Interstitial High-entropy Alloy Research Articles

The Structure Evolution of the Interstitial High Entropy Alloy Fe50Mn30-xCr10Ni10Cx Produced by Mechanical Alloying

The Structure Evolution of the Interstitial High Entropy Alloy Fe50Mn30-xCr10Ni10Cx Produced by Mechanical Alloying

Advanced interstitial high-entropy alloy engineered for exceptional wear resistance via dual nanoprecipitation-induced heterogeneous structure

Despite achieving notable strength and wear resistance in precipitation-strengthened Cantor high-entropy alloy (HEA), the inherent brittleness of precipitates leads to a loss of ductility. In this study, we propose a novel approach to address this issue by introducing nanosized M23C6-combined with Cr2N, termed dual-nanoprecipitation, and heterogeneous structure in a C and N co-doped interstitial HEA (iHEA). The abundant nanoscale M23C6 and Cr2N particles serve not only as reinforcing agents to enhance strength and wear resistance but also hinder the recrystallization process, resulting in a heterogeneous structure containing recrystallized and non-recrystallized zones. This heterogeneity triggers additional strengthening and strain hardening mechanisms, thereby enhancing the deformability of iHEA. Furthermore, the heterogeneous structure effectively mitigates strain localization on the sliding surface during wear, thus improving tribological properties. By overcoming the challenge of poor ductility while maintaining exceptional strength and wear resistance, the dual-nanoprecipitation-induced heterogeneous structure offers promising avenues for the development of alloys with superior strength-ductility synergy and wear resistance.

Mechanical properties and deformation mechanisms of C-doped interstitial high-entropy alloy CrMnFeCoNi: Effects of strain rate and C content

The effects of strain rate on mechanical properties, microstructural evolution and underlying deformation mechanisms of two kinds of interstitial CrMnFeCoNi high entropy alloys (HEAs), (CrMnFeCoNi)99.5C0.5 and (CrMnFeCoNi)99.0C1.0, are investigated under quasi-static and dynamic tension. Dynamic loading is carried out via a split Hopkinson tension bar system. The pre- and post-deformation microstructures are characterized with electron back scatter diffraction and transmission electron microscopy. Both yield strength and work hardening increase remarkably with increasing strain rate. Plastic deformation in both alloys under quasi-static and dynamic loading are dominated by dislocations, stacking faults, kink bands and deformation twins. Twinning is activated easier under dynamic loading, and twin density decreases with increasing carbon content due to the increased stacking fault energy. The Khan-Liu constitutive model can describe the experimental results for these two interstitial HEAs in a wide strain range.

Microstructural evolution and deformation behavior of an interstitial TRIP high-entropy alloy under dynamic loading

The mechanical properties and microstructural evolution under dynamic loading conditions are of paramount importance in the development and utilization of interstitial high-entropy alloys (iHEAs). This study focuses on the investigation of a currently trending material, C-doped TRIP-assisted HEA Fe49.5Mn30Co10Cr10C0.5 (at.%). By subjecting the alloy to uniaxial quasi-static and dynamic tensile loading, the study systematically elucidates mechanical property testing under different strain states, characterizes the microstructural evolution during deformation, analyzes deformation mechanisms, and delves into the essence of mechanical responses. Particularly, by employing digital image correlation (DIC) techniques, local severe deformation regions under high-speed deformation conditions are sampled to investigate microstructural changes under extreme deformation conditions. The results indicated that, in comparison to quasi-static tensile deformation, the dynamic tensile deformation of the HEA demonstrated a slight increase in yield strength, a decrease in ultimate tensile strength, and a trend toward an increase in total elongation. Specifically, at a strain rate of 103 s−1, the tensile strength was ∼825.5 MPa, and at a strain rate of 10 s−1, the elongation was ∼57.7 %. In quasi-static deformation, the TRIP and TWIP effects served as the primary deformation mechanisms. In dynamic deformation, the proportion of ε-martensite phase transformation was significantly reduced, with deformation twinning (DT) and dislocations becoming the dominant mechanisms, originating from the impact of adiabatic heating on the stacking fault energy (SFE). Furthermore, in the locally severe deformation region, multiple twinning systems were activated, coupled with a substantial accumulation of dislocations. A minor presence of dual-oriented martensite also contributed to this complex mechanism. Additionally, the carbides formed through C-doping exerted a significant hindrance on dislocation slip during high-speed deformation and under high-strain conditions. Simultaneously, they promoted the activation of multiple twinning systems, making an indispensable contribution to maintaining the strain hardening rate (SHR) of the HEA.

Hydrogen-assisted fatigue crack propagation behavior of a C–N co-doped interstitial high-entropy alloy

The hydrogen-assisted fatigue crack propagation behavior of a C–N co-doped interstitial high-entropy alloy (iHEA) was investigated under in-situ electrochemical hydrogen charging. The cracking path, deformation microstructure, and fractography were characterized via multiple methods. Results suggest that hydrogen uptake shows a certain acceleration on the fatigue crack propagation rate (FCGR) of the iHEA, and such acceleration increases are intensified when increasing the hydrogen-charging current density. Crack propagation along grain boundaries and a remarkable decrease in the extent of plastic deformation immediately near to the crack surface have also been observed. This decrement of critical plastic strain for fatigue crack propagation primarily accelerates the FCGR.

Effect of Post-Welding Treatment on Corrosion Behavior of Laser and Gas Tungsten Arc-Welded (Fe50Mn30Co10Cr10)99C1 Interstitial High-Entropy Alloy

Effect of Post-Welding Treatment on Corrosion Behavior of Laser and Gas Tungsten Arc-Welded (Fe50Mn30Co10Cr10)99C1 Interstitial High-Entropy Alloy

Achieving excellent mechanical properties and wear resistance in Fe49Mn30Co10Cr10C1 interstitial high-entropy alloy via tuning composition and stacking fault energy by Nb doping

In this study, the effects of Nb doping on the mechanical properties, deformation mechanism, and wear resistance of an Fe49Mn30Co10Cr10C1 interstitial high-entropy alloy (HEA) were studied. Adding 1 at.% Nb resulted in multi-scale NbC particles. The yield strength and elongation remained almost unchanged, and the wear rate was significantly reduced. The precipitation strengthening of nano-NbC particles offsets the decrease in interstitial strengthening. The formation of NbC particles consumed C, reduced the stacking fault energy (SFE) of the alloy, significantly enhanced the transformation-induced plasticity (TRIP) effect, and offsets the deterioration effect of micron-sized NbC particles on plasticity. After adding Nb element, the volume fraction of strain-induced martensitic transformation in the tensile fracture increases from 13.2 % to 30.6 %. The wear mechanism of the two alloys under sliding wear conditions is mainly abrasive wear. Micron-sized NbC blocks the expansion of the groove and protects the matrix. In addition, the Nb-doped high-entropy alloy produced a thicker deformation ultrafine grain layer and a strain-induced phase transition layer below the wear track. The thicker deformation layer and micron-sized NbC particles reduced the wear rate by more than 23.1 %. The rationality of the existing method for calculating the wear rate with the unit load as a parameter was analyzed, and a new method with the unit maximum shear stress as a parameter is proposed.

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.

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.

Enhanced mechanical properties of a carbon and nitrogen co-doped interstitial high-entropy alloy via tuning ultrafine-grained microstructures

C-N co-doped interstitial high entropy alloy (iHEA) was reported to have high strength and ductility. However, iHEA with fully recrystallized ultrafine grains (UFGs) and underlying thermally activated processes associated with dislocation slip, twinning, and solute drag have not been reported yet. In this work, a C-N co-doped iHEA with nominal composition Fe48.5Mn30Co10Cr10C0.5N1.0 (at.%) was prepared, and the microstructures were tuned by cold-rolling and annealing treatments to improve mechanical properties. Upon cold-rolling with a strain of 1.74, the main microstructures in the iHEA are composed of nano-grains, nano-twins, HCP laminates, and high density of dislocations, leading to ultrahigh hardness of 466.7 HV and tensile strength of 1730 MPa at the expense of ductility (2.44%). Both the nanostructures and the high hardness of the iHEA can be maintained up to an annealing temperature of 600 °C (462.5 HV). After annealing at 650 °C for 1 h, the UFG microstructures are obtained in the iHEA, containing recrystallized grains with an average grain size of 0.91 µm and nanoprecipitates with an average diameter of 90.8 nm. The combined strengthening and hardening effects of UFGs, nanoprecipitates, twinning, and solutes contribute to high strain hardening (n = 0.81), gigapascal yield strength (984 MPa), and good ductility (20%). The C-N co-doping leads to a strong drag effect on dislocation slip, resulting in a nano-scale mean free path of dislocation slip λ¯ (1.44 nm) and much small apparent activation volume V* (15.8 b3) of the UFG iHEA.

Nanoindentation creep behavior of diverse microstructures in a pre-strained interstitial high-entropy alloy by high-throughput mapping

Time-dependent plastic deformation, also known as creep, can occur in high-melting-point materials at room temperature when subjected to submicron plastic contact (e.g., components in micro-electronic mechanical systems) due to the extremely high stress conditions. Previous investigations of creep behavior at submicron scale mainly focus on uniform microstructures, whereas recently developed high-performance structural materials usually show complex microstructural heterogeneities. Here, we demonstrate a high-throughput nanoindentation approach to unveil the creep behavior of locally diverse microstructures, via a case study on an interstitial high-entropy alloy (iHEA). The prototype iHEA specimen was firstly pre-strained by severe cold-rolling to achieve diverse microstructure features including dense dislocations, nanotwins and nanograins. Nanoindentation mapping with 144 indents is conducted to collect creep data from all types of diverse microstructures, followed by systematical calculations based on power-law analysis. Then intuitive contour maps of nanohardness (H), strain rate (ε˙) and stress exponent (n) are attained. The individual creep mechanism of each microstructure variant is evaluated by correlating local microstructure features with corresponding creep responses from these contour maps. With the increase of n value for different sample regions, the ε˙ value inversely reduces and the corresponding dominant creep mechanism gradually converts from diffusion-mediated mode to dislocation-controlled one. The presence of nanograins and nanotwins significantly hinders dislocation interactions and promotes stress-induced diffusion. This work verifies that nanoindentation creep behavior is highly correlated with local microstructure features, and the mapping approach is feasible for evaluating creep-resistance of locally diverse microstructures under extremely high-stress conditions.

The determining role of carbon addition on mechanical performance of a non-equiatomic high-entropy alloy

• The role of C on the microstructure and mechanical properties are discussed. • The microstructure of the tensile specimen interrupted at different tensile strains are examined by EBSD and TEM. The deformation mechanism of a C-doped iHEA with excellent mechanical properties is discussed. • The contributions of different strengthening mechanism to yield strength are calculated and the comprehensive strengthening mechanism is discussed. The mechanical properties and deformation mechanism of a C-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe 49.5 Mn 29.7 Co 9.9 Cr 9.9 C 1 (at.%) were investigated. An excellent combination of strength and ductility was obtained by cold rolling and annealing. The structure of the alloy is consisted of FCC matrix and randomly distributed Cr 23 C 6 . For gaining a better understanding of deformation mechanism, EBSD and TEM were conducted to characterize the microstructure of tensile specimens interrupted at different strains. At low strain (2%), deformation is dominated by dislocations and their partial slip. With the strain increase to 20%, deformation-driven athermal phase transformation and dislocations slip are the main deformation mechanism. While at high strain of 35% before necking, deformation twins have been observed besides the HCP phase. The simultaneous effect of phase transformation (TRIP effect) and mechanical twins (TWIP effect) delay the shrinkage, and improve the tensile strength and plasticity. What's more, compared with the HEA without C addition, the yield strength of the C-doped iHEA has been improved, which can be attributed to the grain refinement strengthening and precipitation hardening. Together with the lattice friction and solid solution strengthening, the theoretical calculated values of yield strength match well with the experimental results.

Effects of thermal aging on mechanical properties and microstructures of an interstitial high entropy alloy with ultrasonic surface mechanical attrition treatment

In this work, indentation tests have been carried out on the surface of an interstitial high-entropy alloy, which is treated with ultrasonic surface mechanical attrition treatment and thermal aging treatment at 400 °C. We present and discuss these two treatments on the mechanical behavior of the material by using flat indentation method. Meanwhile, changes in its microscopic morphology, grain size, phase structure, etc. are investigated using X-ray diffraction, electron backscatter diffraction, etc. and its microstructure evolution mechanism under the long-term high temperature condition is revealed. Flat indentation tests and microstructural investigations reveal that the mechanical behavior of interstitial high entropy alloy is significantly affected by surface mechanical attrition treatment and thermal aging time. The gradient refined and strengthened surface layers are demonstrated to appreciably upgrade the mechanical performance of interstitial high entropy alloy, while thermal aging treatment will lead to a reduction of mechanical performance and a phase transformation.

Temperature effect on tensile behavior of an interstitial high entropy alloy: Crystal plasticity modeling

Previous tensile tests showed that the typical coarse-grained (CG) interstitial high entropy alloy (iHEA) with the nominal composition Fe49.5Mn30Co10Cr10C0.5 (at.%) has higher yield stress and better strain hardening at cryogenic conditions compared to that at room temperature. However, the yield stress of the fine-grained (FG) iHEA is little influenced by decreasing temperature, while the strain hardening is significantly enhanced. The fundamental reasons for these observations need to be further investigated. Thus, a micromechanism-based crystal plasticity model is developed to investigate the temperature effect on the tensile behavior of the iHEA. A thermodynamic model is established to calculate the stacking fault energy of the iHEA at different temperatures. The developed constitutive model is verified by comparing the simulated the stress-strain curves and martensite volume fraction of the CG and FG iHEAs at different temperatures with the corresponding experimental results. Moreover, the contributions of different strengthening mechanisms, such as dislocations, grain boundaries, nano-precipitation, and lattice friction, are quantified to reveal the effect of temperature on the iHEA's yield stress. Furthermore, the grain size effects on deformation twinning and martensite phase transformation are considered. The developed constitutive model is further applied to predict the stress-strain curves of the iHEA with various grain sizes at different temperatures. The present study thus provides a useful modeling tool for understanding the underlying mechanisms of the strength and plasticity in the iHEA, paving a way for optimizing the mechanical properties of advanced alloys at various temperatures.

Effect of carbon content, deformation and annealing on the structure and properties of interstitial TRIP high-entropy alloys

In this work, the effect of carbon content on the structure and mechanical properties of Fe(50-x)Mn30Co10Cr10Cx (x=0; 0.5; 1.0 at.%) interstitial high entropy alloys was studied. In the as-cast condition, the alloys were composed of the face centred cubic (fcc) and hexagonal close packed (hcp) phases; the amount of the hcp phase decreased from 46% to 20% as the carbon concentration increased from 0 to 1 at.%. The carbon content had a limited effect on the strength of the alloys, yet the ductility was enhanced significantly as the result of interstitial alloying. Furthermore, Fe49Mn30Co10Cr10C1 alloy was cold rolled to a reduction of 56% reduction and annealed at 700-900°C for 1 hour. After cold working, the alloy has attained almost fully the hcp structure. Annealing resulted in (i) transformation of the deformation-induced hcp phase into fcc; (ii) development of recovery and recrystallization; (iii) precipitation of Cr-rich M23C6 carbides. The cold-rolled alloy was very strong but brittle; annealing resulted in softening and increase of the ductility. The relationships between the chemical composition, structure and mechanical properties of the alloys are briefly discussed.

Awakening the metastability of an interstitial high entropy alloy via severe deformation

The metastability of a typical non-equiatomic FeMnCoCr high-entropy alloy (HEA) system was demonstrated to be dormant upon co-doping with C and N interstitials, i.e., displacive γ-ε phase transformation disappears during tensile deformation. Interestingly, we found in the present study that the displacive phase transformation prevails again in the C-N co-doped HEA upon severe deformation via cold-rolling. The generation of ε-phase lamellae is found to be motivated by strain localization which facilitates the formation of shear bands. Such highly concentrated strains significantly intensify the local elastic fields to neighboring γ matrix and finally eke out the driving force to overcome the interstitials-enhanced energy barrier of ε-martensite transformation. The results suggest that the suppressed phase metastability is awakened, which inspires efforts to tune the phase metastability via modifying the strain state during alloy processing and gives new insights into the development of HEAs with desirable phase stability and mechanical properties.

Cyclic plasticity of an interstitial high-entropy alloy: experiments, crystal plasticity modeling, and simulations

The development of high-entropy alloys (HEAs) comprising multiple principal components is an innovative design strategy for metallic materials from the perspective of thermodynamic entropy. However, despite their potential candidacy for engineering applications, the lack of research on the cyclic loading responses as well as constitutive modeling of the HEAs is a major constraint. Therefore, the present work focuses on the cyclic plasticity of a typical carbon-doped interstitial HEA (iHEA) with nominal composition Fe49.5Mn30Co10Cr10C0.5 (at.%). The results of stress-controlled cyclic tests with nonzero mean stress showed that the iHEA exhibits significant cyclic hardening and stress level–dependent ratcheting. Owing to its improved cyclic hardening, the saturated ratcheting strain rate of the iHEA is lower than that of conventional steels such as the 316L stainless steel. Furthermore, microscopic characterizations revealed that the cyclic deformations caused massive martensitic phase transformation and hierarchical structures in the iHEA. The experimental results were used to develop a physical mechanism-based crystal plasticity constitutive model that is capable of describing the cyclic plasticity of the iHEA, which was implemented into a finite element framework. The simulation results showed that the loading stress significantly affected the microstructural evolutions, leading to a stress level–dependent cyclic plasticity. Thus, this investigation provides a fundamental basis for fatigue tests and service life prediction/optimization of the iHEA in the future, which can promote its engineering applications.

Interfacial nanophases stabilize nanotwins in high-entropy alloys

Nanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nanosized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials.

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.

Interstitial equiatomic CoCrFeMnNi high-entropy alloys: carbon content, microstructure, and compositional homogeneity effects on deformation behavior

While interstitial alloying has been utilized to improve mechanical properties of multi-component high-entropy alloys (HEAs), its effectiveness depends on the interstitial content, microstructure and compositional homogeneity states. Here we present and discuss the influences of these factors on the mechanical behavior of interstitial equiatomic CoCrFeMnNi HEAs at room temperature. Interstitial HEAs containing carbon of 0.2, 0.5 and 0.8 at. % were processed to different compositional homogeneity states and grain sizes. We found that deformation of the various interstitial HEAs at early deformation stages is accommodated by dislocation slip whereas twinning occurs at the later stages. Upon an identical local strain at the later stages of deformation, nano-twin density decreases as the increase of carbon content due to the increased stacking fault energy. Also, the increase of C content leads to significantly higher energy barrier to recrystallization during annealing. Partially recrystallized (∼20 vol %) interstitial HEA with C content of 0.8 at. % shows more than five times higher yield strength compared to the as-homogenized coarse-grained (∼200 μm) reference material, suggesting the significant beneficial effect of interstitials enabled microstructural adjustment on performance of the interstitial HEAs. Further, the compositionally inhomogeneous coarse-grained (∼200 μm) interstitial HEAs exhibit lower work-hardening ability and ultimate strength compared to the homogenized reference material due to that the compositional inhomogeneity promotes the localized plasticity. Some more insights for the design and processing of interstitial HEAs are generalized and discussed.