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

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 (gamma), stable sigma (sigma), and stable epsilon martensite (epsilon) 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 gamma and decreasing the sigma and epsilon content. The strength was further enhanced by FSP at 150 RPM by further refinement of the grain structure and increasing the fraction of epsilon phase. The peak ultimate tensile strength of similar to 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 sigma phase. While the content of diffusion created sigma phase remains constant during subsequent compression, the fraction of epsilon increases due to the diffusionless strain induced gamma ->epsilon phase transformation. The transformation facilitates plastic strain accommodation and rapid strain hardening, which has been attributed to the increase in highly dislocated e 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the tailoring of phase content and grain structure of a high entropy alloy to enhance its strength through rolling, friction stir processing, and compression. The results showed that the adaptive phase stability of the selected HEA, along with grain refinement and manipulation of phase fractions, effectively improved the material's strength. By optimizing the processing parameters, including tool rotation rate during FSP, the alloy achieved a peak ultimate tensile strength of approximately 1850 MPa. Furthermore, the study discussed the role of texture, grain size, and phase content in the transformation behavior during compression.