Metastable High-entropy Alloys Research Articles

Sustaining Strength-Ductility Synergy of Slm Fe50mn30co10cr10 Metastable High-Entropy Alloy by Si Addition

Sustaining Strength-Ductility Synergy of Slm Fe50mn30co10cr10 Metastable High-Entropy Alloy by Si Addition

Modifications of partial-dislocation-induced defects and strength/ductility enhancement in metastable high entropy alloys through nitrogen doping

We identified the role of partial-dislocation-induced defects (PDIDs) on the nanostructural evolution and the resultant macroscopic deformation in high entropy alloys (HEAs) with their phase stability modified via nitrogen doping. We found that deformation proceeds by the extension and expansion of partial dislocation induced defects (PDIDs) such as extended stacking faults (ESFs), ε-martensite and deformation twin bands in these alloys. Simultaneous strength/ductility enhancement was achieved through modifications of PDIDs and phase stability in Fe50Mn25Cr15Co10 by nitrogen addition. We suggest that the formation of closely spaced ESFs reduce the stacking irregularity and therefore closely spaced ESFs is the energetically stable deformation nanostructure. The formation of closely spaced ESFs has the advantage of the deformation homogeneity because of the more uniform slip strain distribution by smaller shear vector of partial dislocations and the large strain rate sensitivity, leading to the enhanced ductility. This intrinsic nature of overlapped ESFs on the effective reduction of stacking irregularities induces the development of closely spaced ESFs and their transition to ε-martensite band and then to deformation twin band with increasing strain. The modifications of various PDIDs in high entropy alloys with their phase stability modified by nitrogen doping is in good agreement with the prediction of the partial dislocation induced deformation nanostrucure based on the energy criteria under quasi-static deformation proposed in this study.

Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

We present a new class of metastable high-entropy alloys (HEAs), triggering deformation-induced martensitic transformation (DIMT) from face-centered-cubic (FCC) to body-centered-cubic (BCC), i.e., BCC-DIMT. Through the ab-initio calculation based on 1st order axial interaction model and combined with the Gibbs free energy calculation, the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy (ISFE) in SixV(9-x)Cr10Mn5Fe46Co30 (x = 2, 4, and 7 at.%) alloy system. The ISFE decreases from -30.4 to -35.5 mJ/m2 as the Si content increases from 2 to 7 at.%, which well corresponds to the reduced phase stability of FCC against HCP. The BCC-DIMT occurs in all the alloys via intermediate HCP martensite, and the HCP martensite provides nucleation sites of BCC martensite. Therefore, the transformation rate enhances as the Si content increases in an earlier deformation range. However, the BCC-DIMT is also affected by the phase stability of FCC against BCC, and the stability is the highest at the Si content of 7 at.%. Thus, the 7Si alloy presents the moderate transformation rate in the later deformation range. Due to the well-controlled transformation rate and consequent strain-hardening rate, the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature. Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.

Aged metastable high-entropy alloys with heterogeneous lamella structure for superior strength-ductility synergy

High-entropy alloys containing multi-principal-element systems significantly expand the potential alloy design space, and offer the possibility of overcoming the strength-ductility trade-off in metallurgical research. However, the gain in ultra-high strength through traditional grain refinement and precipitation-strengthening mechanisms inevitably leads to a drastic loss of ductility. Here, we report on the design and fabrication of heterogeneous-lamella structured, aged bulk high-entropy alloy, which attains gigapascal tensile strength while retaining excellent ductility (UTS ~1.4 GPa, elongation ~30%; UTS ~1.7 GPa, elongation ~10%). Our work shows that the improved strength-ductility synergy arises due to various complementary strengthening mechanisms, including solid-solution, interfaces, precipitation and martensitic transformation, which influence the hardening and deformation processes at different strain levels. In particular, the hetero-deformation that is associated with the formation of microbands as well as the stress-induced martensite promotes additional hardening and hence high ductility. The strategy described here, that is leveraging the concept of heterogeneous microstructure design, provides a practical and novel method for fabricating high-performance structural materials.

The metastable constituent effects on size-dependent deformation behavior of nanolaminated micropillars: Cu/FeCoCrNi vs Cu/CuZr

Metastable high entropy alloys (HEAs) and amorphous metallic glasses (MGs), with the chemical disordered character, are intensively studied due to their excellent performance. Here, we introduce Cu to separately constrain these two metastable materials and comparatively investigate their deformation behaviors and mechanical properties of Cu/HEA FeCoCrNi and Cu/MG CuZr nanolaminated micropillars in terms of intrinsic layer thickness h and extrinsic pillar diameter D. The metastable HEA layers, as the hard phase in Cu/HEA micropillars, are stable and dominate the deformation, while transformation (crystallization) occurs in MG which plays a minor role in deformation of Cu/MG micropillars. The h-controlled deformation mode transits from the D-independent homogenous-like deformation at large h to the D-dependent shear banding at small h in both Cu/HEA and Cu/MG micropillars. Although both Cu/HEA and Cu/MG micropillars exhibit a maximum strain hardening capability controlled by h, the former manifests much lower hardening capability compared with the latter. The intrinsic size h and extrinsic size D have a strong coupling effect on the strength of Cu/HEA and Cu/MG micropillars. The strength of strength of Cu/HEA micropillars exhibits the D-dependent transition from “smaller is stronger” to “smaller is weaker” with increasing h. By contrast, the strength of Cu/MG micropillars exhibits the transition from bulk-like D-independent behavior at large h to small volume D-dependent behavior (smaller is stronger) at small h.

Effect of Strain Rate on Deformation Response of Metastable High Entropy Alloys Upon Friction Stir Processing

Effect of strain rate on the tensile behavior of two friction stir-processed dual-phase high entropy alloys was investigated. The dual-phase high entropy alloys have a high fraction of metastable f.c.c. phase. Engineering the f.c.c. phase metastability through alloy chemistry leads to different levels of transformation and impacts the retained f.c.c. phase after deformation. These metastable high entropy alloys show dominance of transformation-induced plasticity (TRIP) irrespective of strain rates, unlike the observations in TRIP steels.

The tension-compression asymmetry of martensite phase transformation in a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy

The microstructural evolution of a metastable face centered cubic (FCC) Fe40Co20Cr20Mn10Ni10 high-entropy alloy (HEA) under both tension and compression is systemically investigated. The results show much higher level of martensite phase transformation from FCC structure to hexagonal closed packed (HCP) structure under compression than tension, indicating a distinct tension-compression asymmetry. The compressive tests underwent higher true stresses, which further provided stronger driving forces to trigger the phase transformation than those in tensile tests. Except for the martensite phase transformation, dislocation planar slip prevails in both tension and compression, along with the occasional formation of mechanical twins. Dislocation slip dominates the whole tensile deformation, while both dislocation motions and martensite phase transformation play critical roles in the compressive deformation. The martensite phase transformation is preferred to nucleate at grain or subgrain boundaries due to a medium stacking fault energy (SFE) of ∼20 mJ m−2. The formation of HCP phase via partial dislocation emission from low angle grain boundaries offers additional pathways for martensite phase transformation. Our study thus remarkably benefits the understanding of the de formation mechanisms of metastable HEAs.

Tuning δ → γ transformation types to relieve mechanical property degradation in a Co-free face-centered cubic metastable high-entropy alloy

In face-centered cubic Fe-Mn-based high manganese alloys, stacking fault energy could be lowered remarkably by silicon alloying and thus the martensitic transformation of γ-austenite to ε-martensite could be facilitated. Inspired by this, a Co-free face-centered cubic Fe59Mn15Cr11Si9Ni6 metastable high-entropy alloy was developed by replacing cobalt by silicon and adjusting chemical compositions on the basis of Fe-Mn-Cr-Co-Ni high-entropy alloys. The Fe59Mn15Cr11Si9Ni6 alloy solution treated at 1273 K for 30 min (295 MPa yield strength, 776 MPa ultimate strength and 63.6% elongation) exhibited better mechanical properties than a typical Fe50Mn30Co10Cr10 dual-phase metastable high-entropy alloy (~250 MPa yield strength, ~730 MPa ultimate strength and ~50.0% elongation) since their austenitic grains were around 45 µm. When the Fe59Mn15Cr11Si9Ni6 alloy was heat-treated at 1548 K for 30 min, δ → γ massive transformation and δ → γ long-range diffusional transformation would occur after brine quenching and air cooling, respectively. The degradation of ultimate strength and elongation was much weaker after δ → γ massive transformation than after δ → γ long-range diffusional transformation with reference to the solution-treated Fe59Mn15Cr11Si9Ni6 high-entropy alloy. The degradation of mechanical properties derived from δ → γ phase transformation should be relieved through promoting the occurrence of δ → γ massive transformation. It should be a helpful strategy to relieve the degradation of mechanical properties for Fe-Mn-Cr-Si-Ni metastable high-entropy alloys prepared by casting, welding and additive manufacturing.

Atomic deformation mechanism and interface toughening in metastable high entropy alloy

Metastable high entropy alloy (HEA) with excellent properties have attracted extensive attentions recently. However, as a consequence of limited experiments of high-resolution transmission electron microscopy (HRTEM) and the difficulties of molecular dynamic (MD) simulations for the phase transformation process, the detailed atomic deformation mechanisms in the HEA is not well understood. We carry out the in situ HRTEM observation of the martensitic transformation process and find surprisingly wide phase interface between the parent and the martensite in a typical high strength and high elongation metastable HEA. One specific interatomic potential is developed for the metastable HEA and large-scale MD simulation is carried out to investigate the martensitic transformation process from body-centered cubic to hexagonal close packed structures. The whole processes of the stress-induced martensitic transformation (nucleation, incubation, bursting and propagating of the new phase) are well reproduced in the MD simulations, suggesting its good agreements with the HRTEM observations. The width of the phase interface mainly depends on the competition between interfacial energy and lattice distortion energy during the martensitic transformation process. This wide phase interface acts as a buffer to coordinate the martensitic transformation induced strain and as a buffer storage for dislocation gliding and pile-up. As a result, the metastable HEA achieves a high strength combined with a large tensile elongation. The revealed atomic-scale deformation and corresponding interatomic potential should be useful to guide the design in the new series of high-performance metastable alloy.

Carbon and nitrogen co-doping enhances phase stability and mechanical properties of a metastable high-entropy alloy

Abstract To probe the significant interstitial effects coupling with the metastability design of high-entropy alloys (HEAs), we introduced both interstitial C and N into a representative metastable HEA. The mechanical behavior of the novel C-N co-doped interstitial HEA (iHEA) with nominal composition Fe48.5Mn30Co10Cr10C0.5N1.0 (at. %) in different grain sizes has been investigated via digital image correlation (DIC) assisted tensile testing. The microstructure characteristics prior to and after tensile deformation with different local strain levels were probed by various techniques. The C-N co-doped iHEA shows a face-centered cubic (FCC) matrix without hexagonal closed packed (HCP) martensite after annealing plus the subsequent quenching, different from the reference metastable FCC-HCP dual-phase HEA without interstitials. Upon tensile deformation, dislocation slip and twinning prevail, and the new iHEA does not show phase transformation which is the primary deformation mechanism in the reference interstitial-free metastable HEA. This further confirms the enhanced FCC phase stability due to C-N co-doping. The interstitial effect coupling with the multiple strengthening mechanisms including twinning induced plasticity (TWIP) and nano-particle strengthening due to nano-sized carbonitrides leads to significantly higher yield strength (753 MPa) and ultimate tensile strength (1038 MPa) at good ductility (48.0%) compared to that of the reference interstitial-free metastable HEA (∼250 MPa, ∼850 MPa and ∼70%, respectively) at a similar grain size of ∼4 μm. Accordingly, our work demonstrates the significant capability of interstitials including C and N for tuning the phase stability and enhancing mechanical properties of HEAs.

Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy

Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation.

Excellent strength-ductility synergy in metastable high entropy alloy by laser powder bed additive manufacturing

High entropy alloys (HEAs) have attracted scientific interest due to their good mechanical properties and failure resistance, whereas additive manufacturing (AM) has emerged as a powerful yet flexible processing route for advanced materials. However, limitations inherent in both these fields include HEAs display inferior mechanical properties in as cast condition; and AM demands expansion of printable alloys. A new metastable Fe40Mn20Co20Cr15Si5 (CS-HEA) HEA showing stabilized ε-h.c.p. dominated microstructure after laser powder bed fusion additive manufacturing has been evaluated. As-printed CS-HEA showed higher strength due to high work hardenability, whereas substantial uniform ductility is associated with a combination of transformation and twinning induced plasticity during deformation. Additionally, very low volume percent of voids (∼0.1 %) along with high strength-ductility shows excellent printability of the CS-HEA using laser-based additive manufacturing.

Radiation damage tolerance of a novel metastable refractory high entropy alloy V2.5Cr1.2WMoCo0.04

A novel multicomponent alloy, V2.5Cr1.2WMoCo0.04, produced from elements expected to favour a BCC crystal structure, and to be suitable for high temperature environments, was fabricated by arc melting and found to exhibit a multiphase dendritic microstructure with W-rich dendrites and V–Cr segregated to the inter-dendritic cores. The as-cast alloy displayed an apparent single-phase XRD pattern. Following heat treatment at 1187 °C for 500 h the alloy transformed into three different distinct phases - BCC, orthorhombic, and tetragonal in crystal structure. This attests to the BCC crystal structure observed in the as-cast state being metastable. The radiation damage response was investigated through room temperature 5 MeV Au+ ion irradiation studies. Metastable as-cast V2.5Cr1.2WMoCo0.04 shows good resistance to radiation induced damage up to 40 displacements per atom (dpa). 96 wt% of the as-cast single-phase BCC crystal structure remained intact, as exhibited by grazing incidence X-ray diffraction (GI-XRD) patterns, whilst the remainder of the alloy transformed into an additional BCC crystal structure with a similar lattice parameter. The exceptional phase stability seen here is attributed to a combination of self-healing processes and the BCC structure, rather than a high configurational entropy, as has been suggested for some of these multicomponent “High Entropy Alloy” types. The importance of the stability of metastable high entropy alloy phases for behaviour under irradiation is for the first time highlighted and the findings thus challenge the current understanding of phase stability after irradiation of systems like the HEAs.

Reversible dislocation movement, martensitic transformation and nano-twinning during elastic cyclic loading of a metastable high entropy alloy

The present study contends with the room temperature microstructural response of a non-equiatomic metastable high entropy alloy to the elastic cyclic deformation. The stress and strain-induced martensite formation and reversion are recognized as the main microstructural evolutions which are directly correlated with the reversibility of dislocation movement. Two different patterns of reversion for deformation driven epsilon martensite are identified. Full reversion of stress-induced epsilon martensite results in development of nano-twined matrix, the various aspects of which have been described through a dislocation-based model. The strain-induced martensite also goes through partial reversion leading to lath fragmentation which in-turn significantly influences the martensite stability. Interestingly, the presence of a well-developed dislocation substructure is characterized within the martensite bands, which seems to be phenomenal owing to the low imposed strain, low temperature and low stacking fault energy of the experimented material. The development of vein and wall/channel structures is justified through proposing a conceptual based model regarding the interaction of the stacking faults and subsequent generation of the perfect dislocations.

Investigating the deformation mechanisms of a highly metastable high entropy alloy using in-situ neutron diffraction

The present study correlates the effect of enhanced metastability on both the well-understood γ-f.c.c. stacking fault energy (SFE) and deformation mechanisms in the ε-h.c.p. phase of a metastable high entropy alloy (HEA). The SFE of a Fe40Mn20Cr15Co20Si5 alloy (CS-HEA) was experimentally determined to be ∼6.31 mJ m−2 using in-situ neutron diffraction. The relatively low-measured SFE of the CS-HEA results in a high fraction of the ε-h.c.p. phase (58 %) triggering significant stress partitioning to ε-h.c.p. and a marginal fraction of γ-f.c.c. → ε-h.c.p. transformation (∼25 %). The ε-h.c.p. phase accommodated a significant amount of strain marked by the large stress-induced decrease of c/a ratio (from ∼1.619 to 1.588), which was accompanied by activation of non-basal deformation modes, such as deformation twinning and pyramidal slip. Using in-situ neutron diffraction, we show by decreasing SFE and stabilization of large fractions of ε-h.c.p., activation of non-basal deformation modes are responsible for high work hardenability in absence of significant γ-f.c.c. → ε-h.c.p. transformation.

Effects of transformation-induced plasticity (TRIP) on tensile property improvement of Fe45Co30Cr10V10Ni5-xMnx high-entropy alloys

A new metastable high-entropy alloy (HEA) system was suggested by thermodynamic calculations based on the Gibbs free energies of FCC and HCP and the associated stacking fault energy (SFE). The Fe45Co30Cr10V10Ni5-xMnx (x = 0, 2.5, and 5 at.%) alloys were fabricated, and their tensile properties were evaluated at room and cryogenic temperatures. The relationship between the deformation mechanism and strain hardening behavior was investigated to reveal the role of deformation-induced martensitic transformation on tensile properties. The difference in Gibbs energy decreases with increasing Mn content, leading to the decreased SFE in sequence. At room temperature, ~60% of BCC martensite in the 5Mn HEA contributes effectively to the steady strain hardening, suppressing the plastic instability. This TRIP effect achieves much eminence in the cryogenic deformation, enabling the tensile strength to reach over 1.6 GPa due to 100% of BCC and HCP martensite. In addition to the fraction of martensite, the increased Mn content reduces a critical strain required to trigger the martensitic transformation and then raises the transformation rate. The present findings may provide a guide for the design of metastable HEAs to enhance tensile properties for cryogenic applications through adjusting SFE and TRIP effect.

In-situ observation of martensitic transformation in an interstitial metastable high-entropy alloy during cathodic hydrogen charging

We show for the first time that a critical amount of dissolved hydrogen can induce a phase transformation from γ-austenite to ε-martensite in an interstitial metastable high-entropy alloy. This is demonstrated by in-situ hydrogen charging in combination with nanoindentation and scanning probe microscopy, plus further electron channeling contrast imaging, X-ray diffraction, and transmission Kikuchi diffraction techniques. The transformed martensites appear as bands on the surface along γ-{111} habit planes, leading to an irreversible increase of hardness. The hydrogen-induced internal stress together with the intrinsic hydrogen effects are proposed to be responsible for the martensitic transformation upon hydrogen charging.

Interstitial-Free Bake Hardening Realized by Epsilon Martensite Reverse Transformation

By investigating a metastable high-entropy alloy, we report a latent strengthening mechanism that is associated with the thermally-induced epsilon-martensite-to-austenite reverse transformation. We show this reversion-assisted hardening effect can be achieved in the same time-scale and temperature range as conventional bake-hardening treatment, but leads to both improved strength and cumulative ductility. Key mechanisms are discussed considering transformation kinetics, kinematics, strengthening and ductilization modules.

FCC to BCC transformation-induced plasticity based on thermodynamic phase stability in novel V10Cr10Fe45CoxNi35\u2212x medium-entropy alloys

We introduce a novel transformation-induced plasticity mechanism, i.e., a martensitic transformation from fcc phase to bcc phase, in medium-entropy alloys (MEAs). A VCrFeCoNi MEA system is designed by thermodynamic calculations in consideration of phase stability between bcc and fcc phases. The resultantly formed bcc martensite favorably contributes to the transformation-induced plasticity, thereby leading to a significant enhancement in both strength and ductility as well as strain hardening. We reveal the microstructural evolutions according to the Co-Ni balance and their contributions to a mechanical response. The Co-Ni balance plays a leading role in phase stability and consequently tunes the cryogenic-temperature strength-ductility balance. The main difference from recently-reported metastable high-entropy dual-phase alloys is the formation of bcc martensite as a daughter phase, which shows significant effects on strain hardening. The hcp phase in the present MEA mostly acts as a nucleation site for the bcc martensite. Our findings demonstrate that the fcc to bcc transformation can be an attractive route to a new MEA design strategy for improving cryogenic strength-ductility.

Microstructural Evolution and Deformation Behavior of Ni-Si- and Co-Si-Containing Metastable High Entropy Alloys

Two high entropy alloy (HEA) compositions were compared to explore the individual effects of Co and Ni on phase stability and resultant deformation response in Fe-Mn-Cr-Si-containing HEAs. It was observed that Co-Si-containing HEA depicted responsive phase evolution upon friction stir processing owing to decreased γ (f.c.c.) matrix stability as against Ni-Si-containing HEA. As a result, the Co-Si HEA showed the presence of dual-phase microstructure with the dominance of e (h.c.p.) phase (52 pct), whereas the Ni-Si HEA showed single-phase γ microstructure under similar processing condition. Also, the dominant deformation mechanisms were different in the two alloys. Co-Si HEA showed uniform strain partitioning between the f.c.c. and h.c.p. phases. Conversely, single-phase f.c.c. microstructure in Ni-Si HEA accommodated strain by twinning-induced plasticity.