 673 K microbands form in the as-cast HEA, while dislocation clusters were observed in the recrystallized HEA over this temperature range. Only tangled dislocations were found at 973 K for both the as-cast and recrystallized HEA due to thermally-activated deformation. [ABSTRACT FROM AUTHOR]

The effects of Mo and Nb additions on the microstructure and mechanical properties of two FeCrAl alloys were studied. Fine and uniform recrystallized grain structures (~ 20–30 μm) were achieved in both alloys through suitable annealing after warm-rolling. The formation of Fe 2 Nb-type Laves phase precipitates in the Nb-containing FeCrAl alloy effectively stabilized the deformed and recrystallized microstructures. The Mo-containing FeCrAl alloy exhibited strong γ texture fiber after annealing at 650–900 °C, whereas the annealed Nb-containing FeCrAl alloy had much weaker texture. Both strength and ductility decreased as the grain size increased in both alloys. [ABSTRACT FROM AUTHOR]

The enhanced mechanical properties of Ni-based single-crystal superalloys induced by refractory elements such as Re, Wo, Mo, etc. are essential for the manufacture of advanced aero engines' turbine blades. Whereas these elements could promote the formation of the topologically close-packed (TCP) phases, and their growth would have a deleterious effect on the creep rupture life of single-crystal superalloys. Concomitantly, the formation of the brittle phase of the internal nitrides, especially the aluminium nitride (AlN), would be detrimental to superalloys as well. Nonetheless, the formation relationship between AlN and TCP phases has maintained opaque. In this study, the relationship between the TCP phase and AlN phase was analysed by microscopic methods, such as BSE, TEM and EDS. An AlN phase located near the TCP phase in the deep surface of the superalloy was found, and its formation was suggested to be related to nitrogen penetration during the thermal exposure. This accompanying growth of the AlN phase and TCP phase in the deep surface could provide a new perspective on the formation mechanism of these brittle phases in the Ni-based single-crystal superalloys. [ABSTRACT FROM AUTHOR]

One-dimensional glide of loops during ion irradiation at 773 K in a series of Ni-containing concentrated solid solution alloys has been observed directly during experiments conducted inside a transmission electron microscope. It was found that the frequency of the oscillatory motion of the loop, the loop glide velocity as well as the loop jump distance were dependent on the composition of the alloy and the size of the loop. Loop glide was most common for small loops and occurred more frequently in the less complex alloys, being highest in Ni, then NiCo, NiFe and NiCoFeCr. No measurable loop glide occurred in the NiCoCr, NiCoFeCrMn and NiCoFeCrPd alloys. [ABSTRACT FROM AUTHOR]

Recent experiments in notched bulk metallic glasses have found reduced, or insensitive, or improved strengths, while in many of these cases the ductile strain prior to final failure is enhanced. First, although the inverse notch effect is explained by a shift from shear localization to cavitation failure, it is suggested in this work that the synergistic effect between cohesive fracture at the notched area and shear bands emanating from the notch roots may extend the parametric space for the notch insensitive behavior. Second, the dependence of shear band patterns on notch geometric factors is determined by the Rudnicki-Rice theory and the free-volume-based finite element simulations. These results suggest conditions for shear band multiplication to take place and for the shear-localization-induced failure to be delayed. [ABSTRACT FROM AUTHOR]

It has been well documented that plastic deformation of crystalline and amorphous metals/alloys shows a general trend of “smaller is stronger”. The majority of the experimental and modeling studies along this line have been focused on finding and reasoning the scaling slope or exponent in the logarithmic plot of strength versus size. In contrast to this view, here we show that the universal picture should be the thermally activated nucleation mechanisms in small stressed volume, the stochastic behavior as to find the weakest links in intermediate sizes of the stressed volume, and the convolution of these two mechanisms with respect to variables such as indenter radius in nanoindentation pop-in, crystallographic orientation, pre-strain level, sample length as in uniaxial tests, and others. Experiments that cover the entire spectrum of length scales and a unified model that treats both thermal activation and spatial stochasticity have discovered new perspectives in understanding and correlating the strength statistics in a vast of observations in nanoindentation, micro-pillar compression, and fiber/whisker tension tests of single crystals and metallic glasses. [ABSTRACT FROM AUTHOR]

Abstract: Bulk metallic glasses are intriguing candidates for nuclear applications due to their inherent amorphous structure, but their radiation response is largely unknown due to the relatively recent nature of innovations in bulk metallic glass fabrication. Here, microstructural and mechanical property evaluations have been performed on a Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass (BAM-11) irradiated with 3 MeV Ni+ ions to 0.1 and 1.0 dpa at room temperature and 200 °C. Nanoindentation hardness and Young's modulus both decreased by 6–20% in samples irradiated at room temperature, with the sample irradiated to 1.0 dpa experiencing the greatest change in mechanical properties. However, no significant changes in properties were observed in the samples irradiated at 200 °C, and transmission electron microscopy showed no visible evidence of radiation damage or crystallization following ion irradiation at any of the tested conditions. These results suggest that BAM-11 bulk metallic glass may be useful for certain applications in nuclear environments. [Copyright &y& Elsevier]

In this study, we demonstrate the methodology systematically developed for dislocation loop (perfect and faulted loops) imaging and analysis in irradiated face-centered-cubic (FCC) alloys using scanning transmission electron microscopy (STEM). On-zone [001] STEM imaging was identified as the preferred choice for its accuracy and effectiveness based on the comparison with other dislocation loop imaging techniques including: (i) on-zone STEM imaging using other major low-index zone axes, (ii) kinematic two-beam conditions bright field imaging near the [001] zone axis in conventional TEM (CTEM) mode, and (iii) Rel-Rod CTEM dark-field (DF) imaging near the [011] zone axis. The effect of STEM collection angle on the contrast formation of dislocation loops was also investigated. The developed method was confirmed by imaging all populations of perfect and faulted loops of types a/2〈110〉{110} and a/3〈111〉{111} found in an ion irradiated Ni 40 Fe 40 Cr 20 alloy. The proposed STEM-based technique can easily identify said loops with a size greater than 10 nm without any assumptions such as those commonly made using the conventional Rel-Rod CTEM-DF technique. The recommended methodology in this study is developed as a quick and convenient tool that can be generally applied to irradiated FCC-based materials due to their common crystallography. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The elastic-to-plastic transition in single crystal α-silicon nitride was experimentally characterized through a series of nanoindentation experiments using a spherical indenter. The experimental results provide a quantitative description of the critical shear strengths for the transition, as well as estimates of the shear modulus and nanohardness of the material. [ABSTRACT FROM AUTHOR]

Whereas exceptional mechanical and radiation performances have been found in the emergent body-centered cubic (BCC) refractory multi-principal element alloys (RMPEAs), the importance of their complex atomic environment, reflecting diversity in atomic size and chemistry, has been largely unexplored at the atomic level. Here, we adopt a local structure approach based on the atomic pair distribution function measurements in combination with density functional theory (DFT) calculations to investigate a series of BCC RMPEAs. Our results demonstrate that all the analyzed RMPEAs exhibit local lattice distortions (LLD) to some extent, but a severe LLD, a breakdown of the 15% atomic size difference in Hume-Rothery rules, occurs only in the Zr- and/or Hf-containing RMPEAs. In addition, through the DFT calculations we show that charge transfer among the elements profoundly reduces the size mismatch effect in average to stabilize this energy-unfavorable severe LLD. The observed competitive coexistence between LLD and charge transfer demonstrates the importance of the electronic effects on the local environments in RMPEAs. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Chemically-biased diffusion creates Ni segregation near mobile defect sinks in Ni-Fe alloys. • Segregation profiles near voids and interatitial loops suppress their interaction with vacancies. • Interstitial loops grow while void nuclei collapse into Ni-rich precipitates in the damage region. • Ni segregation on voids and interstitial loops lead to Ni depletion of the matrix between sinks. • Vacancies migrate out and form voids beyond the damaged area, inaccessible to SIAs. Recent irradiations of Ni-Fe concentrated solid solution alloys have demonstrated significant improvement of radiation performance. This improvement is attributed to redistribution of the alloying elements near sinks of point defects (voids, dislocations) due to chemically-biased atomic diffusion, where vacancies have preference to migrate via Fe atoms and interstitials via Ni atoms. In Ni-Fe, all sinks are enriched by Ni atoms, which strongly affects further interactions of radiation-produced mobile defects with voids and dislocations, hence void growth and dislocation climb. Ni-decorated sinks interact stronger with interstitial atoms than vacancies, which enhances dislocation loops growth. At the same time, Ni segregation creates Fe-enriched "channels" for vacancy migration out of the damage region to agglomerate in the outer regions, inaccessible to interstitial atoms. Strong effect of chemically-biased diffusion is supported by transmission electron microscope characterization and calls for special attention in designing alloys with desired properties through tuning defect mobilities. [ABSTRACT FROM AUTHOR]

A comprehensive and integrated review of the hypergravity field applied to solidifying alloys is provided. The hypergravity field refers to an environment in which the acceleration of gravity is more significant than the gravitational acceleration through rotation, resulting in significant changes in the solidification behavior of the alloy. The convection mode of the melt is closely related to the level and direction of hypergravity. Obvious floatation or sedimentation of solutes and crystals of different densities is induced. Columnar-to-equiaxed transition, grain refinement, and gradients in composition and microstructure are readily available under hypergravity. Furthermore, the nucleation and growth of crystals and their migration under enhanced convection are gradually revealed through simulations and in-situ observations. This review aims to summarize the universal law of the effect of the hypergravity field on the composition distribution and microstructure evolution of alloys during solidification. Based on the development of hypergravity equipment and characterization methods, it is expected to broaden the theoretical framework of hypergravity solidification. To provide a theoretical reference for developing new solidification processes and high-performance materials. [Display omitted] • A comprehensive review on the principles and development of hypergravity technology is presented. • Microstructure evolution, segregation, and mechanisms are provided during alloy solidification under hypergravity. • Point out the unsettled issues and desirable future work in hypergravity solidification. [ABSTRACT FROM AUTHOR]

Understanding the oxidation of Ni-based single crystal (SX) superalloys is significantly important because oxidation can damage the microstructure and eventually lead to failure of the alloy. Using advanced scanning/transmission electron microscopy, oxidation effects on the microstructure of a SX superalloy during heat treatment and simulated service (stress/temperature coupling) conditions are systematically investigated in order to reveal oxide microstructures and oxidation mechanisms. Heat treatment in argon results in less weight loss, thinner γ′-free layer and fewer internal oxidation than that treated in air. But the oxide layer structures and influence depths of oxidation in both atmospheres are similar. External tensile stress not only accelerates the oxidation of alloy but also affects the oxide microstructures. Moreover, stress effects on the oxidation microstructure depend on temperature. At 750 °C, Ni–Co oxides are formed on the alloy surface, followed by the inner layer of Ni–Co–Mo–Re–W oxides at the location of original γ phase under stress free condition. As the stress increases, an oxide layer mainly containing Al, Cr, Nb, Mo and Re elements is formed between the above two oxide layers. When temperature increases to 1050 °C, as applied tensile stress increases, the thickness of oxide layer increases but the structure of oxide layer is not obviously changed. [ABSTRACT FROM AUTHOR]

To understand the underlying strengthening mechanisms, thermal activation processes are investigated from stress-strain measurements with varying temperatures and strain rates for a family of equiatomic quinary, quaternary, ternary, and binary, face-center-cubic-structured, single phase solid-solution alloys, which are all subsystems of the FeNiCoCrMn high-entropy alloy. Our analysis suggests that the Labusch-type solution strengthening mechanism, rather than the lattice friction (or lattice resistance), governs the deformation behavior in equiatomic alloys. First, upon excluding the Hall-Petch effects, the activation volumes for these alloys are found to range from 10 to 1000 times the cubic power of Burgers vector, which are much larger than that required for kink pairs (i.e., the thermal activation process for the lattice resistance mechanism in body-center-cubic-structured metals). Second, the Labusch-type analysis for an N-element alloy is conducted by treating M-elements (M < N) as an effective medium and summing the strengthening contributions from the rest of N-M elements as individual solute species. For all equiatomic alloys investigated, a qualitative agreement exists between the measured strengthening effect and the Labusch strengthening factor from arbitrary M to N elements based on the lattice and modulus mismatches. Consequently, the Labusch strengthening factor provides a practical critique to understand and design such compositionally complex but structurally simple alloys. [ABSTRACT FROM AUTHOR]

To reveal if universal rules may exist in face centered cubic (FCC) concentrated solid solution (CSS) alloys for strength and plastic deformation at room and cryogenic temperatures, we select a FCC CSS superalloy Haynes 188 to demonstrate the deformation mechanisms from atomic to micrometer scales. In FCC CSS alloys, the yield strength (YS) is intrinsically determined by atomic/modulus mismatches and affected extrinsically by grain size; the tensile strength and elongation to fracture (EF) are governed by the plastic process where low stacking fault energy is beneficial. Moreover, almost all CSS alloys show that YS and EF increase simultaneously with the decrease in the temperature. Our findings may expand alloy application and can be used as a design strategy for stronger and tougher alloys. [ABSTRACT FROM AUTHOR]

Oxidation behaviors of three Ni-based model alloys and pure Ni in the temperature range of 700–1200 °C are investigated to reveal effects of Cr, Al, and Si on the oxidation resistance of Ni-based superalloys. The formation and integrity of consecutive chromia or alumina layers are important for excellent oxidation resistance. The addition of 20 at.% Cr can effectively improve the oxidation resistance of Ni-based alloys by forming a thin chromia film below 1000 °C, while adding 15 at.% Al has a beneficial effect on the oxidation resistance of Ni-based alloys at temperatures above 900 °C. The addition of 2 at.% Si to Ni-Al alloy is insufficient to form a protective SiO2 layer but can accelerate the formation of alumina, which enables Ni-Al alloy to form a consecutive inner alumina layer at a relatively low temperature of 800 °C and further improve the oxidation resistance above 800 °C. [ABSTRACT FROM AUTHOR]

The alloying effects on the microstructure, especially precipitation of harmful phases and solvus temperature of L1 2 -structured γʹ phase, etc., are essential for alloy design and development of precipitation strengthened Co-based superalloys. Based on the newly reported Co30Ni9Al5Mo10Cr2Ta0.03B alloy, alloying elements Mo, Cr, Al, Ta and Ni are systematically changed and alloys are fabricated to investigate their microstructures in two heat treatment statuses, namely homogenization and aging, respectively. Results show that Mo and Cr are γ forming elements and have limited effect on γʹ solvus temperature, but high concentration of Mo and Cr facilitates the formation of harmful μ phase. Al, Ta, and Ni are all γʹ stabilizers and can effectively increase γʹ solvus temperature, but roles of Al, Ta and Ni are different. Increasing Al and Ta would result in other harmful phases, such as the μ phase in the 6Ta and 4Ta alloys, and μ + B2 phases in the 13Al and 11.5Al alloys. However, Ni addition significantly increases solvus temperature and area fraction of γʹ phase without occurrence of harmful phases. The current findings might provide guidelines for design and development of precipitation strengthened Co-based alloys. [Display omitted] • The alloying effects of Mo, Cr, Ta, Al & Ni are systematically investigated. • Mo & Cr are γ formers, which facilitate the formation of μ phase. • Ta is a strong γˊ former, but excessive Ta facilitates the formation of TCP phase. • Al is necessary for γˊ phase, but excessive Al results in B2 & TCP phases. • Ni stabilizes γˊ phase without forming the harmful phases in Co-based alloys. [ABSTRACT FROM AUTHOR]

• Pd/Al/Cu were added to NiCoCrFe to study their effect on ion irradiation resistance. • Copper addition to NiCoFeCr led to Cu-rich precipitates during ion irradiation. • Cu-rich precipitates dramatically improved the irradiation resistance. • L1 2 precipitates from Al addition were not as effective as Cu precipitates. • A model was developed to explain the trend in void swelling during ion irradiation. Commonly studied equatomic single-phase FCC high entropy alloys based on 3d transition metals like NiCoFeCr do not provide adequate strength and radiation resistance at high doses for nuclear structural applications. In the current study, the major alloying effects like lattice distortion, ordering and clustering tendencies were investigated by adding low concentration of Pd, Al, or Cu respectively to study the doping effects on the ion irradiation response of NiCoFeCr alloy. The alloys were irradiated with 3 MeV Ni2+ ions at 500 °C to a fluence of 1 × 1017/cm2 at a beam flux of approximately 2.8 × 1012 ions/cm2/s. The microstructural evolution upon irradiation i.e., formation of dislocation networks, radiation induced segregation and precipitation, and void formation were studied in detail. Post-irradiation characterization results showed that a Pd addition leads to a high void nucleation rate but controlled void growth, which may be attributed to increased lattice distortion. In Al added HEA, our microstructural analysis indicates that radiation induced ordered L1 2 precipitates do not affect void swelling significantly. Cu addition led to Cu precipitation that drastically suppressed dislocation density and void swelling of the alloy. Additionally, a model was developed to qualitatively describe the trend in void swelling of typical FCC alloys under ion irradiation. This model was able to qualitatively explain the suppression and reappearance of void swelling in ion irradiated alloys that generally occurs near the region with peak implanted ion concentration. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: This work developed thermodynamic models for describing phase stability and thermodynamic property of the Fe–Cr–Zr system using the Calphad approach coupled with experimental study. Thermodynamic descriptions of the Fe–Cr and Cr–Zr systems were either directly adopted or slightly modified from literature. The Fe–Zr system has been remodeled to accommodate recent ab-initio calculation of formation enthalpies of various Fe–Zr compounds. Reliable ternary experimental data and thermodynamic models were mainly available in the Zr-rich region. Therefore, selected ternary alloys located in the vicinity of the eutectic valley of β(Fe,Cr,Zr) and (Fe,Cr)2Zr laves phase in the Fe-rich region have been experimentally investigated in this study. Microstructure has been examined by using scanning electron microscope, energy-dispersive X-ray spectroscopy and X-ray diffraction. These experimental results, along with the literature data were then used to develop thermodynamic models for phases in the Fe–Cr–Zr system. Calculated phase equilibria and thermodynamic properties of the ternary system yield satisfactory agreements with available experimental data. [Copyright &y& Elsevier]

[Display omitted] • Elastic moduli of the investigated for high entropy alloys (HEAs) and electronic structure and interatomic bonding have been calculated using OLCAO. • NiFeCoCr has the highest shear modulus, Young modulus, and Vickers hardness compared to other calculated HEAs. Cantor alloy has the larger bulk modulus and total bond order density. • Interatomic analysis reveals the local lattice distortion of HEAs showing the least contribution from Cr-Cr and Pd-Pd bonds. • Clustering HEAs have lower energy in agreement with experimental observation. Exceptional properties of high entropy alloys (HEAs) are attributed to the disordered random solid solution of multiple alloying elements. Despite numerous studies, the fundamental understanding at the electronic and atomic levels is still missing. We report a comparative study of four fcc HEAs of NiFeCoCr and NiFeCoCrX (X = Mn, Cu, or Pd) based on ab initio calculations using large supercells with 500 atoms in equal composition. After fully optimizing their structures using the VASP package, their electronic structure, interatomic bonding, partial charge distribution, and mechanical properties are calculated and compared, revealing the intricate interdependence among them. A novel parameter based on the quantum mechanical metric for internal cohesion, the total bond order density (TBOD), is used to interpret the calculated properties. The highest TBOD is found in Cantor alloy NiFeCoCrMn but lower in NiFeCoCrPd. The atomic radii vary depending on their local chemical environment. The resulting lattice distortions is validated experimentally in NiFeCoCrMn and NiFeCoCrPd. Moreover, modeling of Cu and Pd clustering in the supercell shows they have lower total energy in agreement with the observation of Cu-enhanced nano-participates in NiFeCoCrCu and Pd-induced concentration wave in NiFeCoCrPd HEAs. [ABSTRACT FROM AUTHOR]

Abstract: Nanoindentation creep tests were conducted on nanocrystalline Cu, Cu–2.3Al and Cu–7.2Al (at.%) alloys at room temperature. The stress exponents were calculated from the loading curves. Results show that the stress exponents are much higher than those in the diffusional creep process, ranging from 5 to 50. The stress exponents increase with the indentation load increasing. However, for Cu–Al alloys, the stress exponents are very high even at low loads. The mechanism for the room temperature creep is discussed in the framework of the dislocation dynamics. Grain boundaries may play an important role in the creep behavior. However, there is no clear correlation among the stress exponent, the alloy composition and the grain size. The dislocation histories and solute atoms may affect the creep behaviors, i.e. both the generation and the recovery of dislocations. [Copyright &y& Elsevier]

Elemental specific chemical complexity is known to play a critical role in microstructure development in single-phase concentrated solid-solution alloys (SP-CSAs), including both He bubble formation and irradiation-induced void swelling. While cavity formation and evolution under ion irradiation at elevated temperature are complex nonequilibrium processes, chemical effects are revealed at the level of electrons and atoms herein in a simplified picture, using Ni and a special set of Ni-based SP-CSAs composed of 3 d transition metals as model alloys. Based on Ni and the model alloys with minimized variables (e.g., atomic mass, size, and lattice structure), we discuss the effects of chemically-biased energy dissipation, defect energetics, sluggish diffusion, and atomic transport on cavity formation and evolution under both self-ion Ni irradiation and He implantation. The observed difference in microstructure evolution is attributed to the effects of d electron interactions in their integrated ability to dissipate radiation energy. The demonstrated impact of alloying 3 d transition metals with larger differences in the outermost electron counts suggests a simple design strategy for tuning defect properties to improve radiation tolerance in structural alloys. Image, graphical abstract [ABSTRACT FROM AUTHOR]

To better understand defect structure and the evolution of irradiation-induced damage in single crystal Ni, in situ Rutherford backscattering spectroscopy in channeling geometry (RBS/C) is performed along the <100>, <110>, and <211> axes with different probing beam energies. The RBS/C data reveal that damage evolution occurs in three steps. The first step at low doses (up to 0.2 dpa), characterized by a linear increase with dose, is related to the formation of point defects and small clusters. The second step in the intermediate dose range (0.2–1.0 dpa) shows a sublinear increase in disorder to saturation. This sublinear increase is due to the growth of defect clusters resulting from the interaction of irradiation-induced defects with already existing damage from previous ion impacts. The third step at high doses (1.0–32.4 dpa) exhibits a surprising decrease in the disorder level, which may be attributed to defect evolution from black spot defects to large dislocation loops that leads to strain relaxation. In addition, the damage extends much deeper than the predicted depths and is attributed to long-range defect migration effects confirmed by transmission electron microscopy (TEM) observations. Although similar damage evolution trends have been observed along all channeling directions, the disorder accumulation is largest along the <100> and <211> axes than observed along <110> axis. This "preferential" disordering process along <100> and <211> suggests that more defects are shielded by the <110> atomic rows than the two other axes. The co-existence of both uncorrelated displaced lattice atoms and dislocation loops in Ni irradiated at 1 ion/nm2 is revealed by the energy-dependent RBS/C studies along all three axes. In contrast, dislocation loops and stacking fault tetrahedral are simultaneously present in the crystal structure of Ni irradiated at 100 ions/nm2 that is consistent with previous molecular dynamics simulations and TEM observations. Image 1 • RBS/C show that disordering kinetics exhibit three different regions of defect formation along all the axes studied. • Both uncorrelated displaced lattice atoms and dislocation loops co-exist in Ni irradiated at low doses. • Dislocation loops and SFT are simultaneously present in the crystal structure of Ni irradiated at high doses. • Additional information on the nature of defects can be obtained using multi-axial and multi-energy ion channeling analysis. [ABSTRACT FROM AUTHOR]

Medium entropy alloy NiCoCr draws great attention due to its excellent strength-ductility trade-off mechanical behavior. Its irradiation behavior at elevated temperatures has been investigated using ion beam irradiation in a temperature range of 420–580°Cand transmission electron microscopy. Irradiation induced stacking fault tetrahedra were only observed at 420 °C. With increasing irradiation temperature, all stacking fault tetrahedra vanished, while the size of voids and dislocation loops increased significantly. Nanoindentation-induced structural complexities, including dislocations, stacking faults and twins helped to reduce void swelling. However, at the elevated temperatures, NiCoCr is still much more susceptible to void swelling compared to high entropy alloys such as NiCoFeCrMn and NiCoFeCrPd. Image 1 [ABSTRACT FROM AUTHOR]

The evolution of radiation-induced defects in Ni and the single-phase concentrated solid solution alloy, NiCoCr, were investigated during in situ 2.8 MeV Au ion irradiation and post-irradiation analysis using transmission electron microscopy. Compared to Ni, both the size and area density of defect clusters decreased in NiCoCr under the same irradiation conditions, suggesting that the chemical complexity, i.e., randomness of lattice site occupations, of NiCoCr suppressed radiation-induced damage. One-dimensional glide of defect clusters was observed in Ni but not in the NiCoCr alloy. The structural nature of small defect clusters in NiCoCr were further investigated using high-angle annular dark field scanning transmission electron microscopy. • Smaller and fewer defects were generated in more chemically-complicated alloys. • One-dimensional gild of defect clusters was observed in Ni but not in NiCoCr. • Both vacancy-type and interstitial-type defect clusters were identified in NiCoCr. [ABSTRACT FROM AUTHOR]

Advanced single-crystal superalloys, developed to improve the high temperature capability of gas turbines, often underperform in low-temperature conditions. This limitation compromises the overall reliability of turbine blades under complex service environments. This study introduces a novel strategy to enhance the overall performance of a fourth-generation single-crystal superalloy. Rapid liquid-nitrogen quenching following the aging treatment promotes extensive nucleation of ultrafine γ′ particles (γ′ γ) with an average size of ∼10 nm, precipitated under extremely low supersaturation, as evidenced by high-resolution transmission electron microscopy (HRTEM) and atom probe tomography investigations. The combination of differential scanning calorimetry analysis, density functional theory calculations, and kinetic models verifies the high stability of γ′ γ particles at low-temperature conditions around 900 °C. HRTEM observations reveal Orowan bypassing and dislocation cutting of γ′ γ particles, leading to additional precipitation hardening by limiting dislocation motion in the γ phase and shear of primary precipitates. Consequently, a bimodal precipitation microstructure comprising γ′ γ particles and primary γ′ phase significantly reduces the strain rate of low-temperature creep and nearly doubles the creep rupture life at 800 °C/735 MPa. Simultaneously, high-temperature properties at 900 °C/392 MPa and 1100 °C/137 MPa remain comparable to those of the original microstructure as the primary γ/γ′ microstructure is minimally affected by rapid cooling. This advancement improves overall performance and redefines the importance of small precipitates, broadening microstructure design concepts for future superalloy development. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Temperature significantly affects defect migration and evolution in irradiated materials. However, the effects of temperature on defect evolution in concentrated solid-solution alloys (CSAs), including high entropy alloys, are not well understood, despite their potential as structural materials in advanced nuclear reactors. As an important model system of these CSAs, equiatomic Ni 50 Fe 50 (NiFe) was selected to understand the effects of temperature on defect evolution during irradiation and subsequent thermal annealing. Specifically, defect accumulation and evolution in NiFe alloy under Ni-ion irradiation at 150, 300, and 500 K were studied, and the irradiated specimens were subsequently annealed at higher temperatures. Rutherford backscattering spectrometry along the <100> channeling direction was employed to study damage accumulation and evolution before and after each irradiation and annealing experiment. Here we show that more defects survive and accumulate at 150 K, but more importantly defects can migrate to deeper depths at this low irradiation temperature. Irradiation-induced damage at 150 and 300 K does not recover substantially after post-irradiation annealing at 500 K, but dramatic recovery is observed after post-irradiation annealing at 700 K, indicating an onset temperature of defect recovery between 500 and 700 K. The migration of irradiation-induced defects upon annealing is closely related to the mobility and stress state arising from the surviving defects. With the consideration of five stages of defect recovery in conventional dilute alloys, the underlying mechanisms for temperature-dependent defect accumulation and evolution in NiFe CSA are discussed. Graphical abstract Image 1 Highlights • Temperature effects on defect evolution in NiFe have been systematically studied. • Defects formed at lower irradiation temperature migrate to deeper depth. • Defect recovery is closely related with the size and nature of defects. • Defect evolution in NiFe is compared with defect recovery in diluted alloys. [ABSTRACT FROM AUTHOR]

Dislocation loops in NiCo and NiFe, both being single-phase, equimolar solid solution alloys, are generated by in situ high-voltage electron irradiation and characterized inside a transmission electron microscope. In addition to the different defect growth rates, the two alloys present dislocation loops with distinct shape evolution and element segregation, which reflect the element-sensitive, intrinsic properties (e.g., defect energetics, lattice distortion, and local ordering) of the alloys. These results provide evidence that the structure and dynamics of radiation-induced defects in concentrated alloys depend on not only the number but also the species of alloying elements. Unlabelled Image [ABSTRACT FROM AUTHOR]

A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as 'hard turning'. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ∼50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material. Image 1 [ABSTRACT FROM AUTHOR]

Technologically important mechanical properties of engineering materials often degrade at low temperatures. One class of materials that defy this trend are CrCoNi-based medium- and high-entropy alloys, as they display enhanced strength, ductility, and toughness with decreasing temperature. Here we show, using in situ straining in the transmission electron microscope at 93 K (−180 °C) that their exceptional damage tolerance involves a synergy of deformation mechanisms, including twinning, glide of partials and full dislocations, extensive cross-slip, and multiple slip activated by dislocation and grain-boundary interactions. In particular, massive cross-slip occurs at the early stages of plastic deformation, thereby promoting multiple slip and dislocation interactions. These results indicate that the reduced intensity of thermal activation of defects at low temperatures and the required increase of applied stress for continued plastic flow, together with high lattice resistance, play a pivotal role in promoting the concurrent operation of multiple deformation mechanisms, which collectively enable the outstanding mechanical properties of these alloys. [ABSTRACT FROM AUTHOR]

Abstract Ion channeling is a powerful quantitative technique for studying ion-irradiation induced defect evolution in single crystalline materials. An iterative procedure to determine dechanneling yields has been developed for decades, serving as a major method for analyzing experimental channeling data. The applicability of such procedure is, however, generally limited to the crystalline damage with only point defects and local amorphous domains. For the other cases, such as irradiated metals, the assumption of direct-backscattering free has usually been made. In the present study, Ni, TiAl, MgO, and SrTiO 3 single crystals are selected as four model materials, representing metals, intermetallic alloys, and ceramic compounds with different defect evolution processes under irradiation, to investigate the fidelity of applying dechanneling analysis on various types of defects. The pure dechanneling assumption is shown oversimplified in Ni irradiated with low fluence self-ions and may result in error on the derived damage profile. Moreover, the iterative procedure of dechanneling analysis is shown valid for more general situations than the randomly distributed atoms, including those not exhibiting a peak in channeling spectra. The disappearance of damage peak in channeling spectra is attributed to the combined effects of small (but non-zero) scattering factor, long-range damage effects, and non-ignorable damage level in pristine crystals. Furthermore, the ratio of direct backscattering to dechanneling areas provides information on defect configurations in the materials containing a well-defined damage peak in channeling spectra. The contribution from dechanneling sources increases from SrTiO 3 , TiAl, to MgO, according to the derived scattering and dechanneling factors. Highlights • Dechanneling analysis is valid for materials containing various types of defects. • Pure dechanneling assumption in irradiated metals is oversimplified. • Lack of damage peak does not mean lack of direct-backscattering sources. • Information on defect types can be obtained from dechanneling analysis. [ABSTRACT FROM AUTHOR]

• Ionization from energetic charged particles affects pre-existing defect configurations. • Electronic energy dissipation leads to defect annealing and growth of small clusters. • The pronounced effect observed in complex alloys is attributed to slower energy dissipation. • The impact of electronic energy deposition on damage evolution should to be considered. The effect of energy deposition by energetic particles on Ni and two single-phase concentrated solid solution alloys (NiFe and NiCoCrFe) is investigated through combined experimental and modelling efforts. Damage evolution as a function of increasing ion fluence is monitored via elastic strain developed in the irradiated crystals. We show that damage produced from displacement collision cascades is sensitive to subsequent highly ionizing irradiation that the strain generated by elastic nuclear collisions undergoes partial relaxation upon high-energy irradiation. This finding indicates a change in the damage structure upon electronic energy deposition due to both predominant defect annealing and growth of small defect clusters. Strain relaxation, more pronounced in the alloys than in Ni, is ascribed to both higher thermal conductivity and weaker electron-phonon coupling in Ni. [ABSTRACT FROM AUTHOR]

Abstract Binary single-phase concentrated solid solution alloys (SP CSAs), including Ni 80 Co 20 , Ni 80 Fe 20 , Ni 80 Cr 20 , Ni 80 Pd 20 , and Ni 80 Mn 20 (in atomic percentage), were irradiated with 200 keV He+ ions at 500 °C. He cavity size and density distribution were systematically investigated using transmission electron microscope. Here we show that alloying elements have a clear impact on He cavity formation. Cavity size is the smallest in Ni 80 Mn 20 but the largest in Ni 80 Co 20. Alloying elements could also substantially affect cavity density profile. In-depth examination of cavities at peak damage region (∼500 nm) and at low damage region (∼300 nm) demonstrates that cavity size is depth (damage) dependent. Competition between consumption and production of vacancies and He atoms could lead to varied cavity size. Density functional theory (DFT) calculations were performed to obtain the formation and migration energies of interstitials and vacancies. Combined experimental and simulation results show that smaller energy gap between interstitial and vacancy migration energies may lead to smaller cavity size and narrower size distribution observed in Ni 80 Mn 20 , comparing with Ni 80 Co 20. The results of this study call attention to alloying effects of specific element on cavity formation and defect energetics in SP CSAs, and could provide fundamental understanding to predict radiation effects in more complexed SP CSAs, such as high entropy alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Using ion irradiation and transmission electron microscopy, temperature dependent void swelling and dislocation loop evolution in equiatomic NiCoFeCrMn and NiCoFeCrPd high entropy alloys were investigated. Voids were observed in all tested conditions. An order higher of swelling was observed in both HEAs as temperature increased from 420 to 580 °C. Alloying with Pd was found to have a stronger suppression effect on void and dislocation loop growth than alloying with Mn. Possible contributions to the swelling resistance were explored, including localized lattice distortion and higher defect migration barrier in NiCoFeCrPd. Additionally, no phase decomposition was observed in either alloy. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Transition from long-range one-dimensional to short-range three-dimensional migration modes of interstitial defect clusters greatly reduces the damage accumulation in single-phase concentrated solid solution alloys under ion irradiation. A synergetic investigation with experimental, computational and modeling approaches revealed that both the resistance to void swelling and the delay in dislocation evolution in Ni-Fe alloys increased with iron concentration. This was attributed to the gradually increased sluggishness of defect migration, which enhances interstitial and vacancy recombination. Transition from long-range one-dimensional defect motion in pure nickel to short-range three-dimensional motion in concentrated Ni-Fe alloys is continuum, not abrupt, and within an iron concentration range up to 20%. The gradual transition process can be quantitatively characterized by the mean free path of the interstitial defect clusters. [ABSTRACT FROM AUTHOR]

The influence of hydrogen on the mechanical response of an FCC equi-molar solid-solution alloy, FeNiCoCr, was investigated. Hydrogen caused a reduction in ductility in terms of total elongation of only a few percent in the FeNiCoCr alloy, but the local reduction in area after necking was reduced, and the fracture mode transitioned from transgranular ductile microvoid coalescence to predominantly intergranular. This result demonstrates unequivocally that this equi-molar alloy is susceptible to hydrogen embrittlement. Specifically, hydrogen reduced the elongation of the grains in the loading direction, increased the out-of-plane distortion of grains on the specimen surface, increased the orientation deviations with respect to the mean of individual grains, and induced a more advanced microstructural state in the form of dislocation tangles and dislocation cells. Hydrogen transport by dislocations caused a redistribution of the hydrogen increasing the hydrogen concentration on grain boundaries. Hydrogen accommodated in dislocation cell walls and on dislocations, effectively locked the microstructure in a specific configuration that inhibited strain transfer across grain boundaries. Consequently, an incompatibility constraint across grain boundaries developed. Together these effects explain why the hydrogen-charged alloy did not exhibit necking and transitioned to a grain boundary failure mode. [ABSTRACT FROM AUTHOR]

One of the biggest challenges in the radiation induced defect science is to understand the complex nature of ion-atom interactions under highly extreme conditions. Here, we report the irradiation induced non-equilibrium defect formation in NiCoCr single phase concentrated solid solution alloy due to (i) the extreme inelastic and (ii) the coupled inelastic and elastic ion-atom interactions. These two conditions are achieved at 5 and 30 μm penetration depths along the paths of swift heavy ions (1.542 GeV Bi). In general, the irradiation induced damage consists of interstitial-type dislocation loops and vacancy-type stacking fault tetrahedra (SFT). Near the surface (~5 μm) where electronic energy loss is dominating (~62.5 keV nm −1 ), the atomic motion primarily results in the formation of SFT. A noticeable increase of dislocation loop formation is observed at 30 μm near the maximum energy deposition from elastic interactions (~4.9 keV nm −1 ), as compared to the near surface region (~0.06 keV nm −1 ). Insights on the complex electronic and atomic correlations of extreme energy deposition and dissipation on defect dynamics and structural stability may pave the way for new design principles of radiation–tolerant structural alloys. [ABSTRACT FROM AUTHOR]

Amorphous and nanograined (NG) steels are two categories of strong steels. However, over the past decade, their application has been hindered by their limited plasticity, the addition of expensive alloying elements, and processing challenges associated with producing bulk materials. Here, we report that the surface of a carburized Fe-Mn-Si martensitic steel with extremely low elemental alloying additions can be economically fabricated into an amorphous-nanocrystalline hybrid structure. Atom probe tomography and nanobeam diffraction of a hard turned steel surface together with molecular dynamics (MD) simulations reveal that the original cementite surface structure experiences a size-dependent amorphization and phase transformation during heavy plastic deformation. MD simulations further show that the martensite-cementite interface serves as a nucleation site for cementite amorphization, and that cementite can become disordered if further strained when the cementite particles are relatively small. These graded structures exhibit a surface hardness of ∼16.2 GPa, which exceeds the value of ∼8.8 GPa for the original nanocrystalline martensitic steel and most nanocrystalline steels reported before. This practical and cost-efficient approach for producing a hard surface with retained bulk ductility and toughness can provide expanded opportunities for producing an amorphous-crystalline hybrid structure in steels and other alloy systems. [ABSTRACT FROM AUTHOR]

The effect of alloying elements on radiation-induced microstructural evolution in Ni and Ni-20X (X = Fe, Cr, Mn and Pd) binary alloys was investigated using ion irradiation and cross-sectional transmission electron microscopy. The three-dimensional migration mode is identified to be the dominating migration mechanism for interstitial clusters in these binary alloys, contrary to the one-dimensional mode that dominates in the well-studied dilute alloys. The results reveal that: (1) the average size of defect clusters decreases as the solute atomic volume size factor increases. Smaller void size in Ni-20Cr is attributed to faster vacancy mobility in the near surface region, and weaker vacancy binding energy beyond the irradiation peak than Ni-20Fe. The smaller voids observed in Ni-20Mn and Ni-20Pd beyond the damage peak are due to the stronger Mn/Pd-vacancy binding effect of largely oversized solute atoms. (2) Oversized solutes can act as strong trapping sites for interstitials. The larger the solute atomic volume factor, the stronger the trapping force. This leads to a more significantly sluggish interstitial migration and smaller dislocation loop size. The average dislocation loop size in Ni-20Fe was four times larger than Ni-20Pd (atomic volume factor being 10.6% and 41.3%) but an order of magnitude lower in density. The smaller dislocation loop size in Ni-20Cr is attributed to stronger interstitial binding of Cr-Ni. Overall, the alloying effect on defects is more significant in concentrated binary alloys than in dilute binary alloys, due to the concentration difference of alloying atoms and the interstitial dominant migration mechanisms in the main irradiated region. [ABSTRACT FROM AUTHOR]

Concentrated solid solution alloys have attracted rapidly increasing attention due to their potential for designing materials with high tolerance to radiation damage. To tackle the effects of chemical complexity in defect dynamics and radiation response, we present a computational study on swift heavy ion induced effects in Ni and equiatomic Ni -based alloys (Ni 50 Fe 50 , Ni 50 Co 50 ) using two-temperature molecular dynamics simulations (2T-MD). The electronic heat conductivity in the two-temperature equations is parameterized from the results of first principles electronic structure calculations. A bismuth ion (1.542 GeV) is selected and single impact simulations performed in each target. We study the heat flow in the electronic subsystem and show that alloying Ni with Co or Fe reduces the heat dissipation from the impact by the electronic subsystem. Simulation results suggest no melting or residual damage in pure Ni while a cylindrical region melts along the ion propagation path in the alloys. In Ni 50 Co 50 the damage consists of a dislocation loop structure (d = 2 nm) and isolated point defects, while in Ni 50 Fe 50 , a defect cluster (d = 4 nm) along the ion path is, in addition, formed. The simulation results are supported by atomic-level structural and defect characterizations in bismuth-irradiated Ni and Ni 50 Fe 50 . The significance of the 2T-MD model is demonstrated by comparing the results to those obtained with an instantaneous energy deposition model without consideration of e-ph interactions in pure Ni and by showing that it leads to a different qualitative behavior. [ABSTRACT FROM AUTHOR]

Quantitative analysis of the impact of the compositional complexity in a series of Ni-containing concentrated solid-solution alloys, Ni, NiCo, NiFe, NiCoCr, NiCoFeCr, NiCoFeCrMn and NiCoFeCrPd, on the evolution of defects produced by 1 MeV Kr ion irradiation at 773 K is reported. The dynamics of the evolution of the damage structure during irradiation to a dose of 2 displacements per atom were observed directly by performing the ion irradiations in electron transparent foils in a transmission electron microscope coupled to an ion accelerator. The defect evolution was assessed through measurement of the defect density, defect size and fraction of perfect and Frank loops. These three parameters were dependent on the alloying element as well as the number of elements. The population of loops was sensitive to the ion dose and alloy composition as faulted Frank loops were observed to unfault to perfect loops with increasing ion dose. These dependences are explained in terms of the influence of each element on the lifetime of the displacement cascade as well as on defect formation and migration energies. [ABSTRACT FROM AUTHOR]

Limitations of strength and formability are the major obstacles to the industrial application of magnesium alloys. Here, we demonstrate, by producing the duplex phases and fine intermetallic particles in composition-optimized superlight Mg-Li-Al alloys, a unique approach to simultaneously improve the comprehensive mechanical properties (a strength-ductility balance). The phase components and microstructures, including the size, morphology, and distribution of precipitated-intermetallic particles can be optimized by tuning the Li content, which strongly influences the work-hardening behavior and tension-compression yield asymmetry. [ABSTRACT FROM AUTHOR]

The evolution of irradiation-induced atomic strain in an equiatomic NiFe concentrated solid-solution alloy is investigated using both atomistic simulations and x-ray diffraction analysis. The irradiations are performed using 1.5 MeV Ni ions to fluences ranging from 1 × 10 13 to 1 × 10 14 cm − 2 . The irradiation simulations are carried out by overlapping 5 keV Ni recoils cascades up to a total of 300 recoils. An increase of volumetric strain is observed at low dose, which is associated with production of point defects and small clusters. Relaxation of strain occurs at higher doses, when large defect clusters, e.g., dislocation loops, dominate. [ABSTRACT FROM AUTHOR]

Understanding the effects of chemical complexity from the number, type and concentration of alloying elements in single-phase concentred solid-solution alloys (SP-CSAs) on defect dynamics and microstructure evolution is pivotal for developing next-generation radiation-tolerant structural alloys. A specially chosen set of SP-CSAs with different chemical complexity (Ni 80 Fe 20 , Ni 80 Cr 20 and Ni 40 Fe 40 Cr 20 ) are investigated using 1.5 MeV Mn ions over a wide fluence range, from 2 × 10 13 to 1 × 10 16 ions cm −2 at room temperature. Based on an integrated study of Rutherford backscattering spectroscopy in channeling geometry and molecular dynamics simulations, the results demonstrate that Ni 40 Fe 40 Cr 20 is more radiation tolerant than Ni 80 Fe 20 , Ni 80 Cr 20 and elemental Ni in the low fluence regime. While chemical complexity of this set of SP-CSAs is clearly demonstrated to affect defect evolution through suppressed defect production and enhanced recombination at early stages, the effect of the mixed ferro- and anti-ferromagnetic interactions is not the only controlling factor responsible for the improved radiation performance. The observed strong alloying effect on defect evolution is attributed to the altered defect migration mobilities of defect clusters in these alloys, an intrinsic characteristic of the complex energy landscapes in CSAs. [ABSTRACT FROM AUTHOR]

For hexagonal close-packed (HCP) Hf, Mg and Ti single crystals, indentation Schmid factors, defined as the ratio of maximum resolved shear stress to the maximum contact pressure, were calculated for three typical families of slip systems, including basal 〈 a 〉 , prismatic 〈 a 〉 , and second pyramidal 〈 c + a 〉 slip systems. The dependence of indentation Schmid factors on the crystallographic orientation shows the same hard/soft directions as in the uniaxial tests, but with much reduced variations of magnitudes. The axial ratio, c / a , leads to noticeable effects on the indentation Schmid factors only when the indentation direction deviates from the c -axis by more than about 30°, as shown by the comparison among these three crystals. These predictions are compared to nanoindentation pop-in tests on Mg single crystals in literature and Hf single crystals conducted in this work. The elastic-plastic transition under spherical nanoindentation can be quantitatively measured by a sudden displacement burst (or pop-in) in the load-displacement curve. For defect-free single crystals, the first pop-in results from homogeneous dislocation nucleation, and correspondingly the maximum resolved shear stress (as calculated from pop-in load and indentation Schmid factor) reaches the theoretical strength of the material. Pop-in loads were found to be low for 〈 10 1 ¯ 2 〉 indentation and high for orientations close to the c -axis. While previous experiments in NiAl showed excellent agreement between indentation Schmid factor and pop-in measurements, Mg and Hf works here do not have such an agreement. It is suggested that each of the three sets of slip systems may govern a different range of indentation directions, or stress components other than resolved shear stress dictate the incipient plasticity. [ABSTRACT FROM AUTHOR]

As one of the core effects on the high-temperature structural stability, the so-called 'sluggish diffusion effect' in high-entropy alloy (HEA) has attracted much attention. Experimental investigations on the diffusion kinetics have been carried out in a few HEA systems, such as Al-Co-Cr-Fe-Ni and Co-Cr-Fe-Mn-Ni. However, the mechanisms behind this effect remain unclear. To better understand the diffusion kinetics of the HEAs, a combined computational/experimental approach is employed in the current study. In the present work, a self-consistent atomic mobility database is developed for the face-centered cubic (fcc) phase of the Co-Cr-Fe-Mn-Ni quinary system. The simulated diffusion coefficients and concentration profiles using this database can well describe the experimental data both from this work and the literatures. The validated mobility database is then used to calculate the tracer diffusion coefficients of Ni in the subsystems of the Co-Cr-Fe-Mn-Ni system with equiatomic ratios. The comparisons of these calculated diffusion coefficients reveal that the diffusion of Ni is not inevitably more sluggish with increasing number of components in the subsystem even with homologous temperature. Taking advantage of computational thermodynamics, the diffusivities of alloying elements with composition and/or temperature are also calculated. These calculations provide us an overall picture of the diffusion kinetics within the Co-Cr-Fe-Mn-Ni system. [ABSTRACT FROM AUTHOR]

High-Nb-containing TiAl alloys have been considered an excellent candidate for demanding high-temperature structural components due to their attractive mechanical properties and low density. However, the intrinsic brittleness at room temperature remains a great barrier limiting its processability. Here a novel alloy design strategy was introduced to improve its ductility from the perspective of grain refinement and removal of oxygen within the matrix. A minor content of LaB 6 was introduced into Ti-45Al-8Nb alloy prepared by vacuum arc melting. La 2 O 3 and TiB ceramic phases were in-situ synthesized owing to the decomposition of LaB 6 additive. The effect of LaB 6 addition on the microstructure and compressive properties of TiAl-based alloys was investigated. The LaB 6 inoculation not only refines the lamellar colonies but also promotes the generation of single γ phases, contributing to the transformation from coarse full-lamellar to refined near-lamellar and even dual-phase colonies. The major fracture mode transformed from inter-lamellar to trans-lamellar to inter-granular with LaB 6 addition increasing. With 0.4 wt% LaB 6 addition, both the ultimate compress strength and fracture strain increased from 1065 MPa and 17.1 % to 1503MPa and 26.98 % respectively. • The (TiB+La 2 O 3)/Ti-45Al-8Nb composites were in situ fabricated by vacuum arc melting Ti, Al, Nb and LaB 6 mixed particles. • The effect of LaB 6 on the lamellar colonies size and phase constitution of Ti-45Al-8Nb was identified. • The compress strength of the composites increased by refined lamellar colonies and TiB precipitates. • An optimal addition of 0.4 wt% LaB 6 was preferred with maximum strength and fracture strain. [ABSTRACT FROM AUTHOR]

For further composition optimization, the microstructure evolution of the fourth-generation nickel-based single crystal superalloys, which contains different Co (Cobalt) additions, is studied at three thermal exposure temperatures. Based on Lifshitz, Slyozov and Wagnerz (LSW) theory and trans-interface diffusion-controlled (TIDC) theory, the effects of Co content and aging temperatures on coarsening rate of γ′ phase are investigated in detail. The results show that the diffusion coefficient of solute is accelerated, but the γ/γ′ interfacial energy decreases with the addition of Co, thus changing the coarsening rate. Furthermore, the thermodynamic equilibrium solubility difference of elements between γ′ and γ, and the segregation of elements at the γ/γ′ interface are alleviated with increase of Co, resulting in the suppression of TCP phase after long-time thermal exposure, which guides the economical design of nickel-based single crystal superalloys. • Co additions have a dual effects on microstructure evolution of the superalloys. • The effect of Co on coarsening rate of γ′ depends on temperature. • The accuracy of two coarsening prediction models is tested. [ABSTRACT FROM AUTHOR]