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In this work, interlayer (Cr vs amorphous CuZr) with various thicknesses ranging from 5 nm to 40 nm were utilized for 1000 nm-thick nanostructured Cu films on polyimide substrates. The effects of heterogeneous interface and interlayer thickness on the fatigue behavior of Cu/interlayer bilayers were investigated by in situ synchrotron X-ray diffraction, electrical resistance measurement, in combination with microstructural examinations. The results demonstrated that the fatigue tolerance highly depends on the thickness and constituent of interlayer. The fatigue lifetime monotonically increases as CuZr interlayer thickness increases, coupled to the yield stress. In contrast, the fatigue lifetime first increases and then decreases with increasing Cr interlayer thickness, reaching its maximum value at a thickness of 5 nm. The underlying mechanisms for this discrepancy can be elucidated in terms of heterogeneous constraint and interface voiding, both of which depend on the interlayer thickness. [ABSTRACT FROM AUTHOR]

Abstract: Three agglomerated ammonium molybdate precursors were treated with the same continuous processing, containing such as calcinating, deoxidating, pressing, sintering and deep working. The effects aggregation extent of the precursors on Mo powder preparation and working property of the sintered samples were studied. The results showed that the aggregation of the precursors possesses certain heredity. Agglomerated Mo powder was obtained from the agglomerated precursor. The calcination and deoxidation were delayed obviously because of the aggregation. Increasing of the aggregation extent led to the decreased fluidity of Mo powder, causing the density gradient in performs. The increased aggregation of Mo powder caused the decreased sintering activity and the density of sintered products, as well as the worsen ductility and the mechanical properties. [Copyright &y& Elsevier]

A metastable β-titanium Ti–Al–Mo–V–Cr–Nb alloy exhibits a synergistic increasing in strength and ductility via coupling thermomechanical processing and heat treatment in present work. Synergistic effect of the thermomechanical processing and heat treatment on microstructure evolution and mechanical properties of the metastable β-Ti alloy was systematically investigated. α+β rolled sample sheet consumes less time to finish recrystallization, and leads to more uniformly distribution and smaller size of β grains than β rolled sample after same solution treatment. Primary α phase in α+β rolled sample hinders the growth of β grains during solution treatment and the spacing distance of α lamella in α+β rolled sample is smaller than β rolled sample after aging treatment, enhancing the strength significantly. The small spacing distance of α lamella is due to an increase in precipitation density. The small size and homogeneous distribution of β grains provides moderate ductility. After solution treatment at 770 °C for 2 h and aging treatment at 525 °C for 6 h, α+β rolled samples exhibit higher ultimate tensile strength (1550 MPa) and better ductility (6.5 %) than β rolled sample. The relationship between the average grain size of β grains, the average spacing distance of α lamella, and the yield strength of the alloy is quantitatively calculated using mathematical methods. The mechanisms of synergistic enhancement of strength and ductility in the alloy is qualitatively analyzed. These findings facilitate manufacturing high-performance metastable β-Ti alloys at an industry scale using commercially available thermomechanical processing. [ABSTRACT FROM AUTHOR]

We report the existence of secondary hardening behavior in Ti–10V–2Fe–3Al (wt.%) (Ti-1023) for the first time. Through controlling the ageing temperature window between 550 °C and 575 °C, alloys are found to show the existence of two hardness peaks with aging time. This heat treatment with secondary hardening phenomenon exhibits unusual increase of hardness and strength. Further experimental observations show that the first hardness peak corresponds with the well precipitated α phase at very short time. Further increase of aging time does not influence the morphology of these intragranular α precipitates, but accompanying with a continuous composition change, which could be the origin of the secondary hardness peak. This structure change followed by composition diffusion is the typical characteristic of pseudospinodal mechanism. Phase field and ab-initio calculations based on pseudospinodal mechanism analyze the detailed microstructure/composition evolution and related solid solution strength, which confirm the role of composition on the appearance of secondary hardening behavior. [ABSTRACT FROM AUTHOR]

Abstract The effects of ω precipitates and orthorhombic (α″) martensitic transformation on stress-strain behavior were studied in metastable β Ti-10V-2Fe-3Al alloy single-crystal in submicron scales. We successfully distinguished the size effects induced by ω precipitates and α″ phase transformation through compressing the metastable β Ti-10V-2Fe-3Al along the orientation of <011> β , which is unfavorable for α″ phase transformation. Two individual exponential curves express well the relationship between the strengthening exponents, which were regressed using the flow stresses at different ranges of plastic strains of 0.2–13% and the pillar widths, and the corresponding plastic strain. It is worth noting that the strengthening exponent jumps from 0.256 to 0.293 at 8% plastic strain. Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars. The first (ω precipitate-related) size effect has been predicted with the proposed models, and the second (martensitic transformation-induced) size effect is interpreted with the modified Liu's equation. [ABSTRACT FROM AUTHOR]

Abstract Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along <011> β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity. [ABSTRACT FROM AUTHOR]

Abstract The effects of pre-deformation on the precipitation behaviors and properties in Cu-Ni-Si-Cr alloy were investigated. The results showed the electrical conductivity and the strength of the alloys after peak aging increased with the pre-deformation amount. Quantitative analyses demonstrated the deformation-induced defects improved the precipitation and the number density of precipitates but reduced their average radii. As a result, the conductivity improved due to the slight promotion of precipitation while the strength increased mostly from the enhanced precipitate strengthening and then from the unremoved work hardening. At elevated temperatures, the effects of pre-deformation were greatly weakened and then the difference among the alloys after different treatments disappeared. [ABSTRACT FROM AUTHOR]

A commercial Cu-Ni-Si-Cr alloy used as a model material was annealed and then peak-aged. The detailed microstructure and property analyses were performed to determine the correlations between the microstructure characteristics and properties. Theoretical calculations, which gave results in good agreement with experimental results, demonstrated that solid-solution scattering and Orowan bypass of precipitate strengthening were the key mechanisms for electrical and mechanical properties, respectively. Furthermore, the effects of composition on the aging behaviors and then electrical and mechanical properties for Cu-Ni-Si series alloys were systematically investigated, showing that both the precipitation extent and mechanical properties improved simultaneously with nominal or effective concentration of the precipitates. Quick estimation on the basis of the present analysis procedure indicates that our work is applicable with high accuracy in real cases. [ABSTRACT FROM AUTHOR]

In present work, the localized deformation, crack initiation and propagation behavior of Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) titanium alloy was investigated through the in-situ scanning electron microscope (SEM) test and electron backscatter diffraction (EBSD). The results indicate that deformation in α phase is mostly accommodated by the prismatic slip. The localized deformation is concentrated at the interface of primary α lath. Some parallel slip bands with prismatic and pyramidal slip are prevalent in primary α lath, while the multiple slip bands with prismatic slip are dominant in equiaxed primary α phase. Higher ability to coordinate local plastic strain (nearly to 79.5%) is induced in equiaxed α than the primary α lath (7.9%). Most of the cracks easily initiate at the shear bands in primary α lath, which is a fatal site for the tensile deformation. The main crack propagates quickly along the shear bands or interfaces of α lath and then penetrates into the β trans matrix and also transfers across the equiaxed α phase along the slip bands. The results indicate that the high compatibility of deformation in equiaxed primary α phase delays the initial crack nucleation. Whereas, it does not show satisfied resistance against crack propagation in front of the main crack tip due to the abundant slip bands. [ABSTRACT FROM AUTHOR]

The effect of precipitates on fracture behaviors was comparatively investigated among three kinds of Mg alloys with different precipitates, i.e., Mg-Gd alloy with prismatic plate-shaped precipitates, Mg-Zn alloy with [ 0001 ] α rod-shaped precipitates, and Mg-Gd-Zn-Zr alloy with basal plate-shaped precipitates. By comparing the fracture behaviors of the alloys before and after aging treatment, it was evident that the presence of precipitates greatly promoted the formation of microvoids that were initiated at primary intermetallic particles. Voiding was the most accelerated in the Mg-Gd alloy, where the precipitate-dislocation interactions are the strongest. While in the Mg-Gd-Zn-Zr alloy with the weakest precipitate hardening, the precipitate-facilitated voiding was the least significant. The precipitate-dependent voiding suppressed the twinning behaviors, causing the volume fraction of deformation twins decreased in turn from Mg-Gd-Zn-Zr to Mg-Zn, and finally to Mg-Gd alloys. The tensile ductility of present Mg alloys approximately scaled with the volume fraction of deformation twins, and was highly sensitive to the precipitates. The fracture scenario of present Mg alloys was proposed that the voiding suppressed twinning and the competition between voiding and twinning can be mediated by precipitates. In the Mg-Gd-Zn-Zr alloy with limited voiding, the twinning dominated the deformation process and concomitantly resulted in a great ductility. The formation of microvoids or the fracture of primary intermetallic particles was quantitatively analyzed by applying a Weibull model, where both the alloy strength and the volume fraction of primary intermetallic particles were considered to rationalize the remarkable difference in fracture behaviors among the present three Mg alloys. Furthermore, the coupling contribution of precipitates and twins to the work hardening was modelled in the aged Mg-Gd-Zn-Zr alloy that displayed a large room temperature tensile ductility of ~ 13.5%. [ABSTRACT FROM AUTHOR]

Cyclic plastic deformation, slip characteristics and crack nucleation in Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) with different morphologies of equiaxed and lamellar α phase were systematically analyzed during high-cycle fatigue. The heterogeneous plastic deformation could take place within different α morphologies during high-cycle fatigue even though the cyclic stress amplitude is much less than yield strength. Slip is the dominant deformation mode in the equiaxed primary α, while the slip and ( 10 1 ¯ 1 ) deformation twin are prevalent in the primary α lath. Interactions between slip, twin and interface result in ledges at the primary α lath interface. The relationship between cyclic slip irreversibility, accumulated irreversible strain, and fatigue life is established. A critical parameter, accumulated irreversible strain per area in the crack initiation region (region I), was calculated to be (8.1 ± 2) × 10 −4 μm −2 for initiating fatigue crack. Fatigue cracks will nucleate when the accumulated irreversible strain exceeds the critical value. The primary α lath is the dominant site for crack initiation. The cracks initiate and propagate in interface and slip band, and easily connect each other in the primary α lath. In comparison, most cracks lie within an individual or occupy several equiaxed α phases and often cease in front of the phase boundary, which delays the connection of microcracks. It indicates that the primary α lath is more detrimental than the equiaxed primary α phase during high cycle fatigue. [ABSTRACT FROM AUTHOR]

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be ≥ 400 °C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 ° to 400 °C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 °C (free of DSA), while complex dislocation structure in the samples ratcheted at ≥ 400 °C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 °C to 600 °C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior. [ABSTRACT FROM AUTHOR]

A long-standing conflict concerning metastable β -titanium (Ti) alloys is that high strength can be achieved through precipitation strengthening of α -phase while plasticity is largely compromised and even lost completely. Such strength-plasticity trade-off severely limits their structural applications. Here, an attempt to address the issue has been made in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β -Ti alloy. Heterogeneous β -structure is architected by partial static recrystallization (SRX) through properly manipulating solution treatment in the hot-rolled bars. The resulting microstructure is composed of the recrystallized 'new' equiaxed β -grains (β e) and the remnant elongated β -fibers (β f). This β -morphology dramatically affects the subsequent α s -precipitation behavior during the aging process. α s -precipitation is rather uniform throughout β f -grains because the pre-existing dislocations left by hot rolling provide copious nucleation sites. However, α GB -films are preferentially formed along β e -grain boundaries, which leads to a boundary-affect zone (BAZ) and thereby renders α s -precipitation in a size gradient from β -grain boundaries to grain interiors. As a result, an ultra-high strength of ∼1700 MPa is achieved whilst the ductility can be up to be 5% in the β -hetero aged samples, which is superior to the β -homo aged siblings that have the same strength level but fully succumb to macroscopic brittleness. The enhanced plasticity in the β -hetero aged samples originates from much better deformation compatibility conferred by the aged β f -grains. However, the soft feature of BAZ around β e -grain boundaries in the β -homo aged samples causes strain localization, which eventually induces intergranular brittle fracture. These findings provide an effective strategy for enhancing plasticity of high-strength metastable β -Ti alloys by structural design, where homogeneous α s -precipitates can be easily constructed by combining conventional thermomechanical processing and heat treatment. • Strength-ductility trade-off is evaded by a new structural design in Ti6554 alloy. • Enhanced ductility of ∼5% together with high strength of ∼1700 MPa is achieved. • Heterogeneous β -structure is architected by partial static recrystallization. • α -precipitation is rather uniform whilst α GB -film is effectively suppressed. • The improved ductility is conferred by superior deformation compatibility. [ABSTRACT FROM AUTHOR]

Refractory multi-principal element alloys (MPEAs) have attracted much attention as promising candidate materials for structural applications at elevated temperatures. Several studies have demonstrated that edge dislocation takes an important role in controlling high-temperature strength in refractory MPEAs. Here in order to understand the contribution of edge dislocation to strength, we have studied the effect of local lattice distortion on the core structure of edge dislocation in NbMoTaW MPEAs and the subsystems using atomistic simulations. We found the magnitude of lattice distortion is strongly dependent on the constituent elements and their concentrations. The lattice distortion reaches the maximum at around equiatomic composition. Binary alloys with large atomic size mismatch have a lattice distortion comparable with ternary and quaternary systems. We then have analyzed the core structure of 1/2<111>/{110} edge dislocation in NbMoTaW MPEAs and the subsystems in the framework of Peierls-Nabarro model. The dislocation core half-width decreases with increasing lattice distortion exponentially. As a consequence, the critical shear stress for dislocation motion normalized by shear modulus exhibits a linear increase as the lattice distortion increases. This may help understand the linear correlation of yield strength normalized by Young's modulus with lattice distortion in various refractory MPEAs reported in experiments. Our present work provides insights into the origin of the essential contribution of edge dislocation to strength in refractory compositional complex alloys. [ABSTRACT FROM AUTHOR]

Although the evolution of dislocation substructure during ratcheting deformation has been previously studied in 316LN austenitic steel, studies on ratcheting-induced twinning and de-twinning behaviors are very scarce. In this paper, microstructural evolutions, especially the twinning/de-twinning behaviors, were systematically investigated in a 316LN austenitic steel under ratcheting cycling. Different peak stresses (320, 400, 450, and 500 MPa) were deliberately designed to demonstrate a potential correlation between the twinning/de-twinning and the dislocation pattern. The experimental results showed that, at the low peak stress of 320 MPa, annealing twins were quite stable and the dislocation pattern mainly consisted of planar dislocation arrangement. Raising the peak stress to 400 MPa, most of the annealing twins were gradually de-twinned and the dislocation pattern displayed a different feature prevailed by dislocation veins, walls and cells. As the peak stress reaches 450 MPa and in particular up to 500 MPa, the annealing twins get quickly de-twinned, while stacking faults and deformation twins were induced besides the dislocations. The critical stress to trigger deformation twins in the 316LN austenitic steel was evaluated to be around 500 MPa, which is in broad agreement with the experimental results. This indicates that the nucleation of deformation twins under ratcheting cycles should be controlled by the peak stress rather than the accumulation strain. An additional cycling hardening stage was observed in the ratcheting cycling response under the peak stress of 450 and 500 MPa, which should be closely related to the formation of stacking faults and deformation twins. Moreover, the stress-dependent de-twinning of annealing twins were discussed in terms of dislocation interaction with annealing twins. [ABSTRACT FROM AUTHOR]

As an efficient strategy to design accident-tolerant fuel (ATF) materials, nanostructured metallic multilayers (NMMs) have attracted significant research interest. In this work, Cr 75 Al 25 /Zr (CA/Zr) NMMs were prepared with equal individual layer thickness h spanning from 5 to 75 nm, and subjected to He+ implantation with a total fluence of 1 × 1017 ions cm−2 at room temperature. With decreasing h , the interface changed from crystalline/crystalline to crystalline/amorphous along with size-driven strengthening to softening behavior of CA/Zr NMMs at the critical thickness h = 25 nm. The size-dependent hardness consists well with the evolution of the back stress with respect to h , which is elucidated by the statistical dislocation absorption propensity by the interface. Moreover, notable irradiation hardening emerges in the CA/Zr NMMs, which is caused by He bubble-dislocation interactions at large h and by the cooperation of He bubble hardening and increased back-stress hardening at small h. The hardness of both as-deposited and irradiated CA/Zr NMMs was quantitatively explained by dislocation models operative at different length scales. [ABSTRACT FROM AUTHOR]

Microstructures and mechanical properties of nickel-based superalloy (GH783 alloy) bolts used in a power plant 660 MW Ultra supercritical unit were investigated. The results showed that the material's strength increased monotonically but the elongation and impact toughness both decrease as an exponential decay trend with the service time. Microstructural examinations revealed that γ′ phase within the alloy matrix coarsened significantly leading to an enhancement in strength. Meanwhile, additional γ′ precipitated gradually within the grain-boundary β phase, causing the weakening of the grain boundaries, thus a reduction in ductility. The evolution tendency here obtained could be used to approximately predict the variation tendency of mechanical properties with the service time for GH783 alloy used as bolt material. [ABSTRACT FROM AUTHOR]

Deformed coarse-grained polycrystalline metals always unload elastically where permanent dislocation network well-developed in the loading regime hinders the movement of dislocations and allows only the elastic relaxation of stress. Such elastic unloading behavior is, however, unexpected in nanocrystalline metals because the dislocation network cannot effectively form inside nanometer-scale grains. In this work, we report the experimental finding of significant plastic deformation that emerges in the unloading regime in the compressive cyclic test at room temperature of nanocrystalline Cu. The magnitude of plastic strain produced during unloading depends strongly on loading and unloading rates. This plastic unloading behavior arises from the rapid absorption of dislocations accumulated during loading, which was quantitatively interpreted by performing the incremental unloading test and developing a relationship between the dislocation density and the loading and unloading rates based on the models of the statistical absorption of dislocations by grain boundaries and the dislocation emission from grain boundary ledges. Concurrently, the evolution of deformation structures during the cyclic deformation was also analyzed in terms of the interactions of gliding dislocation–twin boundaries. [ABSTRACT FROM AUTHOR]

A significant sputtering power effect on the microstructure and mechanical properties was investigated in the nanocrystalline Cr coatings that were prepared by employing magnetron sputtering. Experimental results demonstrated that all the Cr coatings exhibited a columnar grain structure with preferred (110) out-of-plane orientation. However, with raising the sputtering power from 50 to 250 W, the surface morphology changed from the faceted pyramid to vermiform-like shape due to the anisotropy of growth rate along different crystallographic plane. Meanwhile, raising sputtering power also increased the grain size and coating thickness, between of which followed a power-law relationship, indicative of an evolutionary overgrowth mechanism. In addition, both the nanoindentation hardness and elastic modulus of Cr coatings exhibited a monotonic increment with reducing the grain size. This size-dependent strengthening behavior was rationally interpreted by the classical Hall-Petch theory, and a grain coalescence model for columnar grain morphology was developed to quantitatively describe the size effect on elastic modulus of nanocrystalline Cr coatings. [ABSTRACT FROM AUTHOR]

Stress induced nanoscale martensitic transformation (SINMT) as well as its effect on mechanical behaviors have been investigated for Ti2448 single crystal in compression along [100] orientation by decreasing sample size to micron-to nano scales which possess high stress due to "smaller, stronger" in metals to trigger SINMT. The transformation process of β (BCC)→O′ (Orthorhombic)→α" (Orthorhombic) involving { 011 } 0 1 ‾ 1 β shuffle followed by { 2 1 ‾ 1 } 11 1 ‾ β shear was directly observed by high resolution transmission electron microscopy (HRTEM). Real-time recording of phase transition by in-situ HRTEM in 90 nm pillar clearly reveals that this SINMT with a critical stress 1206 MPa is high-order-like (continuous) and reversible. Its competition and interaction with dislocation avalanche exhibited a strong size-dependence upon uniaxial compression, inducing a transition from dislocation avalanche to SINMT with decreasing of the pillar size from 2.5 μm to 90 nm, evidenced by scanning electron microscopy (SEM) and HRTEM. According to the results from uniaxial compression, scanning electron microscopy (SEM) and HRTEM, deformation behaviours and mechanical properties of Ti2448 pillars ranging from 2.5 μm to 90 nm exhibit strong size-dependence due to the competition and interaction between the dislocation avalanche and the SINMT. Owing to the "smaller, stronger" size effect, Ti2448 sub-micron pillars possess high stress to induce plenty of nanoscale α" martensites during loading which can effectively impede dislocation avalanche. Ti2448 sub-micron pillars (d < 1 μm) deform in homogenous mode and show an excellent combination of ultrahigh strength (1635 MPa) and plastic stability. By contrast, in micron scale (d ≥ 1 μm) dislocation avalanche dominates the deformation, leading to the plasticity instability. • Mechanical properties and deformation behavior of Ti2448 alloy in micro-to nanoscale is investigated. • Stress-induced nanoscale α" martensitic transformation (SINMT) was recorded by in-situ HRTEM observation. • Phase transition from β (BCC).→ O' (Orthorhombic) → α" (Orthorhombic)was directly observed by HRTEM. • Ultrahigh stress with good plastic stability is achieved by SINMT in sub-micron pillar. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) have attracted considerable attention for their high damping capacities. Here we investigate the damping behavior of Ti 50 (Pd 50− x D x ) SMAs (D=Fe, Co, Mn, V) by dynamic mechanical analysis. We find that these alloys show remarkably similar damping behavior. There exists a sharp damping peak associated with the B2–B19 martensitic transformation and a high damping plateau ( Q −1 ~0.02–0.05) over a wide ambient-temperature range (220–420 K) due to the hysteretic twin boundary motion. After doping hydrogen into the above alloys, a new relaxation-type damping peak appears in the martensite phase over 270–360 K. Such a peak is considered to originate from the interaction of hydrogen atoms with twin boundaries and the corresponding damping capacity ( Q −1 ~0.05–0.09) is enhanced by roughly twice that of the damping plateau for each alloy. Moreover, the relaxation peaks are at higher temperatures for the TiPd-based alloys (270–370 K) than for the TiNi-based alloys (190–260 K). We discuss the influence of hydrogen diffusion, mobility of twin boundaries and hydrogen–twin boundary interaction on the temperature range of the relaxation peak. Our results suggest that a martensite, with appropriate values for twinning shear and hydrogen doping level, provides a route towards developing high damping SMAs for applications in desired temperature ranges. [ABSTRACT FROM AUTHOR]

Different size effects and deformation mechanisms are revealed in α-titanium (Ti) single crystal nanopillars orientated for [112̄0] and [0001] based on molecular dynamics simulations. The strength–size relationship changes from “smaller is stronger” to “smaller is much weaker” when the width of nanopillars reduces from 19 nm to 3 nm. The “smaller is much weaker” is attributed to that the surface effect caused by a thermal vibration of surface atoms leads to the initiation and growth of surface dislocations. [ABSTRACT FROM AUTHOR]

Abstract: Effect of the initial grain size on plastic deformation behavior and grain refinement mechanism has been investigated in Ti–2Al–2.5Zr alloy subjected to multi-impact process. The results show that grain refinement is dominated by dislocation slip in the fine-grained sample. In contrast, deformation twinning becomes prevalent in the coarse-grained one, and contributes to grain refinement by means of twin subdivision and fragmentation. Dynamic recrystallization (DRX) takes place in both samples. But, it is delayed in the coarse-grained one due to relative low fraction of grain boundaries and energy dissipation of deformation twinning. Nevertheless, the final grain size in the refined area does not show an obviously difference between the two samples at a large strain, i.e., 1.61. Moreover, grain refinement is inhomogeneous in both samples. The heterogeneity in the coarse-grained sample is much stronger than that in the fine-grained one. It was ascribed to the superimposed effect of large initial grain size and subsequent deformation twinning. A criterion for achieving an optimum grain refinement was finally suggested. [Copyright &y& Elsevier]

Abstract: Thermal stability of the nanocrystalline Ti–10V–2Fe–3Al alloy produced by cold-forging and corresponding hardness variations were investigated. When the annealing temperature was lower than 400°C, the primary features were the decline of lattice defect density and the increase of lattice ordering level in the vicinity of grain boundaries, named as grain boundary relaxation. The nanocrystalline grains grew slowly. In comparison, when the annealing temperature was higher than 400°C, the decomposition of the deformation-induced α″ martensites and the subsequent rapid grain growth were observed. Vickers hardness measurement shows an initial hardening followed by a softening with the increase of annealing temperature. The hardening peak appeared at 400°C. The critical grain growth temperature, i.e., T cgg, in the nanocrytalline Ti–10V–2Fe–3Al alloy, was slightly lower than those in other nanocrystalline alloys. That could be attributed to the martensitic decomposition-induced grain growth in this alloy. [Copyright &y& Elsevier]

Abstract: Microstructural evolution in primary α phase-containing Ti–10V–2Fe–3Al alloy subjected to cold forging under different applied strains was studied. Experimental results showed that, even at a strain of 0.1, stress-induced α″ martensites were abundantly produced within the β matrix, resulting in alternative α″/β lamellae. Shear bands initiated and grew across α″/β lamellae as the strain increased to 0.35. When the strain increased to 1.2, the volume fraction of shear bands significantly increased and the grains were almost occupied by the shear bands. Interestingly, nanocrystallines were observed inside shear bands. While in the primary α phase, slip was always the predominant plastic deformation mode and dislocations were accumulated to a high density within the strain range from 0.1 to 0.35. When the strain was up to 1.2, the dislocation density was further increased and α/β interface boundary became ill-defined. However, no grain refinement was observed in the α phase. The microstructure refinement in the β matrix could be attributed to that stress-induced α″ martensitic transformation promoted the initiation, thickening and coalescence of shear bands. The plastic deformation combined with martensitic phase transformation could provide a potential effective technique to produce nanocrystalline materials. [ABSTRACT FROM AUTHOR]

The size dependent hardness and strain-rate sensitivity of magnetron sputtered nanocrystalline Mo thin films with film thickness h ranging from 250 to 2000 nm were systematically investigated by using nanoindentation tests. Microstructure examinations manifested that all the as-deposited Mo thin films had a columnar-nanograined hierarchical structure with highly (110)-oriented growth texture, and the lateral columnar size went through a maximum of ~60 nm at h = 1500 nm sample with h. Interestingly, both the hardness and strain-rate sensitivity exhibited a monotonic increment with reducing the columnar size. The underlying strengthening and deformation mechanisms could be interpreted in light of the confined slip of dislocations in the columnar grains, rather than the thermal activation of screw dislocations. [ABSTRACT FROM AUTHOR]

Metastable β -titanium (Ti) alloys have been extensively used in aerospace industry owing to the outstanding mechanical properties. Temperature arising sometimes becomes unavoidable in service and thus it is necessary to evaluate plastic behavior of the alloys at elevated temperatures prior to applications. Here, plastic stability of metastable β -titanium alloys containing the β- spinodal decomposition structures at different temperatures is investigated for the first time using Ti–6Cr–5Mo–5V–4Al alloy as prototypical material. It is found that a serrated plastic flow with little frequency accompanied by overall strain softening appears in the tensile stress-strain curves at 298 K, while the serrated plastic flow occurs much more frequently and overall strain hardening capability is recovered appropriately when the tensile temperature is increased to 573 K. Microstructural characterizations reveal that ordinary dislocation slip dominates the alloy plasticity at all the testing temperatures. Dislocations strongly interact with the β- striations and dislocation debris are left around the striation interfaces. The resulting local stress accumulation destroys the β- striations in the form of shear bands so that sudden release of an avalanche of dislocations in the soft channels leads to stress drop. Subsequent dynamic reconstruction of the β- striations with the assistance of thermal activation especially at elevated temperature hardens the prior soft channels, which results in the following stress rise. The striking difference on deformation characteristics at 298 K and 573 K is that more homogeneous plasticity conferred by dense deformation bands at the latter, which is the direct reason on high serrated frequency and overall strain hardening. These findings offer valuable insights into plastic behavior of metastable β -Ti alloys, which will have important implications for the applications. • Plastic stability in β- Ti alloys containing spinodal structures is firstly studied. • A serrated plastic flow appears at the tensile temperatures of 298 K and 573 K. • Dislocations-spinodal structure interactions result in serrated plastic flow. • A new mechanism regarding plastic flow fluctuation is proposed. [ABSTRACT FROM AUTHOR]

The functional properties of shape memory alloys are strongly sensitive to the composition of alloys. In this study, the effect of Ti/Ni ratio and Hf/Zr ratio on the martensitic transformation behavior, shape memory effect and microscopic structures of two different quaternary TiNiHfZr alloys (Ti x Hf 15 Zr 5 Ni 80-x and Ti 31.5 Hf y Zr 20-y Ni 48.5) have been investigated systematically. It is found that increasing Ti/Ni ratio of Ti x Hf 15 Zr 5 Ni 80- x alloys dramatically increases the martensitic transformation temperature and a maximum value (5%) of the recovered strain during shape recovery on heating is obtained for the alloy with x = 30. On the contrary, changing Hf/Zr ratio of Ti 31.5 Hf y Zr 20- y Ni 48.5 alloys does not result in big change of martensitic transformation temperature or recovered strain. The evolution of macroscopic properties of Ti x Hf 15 Zr 5 Ni 80- x alloys with x can be understood by considering the balanced effect between the Ni content change of the matrix, and the volume fraction of Ti 2 Ni-like precipitates, which are often brittle and not desired for SMAs. Our results suggest that the Ti x Hf 15 Zr 5 Ni 80-x (x > 30.5 at. %) alloys are promising shape memory alloys for high temperature applications above 250 °C. [ABSTRACT FROM AUTHOR]

Face-centered cubic (FCC) high-entropy alloys (HEAs) strengthened by coherent L1 2 -nanoparticles exhibit an excellent strength-ductility balance. However, the strength of previously studied HEAs remains inadequate for the requirements of high-performance structural applications, due to their emphasis on coherency strengthening and the suppression of the formation of incoherent precipitates, which could substantially increase the alloy strength but also leads to significant brittleness. In this study, we propose to employ incoherent precipitates as additional strengthening phases to further improve the mechanical performance of conventional L1 2 -strengthened FCC HEAs without dramatic loss of their ductility. We achieve this using a prototypical (FeCoNiCr) 89 Ti 6 Al 5 (at.%) HEA, in which high-density, fine, and incoherent L2 1 precipitates were introduced and uniformly distributed at the recrystallized grain boundaries through proper thermomechanical processes, including large cold deformation, full recrystallization, and aging heat treatment. A superb combination of strength and ductility in the alloy is confirmed by the uniaxial tensile tests, with a yield strength of 1136 MPa, an ultimate tensile strength of 1597 MPa, and a ductility of 25.3%. This superior mechanical response is caused by the synergistic contribution of the fine and uniformly distributed L2 1 particles and the ultra-ductile and damage-tolerant FeCoNiCr matrix, which are responsible for increasing strength and maintaining ductility, respectively. These findings demonstrate the viability of a new method of using incoherent precipitates to strengthen other FCC HEAs by properly tuning their particle size and distribution. [ABSTRACT FROM AUTHOR]

Abstract: Pure molybdenum and molybdenum alloys doped with two- or three-components rare earth oxide particles were prepared by powder metallurgy. Both the tensile property and fracture toughness of the pure molybdenum and multi-components rare earth oxide-doped molybdenum alloys were determined at room temperature. The multi-components rare earth oxide-doped molybdenum alloys are fine grained and contain a homogeneous distribution of fine particles in the submicron and nanometer size ranges, which is why the molybdenum alloys have higher strength and fracture toughness than pure molybdenum. Quantitative analysis is used to explain the increase in yield strength with respect to grain size and second phase strengthening. Furthermore, the relationship between the tensile properties and microstructural parameters is quantitatively established. [Copyright &y& Elsevier]

In this work, gradient structure is successfully produced in the surface of FCC structured (Fe 40 Mn 40 Co 10 Cr 10)C 3.3 high entropy alloy (HEA) by using surface mechanical rolling treatment (SMRT). The depth of surface gradient layer with higher hardness increases with increasing the SMRT passes. The introducing of gradient layer not only significantly improves the strength of HEAs but also remains sufficient ductility. The ultra-high strength of the gradient structured HEA is mainly related to the synergetic strengthening mechanism, while the comparatively good ductility is ascribed to the back stress strain hardening mechanism. Gradient structure design provides a novel strategy for producing HEAs with high performance. [ABSTRACT FROM AUTHOR]

A novel metastable β-Ti alloy based on Ti–Al–Zr–Mo–Cr system was successfully designed and fabricated. Superior combination of high strength (UTS = 1534-1198 MPa) and ductility (El = 7.0%–20.9%) is achieved in this alloy. After high reduction β transus forging and subsequent heat treatment, a multi-scale distribution of α phase is displayed which contains micro-scale elongated primary α (α p), sub-micro α rod (α r) and nano-sized α platelets (α s). Detailed selected area electron diffraction (SAED) pattern and high resolution-TEM analysis show the production of FCC substructure inside α p and α r with an orientation relationship of <0001> hcp //<001> fcc , {11 2 ‾ 0} hcp //{ 2 ‾ 20} fcc , {10 1 ‾ 0} hcp //{220} fcc. Two variants of FCC lamella are discovered which is related by a 120° rotation around <001> axis. The effect of microstructure on the improved strength-ductility combination is briefly discussed. Both FCC lamella and multi-scale distribution of α phase could create an effective strain partition during plastic deformation and increase the ductility. High strength originates from the distribution of high fraction nano-scaled α s which could effectively hinder dislocation slip in β matrix. [ABSTRACT FROM AUTHOR]

The tensile properties and crack initiation of Ti-55531 alloy were studied by tuning the microstructure from lamellar microstructure (LM) to bimodal microstructure (BM). The results indicate that the mechanical properties change from ultimate tensile strength of 1287 MPa with zero plasticity to ultimate tensile strength of 1310 MPa and ductility of ∼10%. Further transmission electron microscopy and scanning electron microscopy characterization show that the bimodal microstructure have shown a coarsen primary α surrounded by ultra-fine α precipitation, which could be the origin of combination of strength and ductility. TEM images after loading indicate that multiple slip systems have been activated in primary α of BM. Plastic strain localized within primary α due to the low strength, which results about 83.2% voids initiation within or at boundary of primary α. The cracks propagate not only along the interface between primary α and matrix, but also along slip bands in primary α in BM, which results in transgranular fracture. The results show that the micron-scale α phase is necessary to accommodate plastic strain and bring about ductility for β alloys with ultra-fine α precipitation. [ABSTRACT FROM AUTHOR]

As a novel high strength metastable β alloy, Ti–5Al–4Zr–8Mo–7V, it is essential to establish the correlation between its microstructure and mechanical property. In this paper, the microstructure evolution, mechanical properties and work hardening behavior with Bi-modal structure were studied in detail. With the increase of solution treatment (ST) temperature, the volume fraction of primary α phase (α p) decreases, and α platelets (α s) increases during subsequent aging treatment. Aging temperature directly determines the size and volume fraction of α s which significantly influences the strength of the alloy. Low temperature aging (510 °C) treatment produces a high density nano-scale distribution of α s precipitates which induces an extremely high ultimate tensile strength (UTS~1630 MPa) together with considerable total elongation (El~6%). As the ageing temperature increases to 630 °C, the UTS reduces to ~1208 MPa but the El dramatically improves to ~13%. The alloy exhibits a good strength-ductility combination with UTS~1390 MPa and El~10% after ST at 800 °C followed by aging at 570 °C. A Hall-Petch like equation is used to calculate the strengthening effect of α s , which shows a good agreement with the experimental results. It's concluded that the α s spacing length (λ) dominates the free slip length in β matrix, and determines the alloy strength. The ductility evolution is explained by work hardening behavior. Limited elongation of samples aged at 510 °C is attributed to the rapid decrease of work hardening rate after yielding. A slow decrease of working hardening rate of the samples aged at 630 °C results from the slightly dynamic recovery which could effectively postpone the occurrence of necking and increase the alloy's ductility. [ABSTRACT FROM AUTHOR]