Multi-phase High-entropy Alloy Research Articles

A multi-phase CrMnFeCoNiAl0.75 high-entropy alloy with high strength at intermediate temperature

High-entropy alloys with a superalloy-like microstructure show excellent mechanical properties at elevated temperatures. In this paper, a multi-phase high-entropy alloy CrMnFeCoNiAl0.75 was developed. Based on the microstructure characterization by XRD, SEM, TEM and 3D-APT, it was found that the alloy was composed of FCC phase, BCC phase, and B2 phase. The mechanical properties evaluation showed that the alloy had high compression yield strength of 1194 MPa at room temperature and could preserve high yield strength of 1070 MPa at 500 °C. Further increase in the temperature beyond 600 °C resulted in alloy softening. The alloy showed limited strain of 2.72% at room temperature, but it exhibited adequate deformability at elevated temperatures up to 500 °C (strain >20%). The strengthening mechanisms of the alloy at different temperatures were discussed.

Evaluation of microstructure and texture formation during annealing of cold-rolled FeCrCuMnNi multiphase high-entropy alloy

Abstract Microstructure and texture variations of a cold-rolled multiphase FeCrCuMnNi high-entropy alloy were studied after different annealing treatments. Samples were heat-treated at different temperatures and for different time, and then, subjected to different tests including XRD and SEM-EBSD. The results reveal that the FCC1 phase, which goes through more strain, has lower melting temperature, and recrystallizes earlier (lower temperature and shorter time). In addition, it is seen that particle stimulated nucleation is more effective on the recrystallization of this phase compared to FCC2 phase. A significant number of FCC1 nuclei form around the BCC particles. Nucleation of FCC2 phase initiated at 800 °C mostly at the grain boundaries and the inhomogeneities. The FCC1 phase was almost fully recrystallized at this temperature. The annealing process led to the elimination of rolling textures, while the Brass component remained a major component in the recrystallized samples. Increasing annealing temperature as well as annealing time led to the formation of Cube texture component, which is a major component of recrystallized low stacking fault energy materials. Furthermore, D and Rt-Co components formed during recrystallization as a result of the formation of annealing twins.

Strengthening mechanisms in multi-phase FeCoCrNiAl1.0 high-entropy alloy cladding layer

In order to study the strengthening mechanisms in the multi-phase (FCC + BCC) high-entropy alloy cladding layer, FeCoCrNiAl1.0 coating was fabricated by laser cladding technology. For comparison, single phase (FCC) FeCoCrNiAl0.3 coating, pure copper, and pure iron were also considered. XRD analysis, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscope (TEM) were used to investigate the evolution of phases and microstructures in the materials. The hardness and tensile property of the materials were investigated using micro-hardness and small punch tests. It was found that Al element could promote the formation of BCC solid solutions. The solidification pattern of coatings changed from columnar to equiaxed crystals. The FeCoCrNiAl1.0 high-entropy alloy cladding layer (FCC + BCC) exhibited the best mechanical properties. Furthermore, the strengthening mechanisms in the multi-phase high-entropy alloy cladding layer were analyzed systematically. The rapid solidification in the laser cladding process resulted in the formation of fine grains that strengthened the material. In addition, the FCC and BCC lattice structures contain different slip directions and types of dislocation. The lattice distortions of the two crystal structures were determined by calculating the interplanar spacings and the interplanar angles. Under external load, the FCC and BCC phases in the multi-phase high-entropy alloy cladding layer experienced different degrees of displacements, which also played a role in strengthening the material.

High strength and high ductility in a novel Fe40.2Ni11.3Mn30Al7.5Cr11 multiphase high entropy alloy

In the present study, a novel Fe40.2Ni11.3Mn30Al7.5Cr11 high entropy alloy displaying an excellent combination of high strength and high ductility has been developed. The b.c.c.+B2/f.c.c. dendritic-interdendritic microstructure was characterized using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy, and the alloy’s mechanical properties were examined using Vickers hardness measurements and room-temperature tensile tests. The phase stability of Fe40.2Ni11.3Mn30Al7.5Cr11 was investigated by annealing the alloy between 1173 K–1273 K, and the presence of both Ni-rich, b.c.c. needle-shaped precipitates and Cr-rich, σ phase particles was observed for the 1173 K annealed specimen. The σ phase precipitates contributed to the rapid age-hardening effect in the material annealed at 1173 K. A further increase in ageing temperature to 1223 K and 1273 K led to the dissolution of the σ phase and a reduction in the volume fraction of the b.c.c./B2 dendrites, which led to a consequent drop in room-temperature yield strength from the as-cast value of ∼593 MPa–∼486 MPa and ∼228 MPa for the specimens annealed at 1223 K and 1273 K, respectively. An accompanying increase in ductility was observed from ∼22% for the as-cast alloy to ∼27% and ∼40% after annealing the material at 1223 K and 1273 K, respectively. Post-deformation transmission electron micrographs revealed that the f.c.c interdendrites accommodated plastic strain via wavy slip and moreover, deformed before the b.c.c. dendritic regions, which were reinforced by B2 particles and acted as obstacles to moving dislocations. The discovery of a multi-phase HEA that contains 11% Cr and good mechanical properties has relevant high-temperature structural applications especially in cases that require suitable corrosion resistance.

Nanostructuring with Structural-Compositional Dual Heterogeneities Enhances Strength-Ductility Synergy in Eutectic High Entropy Alloy

A lamellar (L12 + B2) AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was severely deformed by a novel hybrid-rolling process. During hybrid-rolling, the deformation was carried out in two stages, namely cryo-rolling followed by warm-rolling at 600 °C. The strain (ε) imparted in each of these steps was identical ~1.2, resulting in a total strain of ε~2.4 (corresponding to 90% reduction in thickness). The novel processing strategy resulted in an extremely heterogeneous microstructure consisting of retained lamellar and transformed nanocrystalline regions. Each of these regions consisted of different phases having different crystal structures and chemical compositions. The novel structure-composition dual heterogeneous microstructure originated from the stored energy of the cryo-rolling which accelerated transformations during subsequent low temperature warm-rolling. The dual heterogeneous microstructure yielded an unprecedented combination of strength (~2000 MPa) and ductility (~8%). The present study for the first time demonstrated that dual structure-composition heterogeneities can be a novel microstructural design strategy for achieving outstanding strength-ductility combination in multiphase high entropy alloys.

Investigation of microstructure, texture, and mechanical properties of FeCrCuMnNi multiphase high entropy alloy during recrystallization

FeCrCuMnNi multiphase high entropy alloy was cold-rolled to 85% reduction and annealed at different temperatures. Recrystallization behavior of the alloy was investigated using XRD, SEM-EBSD, hardness, microhardness, and tensile tests. The results revealed variation in phase distribution due to annealing and earlier recrystallization of the FCC1 phase. Recrystallization of FCC1 and FCC2 phases initiated at 873 and 1073 K, respectively, and a fully recrystallized microstructure was seen after 1273 K. Deformation texture components eliminated slowly with increase in annealing temperature, and D and Cube components formed as a result of recrystallization. Faster decrease in FCC1 microhardness was seen due to earlier recrystallization. In addition, an excellent combination of strength and elongation was achieved in the partially recrystallized samples compared to conventional alloys. The strength was affected more after annealing at temperatures higher than 1073 K. Fracture behavior of FCC2 phase changed from a brittle fracture to a ductile fracture with increase in annealing temperature; however, FCC1 phase revealed a ductile fracture even at low annealing temperatures.

Phase-transition assisted mechanical behavior of TiZrHfTax high-entropy alloys

Recent developments of high-entropy alloys with high strength and high ductility draw attention to the metastability-engineering strategy. Using first-principle theory, here we demonstrate that reducing the Ta level in the refractory TiZrHfTax system destabilizes the body-centered cubic (bcc) phase and leads to the appearance of the hexagonal close-packed (hcp) phase embedded in the bcc matrix. The alloying-induced features of the elastic parameters for the cubic and hexagonal structures are mapped out in details, and strong sensitivity to the crystal lattice and chemistry is revealed. Results show softening of the bcc matrix with decreasing Ta concentration which ensures ductile behavior. However, the elastically nearly isotropic hcp precipitates possess enhanced resistance against shear which promotes strengthening of the TiZrHfTax dual-phase system. The present atomic-level insight provides strong evidence to the experimental observation, and emphasizes the significance of quantum-design for advanced multi-phase high-entropy alloys with excellent strength-ductility combinations.

Extremely high strength and work hardening ability in a metastable high entropy alloy

Design of multi-phase high entropy alloys uses metastability of phases to tune the strain accommodation by favoring transformation and/or twinning during deformation. Inspired by this, here we present Si containing dual phase Fe42Mn28Co10Cr15Si5 high entropy alloy (DP-5Si-HEA) exhibiting very high strength (1.15 GPa) and work hardening (WH) ability. The addition of Si in DP-5Si-HEA decreased the stability of f.c.c. (γ) matrix thereby promoting pronounced transformation induced plastic deformation in both as-cast and grain refined DP-5Si-HEAs. Higher yet sustained WH ability in fine grained DP-5Si-HEA is associated with the uniform strain partitioning among the metastable γ phase and resultant h.c.p. (ε) phase thereby resulting in total elongation of 12%. Hence, design of dual phase HEAs for improved strength and work hardenability can be attained by tuning the metastability of γ matrix through proper choice of alloy chemistry from the abundant compositional space of HEAs.

Simultaneous Strength-Ductility Enhancement of a Nano-Lamellar AlCoCrFeNi2.1 Eutectic High Entropy Alloy by Cryo-Rolling and Annealing

Nano-lamellar (L12 + B2) AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was processed by cryo-rolling and annealing. The EHEA developed a novel hierarchical microstructure featured by fine lamellar regions consisting of FCC lamellae filled with ultrafine FCC grains (average size ~200–250 nm) and B2 lamellae, and coarse non-lamellar regions consisting of ultrafine FCC (average size ~200–250 nm), few coarse recrystallized FCC grains and rather coarse unrecrystallized B2 phase (~2.5 µm). This complex and hierarchical microstructure originated from differences in strain-partitioning amongst the constituent phases, affecting the driving force for recrystallization. The hierarchical microstructure of the cryo-rolled and annealed material resulted in simultaneous enhancement in strength (Yield Strength/YS: 1437 ± 26 MPa, Ultimate Tensile Strength/UTS: 1562 ± 33 MPa) and ductility (elongation to failure/ef ~ 14 ± 1%) as compared to the as-cast as well as cold-rolled and annealed materials. The present study for the first time demonstrated that cryo-deformation and annealing could be a novel microstructural design strategy for overcoming strength-ductility trade off in multiphase high entropy alloys.

In the quest of single phase multi-component multiprincipal high entropy alloys

The design and development of single phase multi-component high entropy alloy (HEA) using conventional hit and trial method is a challenging job due to the large number of experimental trials required to find the right alloy composition from possible combinations of various alloying elements in the multi-component system. Present investigation reports a novel CALculation of PHAse Diagram (CALPHAD) based approach, which can significantly reduce number of experimental trials in a very time effective manner to find single phase HEAs. Here, 1287 equiatomic five-component alloy systems were studied using both parametric and CALPHAD approaches to find single phase fcc and bcc HEAs. The parametric approach reduced the number of trials to 124 whereas; CALPHAD approach reduced the number of alloy compositions to 10. Further, the experimental investigation on the 10 alloy systems reveals that six of them form single phase HEAs. In addition, Thermo-Calc software was also used effectively to draw the isothermal pseudo ternary sections at 1000 K varying all constituents elements from 5 to 35 atomic percent for the CoCuFeMnNi alloy system. Experimental evidences confirmed that the isothermal pseudo ternary diagrams are very useful to design non equiatomic single phase as well as multiphase HEAs in the absence of phase diagram for the five component systems.

Tensile ductility of an AlCoCrFeNi multi-phase high-entropy alloy through hot isostatic pressing (HIP) and homogenization

The microstructure and phase composition of an AlCoCrFeNi high-entropy alloy (HEA) were studied in as-cast (AlCoCrFeNi-AC, AC represents as-cast) and homogenized (AlCoCrFeNi-HP, HP signifies hot isostatic pressed and homogenized) conditions. The AlCoCrFeNi-AC ally has a dendritric structure in the consisting primarily of a nano-lamellar mixture of A2 (disordered body-centered-cubic (BCC)) and B2 (ordered BCC) phases, formed by an eutectic reaction. The homogenization heat treatment, consisting of hot isostatic pressed for 1h at 1100°C, 207MPa and annealing at 1150°C for 50h, resulted in an increase in the volume fraction of the A1 phase and formation of a Sigma (σ) phase. Tensile properties in as-cast and homogenized conditions are reported at 700°C. The ultimate tensile strength was virtually unaffected by heat treatment, and was 396±4MPa at 700°C. However, homogenization produced a noticeable increase in ductility. The AlCoCrFeNi-AC alloy showed a tensile elongation of only 1.0%, while after the heat-treatment, the elongation of AlCoCrFeNi-HP was 11.7%. Thermodynamic modeling of non-equilibrium and equilibrium phase diagrams for the AlCoCrFeNi HEA gave good agreement with the experimental observations of the phase contents in the AlCoCrFeNi-AC and AlCoCrFeNi-HP. The reasons for the improvement of ductility after the heat treatment and the crack initiation subjected to tensile loading were discussed.