Reduction In Stacking Fault Energy Research Articles

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Face-centered cubic (FCC) medium entropy alloys (MEAs) have received considerable attention due to their impressive mechanical properties and responses. However, their practical application is limited by their modest yield strengths. The potential enhancement of the mechanical properties of single-phase MEAs was explored in this study through a synergistic approach combining heterogeneous structure design with subsequent cryo-pre-straining. A heterogeneous lamella structure was produced in a single-phase Fe55Mn20Cr15Ni10 MEA via two-step rolling and annealing. Cryo-pre-straining at varying degrees (6, 12, 21, and 36%) introduced hexagonal close-packed (HCP) phase, high-density dislocations, twins, and stacking faults, leveraging the reduced stacking fault energy at cryogenic temperatures. This process enhanced the alloy's yield strength from 353 MPa to 1.2 GPa (compared to the baseline uniform coarse-grained structure), while maintaining an acceptable total elongation of 8.4%. The impact of cryo-pre-straining on the microstructure and mechanical properties of the MEA was assessed using in-situ synchrotron X-ray diffraction analysis. Cryo-pre-straining (36%) achieved a higher dislocation density (6.1 × 1015 m−2) compared to room-temperature straining (2.5 × 1015 m−2). The stress contribution from HCP-martensite and the evolution of dislocation density during loading were quantified, along with observations of negative stacking fault probability and strain-induced HCP→FCC reverse transformation in cryo-pre-strained samples under loading conditions. Furthermore, the contributions of regulated microstructures to the enhancement of yield strength were quantitatively assessed.

Temperature dependence of the deformation behavior and mechanical properties of a Fe–Mn–Al–C low-density steel for cryogenic application

The temperature dependence of the deformation behavior and mechanical properties of a Fe–20Mn–6Al-0.6C-0.15Si austenitic low-density steel was studied by tensile test and Charpy impact test at −196 °C, −77 °C and room temperature (RT), followed by microstructure analyses. The results indicate that the decreased tensile temperature, which is corresponding to reduced stacking fault energy (SFE), promotes the planar glide of dislocations. This led to larger fraction of low-angle grain boundaries (LAGBs) at lower tensile temperature. Consequently, both the tensile strength and elongation were improved as temperature decreased. At lower impact test temperature, the load and energy of crack initiation were larger, indicating a higher resistance to crack initiation. However, the transition from stable to unstable crack propagation occurred more rapidly at lower temperature, resulting in lower crack propagation energy and reduced resistance to crack propagation. This is because less region experienced planar glide. Due to the rapid unstable crack propagation, the Charpy absorbed energy at −196 °C was the lowest. The smaller fracture surface area, larger fibrous zone and more obvious ductile fracture characteristics in radial zone suggest that the toughness is better at higher temperature. Additionally, the presence of secondary cracks at RT delayed fracture, therefore benefitting toughness.

Temperature Effect on Microstructure Evolution and Mechanical Properties of Fe–28Mn–8Al–1C Lightweight Steel via Supersonic Fine Particle Bombardment

The influence of room temperature (RT) and cryogenic temperature (CR) supersonic fine particle bombardment (SFPB) on the surface, microstructure, and mechanical properties of Fe–28Mn–8Al–1C lightweight steel is investigated in this work. The results indicate that both RT‐SFPB and CR‐SFPB successfully induce gradient nanostructures on the surface of steel, refining the grains to the nanoscale. Furthermore, CR‐SFPB results in a finer grain size and higher dislocation density compared to RT‐SFPB. Additionally, the dominant deformation mechanism shifts from dislocation slip for RT‐SFPB to a combination of dislocation slip and twinning for CR‐SFPB. CR‐SFPB is seen to be superior to RT‐SFPB in terms of surface integrity and strength due to low‐temperature lubrication effect, suppression of dynamic recovery and reduced stacking fault energy of the material. Interestingly, while CR‐SFPB enhances strength, elongation remains comparable to that of untreated material. However, excessive impact times during SFPB treatment promote microcrack formation on the surface, compromising plasticity.

Dynamic response of equiatomic and non-equiatomic CrMnFeCoNi high-entropy alloys under plate impact

A typical Fe-rich non-equiatomic CrMnFeCoNi high-entropy alloy (HEA), Cr10Mn10Fe60Co10Ni10, and the equiatomic Cr20Mn20Fe20Co20Ni20 HEA, are investigated via plate impact experiments along with in situ free surface velocity measurements. Both the initial and postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. These two HEAs contain a single face-centered cubic phase, are texture-free, and have similar grain sizes. Dynamic yield stress and spall strength of the non-equiatomic CrMnFeCoNi are lower by ∼55 % and 15 % than those of its equiatomic counterpart, respectively. After shock compression, dislocation slips, stacking faults, Lomer-Cottrell locks and nanotwins are found in the postmortem samples of both HEAs. Due to the reduced stacking fault energy in the non-equiatomic HEA, more deformation twins and newly formed hexagonal close-packed phases are found. For spallation, intergranular voids are larger in size but smaller in quantity than intragranular voids for both HEAs. Both intragranular or intergranular voids show strong dependence on grain boundary misorientation.

Phase Engineering and Dispersion Stabilization of Cobalt toward Enhanced Hydrogen Evolution.

Phase engineering is promising to increase the intrinsic activity of the catalyst toward hydrogen evolution reaction (HER). However, the polymorphism interface is unstable due to the presence of metastable phases. Herein, phase engineering and dispersion stabilization are applied simultaneously to boost the HER activity of cobalt without sacrificing the stability. A fast and facile approach (plasma cathodic electro deposition) is developed to prepare cobalt film with a hetero-phase structure. The polymorphs of cobalt are realized through reduced stacking fault energy due to the doping of Mo, and the high temperature treatment resulted from the plasma discharge. Meanwhile, homogeneously dispersed oxide/carbide nanoparticles are produced from the reaction of plasma-induced oxygen/carbon atoms with electro-deposited metal. The existence of rich polymorphism interface and oxide/carbide help to facilitate H2 production by the tuning of electronic structure and the increase of active sites. Furthermore, oxide/carbide dispersoid effectively prevents the phase transition through a pinning effect on the grain boundary. As-prepared Co-hybrid/CoO_MoC exhibits both high HER activity and robust stability (44mV at 10mA cm-2, Tafel slope of 53.2mV dec-1, no degradation after 100 h test). The work reported here provides an alternate approach to the design of advanced HER catalysts for real application.

Superior strength and wear resistance of mechanically deformed High-Mn TWIP steel

In the present study, the mechanical and wear behaviour of the surface-mechanically treated high-manganese (high-Mn) twinning-induced plasticity (TWIP) steel were investigated. The TWIP alloy was first designed and fabricated via surface-mechanical attrition treatment (SMAT) system and the mechanical properties including strength, wear behaviour as well as the microstructural evolution were thereafter determined. Transmission electron microscopy (TEM) characterization revealed a typical dislocation as a result of the surface treatment as well as the formation of twin layers with a reduced stacking fault energy (SFE). Due to the ultra-fine grain refinement caused by plastic deformation during surface treatment, a microhardness value of 489 HV can be obtained after treatment. Likewise, the yield strength of the high-Mn TWIP steel could be enhanced from 360 MPa to 813 MPa and a reduction in elongation to failure of about 20 % can be achieved. The wear test showed that the treated TWIP steel possessed a reduced friction coefficient and improved wear resistance at different testing loads, attributed to the nanoscale refinement of grains induced during treatment. The strength, hardness, and wear resistance of the fabricated TWIP alloy improves significantly, thanks to surface treatment by SMAT.

Mechanisms for high creep resistance in alumina forming austenitic (AFA) alloys

Castable alumina forming austenitic (AFA) alloys have demonstrated superior creep life and oxidation resistance at temperatures exceeding 800⁰C. Despite the success in the applicability of these alloys in extreme environments, there is a limited understanding of the deformation modes and the influence of each alloying element guiding the alloy design strategies that could further enhance the creep strength of these AFA alloys, particularly at temperatures at and above 900⁰C. In this study, we reveal the mechanism underpinning the superior creep performance of castable AFA alloys that involves suppressing primary carbide formation through minor compositional modification. This approach results in a three-fold increase in creep strength at 900⁰C and 50 MPa. By employing integrated characterization techniques, we analyzed the microstructures of two AFA alloys, both before and after the creep process. We discovered that the suppression of primary carbides permits the in-situ clustering of now-available interstitial elements such as C, Si, and O during high-temperature creep. This improved solid solution strengthening and reduced stacking fault energy of the alloy. Moreover, it also enabled controlled secondary carbide formation during testing, further improving the creep resistance. These findings underline the important interplay between alloy composition, microstructure, and creep properties, and offer a promising design strategy for developing economical high-temperature Fe-based alloys suitable for advanced applications.

Compositive role of refractory element Mo in improving strength and ductility of face-centered-cubic complex concentrated alloys

Complex concentrated alloys (CCAs) with a face-centered-cubic (FCC) structure exhibit remarkable mechanical properties, introducing the expansion of compositional space in alloy design for structural materials. The formation of a single solid-solution phase is enabled by configuring various 3d-transition elements, while doping other elements even of a small portion generally leads to the formation of brittle intermetallic compounds. Herein, we demonstrate through a systematic investigation of single FCC (CoNi)100-xMox alloys that a wide range of refractory element Mo can simultaneously improve the strength and ductility while sustaining the solid-solution structure. The addition of Mo with a larger atomic size than those of 3d-transition elements introduces severe lattice distortion in the FCC lattice and causes grain-boundary segregation enriched by Mo atoms. In addition, increasing Mo content effectively reduces the stacking fault energy (SFE). The increased lattice distortion with Mo content enhances the solid-solution strengthening of the alloys. Besides, along with reduced SFE and stabilization of the dislocation emission site by grain-boundary segregation, this elevated solid-solution strengthening increases grain-boundary strengthening, reaching a yield strength of ∼1 GPa. Moreover, the reduction of SFE with increasing Mo results in the transition of dislocation substructures and the refinement of deformation twins, allowing for enhanced strain-hardening capability and thus ∼1.3 GPa tensile strength and ∼50% ductility. Such compositive and synergetic effects of refractory element Mo enable the CCAs with a single FCC solid solution to overcome the strength and ductility trade-off.

Ultrasonic Cavitation Erosion Behavior of CoCrxFeMnNi High-Entropy Alloy Coatings Prepared by Plasma Cladding

CoCrxFeMnNi (x represents the atomic percentage of Cr element, x = 20, 25, 30, and 35, denoted as Cr20, Cr25, Cr30, and Cr35 alloys) high-entropy alloy (HEA) coatings were cladded by plasma arc on the surface of 0Cr13Ni5Mo steel. The effects of Cr elements on the cavitation erosion mechanisms were studied by comparing the differences of microstructure, microhardness, cavitation erosion volume loss (CVL), cavitation erosion volume loss rate (CER), and eroded surface morphologies between the coatings. As the Cr content increased, the microhardness of the coatings increased continuously, and the microstructure transformed into fine dendrites. The microhardnesses of Cr20, Cr25, Cr30, and Cr35 were 223.9 HV, 250.5 HV, 265.2 HV, and 333.7 HV, respectively. With structural change, the slip pattern shifted from uniform distribution to distribution along the grain boundary, increasing slip resistance. Additionally, strain hardening capacity increased with reduced stacking fault energy (SFE). The resistance to cavitation erosion (CR) of the HEA increased with the increase in Cr content. The CVL of 20 h cavitation erosion of Cr35 coating was only 26.84% of that of 0Cr13Ni5Mo steel, and the peak CER was only 28.75% of that of 0Cr13Ni5Mo steel. The fracture damage mechanisms of the four HEA coatings were an obvious lamellar structure and fibrous fracture.

Multiscale defects enable synergetic improvement in yield strength of CrCoNi-based medium-entropy alloy fabricated via laser-powder bed fusion

Recently, laser-powder bed fusion (L-PBF) has overcome a shortcoming concerning the relatively modest yield strength in the face-centered cubic structure of CrCoNi medium-entropy alloy (MEA); nevertheless, further enhancement remains challenging because the as-built defects are limited to dislocation cell structures and nano-inclusions. In this study, several the types of hierarchical defect structures are explored (including stacking faults, nano-twins, σ phase, and nano-precipitates) with the addition of Si to reduce the stacking fault energy and promote segregation at dislocation cells with Cr. The CrCoNiSi0.3 alloy is fabricated by the L-PBF process, and the effects of the Si addition and L-PBF processing on the hierarchical multiscale defects and corresponding mechanical responses are unraveled. The highest apparent density above 99.5 % is achieved under optimized conditions, exhibiting a high yield strength of 929 MPa owing to a synergetic effect from the generated defects comprising σ phase, nano-twins, and planar defects with moderate ductility of 14 %. In addition, the reduced stacking fault energy promotes deformation twinning, resulting in steady strain hardening. The alloy ultimately exhibits a tensile strength of 1264 MPa, with a moderate ductility of 14 %. A post-heat treatment induces a morphological change in the σ phase from a film-type at the cell walls to particulates at the cell junctions, leading to a significant increase in ductility without a loss of tensile strength, despite a loss of yield strength. This work provides insights to overcome the pre-existing limitations by imposing and adjusting multiscale defects.

Inherent and multiple strain hardening imparting synergistic ultrahigh strength and ductility in a low stacking faulted heterogeneous high-entropy alloy

Alloys with high yield strength and ductility are attractive for application because of their potential to offer mass reduction, energy savings, and enhanced structural reliability. However, increasing strength usually comes at the expense of ductility, which is commonly known as the strength-ductility trade-off for metal alloys. In this work, we explored a strategy of using a heterogeneous grain size structure and a reduced stacking fault energy in a CoCrFeNiMn FCC high entropy alloy to overcome the limitations of this trade-off. By this approach, the alloy achieved a yield strength of 980 MPa, a tensile strength of 1385 MPa, and tensile elongation to failure of 48% benefiting from cooperative strain hardening via multiple mechanisms, such as hetero-deformation induced (HDI) hardening, deformation twinning, Frank-Read dislocation sources and Lomer–Cottrell dislocation locks instigated by such structures. These micromechanisms of deformation help to harden the alloy via in situ refinement of the mean free paths for dislocations.

High entropy alloys towards industrial applications: High-throughput screening and experimental investigation

Using the Thermo-Calc implementation of the CALPHAD approach, high-throughput screening of the Co–Cr–Fe–Mn–Ni system was implemented to find ‘islands’ of single phase FCC structure within the compositional space in order to reduce the cost of this well-studied alloy system. The screening identified a region centred around Co10Cr12Fe43Mn18Ni17, reducing the material cost compared to the equiatomic alloy by ∼40%. The alloy was experimentally investigated at room and elevated temperatures, including in-situ tensile testing. The alloy was found to possess slightly lower strength compared to the equiatomic alloy at room temperature, however, exhibited excellent thermal strength up to 873K. Deformation twinning was observed after tensile testing at room temperature, primarily attributed to the reduced stacking fault energy (SFE), which was proven by a thermodynamic model for calculating the SFE. The softening behaviour at room temperature can be explained through solid solution hardening (SSH), whereby a modified approach to Labusch's model was used to calculate the SSH in reported alloys in this study within the Co–Cr–Fe–Mn–Ni system. The modified models for SFE and SSH are proposed to be implemented into high-throughput screening algorithms for accelerated alloy design towards specific mechanical properties.

Effects of cryogenic temperature on tensile and impact properties in a medium-entropy VCoNi alloy

Multi-principal element alloys usually exhibit outstanding strength and toughness at cryogenic temperatures, especially in CrMnFeCoNi and CrCoNi alloys. These remarkable cryogenic properties are attributed to the occurrence of deformation twins, and it is envisaged that a reduced stacking fault energy (SFE) transforms the deformation mechanisms into advantageous properties at cryogenic temperatures. A recently reported high-strength VCoNi alloy is expected to exhibit further notable cryogenic properties. However, no attempt has been made to investigate the cryogenic properties in detail as well as the underlying deformation mechanisms. Here, the effects of cryogenic temperature on the tensile and impact properties are investigated, and the underlying mechanisms determining those properties are revealed in terms of the temperature dependence of the yield strength and deformation mechanism. Both the strength and ductility were enhanced at 77 K compared to 298 K, while the Charpy impact toughness gradually decreased with temperature. The planar dislocation glides remained unchanged at 77 K in contrast to the CrMnFeCoNi and CrCoNi alloys resulting in a relatively constant and slightly increasing SFE as the temperature decreased, which is confirmed via ab initio simulations. However, the deformation localization near the grain boundaries at 298 K changed into a homogeneous distribution throughout the whole grains at 77 K, leading to a highly sustained strain hardening rate. The reduced impact toughness is directly related to the decreased plastic zone size, which is due to the reduced dislocation width and significant temperature dependence of the yield strength.

Displacive transformation as pathway to prevent micro-cracks induced by thermal stress in additively manufactured strong and ductile high-entropy alloys

Abstract The micro-cracking behaviors of two high-entropy alloys (HEAs) of the FeMnCoCrNi family prepared by selective laser melting were systematically studied. Residual stresses were also analyzed by X-ray diffraction technique. Results show that the equiatomic FeMnCoCrNi HEAs with a relatively stable single-phase face-centered cubic (FCC) structure suffered from micro-cracking with residual tensile stress after laser melting. In contrast, the metastable non-equiatomic FeMnCoCr HEAs with reduced stacking fault energy are free of micro-cracks with residual compressive stress at various volumetric energy densities (VEDs). The displacive transformation from the FCC matrix to the hexagonal close-packed (HCP) phase during cooling prevents the micro-cracking via consuming thermal stress related internal energy. Further, the displacive transformation during tensile deformation contributes to the higher strength and ductility of the metastable dual-phase HEA compared to that of the stable single-phase HEA. These findings provide useful guidance for the design of strong, ductile, and crack-free alloys for additive manufacturing by tuning phase stability.

Exploiting the synergic strengthening effects of stacking faults in carbon nanotubes reinforced aluminum matrix composites for enhanced mechanical properties

Carbon nanotubes (CNTs) reinforced Al matrix composites containing high-density stacking faults were successfully fabricated through spark plasma sintering (SPS) and subsequently hot rolling with high strain rate. It is found that the inclusion of CNTs facilitate the formation of stacking fault in Al matrix. With increasing the CNTs from 0.5 vol% to 1.0 vol%, the density of stacking fault increases, which is mainly because of the reduced stacking fault energy of Al matrix in Al/CNTs composites, as revealed by the density functional theory simulation. Combined with the effective exploitation of the strengthening effectiveness of CNTs due to uniform dispersion of CNTs and well bonded Al-CNTs interface, the highest tensile strength (378 ± 8 MPa) together with good ductility (17.1 ± 1.5%) can be achieved for 1 vol% CNTs reinforced Al matrix composite. Moreover, the enhanced strength deriving from each strengthening mechanism relies on the CNTs content. When the CNTs content is 1.0 vol%, Orowan looping and stacking fault strengthening are the dominant strengthening mechanisms. The reported microstructure design and control can be informative for improving the mechanical properties of CNTs reinforced metal matrix composites.

Novel Si-added CrCoNi medium entropy alloys achieving the breakthrough of strength-ductility trade-off

A novel CrCoNiSix (x = 0.1, 0.2, 0.3) medium entropy alloys (MEAs) system was designed to achieve enhanced strength and ductility. The CrCoNiSix MEAs are all single-phase face-centered cubic (FCC) structure. The recrystallization rate and average grain size obviously increase with the increase of Si. Compared to CrCoNi MEA, the ultimate tensile strength (UTS) and uniform elongation (UE) of CrCoNiSi0.3 MEA were increased from 790 MPa to 960 MPa and 58% to 92%, respectively, and the product of UTS and total elongation (TE) increased up to 88 GPa% (46 GPa% for CoCrNi MEA), superior to most high strength-ductility alloys. The stacking-fault energy (SFE) in the Si-added MEAs decreases with Si addition, as proved by thermodynamic model. Therefore, the underlying strengthening mechanism is the reduction of SFE and increase in the lattice distortion via Si addition. More concentrated and thinner deformation twins and multiple twinning structures were observed in Si-added MEAs. Furthermore, a nanoscale diffusionless transformation from the FCC to the hexagonal close-packed (HCP) phase occurred at room-temperature tension in Si-added MEAs, which further increased the work hardening and uniform elongation. This work provides a new and significant approach to break through the strength-ductility trade-off in FCC MEAs/HEAs, especially serving as energy-absorbing materials.

High ductility and strong work-hardening behavior of Zn modified as-hot-rolled Al–Mg sheets

High ductility combining with relative high strength is of great significance for the application of Al–Mg alloys. In this study, excellent ductility and moderated strength are obtained in the as-hot-rolled Al-9.2Mg-0.5Zn sheet, with ultimate tensile strength (σb), yield tensile strength (σ0.2) and elongation (δ) are respectively 434 ± 5 MPa, 228 ± 2 MPa and 48 ± 2%. High ductility of the as-hot-rolled Al-9.2 Mg-xZn (0-1.5 wt%) sheets can be ascribed to their strong work-hardening abilities, which increase firstly and then decline with higher Zn content. The highest work-hardening component (n), work-hardening capacity (Hc) and the initial work-hardening rate (θ0) in the Al-9.2Mg-0.5Zn sheet are 0.428, 1.793 and 3496 MPa, respectively. The microstructures of the Al–Mg–Zn sheets are characterized by electron backscattered diffraction (EBSD), transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning electron microscope (SEM). Low density of pre-existing dislocations (PDs), random grain orientations and reduced stacking fault energy (SFE) contribute to the strong work-hardening capacity and lead to desirable ductility in the as-hot-rolled Al–Mg–Zn sheets. However, numerous precipitates with an uneven distribution are harmful for work-hardening and thus lead to the reduced ductility in the Al–Mg sheet with high Zn content.

Cryogenic Mechanical Behavior of a TRIP-Assisted Dual-Phase High-Entropy Alloy

The recently developed dual-phase (DP) non-equiatomic Fe50Mn30Co10Cr10 (at. %) high-entropy alloy (HEA) showed much higher strength and ductility compared to the single-phase equiatomic Fe20Mn20Ni20Co20Cr20 (at. %) HEA at room temperature. Here we probe the cryogenic mechanical properties of the non-equiatomic DP-HEA with different grain sizes and compare with the equiatomic single-phase HEA. Our results show that the cryogenic ultimate tensile strength of the coarse-grained (~200 μm) and fine-grained (~4 μm) DP-HEAs reach up to 1133 MPa and 1342 MPa, respectively, which are significantly higher than that of the equiatomic single-phase HEAs with similar grain sizes. Furthermore, the fine-grained DP-HEA shows substantial improvement in both strength and ductility compared to the coarse-grained counterparts at cryogenic temperatures. Microstructural analysis revealed that the enhanced mechanical properties of the DP-HEA at cryogenic temperatures are attributed to a more extensive displacive transformation from the face-centered cubic (FCC) matrix into the hexagonal close-packed (HCP) phase compared to that at room temperature. Specifically, the HCP phase fraction in tensile tested fine-grained DP-HEAs increases from ~39 % to ~79% with decreasing temperature from 298 K to 77 K. The enhanced transformation behavior is enabled by the reduced stacking fault energy of the material with the decrease of deformation temperatures. The resulting outstanding combination of strength and ductility further suggest that the DP-HEAs are promising candidates as structural materials for cryogenic applications.

Effects of Ga Content on Dynamic Recrystallization and Mechanical Properties of High Strain Rate Rolled Mg–Ga Alloys

The Mg–xGa (x = 1, 2, 3 and 5 in mass%) alloys are subjected to high strain rate rolling (HSRR) at 275 °C with the rolling strain rate of 9.1 s−1 to develop high performance Mg alloy sheets with high plasticity. Effects of Ga content on microstructure and mechanical properties of the Mg–Ga alloys are investigated by SEM, XRD, tensile testing and etc. The Ga addition can reduce the critical strain of DRX in Mg alloys, which is associated with the reduced stacking fault energy, the increased twinning density during deformation and the more DRX nucleation sites during HSRR. With the Ga content increasing from 2 to 3%, the reduced DRX degree is attributable to the hindrance of dynamic precipitates. With the Ga content increasing from 3 to 5%, the slightly increased DRX degree can be ascribed to the relatively coarse precipitates. The Mg–2 Ga alloy sheet, featured with complete DRX, exhibits an ultra-high plasticity (with the elongation to rupture of 36.6%) and a relatively low anisotropy of yield strength and plasticity. The Mg–5 Ga alloy sheet has the best comprehensive mechanical properties, with the ultimate tensile strength of 292 MPa, yield strength of 230 MPa and elongation to rupture of 30.3%, which can be ascribed to the combination of grain refinement strengthening and precipitation strengthening. Effects of Ga content on microstructure, dynamic recrystallization (DRX) and mechanical properties of the Mg-Ga alloys prepared by high strain rate rolling (HSRR) are systematically studied. Among all the alloys, the HSRRed Mg-2Ga alloy exhibits the smallest DRX critical strain, an ultrahigh plasticity (δ up to 36.6%) and a low anisotropy of yield strength and plasticity, due to the complete DRX. The HSRRed Mg-5Ga alloy shows the optimal mechanical properties, with σb of 292 MPa, σ0.2 of 230 MPa and δ of 30.3%. The high strength is mainly attributed to precipitation strengthening and grain refinement strengthening.

On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions

We investigate the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe49.5Mn30Co10Cr10C0.5 (at. %). Extraordinary strain hardening of the iHEA at 77 K leads to a substantial increase in ultimate tensile strength (∼1300 MPa) with excellent ductility (∼50%) compared to that at room temperature. Prior to loading, iHEAs with coarse (∼100 μm) and fine (∼6 μm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to ∼70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. The current investigation thus offers guidance for the design of high-performance HEAs for cryogenic applications.