Impact of gradient microstructure on strain hardening via activation of multiple deformation mechanisms in CoCrNi medium entropy alloy

Nanjing University of Science and Technology in collaboration with Liaoning Academy of Materials, Hohai University and City University of Hong Kong revealed a significant strengthening mechanism related to the core region in a gradient CrCoNi MEA. In this study, the microstructure evolution and mechanical strengthening mechanism of CrCoNi MEA were investigated using advanced techniques such as EBSD, TEM and HRTEM. Through the analysis of microstructure before and after uniaxial tensile plastic deformation along the depth direction, It is revealed that the fault energy characteristics of CoCrNi MEA at the lower level trigger the emergence of nanoscale deformation twins, layer faults, L-C dislocation locks and the phase transformation from FCC to HCP at the twin boundary in the core region to enhance the strain hardening ability of the material.

FCC CoCrNi MEA limits practical application due to its low yield strength at room temperature. Through the " surface nanocrystallization " of gradient materials, severe plastic deformation can be generated on the sample surface to refine the grain size of the material surface, and microscopic structures such as gradient grain size and nanotwin gradient are introduced into the material, thereby improving the mechanical properties and strain hardening ability of the material. In gradient structural materials, initial tensile deformation stages predominantly show plasticity in the core region. Subsequently, plastic deformation shifts towards the surface, introducing a coordinated deformation mechanism and an extra layer of strain hardening. However, the deformation mechanism in MEAs/HEAs remains complex compared to conventional metals. Currently, the microscale intricacies driving strain hardening effects in gradient structures remain unclear. The plasticity of GS materials is mainly affected by the matrix region of the core region, so it is particularly important to explore the deformation mechanism of its core region.

The Solution: Nanjing University of Science and Technology, in cooperation with Liaoning Institute of Materials Research and Hohai University, reported the plastic deformation mechanism in the core region of the CoCrNi MEA. The results reveal two key transitions in the plastic deformation mechanisms at the core of samples: initially, a shift from dislocation slip to deformation twinning occurs, a phenomenon commonly observed in many low stacking fault energy materials, resulting in the nanocrystallization of the core material. Additionally, when the nanoscale twins are refined to a critical size, a local phase transformation occurs at the twin boundaries. This transformation is crucial for maintaining stable plasticity under high flow stress conditions. By empirically validating these observations, this study highlights the potential of engineered gradient structures to reconcile the strength-ductility trade-off in FCC MEAs/HEAs and uncovers the fundamental mechanisms underlying HDI hardening.

The Future: Future studies will explore engineering strategies to induce phase transitions of M/HEAs nanotwins and HCP to significantly improve their strength and ductility at room temperature and extremely low temperatures.

The Impact: This work offers a promising way to understand the anisotropic properties at stepped metal/water interfaces from the prominent water pairs formed at the interface.

The research has been recently published in the online edition of Materials Futures, a new international journal in the field of interdisciplinary materials science research.

Reference
Jiaqi Meng, Yonghao Zhao, Yi Liu, Zongyao Li, Bo Gao, Mengning Xu, Weiheng Xia, Xuefei Chen, Hao Zhou, Yuntian Zhu. Impact of Gradient Microstructure on Strain Hardening via Activation of Multiple Deformation Mechanisms in CoCrNi Medium Entropy Alloy [J]. Mater. Futures 3 041002