Equiatomic CoCrFeMnNi High-entropy Alloy Research Articles

Revealing the inhomogeneous nature of microstructure and texture evolution in the cold-rolled CoCrFeMnNi alloy during static recrystallization

In the present investigation, equiatomic CoCrFeMnNi high entropy alloy (HEA) was subjected to 80% cold rolling (as-rolled) followed by isothermal annealing treatment at 700 °C for different time periods. Microstructural characterization was performed using electron back-scattered diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques on the as-rolled and annealed samples. The as-rolled microstructure consisted of elongated grains mainly composed of strong α-fiber and weak γ-fiber textures. The as-rolled sample showed the formation of ingrain SBs in the γ-fiber grains, which served as preferential sites for grain nucleation. The annealing treatment of as-rolled samples at 700 °C cause static recovery (SRV) before 5 minutes of heat-treatment, whereas static recrystallization (SRX) was observed after 5 minutes of heat-treatment in the CoCrFeMnNi alloy samples. A faster rate of recrystallization kinetics was observed after 15 minutes of heat-treatment. The annealing treatment reduced the intensity of α-fiber and enhanced the Copper, Cube and P ({011}<122>) texture components. Copper components originated from coarse recrystallized grains, which were the results of localized grain growth phenomena. Unusual behavior of banded featured α-fiber grains was observed during the annealing process. These banded grains in the partially annealed samples consisted of subgrain-boundaries and showed a delayed response to recrystallization as compared to grains with other orientations. Stored energy (SE) and Taylor factor (M) calculations were also used to understand the delayed recrystallization response of the banded featured/retained deformed grains. The microhardness value of the as-rolled sample was approximately 450 Hv, which decreased to around 230 Hv for the fully recrystallized grains.

Strength-Ductility Mechanism of CoCrFeMnNi High-Entropy Alloys with Inverse Gradient-Grained Structures.

The microstructures and mechanical properties of equiatomic CoCrFeMnNi high-entropy alloys (HEAs) treated with various processing parameters of laser surface heat treatment are studied in this paper. The typical inverse gradient-grained structure, which is composed of a hard central layer and a soft surface layer, can be obtained by laser surface heat treatment. A much narrower gradient layer leads to the highest yield strength by sacrificing ductility when the surface temperature of the laser-irradiated region remains at ~850 °C, whereas the fully recrystallized microstructure, which exists from the top surface layer to the ~1.05 mm depth layer, increases the ductility but decreases the yield strength as the maximum heating temperature rises to ~1050 °C. Significantly, the superior strength-ductility combination can be acquired by controlling the surface temperature of a laser-irradiated surface at ~1000 °C with a scanning speed of ~4 mm/s due to the effect of hetero-deformation-induced strengthening and hardening, as well as the enhanced interaction between dislocation and nanotwins by the hierarchical nanotwins. Therefore, retaining the partial recrystallized microstructure with a relatively high microhardness in the central layer, promoting the generation of hierarchical nanotwins, and increasing the volume proportion of gradient layer can effectively facilitate the inverse gradient-grained CoCrFeMnNi HEAs to exhibit a desirable strength-ductility synergy.

Suppressed Plastic Anisotropy via Sigma-Phase Precipitation in CoCrFeMnNi High-Entropy Alloys.

The effect of sigma-phase precipitation on plastic anisotropy of the equiatomic CoCrFeMnNi high-entropy alloy was investigated. Annealing at 700 °C after cold-rolling leads to the formation of the Cr-rich σ phase with a fraction of 2.7%. It is experimentally found that the planar anisotropy (∆r = -0.16) of the CoCrFeMnNi alloy annealed at 700 °C is two times lower than that of the alloy annealed at 800 °C (∆r = -0.35). This observation was further supported by measuring the earing profile of cup specimens after the deep drawing process. The plastic strain ratio, normal anisotropy, and planar anisotropy were also predicted using the visco-plastic self-consistent model. The results indirectly indicated that the reduction of plastic anisotropy in alloy annealed at 700 °C can be attributed to the formation of the σ phase.

Unprecedented bake hardening responses of interstitial high-entropy alloy by synergistic effect with lattice distortion

High-entropy alloys (HEAs) composed of multiple principal elements have attracted considerable interest in recent decades as a new class of metallic materials. Among them, the equi-atomic CoCrFeMnNi HEA system has great potential for application as a structural material due to its excellent properties in various fields, but it has a drawback of relatively low yield strength (YS) at room temperature. Here, for the first time, we propose the bake hardening (BH) as a breakthrough to overcome the relatively inferior YS of the CoCrFeMnNi HEA system. We found that the carbon-added CoCrFeMnNi HEAs are not only bake-hardenable, but also exhibit unprecedented BH responses than conventional bake-hardenable materials, with BH responses up to 120.0 MPa depending on the carbon concentration in the matrix. It is the outcome of the mutual synergistic effect between the massive solid solution strengthening characteristic from lattice distortion of the HEAs and the Cottrell atmosphere generated during the BH process. This study demonstrates that the carbon-added CoCrFeMnNi HEAs are more advantageous to apply the BH effect than conventional bake-hardenable materials, which suggests the possibility of industrialization of various interstitial HEA systems as well as carbon-added CoCrFeMnNi HEAs.

Strong yet strain-hardenable equiatomic CoCrFeMnNi high-entropy alloys by dynamic heterostructuring

Heterostructuring gives solutions to achieve better mechanical performance of metallic materials. For instance, partial static recrystallization by short-term annealing induces bimodal grain size distribution. Distinct from the well-studied ‘static heterostructuring’ by cold rolling followed by annealing, this study introduces ‘dynamic heterostructuring’ by hot rolling. Dynamic recrystallization (DRX) during the hot rolling as a softening mechanism leaves room for dislocation accumulation. With the expectations of enhanced strain hardening of the as-rolled materials with the aid of the DRX, we monitored tensile responses of the as-hot-rolled equiatomic CoCrFeMnNi high-entropy alloys (HEAs). Among the hot rolling temperatures from 800 ℃ to 900 ℃ with a thickness reduction ratio of 78.6% for the HEAs, the hot rolling at 850 °C results in a bimodal grain size distribution, i.e., partial DRX. It comprises relatively fine DRXed grains and coarse unrecrystallized grains: the latter is harder than the DRXed ones with relatively low dislocation density. The microstructural heterogeneity results in uniform elongation of ∼13% due to the accumulation of dislocations at the grain boundaries inside the soft DRXed grains. This endows the as-hot-rolled single-phase HEAs with strain hardenability without the aid of phase transformation, deformation twinning, or precipitates. In addition, the hot-rolled HEAs with high dislocation density have doubled yield stresses of those of conventionally cold-rolled and annealed counterparts. Considering the industrial benefits of the simplified thermo-mechanical control processes, this work extends the academic and practical value of DRX by dynamic heterostructuring through annealing-free hot rolling.

Mechanical properties and deformation behavior of equiatomic CoCrFeMnNi high-entropy alloy foam: A molecular dynamics study

In recent years, the aerospace and mechanical industries have employed metal foams for High Energy Capacity applications. However, the porous nature of such metallic foams results in a drop in tensile properties. High Entropy Alloys (HEAs) have been reported to overcome the strength-ductility trade-off as opposed to conventional metals and their alloys. Based on this, an equiatomic nano porous CoCrFeMnNi HEA is investigated using molecular dynamics simulation to elucidate the deformation behavior and mechanical performance at different atom distributions, percent porosity, and pore locations. The findings indicated that the mechanical properties are independent of atom distributions. However, with an increase in percent porosity, a decline in the ultimate tensile strength was observed. Up to 66% reduction in tensile strength was recorded. A similar trend was observed under temperature variations. By varying pore location, either within or on the structure, the pores within the foam structure led to a significant reduction in properties. The deformation mechanism of the HEA foam was established via analyses of the dislocation distributions, stacking faults, crack nucleation, and propagation. The findings offer new insights for material design to produce high-performance HEA foams and a comprehension of its failure mechanism under loading.

Early-stage surface oxidation of the equiatomic CoCrFeMnNi high entropy alloy studied in situ by XPS

The mechanisms of initial oxide growth on the equiatomic CoCrFeMnNi high entropy alloy were studied in situ by X-ray photoelectron spectroscopy on clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides were formed at 100 °C, and Mn(II) at 200 °C. Data quantification revealed a transition mechanism from selective initial oxidation to 3D kinetically-controlled growth, leading to stratification of the oxide film. Formation of the Co-rich outer layer above the Cr-rich inner layer first formed implies predominant cationic outward oxide growth supported by the overall stoichiometry of the film.

Precipitation growth of σ-phase along the interface in CoCrFeMnNi high-entropy alloy

The equiatomic CoCrFeMnNi high-entropy alloy (HEA) has a face-centered cubic (FCC) single-phase structure with a good balance of yield strength and elongation due to the twinning-induced plasticity (TWIP). In order to increase the usability of FCC-structured HEA, it is necessary to develop its mechanical properties. The hard precipitation of the σ-phase might be a promising strengthening mechanism. Furthermore, developing an advanced manufacturing process is necessary to apply the σ-phase-containing HEAs. This study focused on σ-phase formation during annealing after hot compression of a CoCrFeMnNi HEA. The σ-phase was primarily observed near the high-angle grain boundaries and acted as the obstacle for the boundary migration, resulting in grain growth during annealing. In addition, the development of the σ-phase according to the interface type was investigated. This microstructure increased the hardness of the alloy and may contribute to the further development of high-strength HEAs.

Fabrication of Equiatomic CoCrFeMnNi High-Entropy Alloy by Metal Injection Molding Process Using Coarse-Sized Powders

Fabrication of Equiatomic CoCrFeMnNi High-Entropy Alloy by Metal Injection Molding Process Using Coarse-Sized Powders

The effect of Si addition on the structure and mechanical properties of equiatomic CoCrFeMnNi high entropy alloy by experiment and simulation

The effect of Si addition on the mechanical properties and deformation behaviour of the Cantor alloy in varying compositions from 1 to 5 atomic % has been investigated by experiments and first principle calculations. All alloy compositions exhibit single-phase FCC crystal structures with average grain size ranges from 125–140 μm. Cantor alloy with 1 and 2 atomic % of Si shows an excellent combination of strength and ductility while 5 atomic % Si exhibits the lowest strength and highest ductility (⁓100 %). Detailed TEM analysis of deformed samples revealed the formation of high densities of closely spaced deformation twins in 1 and 2 atomic % Si at room temperature which was observed in Cantor alloy at cryogenic temperatures, while 5 atomic % Si shows the slip-mediated deformation with no observations of the twins. The dependency of the generalized stacking fault energy as a function of Si concentration was investigated by the first principle calculation for Si-containing alloys. The change in deformation mode from twinning for low Si content ( < 4 atomic %) to the dislocation-mediated slip for high Si content ( ≥ 4 atomic %) can be explained based on effective energy barrier, average interlayer distance, and interlayer distance distortion, which is in good agreement with experiments. The study showcases the unique contribution of Si in triggering multiple strengthening and strain hardening mechanisms by stacking fault mediated plasticity in Cantor alloy, thus highlighting the potency of minor alloying additions to single phase HEAs to achieve a large spectrum of strength-ductility paradigm.

Exceptional strength–ductility combination of CoCrFeMnNi high-entropy alloy with fully recrystallized structure by selective laser melting after post-deformation annealing

The low yield strength of the equiatomic CoCrFeMnNi high-entropy alloy (HEA) at ambient temperature severely limits its future application as engineering materials. Here, a CoCrFeMnNi HEA with a fully recrystallized structure and an excellent strength-ductility combination was prepared by post-deformation annealing (PDA) process (i.e., 773–1173 K for 2 h) after selective laser melting. The evolution of the microstructures and mechanical properties of the as-printed and PDA-processed samples were systemically studied, while the strengthening mechanism of PDA-processed samples was clarified. The high density of dislocations and anisotropic columnar grains in the as-printed samples improved the rate of stored deformation energy, thus effectively promoting grain refinement during the PDA process. Compared to the average grain size of 21.26 μm of the as-printed sample, the average grain size of the PDA-873 sample was decreased to 1.20 μm after full recrystallization. Especially in the PDA-873 sample, we found a large amount of high angle grain boundaries (HAGBs) (∼98.3%) and twin boundaries (TBs) (∼46.6%). Besides, some Cr and Mn-rich oxide particles also precipitated in the PDA-processed samples. The main strengthening mechanisms of PDA-processed samples can be attributed to grain boundary strengthening. The PDA-873 sample exhibits a high yield strength, which is twice as large as that of the as-printed samples, without sacrificing ductility significantly. The present work provides an effective way to manipulate grain size and precipitates in HEAs.

Cryogenic tensile behavior of laser additive manufactured CoCrFeMnNi high entropy alloys

In the present work the microstructural evolution, tensile properties, and deformation behavior of additively manufactured equiatomic CoCrFeMnNi high entropy alloy (HEA) were investigated at a cryogenic temperature (77 K). Metal additive manufacturing components are known to have exceptional yield strength, without compromising ductility. Here, laser powder bed fusion (LPBF) and direct energy deposition (DED) additive manufacturing processes were chosen as the fabrication techniques and wrought CoCrFeMnNi HEA was also fabricated for comparison. The tensile behaviors at different temperatures (298 K and 77 K) indicated temperature-dependent strength and ductility in all the samples. In particular, the HEA processed by DED exhibited excellent strength and ductility, with exceptional strain hardening, compared to the LPBF and wrought samples at 77 K. Post-tensile microstructures revealed the formation of deformation twinning in addition to dislocation slip as deformation mechanism at 77 K. Our results suggest that cellular structure plays an essential role in yield strength. Moreover, the interactions of the deformation twins and cellular dislocation structures increase strain hardening and help retard plastic instability, thereby improving ductility at 77 K.

“Non-equilibrium” grain boundaries in additively manufactured CoCrFeMnNi high-entropy alloy: Enhanced diffusion and strong segregation

Grain boundary diffusion in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy is systematically investigated at 500 K under the so-called C-type kinetic conditions when bulk diffusion is completely frozen. In the as-manufactured state, general (random) grain boundaries are found to be characterized by orders-of-magnitude enhanced diffusivities and a non-equilibrium segregation of (dominantly) Mn atoms. These features are explained in terms of a non-equilibrium state of grain boundaries after rapid solidification. The grain boundary diffusion rates are found to be almost independent on the scanning/building strategy used for the specimen’s manufacturing, despite pronounced microstructure differences. Grain boundary migration during diffusion annealing turned out to preserve the non-equilibrium state of the interfaces due to continuous consumption of the processing-induced defects by moving boundaries. Whereas the kinetic “non-equilibrium” state of the interfaces relaxes after annealing at 773 K, the non-equilibrium segregation is retained, being further accompanied by a nano-scale phase decomposition at the grain boundaries. The generality of the findings for additively manufactured materials is discussed.

Additively manufactured equiatomic CoCrFeMnNi high entropy alloy: Precipitation-induced heterogeneity by mechano-chemical coupling

Additive manufacturing via selective laser melting of gas-atomized equiatomic powder was used to fabricate a CoCrFeMnNi high-entropy alloy. Analytical transmission electron microscopy, nanobeam diffraction, atom probe tomography and nanoindentation were employed to provide a comprehensive overview on the evolution of microstructure and nano-hardness upon annealing at a moderate temperature of 550∘C motivated by a maximum heat release at that temperature. A complex mechano-chemical coupling was observed, which leads to segregation and phase separation at grain boundaries. The as-manufactured material contained alternating regions of equiaxed and columnar grains. The corresponding microstructure is composed of high-angle grain boundaries and intrinsic dislocation networks, which displayed heterogeneous segregation of Mn and to some extent Ni. Longer annealing led to Cr enrichment at high-angle grain boundaries, and later to a phase separation with neighboring Cr-rich and MnNi-rich regions. Synergetic effects of segregation, nano-precipitation and dislocation accumulation at high-angle grain boundaries give rise to built up stresses which increase and homogenize hardness in the 3D-printed CoCrFeMnNi alloy.

Investigation of accelerated recrystallization behavior via electropulsing treatment in CoCrFeMnNi high-entropy alloy

In this work, the effects of annealed temperatures (700 °C, 800 °C) and heating rates (low heating rate (LR) of 0.17 °C/s, medium heating rate (MR) of 500 °C/s and high heating rate (HR) of 4000 °C/s) on recrystallization behavior in equiatomic CoCrFeMnNi high-entropy alloy (HEA) were investigated. The mechanism of accelerated recrystallization behavior induced by electropulsing treatment (EPT) was elucidated through the evolution analysis of microstructure and geometrically necessary dislocations (GNDs). The results showed that the average GNDs density and grain size of the recrystallized region decreased with increasing annealed heating rate. At 700 °C, the average recrystallized grain size decreased from 6.1 μm to 2.2 μm with increasing annealed heating rate. At 800 °C, the HR material exhibited a higher recrystallization percentage and the corresponding decrease in average grain size from 12.5 μm for LR to 4.7 μm for HR. EPT promoted the recrystallization nucleation rate by reducing the recrystallization nucleation barrier. The better combination of grain refinement and dislocation strengthening due to heterogeneous deformation led to the yield strength (772 MPa) and fracture elongation (0.30) of the MR sample increased by ∼31% and ∼25% respectively compared with LR after annealing at 700 °C. This work provides essential insights into accelerating the recrystallization behavior and further improving the degree of grain refinement of CoCrFeMnNi HEA via EPT under the lower rolling amounts.

Global understanding of deformation behavior in CoCrFeMnNi high entropy alloy under high-strain torsion deformation at a wide range of elevated temperatures

A number of recent studies have investigated deformation behavior of CoCrFeMnNi (Cantor) alloy at elevated temperatures by using plastic deformation to relatively small strains such as tensile testing. Therefore, little has been known about the deformation behavior of this typical FCC high-entropy alloy (HEA) in case that the material is subjected to ultra-high strains at various temperatures. In the present study, the equi-atomic CoCrFeMnNi HEA was successfully deformed over a wide range of strains (von Mises equivalent strains (ε) of 1∼5.5) by torsion at various temperatures ranging from 25 °C to 1100 °C. Deformation twinning was extensively activated at moderate to high strains (ε ≥ 1) and even found in the deformation at elevated temperatures as high as 600 °C where deformation twinning is not normally expected in Cantor alloy. The HEA showed outstanding deformability and the highest strains to fracture reached 4.0∼5.5 at low temperatures below 400 °C. The excellent deformability was attributed to the extensive twin activities including the formation of twin bundles and thin nanotwins as well as microbands formation. However, localized shear deformation that was promoted by the high straining at low temperatures could negatively affect the deformability. The heavy deformation led to a significant reduction of the grain sizes down to 50 nm∼150 nm. A sudden shortage of ductility occurred at intermediate temperatures, where small strains to fracture (1.2∼1.3) were realized at 600 °C∼700 °C. The embrittlement was accompanied by the formation of micro-voids at grain boundaries and intergranular fracture. The susceptibility to the embrittlement was caused by the precipitation of Cr-rich σ-phases at grain boundaries. Dynamic recrystallization (DRX) of the FCC matrix led to an accelerated softening at high temperatures above 600 °C. Nucleation and growth of DRX grains in Cantor alloy were not fundamentally different from those in conventional FCC alloys. This study gives an insight into the microstructure evolution and mechanical response in Cantor alloy under shear deformation over a wide range of strains and temperatures.

Selective laser melting of in-situ CoCrFeMnNi high entropy alloy: Effect of remelting

In-situ multicomponent alloying in additive layer manufacturing shows inhomogeneous elemental distribution. This study aimed to minimize this inhomogeneity in equiatomic CoCrFeMnNi high entropy alloy (HEA) produced by selective laser melting (SLM) from a mixture of elemental powder. A detailed study was undertaken regarding the effects of remelting scan strategy on microstructural and mechanical properties, melt pool geometry, and elemental distribution of CoCrFeMnNi HEA manufactured using the SLM process. The SLM sample with remelting scan strategy showed a homogeneous elemental distribution. The remelted SLM sample showed a homogeneous hardness distribution. The X-ray diffraction of the as-built and remelted SLM showed a single-phase FCC structure, typical for a HEA. Prediction and observation of melt pool depth exhibited a lower depth in the remelted SLM samples. The microstructure of the melt pools of both as-built and remelted SLM showed a fine grain morphology, typical of rapid solidification. The boundary of melt pools in the as-built SLM and the inter-space of melt pools in the remelted SLM consisted of both coarse and equiaxed/columnar dendritic grains. The present results confirmed the possibility of fabricating a homogenized CoCrFeMnNi HEA from mixed elemental powders using SLM with remelting scan strategy.

Effect of crystallographic orientations on shock-induced plasticity for CoCrFeMnNi high-entropy alloy

Using nonequilibrium molecular dynamics simulations, we studied the crystallographic-orientation-dependence shock-induced plasticity for the face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy (HEA). The local FCC-BCC (body-centered cubic) structural transition was identified by shock loading in the [001] direction, which is attributed to the compression along the <001> direction of original body-centered tetragonal (BCT) structure, and then the BCC structures trend to activate the dislocation nucleation. Further, dislocations slip to produce HCP (hexagonal close-packed) structures in the FCC lattice. In contrast, for the shock loading along the [110] and [111] directions, numerous disordered structures were found due to the insufficient lattice displacement along the <112> direction on the {111} crystal plane of FCC structure and the nonsynergistic behavior of the deformation between the <001>, <110> and <111> directions of BCT structure. In particular, both BCC structures and disordered structures with the Mn-rich composition characteristics play a significant role to promote the development of localized plastic deformation, which is related to the stress concentration around Mn caused by the inherent local inhomogeneity in the CoCrFeMnNi HEA. Moreover, under shock loading, the high atomic fraction of the local BCC structures and disordered structures can further promote chemical composition heterogeneity in these structures. Our results provide some significant insights for understanding the shock-induced plasticity of the CoCrFeMnNi HEA.

Hydrogen-assisted failure in partially recrystallized carbon alloyed equiatomic CoCrFeMnNi high-entropy alloy

We investigated the hydrogen-assisted failure behavior of partially recrystallized carbon alloyed equiatomic CoCrFeMnNi high-entropy alloys. Microstructural evolution in the vicinity of the crack initiation sites and propagation pathways was observed. The results showed that fracture of the samples in the presence of hydrogen was dominated by localized plastic deformation. Hydrogen enhanced nanotwinning and the formation of nanovoids were observed during tensile deformation. Furthermore, stress assisted intergranular and transgranular cracking in hydrogen-charged samples was caused by slip localization at boundaries. Observations confirmed that the resistance to hydrogen embrittlement was increased by the presence of non-recrystallized zones and nanotwins.

Strengthening the mechanical properties and wear resistance of CoCrFeMnNi high entropy alloy fabricated by powder metallurgy

In this study, an equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by a rapid solidified gas atomization process. Subsequently, the high-energy mechanical milling was carried out to further refine the microstructure of pre-alloyed powder to improve the sintering ability and strengthening of HEAs. The microscopic results show that the powder morphology significantly changed from spherical to flatten, flake, irregular, and partially spherical shape with increasing milling time. The XRD results exhibited HEA bulks consisting of major FCC and minor Cr7C3 phases. The hardness of HEA bulks increased from 270±10 Hv to 450±10 Hv with increasing milling time, while the compressive yield strength increased from 370 MPa to 1050 MPa due to grain boundary strengthening and dislocation strengthening. Meanwhile, the lowest coefficient of friction ∼0.283 and specific wear rate ∼1.03×10-5 mm3/Nm were obtained for the 60 min milled HEA due to increased surface hardness and oxidation behavior. The developed powder metallurgy approach could be considered as a promising way to improve the strength and wear resistance when compared to the conventional processed CoCrFeMnNi HEAs.