L12 Precipitate Research Articles

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

Carbon-microalloying enhances strength-ductility synergy of (FeCoNi)90Al10 medium-entropy alloy via tailoring precipitation

The metastable high/medium-entropy alloys (H/MEAs) composing L12 nano-precipitates exhibit outstanding mechanical properties. In this work, the effect of carbon (C)-microalloying on the precipitation behavior and mechanical properties of (FeCoNi)90Al10 MEA was systematically investigated. With the addition of 0.2 at. % C, the yield strength of the alloy increased from 290.4 ± 1.3 MPa to 343.1 ± 2.8 MPa, the ultimate tensile strength increased from 648.5 ± 3.5 MPa to 686.1 ± 3.2 MPa, and the uniform elongation slightly increased after ageing for 45 min. C-microalloying resulted in a decrease of 0.85 kJ/mol and 0.26 kJ/mol in Gibbs free energy for face-centered cubic matrix to L12 and B2 phase transitions, facilitating the segregation of Al atom. This promoted the precipitation of L12 and B2 phases during ageing treatment, which enhanced ordering strengthening. The discontinuous precipitation was promoted at grain boundary after C-microalloying, providing an impetus for the movement of grain boundary. Meanwhile, the movement of grain boundary and the pinning of grain boundary by body-centered cubic precipitates resulted in the formation of grain boundary serration, which could release intergranular strain localization during deformation. Moreover, C-microalloying promoted the formation of planar slip and Taylor lattice during the deformation process, thus improving the plasticity. This finding provides a significant reference for C-microalloying and precipitate control of metastable MEAs.

Effects of L12 precipitates containing Zr, Er, and Y on the precipitation of Al–Zn–Mg alloys at elevated temperatures

Microalloying significantly enhances the high-temperature properties of nano-precipitates through solute partitioning at elevated temperatures. In this study, we investigated the effects of L12 crystal structural precipitates containing Zr, Er, and Y influence the precipitation of Al–Zn–Mg alloys using transmission electron microscopy and atom-probe tomography. The results demonstrate that the multi-addition of Zr, Er, and Y improves both ambient and high-temperature hardness of Al–Zn–Mg alloys by increasing the number density of precipitates and preventing coarsening of L12/η-MgZn2 type nano-precipitates during over-aging period at elevated temperatures. Particularly, Y forms a thermally stable L12 precipitate through multiple additions with Zr and Er, a finding supported by atom-probe tomography, which reveals a synergistic interaction between Er and Y. The Er and Y multi-addition increases the number density of fine L12-Al3(Zr, Er, Y) precipitates, providing heterogeneous nucleation sites for η-type nano-precipitates at the interfaces between L12/η precipitate, thereby increasing the formation of fine η-type nano-precipitates during the over-aging period.

Discontinuous coarsening behavior and kinetics of L12 precipitate in high-entropy alloys

Coarsening is a metallurgical process that is related to solute transmission and system energy decrease. However, for the coarsening behavior of the L12 precipitate in high-entropy alloys, the influence of moving boundary is still an open question. Here, we found the moving boundary accelerates the discontinuous coarsening of L12 phase. The multiple defects-accelerated diffusion and continual particle-capturing ability of the moving boundary provide a high-efficiency solute transport for the fast-coarsening kinetics of L12 phase. In addition, a boundary-encountered assisted precipitate formation mechanism is also validated in the recrystallized area.

Superior tensile properties at cryogenic temperature induced by dual-graded structures in medium entropy alloys

The dual-graded structure with both gradients of grain size and coherent L12 precipitation was applied to a (CoCrNi)96Al3Ti3 medium-entropy alloy for achieving superior tensile properties. Simultaneous improvement on yield strength and uniform elongation is observed in the dual-graded structure at cryogenic temperature, compared to those at room temperature. Stronger hetero-deformation-induced hardening and higher density of geometrically necessary dislocations are observed at cryogenic temperature in both single-grain-size-graded and dual-graded structures. The hardening mechanisms at room temperature are characterized by low density of deformation twins and stacking faults (SFs) on one slip system, while are dominated by high density SFs at two slip systems and formation of Lomer-Cottrell (L-C) locks at cryogenic temperature for the single-graded structure. The hardening mechanisms at both room and cryogenic temperatures are revealed by high density of SFs and L-C locks in the dual-grade structure. These SFs and L-C locks can provide strong hardening themselves as dynamic Hall-Petch effect on one hand, and can interact with coherent L12 precipitates for intense precipitation hardening on the other hand. The better tensile properties for the dual-graded structure at cryogenic temperature should be attributed to a special SF-induced plasticity, i.e., the formation of SF networks and their interactions with nanoprecipitates.

Combining strength and ductility in a precipitation (γ’) hardened Fe-rich compositionally complex alloy via recrystallisation

Fe-rich alloys are the most attractive among the compositionally complex alloys (CCAs) owing to the excellent combinations of mechanical properties combined with low cost. The kinetics of recrystallisation and grain growth of an austenitic Fe-35.5Ni–4Cr-6.3Al-3.2Ti alloy (in at.%) is systematically studied to balance the combination of high strength and good ductility through grain refinement and the precipitation of L12 (γ′) phase. The yield strength of the Fe-35.5Ni–4Cr-6.3Al-3.2Ti alloy can be tuned by thermo-mechanical processing from 294 MPa in the coarse-grained FCC (γ) phase structure to 1142 MPa through inducing a heterogeneous grain structure consisting of fine recrystallised grains, subgrains/deformed grains and precipitates. The fully recrystallised alloy showed a balanced combination of strength (∼1.35 GPa) and uniform elongation (∼24%). The alloy showed good strengthening by grain refinement (Hall-Petch coefficient of 581 MPa μm0.5) and displayed an activation energy of 385 kJ/mol for static recrystallisation. The present Fe-rich CCA exhibited similar mechanical properties offered by Ni and Co-rich CCA compositions with a γ+γ′-phase structure, but at a substantially low alloying cost (<50%).

Effect of transition-metals (Fe, Co and Ni) on the L12-phase strengthening in the Al0.3CrFeCoNi high-entropy alloy

L12-strengthened high-entropy alloys have become research of interest for their great potential to achieve a superb balance between ductility and strength. While much attention has been paid to strong L12-phase forming elements, little discussion has focused on the role of transition-metals. The concentration of Fe, Co, and Cr in the Al0.3CrFeCoNi alloy was decreased to investigate how transition-metals affect the stability and distribution of the L12 precipitate. The transmission electron microscope was used to characterize the precipitation behavior in the four alloys. The results show that reducing the level of transition-metals promotes the stability of the L12 phase. In the case of the base Al0.3CrFeCoNi alloy, B2 phase replaces nanosized L12 particles as the main precipitates when aging at 700 °C. In contrast, uniformly dispersed L12 nanoparticles remain stable after 700 °C aging, with almost no B2 phase observed. Adjusting the content of these transition-metals can also alter the solid solubility of Al in the fcc matrix and change the site occupancy of Al sites in the L12 phase, enabling a shift in the volume percentage of the L12 phase. Of the three elements tested, Fe reduction effectively increases the fraction of the L12 phase, while Cr reduction even have a little negative influence on the density of the L12 particles. As a result, the microhardness and strength decrease in the following sequence: Al0.3CrFe0·6CoNi1.4, Al0.3CrFeCo0·6Ni1.4, Al0.3CrFeCoNi and Al0.3Cr0·6FeCoNi1.4. This study provides significant insights into the role of transition-metals and facilitates the design of L12-strengthened high-entropy alloys with excellent mechanical properties.

Investigation of nanoscale strain fields induced by the L12–Al3Zr phase in Al–Zn–Mg–Cu alloys

Al–Zn–Mg–Cu alloys are widely used in aerospace as an advanced lightweight structural material. The precipitation of the nanoscale L12–Al3Zr phase enhances the mechanical properties of the alloy. This study employs scanning transmission electron microscopy and geometric phase analysis techniques to investigate the strain fields induced by the nanoscale L12–Al3Zr phase. The findings demonstrate that after the precipitation of the L12–Al3Zr phase, the strain within the L12–Al3Zr phase is negligible, and a coherent strain field is developed near the L12–Al3Zr/Al interface. Characterizing the coherent strain fields of the nanoscale L12–Al3Zr phase provides a valuable reference for optimizing the mechanical properties of Al alloys.

Hot isostatic pressing induced precipitation strengthening at room and high temperature of Ni-Fe-Cr-Al-V high-entropy alloy manufactured by laser powder bed fusion

The highly adjustable properties of high-entropy alloys (HEAs) offer great potential for developing superior materials for critical structural applications in high-temperature conditions. In this study, Ni-Fe-Cr-Al-V HEA was manufactured by laser powder bed fusion, which has a unique microstructure composed of dislocation substructure and Face-centered cubic + L12 coherent structure and achieves the desired strength-ductility combination. Hot isostatic pressing post-treatment was applied on the laser powder bed fusion-processed HEA to further improve the relative density and optimize the mechanical properties. During the hot isostatic pressing process, the precipitation of L12 and B2 phases can be ascribed to the precipitation modes dominated by continuous precipitation and discontinuous precipitation, respectively. With the tensile deformation temperature increasing from 773-1,173 K, the softening degree of HEA increases continuously, and the dominant deformation mechanism evolves from intragranular dislocation slip to grain boundary sliding. At temperatures below 773 K, precipitation strengthening significantly improves tensile strength and ductility. At 1,173 K, the grain boundary strength decreases and grain boundary area increases, which promotes grain boundary sliding and contributes to plastic deformation, resulting in significant softening.

Magnificent tensile strength and ductility synergy in a NiCoCrAlTi high-entropy alloy at elevated temperature

The study reported a novel L12-strengthening NiCoCrAlTi high-entropy alloy (HEA) with an outstanding synergy of tensile strength and ductility at both ambient and high temperatures. The HEA was prepared by arc melting and cold-rolling, followed by isothermal aging at designed precipitation temperatures to achieve coexistence of recrystallized and non-recrystallized grains. Transmission electron microscopy (TEM) characterization revealed a high density of rod-like and spheroidal L12 precipitates distributing in the micro/nanograins and non-recrystallized regions in the annealed specimens. The yield stress, ultimate tensile stress, and ductility of the HEA were ∼1300 MPa, 1610 MPa, and 14 % at room temperature, respectively. Such excellent mechanical properties of ∼1060 MPa, 1271 MPa, and 25 % were maintained to 600 °C, which was superior to most reported HEAs and Co- and Ni-based superalloys to date. Systematic TEM analysis unveiled at low strain, the deformation mechanism was mainly controlled by dislocations and stacking faults (SFs). With the increase in strain, the deformation mode gradually transferred to dislocations, deformation twins (DTs), and SFs. Moreover, the complex interaction between SFs and L12 precipitates, including the shearing and blocking effects, was frequently observed, contributing to the high tensile strength. Thus, the extremely high tensile strength and sustained ductility at 600 °C mainly originate from the cooperation among interaction between L12 precipitation and dislocations and extensive SFs, DTs, immobile Lomer-Cottrell (L-C) locks formed from interactions between SFs and SFs/DTs, hierarchical SFs/DTs networks, as well as hetero-deformation-induced strengthening. Such a unique deformation mechanism breaks the strength-ductility trade-off of the HEA at high temperatures.

Achieving excellent strength-ductility combination in Co60.8-xNi39.2Alx eutectic medium-entropy alloys

In this study, we design and prepare three eutectic medium-entropy alloys (EMEAs) with the composition of Co60.8-xNi39.2Alx (x = 19, 21.6 and 24). All three alloys exhibit a FCC-B2 dual-phase structure, which evolves from hypoeutectic to eutectic and then to hypereutectic with increasing Al content. The Co39.2Ni39.2Al21.6 EMEA demonstrates a complete lamellar eutectic structure with fine and disperse ordered L12 precipitations in the FCC phase, while the nano-scale strip L10 martensite is present in the B2 phase. The Co39.2Ni39.2Al21.6 EMEA exhibits an exceptional combination of strength and ductility, with high yield strength and elongation that are superior to other FCC-B2 dual-phase EMEA reported in the past five years. The superior ductility was related to the stable K–S interface orientation during the deformation process and the transformation-induced plasticity effect in B2 phases. The high strength is derived from the interface strengthening effect of the FCC-B2 interfaces. Moreover, the hindering of dislocation slipping by the ordered L12 precipitates and the Lomer-Cottrell locks in the FCC lamellar can also enhance the strength of the alloy. These findings have important implications for the structural design of eutectic high/medium-entropy alloys.

Influence of chemical composition on coarsening kinetics of coherent L12 precipitates in FCC complex concentrated alloys

In this study, we report the experimental coarsening kinetics at 850, 900 and 950 °C of four complex concentrated alloys in the Al–Ti–Cr–Fe–Co–Ni senary system with different chemical compositions but a similar γ’ (L12) volume fraction (∼35 % at 950 °C) in a face-centered cubic (γ, FCC) matrix. The selected alloys were specifically designed to investigate the influence of Fe additions and Ni–Co substitutions on Ostwald ripening kinetics. Atom Probe tomography (APT) was used to determine the compositions of the FCC and L12 phases, which agree very well with Calphad calculations at thermodynamic equilibrium. Thermo-kinetic modeling of L12 precipitation was carried out using the Prisma module developed by Thermo-Calc and compared with experimental results. Apparent activation energies were determined and discussed in light of diffusion-controlled coarsening models to identify the key parameters affecting Ostwald ripening. We suggest that the abnormally high apparent activation energies results from composition-dependent parameters. When the latter are accounted for, the corrected activation energies for coarsening are in better agreement with available diffusion data.

Ultrastrong High‐Ductility Ni35Co35Fe10Al10Ti8B2 High‐Entropy Alloy Strengthened with Super‐High Concentration L12 Precipitates

L12 phase hardening alloys with excellent mechanical properties are of great significance for structural applications. However, low volume fractions of L12 precipitates in conventional alloys (nearly lower than 60%) tend to limit their practical usage, while the strengths of the alloys generally increase with L12 precipitation contents. Herein, a novel high‐entropy alloy (HEA) Ni35Co35Fe10Al8Ti10B2 with ultrahigh concentration L12 precipitates is successfully designed aided by the calculation of phase diagrams (CALPHAD). The volume fraction of L12 precipitates in this HEA is up to 75% and outperforms that of most of traditional superalloys. The novel L12‐strengthened Ni35Co35Fe10Al8Ti10B2 has an ultrahigh tensile yield strength of ≈1.45 GPa, ultimate tensile strength of ≈1.9 GPa, and great ductility of ≈23% at room temperature. The desirable strength–ductility combination is superior to most of conventional superalloys and reported HEAs, mainly due to the presence of ultrahigh concentration L12 precipitates that act as dislocation obstacles and the formation of numerous stacking faults and deformation twining. This work is expected to provide guidance for developing new high‐performance HEAs with an excellent combination of strength and ductility.

Non-classical nucleation of ordered L12 precipitates in the FCC based Al0.25CoFeNi high entropy alloy

The complex interplay between competing phase stabilities of FCC, L12, BCC, and B2 phases in the Al0.25CoFeNi (7Al-31Co-31Fe-31Ni in at. %) high entropy alloy (HEA) leads to non-classical phase transformation pathways and resultant novel microstructures. Specifically, the competition between the homogenous precipitation of L12 and heterogenous precipitation of BCC/B2 can be studied at a temperature of 500 °C in the Al0.25CoFeNi alloy. Upon isothermally annealing the single FCC phase microstructure of this HEA at 500 °C up to 50 h, the transformation initiates with the formation of a transient ordered L12 phase with minor Ni–Al enrichment, which is far-from equilibrium, as revealed by atom probe tomography, and can be considered non-classical nucleation. The near equilibrium L12 phase eventually replaces the transient L12 during continued annealing at the same temperature. However, the resultant FCC + L12 microstructure is metastable because the true equilibrium for the Al0.25CoFeNi alloy at 500 °C is a mixture of L12 + B2 phases, as revealed by solution thermodynamics modeling. The higher nucleation barrier for the BCC-based ordered B2 phase coupled with the slower kinetics at 500 °C leads to the homogeneous precipitation of L12, while the B2 phase appears to sluggishly grow from grain boundaries acting as heterogeneous nucleation sites.

Study on the Microstructure and Mechanical Properties of Non-Equimolar NiCoFeAlTi High Entropy Alloy Doped with Trace Elements

The method of improving the microstructure and thus the properties of alloys by adjusting their composition has been widely used in the study of high entropy alloys (HEAs). However, most studies have focused on improving the properties of HEAs with face-centered cubic (FCC) or body-centered cubic (BCC) structures by adjusting the contents of elements such as Ni, Al, Ti, Cr, Mn and Mo. The doping of B, Mg and Zr also has a certain effect on the mechanical properties of HEAs. In this paper, the phase structure, microstructure, and mechanical properties of Ni45.5Co22Fe22Al5Ti5 HEA doped with B, Mg, and Zr were investigated. The results demonstrated that the three-phase structures of FCC matrix, L12 precipitate, and BCC phase were present in all the as-cast HEAs of Ni45.5Co22Fe22Al5Ti5×0.5 (X = B, Mg, and Zr). The microstructures of the as-cast alloys showed typical dendritic and inter-dendritic architecture. The maximum hardness was found in the alloy doped with B element, with a value of 433 HV. During the compressive test at room temperature, neither the Mg0.5 HEA nor the Zr0.5 HEA cracked until the load limit, but the B0.5 HEA cracked at a compressive strain of about 12%. B0.5 HEA had the highest compressive yield strength of the three alloys, followed by Zr0.5 HEA, while Mg0.5 HEA had the lowest, with values of 1030 MPa, 754 MPa, and 628 MPa, respectively. The work is expected to provide a boost for the research on the optimization of the properties of new HEAs reinforced by precipitation of L12 phase by providing a simple solution-microalloying method.

Microstructures and mechanical properties of the precipitation strengthened Al0.4Cr0.7FexNi2V0.2 high entropy alloys

In this study, a series of Al0.4Cr0.7FexNi2V0.2 (x: molar ratio, x = 0.5, 1, 2) high entropy alloys were cold-rolled and annealed. The microstructures and mechanical properties of the alloys were systematically investigated. The results revealed that all the solution-treated alloys with different Fe contents exhibited solo FCC phase. After annealing, the coherent L12 phases and hard BCC phases were formed in the Fe0.5 and Fe1 alloys, while the Fe2 alloy only contained the L12 phases. The precipitation of L12 and BCC phases increased the hardness of the alloys. The volume fraction of BCC phase was found to decrease as the Fe content increased, the excessive BCC phases led to the decrease of plasticity. The mechanical properties of alloys were mostly determined by the degree of recrystallization and the amount and type of precipitates. The precipitation of the second phase was regulated by tailoring the alloy composition, so that the AN Fe1 alloy can obtain excellent mechanical properties.

Microstructure and mechanical property of gas tungsten arc and friction stir welds of L12 precipitate FCC high-entropy alloy

The welding technology is significant for application of high-entropy alloys (HEAs) in the industry. In this study, the mechanical properties and microstructures of Al0.2Co1.5CrFeNi1.5Ti0.3 after welding by gas tungsten arc (GTA) welding and friction stir welding (FSW) are discussed, respectively. GTA welding of precipitated HEAs resulted in the formation of dendrites in the fusion zone; the hardness and tensile strength of the GTA weld decreased to 68% and 51% compared to the base metal, respectively. However, FSW exhibited excellent mechanical properties, which were still over 94% of the hardness value and tensile strength of the base metal. The microstructure was characterized by discontinuous dynamic recrystallization and the grain refinement effect in the stir zone. The microstructure of the two welds resulted in different mechanical properties. The weld after FSW was strengthened by the grain refinement strengthening, which almost compensates the decrease in hardness caused by the re-dissolution of all precipitates in the stir zone, while the dendritic structure strongly affected the mechanical properties and softened the fusion zone after the GTA process. During the tensile test, the digital image correlation was conducted simultaneously. It shows that the GTA weld had lower strength with nonuniform deformation in the fusion zone, while the FSW weld showed higher strength with uniform deformation.

Microstructure and mechanical properties of cast Al-Ce-Sc-Zr-(Er) alloys strengthened by Al11Ce3 micro-platelets and L12 Al3(Sc,Zr,Er) nano-precipitates

The microstructure and mechanical properties of two die-cast hypoeutectic Al-Ce-based alloys modified with Sc, Zr and Er micro-additions – Al-1.5Ce-0.14Sc-0.03Zr with high Sc content and Al-1.4Ce-0.02Sc-0.06Zr-0.003Er (at%) with low Sc content – are studied, with focus on the two strengthening phases: micron-scale Al11Ce3 platelets formed on solidification and L12 Al3(Sc,Zr,Er) nanoprecipitates formed during aging. The evolution of microstructure and microhardness upon isothermal aging at 350 and 400 °C, and the creep resistance at 300 °C, are studied and compared to alloys containing only one of these strengthening phases. The Al11Ce3 and L12 phases form mostly independently of each other, except in the low-Sc alloy where scavenging of Er and Si by Al11Ce3 on solidification reduces the kinetics of precipitation and number density of the L12 precipitates formed on subsequent aging. The two strengthening phases, while mostly independent of each other chemically, interact synergistically in terms of strengthening. At ambient temperature, the hardness of the Al11Ce3- and L12-strengthened alloys is higher than that of similar alloys containing only one strengthening phase. At 300 °C, the creep strain rate is reduced by as much as five orders of magnitude in the Al11Ce3- and L12-strengthened alloys compared to alloys containing only L12 precipitates. This improvement in both room and elevated-temperature mechanical properties is modeled through a combination of load transfer from the Al11Ce3 micro-platelets, and precipitation strengthening from the Al3(Sc,Zr,Er) nanoprecipitates.

The interplay of precipitation of ordered compounds and interfacial segregation in Al‐Cu‐Hf‐Si alloys for high-temperature strength

Achieving high yield strength (250-270 MPa) at elevated temperatures (250 °C and above) for precipitation strengthened aluminium-copper (Al-Cu) alloys still remains a challenge for the alloy designers. This paper presents a systematic progression of the microstructure evolution in a binary Al-0.3Hf, ternary Al-0.3Hf-0.3Si and quaternary Al-2Cu-0.3Hf-0.3Si (in at%) alloys. A quantitative understanding of the role that Hf and Si plays in promoting ordered precipitates and the metastable Al-Cu intermetallic compounds θ"/θ' has been attempted. The initial microstructure is guided by the initial precipitation of primary phases, triggering the formation of Hf-rich dendrites and finally, the Si/Cu segregation at boundaries. Ageing of binary Al-Hf alloy results in precipitation of L12 ordered phase via discontinuous (cellular) process. Si, however, promotes a continuous precipitation process, resulting in the formation of coherent and spherical L12 ordered phase in the matrix. The role of these ordered precipitates in influencing the precipitation of θ"/θ' compounds in quaternary copper containing alloy was explored in detail, including their coarsening and mechanical properties at elevated temperatures. The compact morphology developed in the alloy results in yield strength of 445 ± 8 MPa at room temperature (25 °C). The yield strength evolves through the interaction of L12and θ"/θ' ordered phases with the α-Al matrix. The coarsening of θ' phase was slowed down due to Hf segregation at a semi-coherent interface and led to a thermally stable microstructure that resulted in yield strength of 256 ± 8 MPa at 250 °C.

Elimination of room-temperature brittleness of Fe–Ni–Co–Al–Nb–V alloys by modulating the distribution of Nb through the addition of V

Compared with Ti–Ni-based and Cu–Al-based alloys, Fe–Ni–Co–Al-based superelastic alloys have received great attention in recent years because of their low cost and excellent performance. However, the precipitation of brittle precipitates on grain boundaries in Fe–Ni–Co–Al-based polycrystalline alloys significantly reduces the mechanical properties. In this paper, the effects of V on the formation of brittle precipitates on both grain boundaries and inside grains and mechanical properties in Fe–Ni–Co–Al–Nb alloy were investigated systematically. The results show that γ′(Ni3Al) precipitate with L12 structure precipitate out on grain boundaries and inside grains in the Fe–Ni–Co–Al–Nb alloy during aging without V addition. The γ′ precipitate with L12 structure on the grain boundaries grew to form a heavily-coarsened L12 precipitate (HCLP γ′ precipitate) due to the accumulation of Nb on the grain boundaries, resulting in a significant decrease in elongation. With the addition of V, intragranular Nb and V can combine together with γ′ to form Ni3(Al, Nb, V) (nano-sized γ′ precipitate) composite precipitates, which improves the strength. More important, since the diffusion coefficient of V was larger than that of Nb, V preferentially occupied the lattice positions on grain boundaries, repelling Nb from the grain boundaries to the matrix. Thus, the coarsening of HCLP γ′ precipitate on the grain boundaries was reduced, and the elongation of Fe–Ni–Co–Al–Nb–V alloy was significantly improved.