Deformation Of High-entropy Alloy Research Articles

In-situ TEM Deformation of High Entropy Alloys.

In-situ TEM Deformation of High Entropy Alloys.

Micro-characteristics of High-Entropy Alloy Weld Formed by Friction Stir Lap Welding of 1060 Al and Al0.1CoCrFeNi

1060 Al and Al0.1CoCrFeNi are friction stir lap welded in this study. The limited flow of high-entropy alloy avoids the occurrences of hook and cold lap defects in the lap joint. Coarsening of large-size Al(Cr)-rich particles and dissolution of small-size Al(Cr)-rich particles simultaneously take place in the heat-affected zone of high-entropy alloy during the welding. The plastic deformation of high-entropy alloy induced by welding tool refines and homogenizes the microstructures in thermo-mechanically affected zone and stir zone. Friction stir lap weld of high-entropy alloy presents higher hardness than the base metal in each zone. The hardness of stir zone exhibits a maximum of 600 Hv, higher than the maximum hardness of AlxCoCrFeNi traditionally obtained. This should be attributed to the microstructure refinement as well as the enhancement of solid-solution strengthening and the occurrence of body-centered-cubic structures due to Al content improvement.

Molecular dynamics simulations for nanoindentation response of nanotwinned FeNiCrCoCu high entropy alloy

Twin boundary (TB) plays an important role on the plastic deformation of high entropy alloy (HEA). The strong effects of TB on the deformation response of HEA are revealed from atomic level based on the defect structure, shear strain and surface morphology, by comparing the nanoindentation behavior of nanotwinned FeNiCrCoCu HEA (nt-HEA) and single-crystal FeNiCrCoCu HEA (single-HEA). The plastic deformation of nt-HEA is mainly dominated by the dislocations slip confined by first twinning layer, the TB migration, the dislocation nucleation at TB and the stacking fault tetrahedron (SFT) formation, while the dislocation loop emission is the main plastic deformation feature of single-HEA. Compared to the case for single-HEA, the nanoindentation induces more dislocations in nt-HEA. The shear strain in nt-HEA mainly distributes in the first twinning layer, due to the obstacle effect of TB. The shear zone is larger in nt-HEA, and the distribution of shear strain on the nt-HEA surface is more symmetric. The nanoindentation generates fewer steps on the nt-HEA surface, and then brings about a relatively smooth surface for nt-HEA. These findings provide an insight into the TB effect on the nanoindentation response of FeNiCrCoCu HEA, and develop the application of nanotwinned HEA systems.

Micromechanical modeling of work hardening for coupling microstructure evolution, dynamic recovery and recrystallization: Application to high entropy alloys

A dislocation density-based multiscale constitutive model is developed to describe the hot deformation of high entropy alloys. The work hardening process of high entropy alloy is divided into several stages according to the change of hardening rate. As crystalline material, high entropy alloys are regarded as two-phase materials. In the current study, a physical-based constitutive model with three key microstructural state variables, cell mobile dislocation density, cell immobile dislocation density and wall immobile dislocation density, is proposed to simulate the phases deformations. The physical mechanisms including strain hardening, dynamic recovery, and recrystallization are considered in describing the evolution of dislocation densities. It is noted that the competition and interaction among the three mechanisms result in a typical hardening behavior of high entropy alloys. The dislocation density-based constitutive model is embedded in the modified Mori-Tanaka scheme to describe the hardening behaviors of two-phase materials with an elastic phase cell wall and an elastic-inelastic phase cell interior. The model is applied for simulating the hardening curves of CoCrFeMnNiC0.5 high entropy alloy at different temperatures and strain rates. Compared with experimental data, it shows that the developed model can describe hot deformation of high entropy alloys with reasonable accuracy. The macroscopic inelastic strain in the Mori-Tanaka scheme is calculated by a classical plasticity method. The simulation results show that the modified method can give a more accurate prediction compared to the average inelastic strain of all phases.

Dislocation avalanche mechanism in slowly compressed high entropy alloy nanopillars

Crystals deform by the intermittent multiplication and slip avalanches of dislocations. While dislocation multiplication is well-understood, how the avalanches form, however, is not clear, and the lack of insight in general has contributed to “a mass of details and controversy” about crystal plasticity. Here, we follow the development of dislocation avalanches in the compressed nanopillars of a high entropy alloy, Al0.1CoCrFeNi, using direct electron imaging and precise mechanical measurements. Results show that the avalanche starts with dislocation accumulations and the formation of dislocation bands. Dislocation pileups form in front of the dislocation bands, whose giveaway trigs the avalanche, like the opening of a floodgate. The size of dislocation avalanches ranges from few to 102 nm in the nanopillars, with the power-law distribution similar to earthquakes. Thus, our study identifies the dislocation interaction mechanism for large crystal slips, and provides critical insights into the deformation of high entropy alloys.