The predicted rate-dependent deformation behaviour and multistage strain hardening in a model heterostructured body-centered cubic high entropy alloy

J Peng and L Li and F Li and B Liu and S Zherebtsov and QH Fang and J Li and N Stepanov and Y Liu and F Liu and PK Liaw, INTERNATIONAL JOURNAL OF PLASTICITY, 145, 103073 (2021).

DOI: 10.1016/j.ijplas.2021.103073

Heterostructured high-entropy alloys (HEAs) exhibit a good combination of strength and ductility. However, the deformation mechanisms and related mechanical properties of the heterogeneous nanostructured body- centred cubic (BCC) HEAs remain largely unrevealed. Here, we report the effect of the strain rate on the mechanical properties and microstructural evolution in the heterostructured Al3CoCrCuFeNi HEA with the gradient grain size ranging from 9 nm at the surface to 45 nm in the center using the large-scale atomic simulations. Based on the characterization of microstructure evolution from the atomic simulation, a microstructure-based constitutive model that utilizes a single parameter set is established to study the effects of the multiple strengthening mechanisms at various deformation stages on the strength and strain hardening of the heterostructured HEA under different strain rates. The results show that the plastic deformation mechanisms are strongly influenced by the strain rate in the range of 1 x 10(6) to 1 x 10(10) s(-1). The dislocation and deformation twin are the main deformation mechanisms at the strain rate less than 1 x 10(8) s(-1). The deformation twinning cooperated with multiple phase transformations plays a pivotal role at the strain rate of 1 x 10(9) s(-1). The multiple phase transformations are the dominant plastic deformation model at the strain rate of 1 x 10(10) s(-1), which is different from the classic plastic-deformation mode, including the dislocation and deformation twin in traditional alloys. This phenomenon is ascribed to various types of atomic interactions and atomic-size mismatches, which produce strong lattice distortion. In particular, a large number of edge dislocations are observed, which is extremely different from screw dislocations invariably generated in traditional BCC alloys, and thus, could be responsible for high strength. Furthermore, the stress-strain response of the Al3CoCrCuFeNi HEA is derived from the individual contributions of the lattice distortion, dislocation, grain boundary, deformation twin, phase transformation, and back-stress strengthening mechanisms. The heterogeneous grain boundaries and lattice distortion contribute the most to the yield strength, and the dislocation, deformation twin, phase transformation, and back stress dominate the strain hardening. Therefore, these results highlight a new design strategy of HEAs to tailor their mechanical properties based on the partition of the multistage strengthening mechanisms by the heterostructure.

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