Metastable High-entropy Alloys Research Articles

Enhanced deformation-induced FCC-HCP transformation of a metastable high-entropy alloy induced by a smaller grain size

This study reports the grain-size-dependent deformation-induced face-centered cubic (FCC)–hexagonal close-packed (HCP) transformation behavior of Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 (at. %) high-entropy alloy in the medium grain size range (10–20 μm), their mechanical properties, and deformation mechanisms. The materials annealed at 900 °C for 1 h (A900-1) and 10 h (A900-10) showed a fully FCC, recrystallized microstructure with a larger grain size in the latter owing to grain growth during annealing. The A900-1 sample exhibited a larger strength–ductility balance and a higher strain hardening rate than the A900-10 sample. The A900-1 sample, with a larger number of grain boundary nucleation sites of the deformation-induced HCP phase, exhibited a higher HCP transformation rate than the A900-10 sample in the early stage of deformation below 10 % strain, whereas both materials showed similar HCP transformation rates at strain levels higher than 30 %. The emission of deformation-induced HCP plates in both neighboring grains from the same grain boundary (A900-1) and the activation of the deformation-induced HCP phase in a single grain and only dislocations in the neighboring grains (A900-10) suggest the importance of grain boundary stress in the observed phenomena. The observed additional mechanisms other than dislocation slip and deformation-induced HCP phase, that is, reverse FCC-HCP transformation, HCP {101‾2} twinning, and kink banding, could contribute to deformation accommodation and stress relaxation, thereby leading to the excellent ductility of the investigated materials.

Cooperative roles of twinning, transformation and strain partitioning on the excellent cryogenic strength-toughness synergy of dual-heterogeneous metastable high entropy alloys

In a metastable Fe50Mn30Co10Cr10 high entropy alloy (HEA), a dual-heterogeneous structure, encompassing both grain size and dislocation heterogeneity, has been constructed to obtain excellent cryogenic mechanical performance by varying the annealing temperature (630 and 740 °C) following severe deformation. The HS-630 sample, consisting of recovered coarse-grains (CG), recrystallized fine-grains (FGs) and un-recrystallized ultrafine-grains (UFGs), possesses higher yield strength (YS) of ∼965 MPa, ultimate tensile strength (UTS) of ∼1680 MPa and uniform elongation (UEL) of ∼45% when compared to the fully-recrystallized HS-740 sample at liquid nitrogen temperature (LNT). The difference in YS is primarily caused by the grain boundary and dislocation strengthening of UFGs, whereas the higher ductility of the HS-630 sample is associated with the hierarchical activation of transformation- and twinning-induced plasticity (TRIP and TWIP) effects, and compatible strain partitioning between heterogeneous domains. The dual-heterogeneity results in different critical stresses of ε-phase nucleation in heterogeneous domains, and promotes sustainable TRIP effect in CGs and the emergence of TWIP effect in UFGs with strain progressing. Moreover, the strain of the ductile transformed ε-phase is mainly accommodated by non-basal slip, which contributes to the strain hardening in the later deformation stage. Furthermore, the higher cryogenic impact toughness of the HS-630 sample can be attributed to the synergistic effects of the fracture mode, the transformation characteristics, and the heterogeneous strain partitioning.

Modulating hierarchical microstructural components to enhance strength-ductility synergy of a high-entropy alloy containing brittle sigma phase

Although brittle intermetallic phases are usually unwanted in the design of strong and ductile alloys, high strength and good ductility can be achievable in high-entropy alloys (HEAs) containing brittle phases. Here, we present an investigation on modulating the hierarchical microstructural components in a prototype metastable HEA (Fe30Cr25Co25Mn10Ni10, at. %) containing sigma (σ) phase to optimize the mechanical properties. An excellent combination of tensile strength (999.3 MPa) and ductility (57.7 %) is achieved in alloy sample containing recrystallized grains of various sizes, which is mainly attributed to the continuous transformation-induced plasticity (TRIP) effect and the back stress effect caused by the pile-up of geometrically necessary dislocations at various interfaces in the hierarchical microstructures. The decrease in fraction of hard domains (σ particles, non-recrystallized and small-sized grains) can lead to the decrease of tensile strength due to the weakened back stress effect. However, an excessively high fraction of hard domains (particularly with small-sized grains and σ particles) suppresses the TRIP effect, facilitating crack propagation and reducing ductility. The back stress effect can be rationally controlled through tuning the fraction and size of small-sized grains (accompanied with fine σ particles) by thermo-mechanical treatments, which can also enable the activation of TRIP effect in grains of various sizes to render high strain-hardening rate and good strength-ductility synergy. This provides a strategy for achieving an optimum combination of strength and ductility for metastable HEAs containing fine brittle phases via alleviating the embrittling effect of brittle phases and retaining the precipitation strengthening effect.

Formation mechanisms of two types of sigma phases and the effects on mechanical behavior in a quaternary metastable high-entropy alloy

Metastable non-equiatomic FeMnCoCr high-entropy alloys (HEAs) exhibit excellent strength-ductility synergy mainly due to the transformation-induced plasticity (TRIP) effect. However, at relatively high Cr content and certain temperatures, the brittle sigma (σ) phase is prone to be induced, which may deteriorate the mechanical properties. In this work, we reveal two types of σ phases formed in a quaternary Fe40Mn20Co20Cr20 (at.%) HEA with a metastable face-centered cubic (FCC) matrix and illustrate their effects on the deformation behavior of the material. The first type of Cr-rich σ phase (T-type) is formed by the solid-state transformation of the BCC-δ phase and thus they have similar sizes (∼20 μm). The second type of σ phase (P-type) with an average size of ∼500 nm is formed by precipitation with the segregation of Cr at grain boundaries and grain-interior dislocations during recrystallization. Upon deformation, the propagation of cracks in the P-type σ phase particles can be suppressed by martensitic transformation and 9R structure formation in the alloy matrix, leading to a crack buffering effect and contributing to a high ultimate tensile stress of ∼1.2 GPa at a total elongation of ∼15%. Yet, the crack buffering effect can hardly develop in the T-type σ phase with larger sizes and irregular shapes, which accelerates the fracture of the material. This work thus provides insights for controlling the formation of σ phases and the associated design strategies of strong and ductile complex alloys.

Evolution of microstructure and deformation mechanisms in a metastable Fe42Mn28Co10Cr15Si5 high entropy alloy: A combined in-situ synchrotron X-ray diffraction and EBSD analysis

In this work, a combination of in-situ high synchrotron X-ray diffraction and electron backscattered diffraction were used to systematically investigate the activation and evolution of the deformation mechanisms in an as-cast Fe42Mn28Co10Cr15Si5 metastable high entropy alloy deformed until fracture at room temperature. This work unveils the critical role of the dual-phase γ-f.c.c. / ε-h.c.p. microstructure on the deformation response of the alloy. The different deformation modes, i.e., slip, transformation induced plasticity (TRIP) and transformation induced twinning (TWIP), were seen to initiate at different loading stresses and then to overlap. Quantitative microstructural characterization, which included the evolution of the phase fraction, stress partitioning, dislocation density, c/a ratio and lattice strain for different planes, was performed to elucidate the role of each phase on the macroscopic mechanical response of the metastable high entropy alloy. Furthermore, the magnitude of the different strengthening contributions has been quantified for the first time.

Strength-ductility synergy of an additively manufactured metastable high-entropy alloy achieved by transformation-induced plasticity strengthening

This study investigated the microstructures and mechanical properties of a metastable high-entropy alloy (HEA) Fe34Co34Cr20Mn6Ni6 produced by laser powder bed fusion (LPBF) and compared them with those of an as-cast one. The LPBF-processed HEA exhibited a face-centered cubic (FCC) structure due to the high cooling rate of the laser-induced melt pools. In contrast, the as-cast HEA featured a mass of hexagonal close-packed (HCP) phase within the FCC matrix due to the elemental segregation resulting from low cooling rates. The LPBF-processed HEA exhibited superior strength-ductility synergy when compared to its as-cast counterpart. The yield strength, ultimate strength, and plasticity of the LPBF-processed HEA were 305 MPa, 808 MPa, and 18.9 %, respectively, which were much higher than those of the as-cast HEA (171 MPa, 463 MPa, and 7.3 %). This strength-ductility synergy was attributed to the in-situ formation of a fine HCP phase through the stress-induced phase transformation (TRIP) effect. The fine HCP phase provided abundant FCC/HCP interfaces, thus enhancing strong back stress hardening and facilitating the deformation uniformity. In contrast, the as-cast HEA displayed coarse and straight FCC/HCP interfaces that hindered back stress hardening. Moreover, the interaction between the pre-existing HCP phase and the stress-induced HCP phase in the as-cast HEA tended to cause stress concentration and subsequent crack initiation, leading to reduced ductility in the as-cast HEA. These findings are expected to provide valuable insights for a better understanding of additively manufactured TRIP-assisted HEAs.

Synergistic effects of grain sizes on the corrosion behavior and mechanical properties in a metastable high-entropy alloy

Synergistic effects of grain sizes on corrosion behavior and mechanical properties of metastable high-entropy alloys (HEAs) were studied. The results exhibit that the corrosion resistance of metastable HEA increases first and then decreases with grain size increases. This difference in corrosion behavior is manifested in different degrees of passive film dissolution and pitting corrosion, and ascribed to characteristics of passive film induced by different grain boundary densities. Moreover, increasing the grain size decreases mechanical strength, suggesting a strong trade-off in matching corrosion resistance and mechanical properties. The potential corrosion mechanism and trade-off between corrosion resistance and mechanical properties were discussed.

Evolution of microstructure and strength of a high entropy alloy undergoing the strain-induced martensitic transformation

In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage.

Simultaneously enhancing the strength and strain hardenability by Si substitution in metastable high-entropy alloys

We have demonstrated the potential to simultaneously enhance the strength and strain hardenability of metastable dual-phase high-entropy alloys (HEAs) by substituting the constituent transition metal element with Si. This leads to the development of a face-centered cubic single-phase Fe50Mn30Cr10Si10 HEA with excellent strength-ductility synergy, low density and low alloying costs. The substantial solid-solution hardening effect achieved through Si-alloying leads to a higher yield stress for this HEA compared to its dual-phase counterpart when their grain sizes are comparable. In addition, the Si-alloying imparts a low stacking fault energy (∼12.8 mJ/m2) to this HEA, facilitating the progressive formation of hexagonal close-packed martensite plates during plastic deformation. Furthermore, the Si-alloying increases the volume fraction of the deformation-induced martensite plates (up to 75.5%) and renders their thickness ultra-thin (<20 nm), thereby leading to remarkable strain hardenability.

Autonomous Materials Synthesis Strategies Towards Accelerated Discovery of Novel Functional Nanomaterials for Green Energy Production

Electrocatalysts designed by alloying two or more elements (e.g., transition metals) showed promising enhancement in catalytic activity and durability due to synergistic effects from alloying, such as strain engineering or electron exchange. The potential of the alloying approach is constrained by standard synthesis methods (e.g., wet chemistry and thermal decomposition techniques) due to the governing thermodynamics rules (e.g., miscibility of the different alloying elements). That renders the synthesis of high entropy alloys (HEA), with more than 3 elements, to be complicated and tedious to achieve by those conventional methods.In the presented talk we are presenting different strategies to synthesize metastable high entropy alloys (HEA) and their outstanding catalytic performance towards OER, in correlation to their unique chemistries and structures. Rapid cycling of temperature from room 25 °C to highly elevated levels (1700 °C), in conjunction with ultra-rapid cooling, within milli-second range have been demonstrated by different methods including rapid Joule heating, Intense light flashing, and microwave plasma shock. Our results show successful synthesis of non-noble metals HEA NPs, possessing higher catalytic activity than IrO2 catalyst for OER. The nature of the alloying elements dictates OER activity by promoting different oxidation states of the catalytically active transition metals (e.g., Fe, Ni and Co). In addition, a phenomenal in-situ chemical welding of HEA NP to carbon support, yielded HEA catalysts with two orders of magnitude longer durability than IrO2. Our developed techniques resulted in formation of super-strong metal-carbide bonds “chemically welding” HEA NP to the catalyst support.Synthesis of various HEA catalyst and their electrocatalytic performance screening has been carried out by using liquid handler robots and multi-axes robotic arm systems. Moreover, to optimize the design of materials (structure and chemistry), we are using active learning techniques to evaluate the achieved performance results by applying Gaussian process, followed by applying Bayesian Optimization techniques to propose novel chemistries and structures with a higher probability to achieve further improved performances. The presented the study does not open the door only towards developing novel catalysts for energy production. However, it presents a strategy with multiple successfully demonstrated platforms towards accelerated discovery of novel materials. The high-throughput nature of the synthesis protocols presented in this talk can produce tens of thousands of different novel chemistries, at the same time taken by conventional methods to synthesize just a few, at a minimal energy consumption, cost, and manpower.

Mechanical behavior and microstructure-property correlation of a metastable interstitial high entropy alloy with hierarchical gradient structures

Metastable high entropy alloys (MHEAs) are attractive materials due to their exceptional mechanical properties, particularly in addressing the challenge of the strength-ductility trade-off. However, one of the major hurdles of MHEAs is that their yield stresses are still relatively low compared to the engineering requirements, even with carbon-doped treatment and grain refinement. To further improve the strength of MHEAs, surface mechanical attrition treatment (SMAT) was applied to an interstitial MHEA (iHEA) with Fe49.5Mn30Co10Cr10C0.5 (at.%) compositions. Mechanical tests demonstrated that the yield stress of the iHEA had been improved by 32.6% after only 15 min of treatment without sacrificing ductility, and a 60-min treatment made the ultimate tensile strength approach 1 GPa. In-depth quantitative analysis indicated that hierarchical gradient microstructures were generated by SMAT, including grains, dislocations, twins, and dual phases. The theoretical analysis confirmed the synergetic extra-strengthening caused by the gradient distributions of various microstructures. Furthermore, the microstructural heterogeneity resulted in the hetero-deformation induced (HDI) hardening, contributing to strain hardening and maintaining sufficient ductility. The microstructure-property correlation revealed by this study provides guidance for the mechanical property design of MHEAs via gradient structure.

Metastable high entropy alloys of TiZrHfTa with glass-like characteristics at low temperatures

Metastability engineering of high entropy alloys can widen the alloy-design window through the access of a potentially huge configuration space. At the same time, it has the great capacity to overcome the strength-ductility trade-off, where often one witnesses an actual performance improvement at low to cryogenic temperatures. An in-depth microscopic understanding of the underlying mechanism is now needed to drive the design effort. Here we investigate in detail the energetic, structural and chemical complexity of Ti–Zr–Hf–Ta system. The large configuration space with local chemical interaction heterogeneity is explored by a combination of structure-searching techniques and accurate energy models constructed by cluster expansion based on density functional theory data. We find that the potential energy landscape evolves towards displaying shallow mega-basins when decreasing the Ta content. This is accompanied by an expansion of the configurational density of states, which evolves to be clearly distinct from that of a homogeneous random solid-state solution. Furthermore, the energy-state variability and local configuration flexibility are identified from high throughput analysis of atomic structures and their energies. Such features remind closely what found in glass-like systems. This work suggests a theoretical connection between glass physics and metastability engineering of HEAs is quite promising.

Tailored unstable fault energies accurately predict deformation in metastable high-entropy alloys

Tailored unstable fault energies accurately predict deformation in metastable high-entropy alloys

Microstructure and tensile properties of metastable Fe50Mn30Co10Cr10 high-entropy alloy prepared via powder metallurgy

With the characterization of microstructure and tensile properties, the feasibility of preparing Fe50Mn30Co10Cr10 high-entropy alloys (HEAs) via powder metallurgy was explored, and the effect of sintering temperature and deformation temperature on the microstructure and tensile properties of the alloy was investigated. The alloys sintered at different temperatures possessed a dual-phase structure with face-centered cubic (FCC) and hexagonal close-packed (HCP). The average grain size, HCP phase fraction, and twinning boundary fraction increased with the sintering temperature. The best combination of strength and ductility was obtained in the alloy sintered at 1000 °C, which exhibited a strongly temperature-dependent mechanical behavior, i.e., when the deformation temperature decreased from 298 to 77 K, the yield strength and ultimate tensile strength increased from ∼287 to ∼490 MPa and from ∼745 to ∼1107 MPa, respectively, with only a tiny loss of uniform elongation; this is mainly attributed to the deformation mode of the alloy varied with deformation temperature. During tensile deformation at 298 K, dislocation slip, phase transformation, detwinning of annealing twins, and mechanical twinning were the dominant mechanisms, while no mechanical twinning was activated at 77 K. The excellent combination of strength and ductility for the alloy at 77 K relative to 298 K is mainly attributed to a more significant TRIP effect. Powder metallurgy can be used as a promising way for manufacturing metastable high entropy alloys with excellent tensile properties.

The astonishing effect of Si addition on low-cycle fatigue life in a metastable high-entropy alloy

The effect of Si content on the tensile and cyclic deformation behavior of Fe50-xMn30Co10Cr10Six (x = 0–6) as a metastable high-entropy alloy (HEA) was investigated in this study. Heat treatment at 1100 °C for 30 min led to a single-phase microstructure (γ) with a face-centered cubic crystal structure and an average grain size of ∼50 μm in the cold-rolled specimens. The similar tensile properties and the same amount of ε after tensile tests in 0Si and 4Si alloys show that up to 4 at. % of Si the deformation mechanisms were not significantly affected. However, the cyclic deformation behavior was very sensitive to the Si content. The initial cyclic hardening decreased by adding Si and the cyclic softening stage was replaced by stress saturation or secondary hardening. Moreover, the γ to ε-martensite phase transformation that is accompanied by contraction in the unit cell volume of γ was facilitated by adding Si in cyclic loads. These phenomena resulted in the fatigue lives more than 10000 cycles at Δεt= 0.7% that were not reported up to now at this strain amplitude in HEAs. Although the stacking fault energy might be lowered by adding Si, the fatigue life enhancement by adding Si attributes more to the beneficial effect of Si on the reversibility of γ↔ε phase transformation.

Micromechanical origin for the wide range of strength-ductility trade-off in metastable high entropy alloys†

Through different annealing temperatures and times, it is reported here that interstitial metastable high entropy alloys (HEAs) can exhibit a wide range of strength-ductility trade-offs. The underlying mechanisms were investigated via in situ neutron diffraction, electron backscattered diffraction, and electron channel contrast imaging analyses. These techniques revealed that the phase transformation process could be tuned by various annealing processes, resulting in different degrees of load partitioning and sharing among different phases and grain families on the commensurate microstructural length scales. Therefore, the microstructures generated by thermal treatments and phase transformation from face-centered-cubic to hexagonal-close-packed phases could efficiently improve the ductility of the studied alloys.

Additive manufacturing of a metastable high entropy alloy: Metastability engineered microstructural control via process variable driven elemental segregation

Compositional paradigm shift in high entropy alloys (HEAs) provided new opportunities for microstructural engineering, whereas process control during laser powder bed fusion (LPBF) additive manufacturing (AM) enable fine microstructure tailoring. Metastability engineering in transformation induced plasticity (TRIP) HEAs by addition of minor alloying elements is an attractive strategy for fine microstructural tuning. This study explored in detail the microstructural evolution during LPBF AM of a metastable Fe40Mn20Co20Cr15Si5 (at.%) dual phase HEA (CS-HEA). LPBF processing window for CS-HEA was established based on quantitative analysis and experiments. Based on melt pool overlap lack of fusion pores were observed at lower energy densities (J) of J ≤ 31.25 J/mm3 and key-hole formation by melt pool destabilization in case of J ≥ 75 J/mm3. The microstructure of CS-HEA consists of metastable FCC-γ and HCP-ε phases; LPBF process parameters governed the final phase fraction in the alloy which has been correlated to metastability alteration of the high temperature γ phase. Final microstructural engineering was devised by LPBF process control which enabled cooling rate manipulation to guide Mn and Si segregation at the cell boundaries, thereby controlling the matrix metastability and final phase fraction. Additionally, high resolution transmission electron microscopy (TEM) revealed disparity in stacking fault morphology in CS-HEA with LPBF process variable alterations associated with local variations in chemical composition and stacking fault energy. The phase evolution with process parameters also affected the nanomechanical behavior of the alloy.

Bauschinger effect in Gd micro-alloying metastable high-entropy alloy

To investigate Bauschinger effect in metastable high-entropy alloys with strain-induced FCC → HCP phase transformation, we designed a series of Gd micro-alloying non-equiatomic metastable high-entropy alloys. The results show that all tested alloys have asymmetrical tension-compression properties exhibiting the obvious Bauschinger effect, which is related to the different levels of phase transformation in compression and tension owing to different deformation textures. Excessive phase transformation suppresses the increase in pile-up density of geometrically necessary dislocations (GNDs), resulting in a back stress relaxation effect. The inverse phase transformation observed by quasi in-situ EBSD proves the instability of the HCP phase. The established kinetic model points out limitation of the classical back stress measurement method in metastable high-entropy alloys.

Microstructure evolution and mechanical properties in a gas tungsten arc welded Fe42Mn28Co10Cr15Si5 metastable high entropy alloy

Weldability studies on high entropy alloys are still relatively scarce, delaying the deployment of these materials into real-life applications. Thus, there is an urgent need for in-depth studies of the weldability of these novel advanced engineering alloys. In the current work, an as-cast Fe42Mn28Co10Cr15Si5 metastable high entropy alloy was welded for the first time using gas tungsten arc welding. The weld thermal cycle effect on the microstructure evolution over the welded joint was examined using electron microscopy in combination with electron backscatter diffraction, synchrotron X-ray diffraction analysis, and thermodynamic calculations. Furthermore, tensile testing and hardness mapping were correlated with the microstructure evolution. The microstructure evolution across the joint is unveiled, including the origin of the ε-h.c.p. phase at different locations of the material. Different strengthening effects measured throughout the joint are associated with the weld thermal cycle and resulting microstructure. A synergistic effect of smaller grain size of the ε-h.c.p. phase in the fusion zone, overturns the reduced volume fraction of this phase, increasing the local strength of the material. Moreover, the brittle nanosized σ phase was also found to play a critical role in the joints’ premature failure during mechanical testing.

Excellent strength-ductility synergy and corrosion resistance in a metastable high entropy alloy via heterogeneous structure design

For metastable high entropy alloys (HEAs), the balance between mechanical properties and corrosion resistance is rarely achieved. Here, metastable HEAs with a heterogeneous structure (HS) were fabricated by innovatively combining cold rolling and partial recrystallization annealing, showing excellent corrosion resistance and strength-ductility synergy. HS exhibits heterogeneous distributions of dislocation density and grain size. Extraordinary strength-ductility is mediated by prominent hetero-deformation-induced strengthening, high-density dislocations, grain refinement, and transformation-induced plasticity. Such HS samples show higher corrosion resistance than the homogeneous coarse-grained sample, which is due to the low coincidence site lattice grain boundaries, substructures and the formation of stable passive films.