Effects of plasticity-induced martensitic transformation and grain refinement on the evolution of microstructure and mechanical properties of a metastable high entropy alloy

Shubhrodev Bhowmik, Jianzhong Zhang, Sven C. Vogel, Saurabh S. Nene, Rajiv S. Mishra, Brandon A. McWilliams, Marko Knezevic

This paper describes the main results from an experimental investigation into tailoring the phase content and grain structure for high strength of a microstructurally flexible high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%), using rolling, friction stir processing (FSP), and compression. Optical microscopy, neutron diffraction, and electron backscatter diffraction were employed to characterize microstructure and texture evolution. The material upon rolling was found to have triplex structure consisting of metastable gamma austenite (γ), stable sigma (σ), and stable epsilon martensite (ε) phases. The adaptive phase stability exhibited by the selected HEA of very low stacking fault energy with strain, strain rate, and temperature was used along with grain refinement to enhance the strength. To this end, the complex structure was refined by FSP at 350 revolutions per minute (RPM) tool rotation rate, while increasing the fraction of γ and decreasing the σ and ε content. The strength was further enhanced by FSP at 150 RPM by further refinement of the grain structure and increasing the fraction of ε phase. The peak ultimate tensile strength of ~1850 MPa was achieved by double pass FSP (350 RPM followed by 150 RPM), the sequence which even more refined the microstructure and increased the fraction of σ phase. While the content of diffusion created σ phase remains constant during subsequent compression, the fraction of ε increases due to the diffusionless strain induced γ→ε phase transformation. The transformation facilitates plastic strain accommodation and rapid strain hardening, which has been attributed to the increase in highly dislocated ε phase fraction and transformation induced dynamic Hall-Petch-type barrier effect. Interestingly, a great deal of hardening ability was exhibited by the HEA even at very high strength. Roles of texture, grain size, and phase content on the transformation during compression have been rationalized and discussed in this paper.

Keywords: High entropy alloys; Friction stir processing; Phase transformations; Grain structure; Texture

Journal of Alloys and Compounds
Volume 891, 2022, 161871, ISSN 0925-8388,