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The initial microstructures with the unimodal and bimodal size distribution of γ′ precipitates (referred to as UD and BD, respectively) in CoNi-based superalloy are obtained utilizing different heat treatment regimes. In the present work, compression tests are carried out to investigate the anomalous temperature dependence of strength and the effect of initial microstructure on the deformation mechanism of CoNi-based superalloys. Results show that the anomalous yield occurs above 600 °C, and BD specimen exhibits higher yield stress than UD specimen. In UD specimen, the major deformation mechanisms are pairs of a/2 < 110 > dislocations coupled by APB (antiphase boundary) shearing γ′ precipitates (600–700 °C), and dislocations bypassing the γ′ precipitates accompanied with the SISFs (superlattice intrinsic stacking faults) shear above the peak temperature (900 °C). In BD specimen, the predominant deformation mechanism transitioned from the APB shearing (600 °C) to APB shearing and SISFs shearing (700 °C), and then to the dislocations bypassing the γ′ precipitates together with the SISFs shearing (900 °C). Meanwhile, the relationship between the initial microstructure and deformation mechanism is discussed. Moreover, an attempt is made to quantitatively assess the contribution of secondary γ′ precipitates to the strength at the peak temperature. [ABSTRACT FROM AUTHOR]

Precipitation and growth behavior of γ′ (Ni3Al) phase are crucial to evaluate the service stability of superalloys. In this work, precipitation and growth mechanisms of γ′ phase in different areas of a newly designed Ni3Al-based superalloy were investigated under short-term thermal exposure conditions. Results showed that twinning γ′ phase and foliated γ′ phase precipitated in eutectic area and interface area, respectively. γ′ phase went through from sphere to precursor plate and plate twinning eventually in eutectic area, which was related to preferred orientation of γ′ phase and local segregation of Al element. Dislocation and sidestep interface between matrix and the precipitation were the main reasons for the formation of twinning γ′ phase. Meanwhile, the morphology of γ′ phases in interface area went through a process from sphere to rod-like, and foliated γ′ phases formed eventually which grew toward eutectic areas with thermal exposure time increasing. Vacancies, distortions caused by different thermal expansion coefficients, lattice mismatch between dual-phase area and eutectic area contributed to the formation and growth of spherical phase. Moreover, reduction in interface energy promoted the transformation of spherical phase to rod-like and foliated γ′ phases. In addition, concentration gradient of Al element and adequate holding time were responsible for the directional growth of foliated γ′ phase. [ABSTRACT FROM AUTHOR]

In this paper, the evolution of precipitate phases in 9CrMoCoB (CB2) steel after tempering at 650 °C for different times has been investigated using transmission electron microscopy. As the tempering time prolonged from 0 to 100 h, four types of secondary phases consisting of M 3 C, M 2 X, M 7 C 3 and M 23 C 6 were identified in the experimental steel. It was found that the needle-like M 3 C carbides could be precipitated during both the air cooling process after austenization and the early stage of tempering. After tempering for 15 to 120 min, fine M 2 X particles were observed. The precipitation and growth of M 2 X were derived from the partially consumption of M 3 C carbides. After tempering for 50 h, M 2 X disappeared, while the rod-like M 7 C 3 carbides were observed to be precipitated independent of the early formed M 3 C precipitates. After tempering for 100 h, only M 23 C 6 carbides persisted, of which the number density and size kept increasing throughout 30 min to 100 h owing to their high thermostability. In addition, the precipitation mechanisms of the four types of precipitates were explained, and the relationship between the measured hardness values and microstructure was discussed. • Precipitates evolution in 9CrMoCoB (CB2) steel during tempering has been investigated. • M 3 C precipitated during both the air cooling process after austenization and the early stage of tempering. • The precipitation sequence can be given as follows: M 3 C → M 2 X → M 7 C 3 → M 23 C 6. • Hardness reduction rate of specimens decreased with the tempering time increasing. [ABSTRACT FROM AUTHOR]

To study the hot deformation behavior of Ni 3 Al-based alloy, hot compression tests were conducted in the temperature range of 1050–1250 °C with the strain rates from 0.01 to 10 s −1 . With the increase in deformation temperature and the decrease in strain rate, flow stress of the Ni 3 Al-based alloy would be decreased. Based on the obtained constitutive equation, the calculated values of peak flow stress are in good agreement with the experimental ones, and the activation deformation energy is determined as 802.71 kJ/mol. Moreover, by dynamic material model (DMM), processing maps of the hot-deformed Ni 3 Al-based alloys are established. It is indicated that the optimum processing parameters for the studied alloy correspond to deformation temperature of 1250 °C and strain rates from 0.01 to 0.1 s −1 . Specimens deformed under the optimum processing conditions exhibit fine and uniform grains, which is a typical dynamic recrystallization (DRX) microstructure. The DRX degree could be effectively enhanced with the increase of deformation temperature and the decrease of strain rate. [ABSTRACT FROM AUTHOR]

The thermal dilation experiment and the martensite transformation features of modified high Cr ferritic heat-resistant steel upon continuous cooling were explored at various cooling rates. The "spread" martensite transformation model was introduced to investigate the influence of the cooling rate applied on the martensite transformation behaviors. The martensite fraction, martensite formation rate, and the density of martensite laths were obtained as a function of cooling rate. Both the onset and offset temperatures of the martensite transformation decrease with the increase of cooling rate, and the martensite formation rate bursts at the beginning of transformation and then reaches a peak rapidly. The fitted data based on the proposed kinetic model indicated that the aspect ratio of martensite lath decreases, instead the density of martensite laths increases, with the increase of cooling rate. [ABSTRACT FROM AUTHOR]

Abstract: Oxide dispersion strengthening ferritic steels are fascinating materials for future high temperature energy production technologies. Mechanical milling with the aim of a fine dispersion of oxides in the metal matrix becomes the main process for the production of ODS steels. The mixed powder which is composed of iron, chromium and yttria (Fe-9Cr-15%Y2O3) was mechanically milled for a maximum period of 100h. Size and structure evolution of Y2O3 and the microstructure changes of the mixed powder during mechanical milling and subsequent annealing were studied. The powder is fractured and welded with rotation and vibration of container during mechanical milling. The results show that the particle size and the grain size decrease with increasing milling time. Nanocrystalline of Y2O3 is gradually formed by severe plastic deformation. It can be explained that the long-range order structure of Y2O3 is damaged by mechanical milling. The formation processes of nanocrystalline in ordered oxides may follow the sequence: ordered phase→disordered phase (loss of long-range order)→fine-grained (nanocrystalline) phase. Growth of nanocrystalline Y2O3 occurs at about 891K during subsequent annealing and the nanostructure of Y2O3 after mechanical milling and annealing was observed by TEM and HRTEM. [Copyright &y& Elsevier]

γ′-strengthened CoNi-based superalloys containing Ti and Ta are studied to investigate the γ′ stability after long-term aging at 900 °C. Both thermodynamic predictions and experimental observations suggest that there is no formation of CoAl or Co 3 W phases, indicating that the γ′ phase remains stable in the investigated CoNi-based superalloys. Typical γ/γ′ structure with a relatively high amount of γ′ phase remains in the microstructures of explored CoNi-based superalloys containing Ti and Ta after aging for 3000 h. The secondary MC carbides separate out from γ matrix during aging and different orientation relationships between MC and matrix exist for the explored CoNi-based superalloys: (100) γ // (100) MC , [ 001 ] γ // [ 001 ] MC for the alloy with Ti; (11 1 ¯ ) γ // ( 1 ¯ 1 1 ¯ ) MC , [ 011 ] γ // [ 011 ] MC for the alloy with Ta. Nanoindentation was performed to evaluate the mechanical properties of individual γ channel and γ′ precipitate. It is found that indentation modulus of γ′ phase in the explored CoNi-based superalloys locate between those of Ni 3 Al and Co 3 (Al, W) alloys, and Ta exhibits stronger strengthening effects as compared to that of Ti. • γ′ phases are stable in CoNi-base alloys after long-term aging at 900 °C. • Young's modulus of γ′ in CoNi-base alloys are between those of Ni 3 Al and Co 3 (Al,W). • Ta has a stronger strengthening effect on γ′ than Ti. [ABSTRACT FROM AUTHOR]

Deformation mechanism of L12‐γ′ phase in bimodal γ″‐γ′ precipitation hardened Inconel 718 alloy is characterized through high resolution transmission electron microscopy (HRTEM) observations. The spherical γ′ particle exhibits morphological transition toward ellipsoid along the <110> direction and shows deformation anisotropy leading to orientation‐dependent lattice misfitting status with γ‐matrix. The deformation process of γ′ phase is determined to be of APB (anti‐phase boundary)‐type shearing induced by gliding of paired {100}a/2<1¯10> superpartial dislocations, thus γ′ phase plays a APB‐strengthening role in Inconel 718 alloy. Deformation mechanism of γ′ phase in Inconel 718 alloy is characterized through high‐resolution TEM observations. The spherical γ′ particle exhibits deformation anisotropy leading to orientation‐dependent lattice misfitting status with γ‐matrix. Based on the emergence of symmetrical superlattice reflections on FFT patterns, the L12‐γ′ lattice distortion is determined to be induced by gliding of APB‐paired {100}a/2<1¯10> dislocations. [ABSTRACT FROM AUTHOR]

Abstract The microstructure evolution and creep behavior of a novelly designed as-cast Ni 3 Al-based superalloy with multiphase configuration are investigated by high-temperature annealing treatments. The as-cast microstructure comprises dominant γ′+γ dendritic and less (~19.37 vol%) interdendritic β areas, annealing treatments at 1160–1280 °C promoted the growth of γ' Ⅰ phase in the γ'+γ dendrite, during which the interdendritic β phase aggregated and coarsened rapidly in width with keeping its relatively constant volume fraction. Meanwhile, the annealing treatments significantly promoted the precipitation of quasi-spherical α-Cr phase particles within the β phase. The original as-cast microstructure exhibited inferior creep resistance at 800 °C/200 MPa with the shortest creep rupture life of 194 h, however, the annealing treatments at 1160–1280 °C prolonged the creep rupture life to beyond 611 h. Although the steady-state creep rate was gradually reduced with increasing annealing temperatures, the creep ductility was degraded by higher-temperature annealing at 1240 and 1280 °C compared with original as-cast microstructure. The 1160 °C annealed microstructure exhibited the longest creep rupture life of 665 h and the maximum creep strain to fracture of 3.21%, while the minimum steady-state creep rate was obtained on account of the largest γ' Ⅰ phase sized 0.91 µm after annealing at 1280 °C. Despite its positive role in the castability, thermoplasticity and weldability of the explored multiphase Ni 3 Al-based superalloy, interdendritic β phase has negative effects on the creep properties due to its incoherent existence with the dominant γ′+γ dendrite. [ABSTRACT FROM AUTHOR]

Additive manufacturing was currently widely used to fabricate complex-shaped superalloy components for extreme environment applications. To replace the components prepared by the traditional approaches, it is crucial to ensure the reliability of AMed superalloys, especially in a thermal-stress environment. In this work, the high-temperature tensile properties of AMed GH4099 nickel-based superalloy are studied to establish the relationships among deformation temperatures, mechanisms, and failure responses. The results show that the deformation mechanisms at 600 °C are mainly cross-slip of the slip bands and arrays, which induced a ductile fracture. Increasing the tensile temperature will cause a shift of the dislocation movement from cross-slip to Orowan looping and stacking fault shearing to dislocation entangles-recovery-recrystallization, and the fracture mode transformed from pure ductile to mixed ductile-brittle. Additionally, the neighboring grain boundaries and the carbide-grain boundary bondings will also change with the temperature, which is further linked to the mediate-temperature brittleness and high-temperature abnormal plasticity of this superalloy. [ABSTRACT FROM AUTHOR]

Powder metallurgy generally is a preferred method to consolidate and prepare some refractory metals considering their high melting temperatures. However, sintering such materials to high density while maintaining fine grain structure is challenging, and generally require ultrahigh temperature or need to be assisted by external pressure. Especially, the preparation of nanostructured oxide dispersion strengthened (ODS) refractory metals at low temperature is more difficult because grain coarsening and densification are simultaneously hindered by the common oxide addition. Thus, it is of great sense to explore a kind of oxide that can be used as both strengthening phase and sintering accelerator. In this work, after extensive theoretical calculation and experimental observation, it is found that the accelerated sintering at low temperature can be achieved in W-HfO 2 system. In the absence of external pressure or activator, this system can be rapidly densified without undesirable grain coarsening at a relative low temperature of only 1480 ˚C. Through kinetic analysis and density functional theory (DFT) calculation, it was confirmed that the intergranular HfO 2 particles provide a rapid diffusion transport pathway for W atoms in support of densification at temperature of interest. As a result, this alloy exhibits a homogeneous microstructure with ∼97% relative density and ∼200 nm grain size, which renders our alloy with an ultrahigh compressive strength of ∼3 GPa, much higher than the reported values. We believe that the accelerated sintering mechanism found in our W-HfO 2 system can be applicable to other refractory metals to prepare high-density nanocrystalline alloys with excellent properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Allvac 718Plus alloy is a promising Ni-based superalloy with excellent thermal stability and mechanical properties up to 704 °C. The γ′ coarsening, σ precipitation and the resulting degraded tensile properties at room and elevated (704 °C) temperatures during long-term thermal exposure at 704 and 760 °C are systematically investigated in this work. The results showed that a higher exposure temperature and a longer exposure time significantly accelerate the γ′ coarsening and σ precipitation. The σ particles were detrimental to the alloy by acting as the preferential nucleation sites for cracks, and the reduction in tensile strengths was mainly caused by the increasing size of γ′ precipitates during thermal exposure. Furthermore, there were significant differences between the deformation mechanisms of room-temperature and elevated-temperature (760 °C) tensile tests. Concerning the deformation mechanism of γ-matrix, planar slipping dominates during tensile testing at room temperature, while the formation of stacking faults and microtwinning are primary during tensile testing at elevated temperature (760 °C). The dominant precipitation strengthening mechanism depended on the γ′ characteristics of different thermal exposure conditions and the tensile temperature. The 704 °C/100 h exposure sample shows a dislocation shearing mechanism when tested at room or elevated (760 °C) temperature. Notably, the 760 °C/1000 h exposure sample exhibits a mixed precipitation strengthening mechanism of Orowan bypassing and superlattice stacking faults shearing when tensile tested at room temperature while showing a dislocation and superlattice stacking faults shearing mechanism when tensile tested at elevated (760 °C) temperature. • The Cr-rich σ particles nucleate at γ/η interfaces during long-term exposure in 718Plus alloy. • The σ formation is detrimental to room-temperature tensile ductility. • During tensile tests, planar slipping dominates at 25 °C while microtwinning and stacking faults dominate at 704 °C. • The dominant precipitation-strengthening mechanisms are predicted by calculating the critical γ′ size. [ABSTRACT FROM AUTHOR]

In this work, we creatively introduced MAX additives (Ti 2 AlC) into W alloy system to break the development bottleneck of ball milling technology: a large amount of oxygen impurities and large-sized second phase particles at grain boundaries (GBs). Unlike traditional oxides/carbides, Ti 2 AlC can undergo self-decomposition during high-temperature sintering stage (>1400 °C) on the basis of hydrogen sintering, and continue to adsorb a number of stubborn oxygen impurities to form various smaller, more dispersed, and stronger in-situ Ti x C y , TiO x , Al 2 O 3 , and Al x Ti y O z nanoparticles. Furthermore, these in-situ nanoparticles could maintain stable coherent interfaces with W, thereby more effectively refining, purifying, and strengthening matrix. Compared to traditional W-carbide/oxide alloys, MAX additives render our alloy with high hardness (521 ± 24 HV 0.2), a remarkable combination of high compressive strength (1262.9 ± 35.1 MPa) and large ductility (9.88%). This work provides a new perspective for the preparation of high-performance refractory metals and relevant engineering application. • The MAX phase (Ti 2 AlC) is innovatively added into W alloys. • The Ti 2 AlC phase decomposes at above >1400 °C and forms in-situ nanoparticles. • The Ti 2 AlC phase continues to adsorb stubborn oxygen impurities at sintering stage. • In-situ nanoparticles could maintain stable coherent interfaces with W. • The W-Ti 2 AlC alloy possess excellent mechanical properties. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) refractory metals, represented by tungsten and molybdenum alloy, possesses many excellent properties such as high-temperature strength, outstanding thermal microstructure stability and good resistances to oxidation and corrosion. However, in traditional preparation technologies, these ex-situ oxides usually form poor oxide/matrix interface relationships and keep coarse sizes at grain boundaries (GBs), which prominently limit the improvement of alloy mechanical properties until now. In this work, we skillfully employed novel phase interface engineering to successfully construct completely coherent oxide/matrix interfaces in ODS Mo alloys through the newly formed ternary phase oxide nanoparticles or diffusion layers. Furthermore, these ODS Mo alloys are characterized by ultrafine grains, ultrafine subgrains, small and dispersed intragranular and intergranular particles. An amount of oxygen impurities can also be depleted via phase interface engineering, thus purifying Mo matrix. The phase interface engineering endows ODS Mo alloys with ultrahigh strength, excellent ductility and excellent hot machinability, which illuminates a new research direction for those ex-situ oxide dispersion strengthened alloys with excellent mechanical properties. [ABSTRACT FROM AUTHOR]

Coarse columnar grains are always present in nickel-based superalloys fabricated by laser powder bed fusion (LPBF), which not only causes anisotropy in mechanical properties, but also increases the hot cracking susceptibility of the alloys. Generally, the addition of nucleating agents is a common consideration for grain refinement. However, the intrinsic ultra-high thermal gradient and cooling rate of the LPBF process result in significant differences in grain nucleation and growth compared to conventional casting. The ultra-high thermal gradient of the LPBF process severely inhibits constitutional supercooling, which significantly reduces the heterogeneous nucleation efficiency of nickel-based superalloys. In this work, we introduced Zr into Haynes 230 alloy to improve constitutional supercooling of the LPBF molten pool during solidification and investigated the individual/synergistic effects of nucleating agent (TiC particles) and solute (Zr atoms) addition on the grain refinement of Haynes 230 alloy. It was found that the columnar grain size and texture intensity of LPBF fabricated Haynes 230 alloys were dramatically decreased with Zr and TiC co-addition, which obviously improved the mechanical properties as well as weakened its anisotropy. Our work revealed that inhibition of cell epitaxial growth and promotion of cell nucleation are necessary conditions to refine the grain size of LPBF fabricated nickel-based superalloys. Solutes with high growth restriction factor (Q value), such as Zr in our work, can rapidly generate constitutional supercooling, thereby obviously improving the heterogeneous nucleation efficiency of nucleating agents in nickel-based superalloy during the LPBF process. [Display omitted] [ABSTRACT FROM AUTHOR]

The evolution of η phase and its effect on the tensile properties at 700 °C in 718Plus superalloy after three solution (at 940 °C for 1 h) and single aging (at 870 °C for 25 h, 50 h and 100 h) heat treatments, were investigated. The results showed that there were several various morphologies of η phase in 718Plus samples. As aging time increased, the volume fraction of needle-like/plate-like increased continuously from 5.32%, 7.78% to 29.52%, while the volume of granular/short rod-like η phase firstly increased from 0.46% to 0.68% then decreased to 0.56%. The high temperature tensile properties were affected by η phases with various morphologies. With aging time increasing, the ultimate tensile strength was obviously improved from 797.11 MPa to 860.57 MPa, which could be attributed to a certain number of plate-like/needle-like η phases at the grain boundaries which could hinder the dislocations motion near the grain boundaries. And the elongation weakly increased firstly from 16.84% to 18.76% and then decreased to 18%, which was closely related to granular/short-rod η phases. Enhancing tensile strength and acceptable ductility could be obtained by morphology control of η phase applying SA heat treatments in the study: promoting the simultaneous precipitation of an appropriate amount of plate-like/needle-like η phases and a small amount of granular/short rod-like η phases. • The morphologies of η phases found in studied 718Plus superalloy samples are plate-like/needle-like, granular/short rod-like. • With aging time extending, the volume fraction of needle-like/plate-like increased, resulting in improving tensile strength. • By applying SA aging heat treatments, an enhancing tensile strength could be obtained by morphology control of η phase [ABSTRACT FROM AUTHOR]

• Rapid solidification rate promotes high nucleation burst of α-Cr precipitates (sources for dislocations). • Dislocations act as precursor to stacking faults and microtwinning. • Martensite plates with a high density of stacking faults and multiple microtwins are formed. Influence of solidification rate on the interdendritic microstructure evolution of a multiphase Ni 3 Al-based superalloy was investigated. Compared to the original as cast sample, rapid solidification rate leads to high nucleation burst of α-Cr precipitates in spray casting alloy, which promotes the transformation from b.c.c. B2 β phase to f.c.t. L1 0 martensite plates with high density of stacking faults and microtwins in interdendritic regions. [ABSTRACT FROM AUTHOR]

Vacuum diffusion bonding at a relatively low temperature is a recognized method for restraining intermetallics and optimizing joint microstructure. The surface self-nanocrystallization (SSN) treatment by shot peening technology is applied on the surface before bonding a couple of Ni 3 Al-based superalloy and S31042 steel, which lowers the bonding temperature for achieving a sound joint by 100 °C. Moreover, the joint strength of samples bonding at 1000 °C after SSN treatment is 178 MPa higher than that of the bonded samples at 1100 °C without SSN treatment. SEM, XRD and TEM analyses indicate SSN treatment improves bonding efficiency by increasing surface micro-deformation and introducing nanocrystalline and deformed layer. The formation of disordered Ni-solution on the surface makes γ' strengthening phase re-precipitates in the joint and hard-brittle phase of AlN is restrained by diffusion bonding at 1000 °C. The good metallurgical bonding, re-precipitated γ' phase and the formation of refining grains zone near the bonding interface contribute to the good performance of 1000 °C-bonded samples after SSN treatment. • SSN technology realized the disordering in surface of Ni 3 Al-based superalloy. • The sound bonded joint was achieved at a lower temperature by SSN technology. • The low-temperature bonding restrained the formation of AlN phase. [ABSTRACT FROM AUTHOR]

Abstract Inconel 718 is a widely used nickel-based superalloy in aircraft engines during the past decades, whose elevated temperature strength derives from disc-shaped gamma double prime (γ″) in three perpendicular variants plus less spherical gamma prime (γ′) precipitates. In present work, we observed an abnormal γ″ variant with tailored orientation and shape in directly aged cold-rolled Inconel 718, creep tests proved its substantial benefits in improving creep resistance at 650 °C superior to commonly perpendicular γ″ variants. The results can provide new insights for processing optimization and second-particle configuration design to maximize the strengthening effect of γ″ phase in 718-type superalloys. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

It is always a great challenge to manufacture defect-free and high-performance pure tungsten by laser additive manufacturing until now. In this work, using nanopowder as precursors, the pure tungsten green parts were successfully manufactured by indirect additive manufacturing technology, powder extrusion printing. The subsequent debinding-sintering process of the green parts has been optimized to obtain high-relative-density, fine-grain and defect-free tungsten samples. The relative density of the sample increased with the increase of sintering temperature. The microstructures of pure tungsten prepared by direct and indirect additive manufacturing techniques were studied comparatively, including laser powder bed fusion and powder extrusion printing. The formation of microcracks in pure tungsten samples is mainly due to the high residual stress provided by laser powder bed fusion and the high ductile-to-brittle transition temperature characteristics of tungsten. The sintering temperature of the pure tungsten samples with the highest compressive strength (1290 ± 10 MPa) was determined to be 2000 °C. Compared with other sintered samples, defect-free samples sintered at 2000 °C had finer grains (9.8 μm) while maintaining high relative density (99.1 ± 0.4%). The higher strength of the sintered samples at 2000 °C is related to the fine-grain strengthening and high relative density compared to samples obtained from other direct additive manufacturing technologies and sintered at other temperatures. The indirect additive manufacturing technique based on the use of nanopowder precursors developed in this work provides a new technical approach for the additive manufacturing of high-performance tungsten and/or other refractory metals. • The application of nano-powders for indirect additive manufacturing was innovative. • A defect-free and high-performance pure tungsten parts were fabricated. • The difference in microstructure of pure tungsten prepared by LPBF and PEP was investigated. • The mechanism of mechanical properties of pure tungsten parts was revealed. [ABSTRACT FROM AUTHOR]

Allvac 718Plus alloy is a newly-developed Ni-based superalloy with excellent thermal stability and mechanical performance at elevated temperatures up to 704 °C. The precipitation behavior of the rapidly-solidified Allvac 718Plus superalloy during aging is systematically investigated in this work. The microstructure evolutions of the strengthening γ' phase and the γ'/γ″ co-precipitates during aging are examined. During aging, the "duplet" γ'/γ″ and "triplet" γ″/γ'/γ″ occur and then disappear due to the dissolution of γ″ phase, which is promoted by the δ precipitation. On the contrary, the "triplet" γ'/γ″/γ' forms and remains after aging, resulting from the low diffusion rate of solute atoms through the spheroidal γ' precipitates. The lamellar δ-Ni 3 Nb phase occurs at grain boundaries. Its size and content increase with the prolongation of aging time. After long-term thermal exposure, the hexagonal η phase forms at the δ-γ interface due to the variations of local chemical composition, and then the tetragonal Cr-rich σ phase nucleates adjacent to the δ and η particles because of the segregation of Cr atoms rejected from the δ and η particles. • The rapidly-solidified γ-matrix of 718Plus is obtained with a supersaturated solid solution and a high density of defects. • The dissolution of ellipsoidal γ″ results in the disappearance of the "duplet" γ'/γ″ and "triplet" γ″/γ'/γ″. • The "triplet" γ'/γ″/γ' is remained due to the low diffusion rate of solute atoms through the spheroidal γ' precipitates. • The lamellar δ forms first and the η and σ phases successively nucleate at the δ-γ interface after long-time thermal exposure. [ABSTRACT FROM AUTHOR]

A polycrystalline Ni 3 Al-based superalloy was joined through transient liquid phase (TLP) bonding technique. A post bond heat treatment (PBHT) was successively conducted. Creep tests of the TLP and TLP + PBHT specimens were then carried out under 800 °C and 160 MPa. The creep deformation and fracture behaviors were further investigated through interrupted creep tests. During the PBHT procedure, microstructure evolution of the joint occurred with creep life prolonged from 64 h to 368 h. The superdislocations in TLP and TLP + PBHT specimens were determined to study the γ′ phase shearing modes. Both individual superdislocations and APB-coupled partial dislocation pairs are observed. The variation of dominate superdislocation configurations among different joint zones and the evolution of the superdislocations during the creep process are discovered. Besides, the formation of different γ′ rafting microstructures and the initiation of intergranular microcracks in the polycrystalline joint are both studied. [ABSTRACT FROM AUTHOR]

The roles of the alloying elements, Ti and Ta, on the γ' strengthening phase and the corresponding high temperature deformation responses of the CoNi-based superalloys have been systematically investigated. The results indicate that addition of either 4 at.% Ti (4Ti) or 2 at.% Ta (2Ta) can significantly increase the volume fraction of the γ' phase. Detailed analysis of the deformation mechanisms and Gibbs free energies of each phase in the two alloys at elevated temperatures were carried out by using the transmission electron microscopy (TEM) characterization and the thermodynamic calculation, respectively. The dislocations shearing into the γ' precipitates is the primary deformation mechanism in both alloys when deformed at 600 °C. However, when increasing the deformation temperature to 800 °C, the dislocation cross-slipping and by-passing mechanism dominants the 4Ti alloy's deformation, while the additional stacking faults (SFs) were introduced to the 2Ta alloy due to the lower barrier energy of the SFs formation. Continuing increasing the deformation temperature to 1000 °C will promote the dislocation-based mechanisms by lower the dislocation propagation resistance in both alloys. The process will also induce the mechanical twins in the 2Ta alloy which can further strengthening the alloy. The alloying induced deformation mechanism transition and the related high-temperature mechanical performance of the CoNi-based superalloys have been further discussed. • Adding Ti or Ta to the CoNi-based alloy will improve its γ' volume fraction to 60% • Ti and Ta will improve the flow strength at medium and high temperature, respectively • Ta can reduce the energy barrier for stacking fault formation at high temperature • Deformation mechanisms at elevated temperatures can be altered by the alloy elements [ABSTRACT FROM AUTHOR]

Avoiding the formation of cracks to ensure a reliable printability and a good stability is crucial in the laser additive manufacturing of alloys. Contrary to previous studies that have generally tried to decrease the liquid film and solidification range, in this work, we innovatively utilized segregation engineering and abundant cell boundaries to introduce liquid backfilling as well as a network of segregation phases to alleviate thermal stress, consequently eliminating hot cracking. More specifically, zirconium was introduced into a nickel-based superalloy to form a continuous interdendritic liquid film during the laser additive manufacturing process. It was found that the continuous intermetallic Ni 11 Zr 9 segregation phase decorated the cell boundaries, and cracks were completely eliminated in the as-printed Haynes 230 alloys when their Zr content reached 1 wt.%. Moreover, this continuous Ni 11 Zr 9 network layer was able to act as a "skeleton" to significantly improve the yield strength of the as-printed samples. Following appropriate heat treatment, these Zr-modified Haynes 230 alloys exhibited an extraordinary combination of strength and plasticity, which were superior to those of the previously reported Haynes 230 alloy. These findings provide a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The correlation of microstructure-properties of the Ni-based superalloy with coprecipitation of γ'/γ'' phases has been studied in details. To obtain the different γ'/γ'' microstructures, various heat treatments were attempted. The experimental results show that high amounts of the compact γ'/γ'' phases contribute to the tensile strength of Ni-based superalloy, which decreases with the coarsening of γ'/γ'' structures. The γ'/γ'' phases can restrain the motion of the dislocations. Further increasing the size of the γ' and γ'' phases, the deformation mechanism transforms from dislocation pairs shearing to Orowan bypassing, and then to dislocation shearing. Stacking faults (SFs) also appears in γ'/γ'' phases. Additionally, the moderate η/δ phases effectively pin the grain boundary, which enhances the alloy's ductility. [ABSTRACT FROM AUTHOR]

A CoNi-based γʹ phase strengthened superalloy was designed with alloy elements Al, W, Cr, Ti, and Ta added. The size and volume fraction of the γʹ phase can be tailored by the post-heat-treatment, which will then affect mechanical behavior of this alloy. The aged alloy was compressively tested at a wide temperature range, from room temperature to 1000 °C. Detailed microstructural analysis of the tested alloy was carried out using transmission electron microscopy. At 600 °C, the primary deformation mechanism is pairs of a/2 <110> type dislocations and stacking faults shearing in the alloy. The deformation mechanism changed to dislocation pile-ups and climbing in the γ phase when deformed at 800 °C. Further increasing the deformation temperature to 1000 °C, both nanotwins and dislocations were formed and their interactions will be the main strengthening mechanism of the alloy. The deformation mechanism transition and correlated mechanical behavior of this alloy was further discussed based on the defect analysis. [ABSTRACT FROM AUTHOR]

A CoNi-based alloy (various cooling rates after solution heat treatment) was utilized to investigate the microstructure-dependent coarsening behavior of γ′ precipitates. The fast cooling rate leads to the formation of initial fine γ′ precipitates with high number density. The different initial microstructures of CoNi-based alloy lead to various coarsening behaviors of γ′ precipitates during subsequent aging treatment at 900 °C. The coarsening behavior of large γ′ precipitates with low density (formed in slow cooling rate, 0.1 °C/s) is in accordance with the Lifshitz-Slyozov-encounter modified (LSEM) model. Whereas for the fine γ′ precipitates with high density (formed in fast cooling rate, 60 °C/s or 240 °C/s), its coarsening behavior is consistent with the TIDC model at early stage and LSEM model at later stage, respectively. • The different initial microstructures of Cobalt-based alloy lead to various coarsening behaviors of γ′ precipitates. • Fast cooling rate leads to the formation of fine γ′ precipitates with high density. • Coarsening of fine γ′ precipitates is consistent with the TIDC model in the early stage and LSEM model in the later stage. • Coarsening of large γ′ precipitates is in accordance with the LSEM model. [ABSTRACT FROM AUTHOR]

Nowadays, applying the SLM technology to produce crack-free high-performance solution strengthened superalloys or refractory metal is still a big challenge. In this work, an innovative method via in-situ chemical doping of Y 2 O 3 was used to solve the above problem. The results show that the Y 2 O 3 introduced by this method forms a smooth and uniform coating on the surface of Hastelloy X spherical powders. Using this powder as the precursor, cracks in the SLM manufactured Hastelloy X can be completely eliminated. More heterogeneous nucleation sites provided by doped Y 2 O 3 significantly reduce the grain and cell size of SLMed Hastelloy X, thereby inhibiting crack initiation and propagation. Based on the microstructural optimization, the ultimate tensile strength of Y 2 O 3 -doped Hastelloy X alloy increases significantly from 982 ± 66 MPa to 1405 ± 26 MPa, along with its yield strength increasing from 820 ± 46 MPa to 1130 ± 25 MPa. Besides, its total elongation also increases from 22.7 ± 1.7% to 27.7 ± 0.5%. The increase of strength is attributed to the strengthening effect of Y 2 O 3 nanoparticles and the refinement of cells and grains, while the increase of total elongation is attributed to the elimination of internal cracks. The strategy of introducing uniform rare earth oxide coating on the surface of metal powders by in-situ chemical method proposed in our work provides a new perspective for SLM manufacturing high-performance solution strengthened superalloys or refractory metal prone to crack. [ABSTRACT FROM AUTHOR]

• The major highlight of this paper is how to improve the thermal stability of γ I ′ phase by investigating its directional coarsening behavior. • The second highlight is clarifying the kinetics of γ I ′ directional coarsening and its connection mode. • The third highlight is clarifying the influence of initial radius, number, volume fraction and γ channel distance on the kinetics of γ I ′ directional coarsening. • The forth highlight is clarifying the influence of lattice misfit, elemental diffusion and interfacial composition width on γ I ′ directional coarsening. Polycrystalline Ni 3 Al-based superalloy with up to 70% γ ′ phase owns excellent strength at around 1100 °C. It has been successfully used as a substitute for turbine blade crown, which is one of the aircraft components working in the severest environment (i.e., sometimes nearly overheating). However, few studies concentrate on the coarsening resistance and phase thermal stability in this newly designed polycrystalline superalloy at the working temperature, hence, directional coarsening behavior of primary γ ′ (γ I ′) has remained restricted. This paper investigated directional coarsening behavior of γ I ′ during aging treatment (1100 °C) after overheating at 1270 °C with furnace cooling (SFA), air cooling (SAA) and water cooling (SWA) respectively. It clearly showed that γ I ′ grew into lamellar and gradually became irregular during the directional coarsening process. Moreover, its morphological combination mode was illustrated. Since parallel short lamellar γ I ′ connected each other, long lamellar formed. When perpendicular long lamellar γ I ′ connected each other, it became L -type or T-type, i.e., precursor of irregular γ I ′. The kinetics of directional coarsening regime of γ I ′ was in line with the matrix-diffusion - controlled (MDC) theory according to the evaluation of γ I ′ size. This was caused by higher migration of γ I ′ interfaces than the diffusion of solute elements. It was also confirmed that driving force of directional coarsening mainly depended on the competition of elemental diffusion, lattice misfit distribution and γ channel distance. [ABSTRACT FROM AUTHOR]

In the present study, a two-phase Ti 2 AlNb-based alloy with a nominal composition of Ti–22Al–25Nb (at%) comprising the dominant O + B2/β phase was explored. The microstructural evolution and tensile properties are studied by applying different heat treatment methods of water-cooling (WC, 140 °C/s) and furnace-cooling (FC, 0.08 °C/s) after heat treatment at 1015 °C for 40 min, followed by aging treatment at 800 °C for 1, 5, and 100 h. The results indicated that the morphologies of solution-treated alloys after different cooling methods have a significant distinction. The WC alloy consists of a continuous B2/β phase and a few remnant O phase, whereas the FC alloy comprises a mass of lath O phase and discontinuous B2/β phase. The Widmanstätten O + B2 structures were generated in the following aging treatment. The α 2 phase was not detected during the heat treatments in the present work. The B2 phase and O phase in the aged WC and FC alloys follow the classical orientation relationship: (110) B 2 / / (001) O , [ 1 ‾ 11 ] B 2 / / [ 1 1 ‾ 0 ] O. The room-temperature ultimate tensile strength (UTS) and elongation to failure (EL) of the FC and aged FC alloys are higher than the WC and aged WC alloys, except for the 100 h-aging condition. On the contrary, the high-temperature UTS and EL of the aged WC alloys are higher than the aged FC alloys. The existence of the O/O phase boundaries, the coarsening of the B2 grain boundary and discontinuous precipitation harmed the performances of the studied alloys. Beyond that, the fracture mode of the explored alloys with different heat treatment methods was also analyzed. • The morphologies of solution-treated Ti 2 AlNb-based alloys after different cooling methods have a significant distinction. • The B2 phase and O phase in the studied alloys follow the classical orientation relationship: 110B2//001O, [111]B2//[110]O. • Tensile properties and fracture mode are significantly influenced by different cooling rates and aging treatments. • The O/O boundaries, coarsening of the grain boundary and discontinuous precipitation harmed the properties of the alloys. [ABSTRACT FROM AUTHOR]

The effects of solution cooling rate on microstructure, mechanical properties and the corresponding deformation mechanisms of ATI 718Plus superalloy were investigated. With solution cooling rate decreasing, the volume fraction and the size of primary γ′ phase both increase, which is attributed to that the lower solution cooling rate is beneficial to the migration and interaction of alloy elements in γ′ phase. The tensile strength and microhardness of sample were improved by decrease of solution cooling rate, which is mainly attributed to increase of volume fraction of primary γ′ phase. Specially, the elongation of sample with the lowest solution cooling rate (furnace cooling) was also higher than other samples probably due to large size of primary γ′ phase, which is beneficial to plastic deformation by helping promote active movement of dislocations. In general, three samples exhibit the mixed deformation mechanisms characterized by shearing and bypassing, but primary deformation mechanism of three samples was different, which depends on different precipitation behavior of γ′ phase resulting from different solution cooling rate. The sample that was furnace cooled from solution temperature exhibits a typical shearing deformation mechanism, including stacking faults (SFs) and dislocations shearing, while other two samples with the higher solution cooling rate primarily show an Orowan looping deformation mechanism. The result of calculating critical resolved shear stress (CRSS) exhibits an increasing tendency of SFs shearing while decreasing tendency of Orowan looping with solution cooling rate decreasing, which is corresponding to experimental results. [ABSTRACT FROM AUTHOR]

The isothermal oxidation behavior of micro-regions in multiphase Ni 3 Al-based superalloys (high Fe and Cr contents) was investigated at 600 °C in air. The oxide scale is mainly composed of the outer mixed oxide layer (NiO, NiFe 2 O 4 and Al 2 O 3) and the inner single Al 2 O 3 layer. In particular, Cr 2 O 3 is formed in the outer oxide scale of γ' + γ dendrite with enrichment in Cr element. The relative dense continuous Al 2 O 3 layer is formed in the interdendritic region (β phase with high Al content). In addition, γ' envelope (connecting the γ' + γ dendrite and interdendritic β phase) shows preferential oxidation featuring significant cellular bulge (NiO and NiFe 2 O 4 mixed oxides). Unlabelled Image • The micro-regions in multiphase Ni 3 Al-based superalloy present significant different oxidation behaviors at 600 °C. • The γ' envelope shows preferential oxidization featuring obvious cellular bulge (NiO and NiFe 2 O 4 mixed oxides). • Cr 2 O 3 is formed in the outer oxide layer of γ' + γ dendrite enriching in Cr element. • The dense protective alumina layer is formed on interdendritic region (β phase) with relative high Al content. [ABSTRACT FROM AUTHOR]

In this work, hot compression deformation behavior and processing maps of ATI 718Plus superalloy were studied by isothermal compression test on Gleeble-3800 thermal simulation testing machine. The results showed that under the present conditions (with deformation temperature range of 1030–1180 °C, strain rate of 0.01–10 s−1 and deformation amount of 50%), flow stress decreased with the increasing of deformation temperature and decreasing of strain rates. Meanwhile, simplified and strain-compensated classical Arrhenius constitutive equation models were established respectively, and strain-compensated constitutive equation improves the accuracy of flow stress prediction. In addition, processing maps showed that the optimal hot processing parameter ranges were 1140–1180 °C with strain rate of 0.01–0.1 s−1 respectively. The effects of strain rate and deformation temperature on microstructure evolution was investigated and low strain rate and high deformation temperature are conductive to DRX. • The strain compensation constitutive equation for 718Plus was established and accuracy was compared with simplified equation. • The optimal parameter range of hot working is determined by the establishment of hot processing maps. • Combining the processing maps and microstructure observation the evolution behavior of DRX structure is described. [ABSTRACT FROM AUTHOR]

The microstructure evolutions during 1000 °C-1000 h long-term aging and relevant 800 ~ 840 °C/220 ~ 300 MPa creep behaviors of a novelly designed multiphase Ni 3 Al-based intermetallic alloy were investigated. Results showed this alloy presented excellent intermediate temperature creep properties even after 1000 °C-1000 h long-term aging. A kind of bimodal R-type γ′ rafting structures were observed in the 800 °C/220 MPa creep process of the long-term aged alloy, and the creep resistance levels of three kinds of γ′ rafting structures were evaluated to be in the order of bimodal R-type γ′ rafting structures > R-type γ′ rafting structure > N-type γ′ rafting structure. The 1000 °C-1000 h long-term aging can promote the inhomogeneous precipitation of Cr 4.6 MoNi 2.1 phase at the edge γ′+γ dendrite area, which can play the role of pinning dislocations and inhibiting the further rafting and coarsening of the R-type γ′ rafts during creep. Differed from Ni-based alloys and other Ni 3 Al-based alloys, this long-term aged alloy exhibited abnormal four-stage creep characteristics, which consisted of atypical short steady-state creep stage with step fluctuant feature and atypical long accelerated creep stage with creep rate increasing at a constant rate. [ABSTRACT FROM AUTHOR]

The microstructure, element segregation and γ/γ′ interface characters of polycrystalline Ni 3 Al-based superalloys under conventionally solidified (~10 K/s) and rapidly solidified (~103 K/s) conditions were investigated. Cr and Fe are primarily partitioned into the γ phase while Al and Ni are into γ′ phase. However, rapid solidification rate of spray casting alloy improves the solubility of Cr and Fe elements at the expense of Al in γ′ phase, which decreases the lattice parameters of γ and γ′ phases. Moreover, it further increases the absolute value of γ/γ′ lattice misfit and contributes to the formation of (100) plane misfit dislocations in γ/γ′ interface. As a result, spray casting specimen possesses higher hardness (6.5 ± 1.0 GPa) and elastic moduli (166.4 ± 15.9 GPa) than that in as-cast alloy with lower solidification rate. • Rapid solidification rate of spray casting alloy improves the solubility of Cr and Fe elements in γ′ phase. • It further increases the absolute value of γ/γ′ lattice misfit. • The rapidly solidified spray casting alloy possesses high hardness and elastic moduli. [ABSTRACT FROM AUTHOR]

The microstructural evolution and phase transformation behavior of a designed multiphase Ni 3 Al-based superalloy in as-cast and spray casting states were investigated under thermal exposure at various temperatures. For the exposure temperatures range from 600 °C to 900 °C, the average size of γ′ precipitates in the dendritic dual (γ+γ′) areas of the two states alloys increases, and the morphology of γ′ changes to irregular shapes due to merging of neighboring particles and Ostwald ripening. After thermal exposure at 600 °C or 900 °C, bulky γ′ precipitates generate in the interdendritic regions of as-cast alloy, and the γ′ envelope develops a curved interface by aggregation. For the spray casting sample, high density of microtwins in γ′ precipitates are 'inherited' from the preceding L1 0 -type martensite phases after thermal exposure at 600 °C. However, the γ′ precipitates without any microtwins form after thermal exposure at 900 °C. • For as-cast alloy, bulky γ′ precipitates generate in β phase after thermal exposure at 600 °C or 900 °C. • For the spray casting sample, high density of microtwins in γ′ precipitates are 'inherited' after thermal exposure at 600 °C. • The γ′ precipitates without microtwins form in spray casting alloy after thermal exposure at 900°C. [ABSTRACT FROM AUTHOR]