Microstructure and Texture Evolution of a Dynamic Compressed Medium-Entropy CoCr0.4NiSi0.3 Alloy

Abstract

Focal research has been conducted on medium-entropy alloys (MEAs) that exhibit a balanced combination of strength and plasticity. In this study, the microstructure, dynamic mechanical properties, and texture evolution of an as-cast medium-entropy CoCr0.4NiSi0.3 alloy were investigated through dynamic compression tests at strain rates ranging from 2100 to 5100 s−1 using the Split Hopkinson Pressure Bar in order to elucidate the underlying dynamic deformation mechanism. The results revealed a significant strain rate effect with dynamic compressive yield strengths of 811 MPa at 2100 s−1, 849 MPa at 3000 s−1, 919 MPa at 3900 s−1, and 942 MPa at 5100 s−1. Grains were dynamically refined from 19.73 to 3.35 μm with increasing strain rates. The correlation between adiabatic temperature rise induced by dynamic compression and dynamic recrystallization was examined, revealing that the latter is not associated with adiabatic heating but rather with phase transition triggered by the dynamic stress during compression. The proportion of Σ3n (1 ≤ n ≤ 3) grain boundaries in deformation specimens increases with increasing strain rates during dynamic compression. The formation of specific three-node structures enhances both strength and plasticity by impeding crack propagation and resisting higher mechanical stress. In the as-cast state, significant anisotropy was observed in the MEA. As strain rates increased, it transited into a stable {111}<112> F texture. The exceptional dynamic properties of strength and plasticity observed in the as-cast state of the MEA can be attributed to a deformation mechanism involving a transition from dislocation slip to the formation of intricate arrangements, accompanied by interactions encompassing deformation nanotwins, stacking faults, Lomer–Cottrell locks, stair-rods, and displacive phase transformations at elevated strain rates.

Investigation of the Hot Deformation Behavior and Mechanism of a Medium-Entropy CoCr0.4NiSi0.3 Alloy

“…Additionally, under a strain rate of 0.001 s −1 , deformation causes the transformation of SFs in the FCC matrix into the HCP phase, subsequently leading to LPSO formation, as depicted in Figure 13i-k. Following the FCC-HCP phase transition law in this MEA [55], the generated LPSO phase can nucleate through either process due to low SFE [56] of this CoCrNi-based MEA followed the process of FCC→SFs→HCP→LPSO produced by SFs or FCC→Twins→HCP→LPSO produced by twinning. The LPSO phase, being a hard precipitated phase, exhibits a dispersion-strengthening effect during deformation.…”

“…The volume fraction of DRX and average grain diameter were calculated and analyzed under various deformation conditions, as depicted in Figures 7-9. The phase composition of the initial state (as-cast CoCr0.4NiSi0.3 alloy) of the specimens prior to hot deformation was confirmed to consist of an FCC matrix with a minor presence of lamellar Ni31Si12 and Ni2Si eutectic structures at subgrain boundaries, as characterized by XRD and TEM [55]. The influence of deformation temperature and strain rate on the behavior and growth of DRX during hot compression of the MEA was investigated using EBSD analysis technology.…”

“…Figure 7a,b depicts the changes in grain morphology at strain rates of 1 s −1 and 0.001 s −1 , respectively, at a temperature of 850 • C. Figure 7c,d illustrates the grain boundary volume fraction and distribution of Σ3 in regions A and B shown in Figure 7a,b, respectively. Figure 7e presents the contrast between the volume fraction of recrystallized, substructured, and deformed grains at strain rates of 1 s −1 and 0.001 s −1 at a temperature of 850 • C. Figure 7f demonstrates the changes in average grain diameter (by equivalent circular diameter) at strain rates of 1 s −1 and 0.001 s −1 at a temperature of 850 • C. The as-cast average grain diameter of the MEA measures 19.73 µm [55]. Upon hot deformation at 850 • C/1 s −1 , a significant number of fine necklace-like dynamically recrystallized (DRXed) grains form along the grain boundaries.…”

“…Moreover, under a strain rate of 0.001 s −1 , the SFs in the FCC matrix transform to the HCP phase during deformation and subsequently form LPSO, as depicted in Figure 13i-k. Following the law of FCC-HCP phase transition of this MEA [55], the generated LPSO phase can follow either process due to low SFE: FCC→SFs→HCP→LPSO produced by SFs, or FCC→Twins→HCP→LPSO produced by twinning. The recrystallization process at high strain rates results in the formation of the lamellar LPSO phase within the DRXed grains as shown in Figure 12d.…”

A Review: Design from Beta Titanium Alloys to Medium-Entropy Alloys for Biomedical Applications

“…In recent years, researchers have developed some non-equiatomic medium-entropy alloys (MEAs) [45][46][47] and ternary MEAs [48][49][50]. Although the ∆S mix of these new MEAs does not meet the conditions mentioned earlier (11 ≤ ∆S mix ≤ 19.5 J/K•mol), the ∆H mix of the alloys is close to or less than zero, allowing for the free energy (∆G mix = ∆H mix − T∆S mix ) of the alloys to be at a lower value, facilitating the formation of a solid solution.…”