High entropy alloys (HEA) with improved properties are sought to be developed for structural applications. In this work, a metastable, non-equiatomic, dual-phase, Fe_49.5Mn₃₀Co₁₀Cr₁₀C_0.5, HEA Is subjected to Severe Plastic Deformation (SPD) technique of Friction Stir Processing (FSP). The microstructure of the Fe₅₀Mn₃₀Co₁₀Cr₁₀ alloy, in the annealed condition consists of FCC phase and Hexagonal Closed Pack (HCP) laminates [1]. Different strengthening mechanisms viz., solid solution strengthening (SSS), dislocation strengthening, grain boundary strengthening, twinning induced plasticity (TWIP), transformation induced plasticity (TRIP) are incorporated in this alloy [2]. Li et al. [3] have further improved the dual-phase Fe₅₀Mn₃₀Co₁₀Cr₁₀, HEA by adding small amount of C (~0.5%), as an interstitial solid solution strengthening element. Grain refinement achieved via friction stir processing (FSP) is known to enhance the mechanical properties as well as the corrosion resistance of the alloy. Systematic investigations are carried out to determine the microstructural changes occurring during different passes of FSP and to evaluate the mechanical and corrosion behaviour of the material with each progressive pass. A reduction in the grain size due to dynamic recrystallization and an increase in the volume fraction of the HCP phase due to the TRIP effect, was observed after each pass. The one-pass FSPed material exhibits a higher work hardening rate, a higher ultimate tensile strength and improved corrosion resistance compared to annealed and two-pass FSPed material. This was attributed to a combination of two factors viz., a small grain size and a higher fraction of FCC phase retained in the microstructure after FSP. While the smaller grain size in the one-pass FSP material leads to increased resistance to deformation; the higher amount of FCC phase retained after FSP in one-pass material, during subsequent tensile loading, undergoes strain induced transformation resulting in a higher value of UTS. While the smaller grain sized material with larger fraction of high-angle grain boundary promotes rapid diffusion of Cr leading to a thicker passive film formation on the surface, resulting in decreased corrosion rate; the decreased fraction of strain-induced HCP phase in the microstructure of the one-pass FSP material, engenders lower residual tensile stress in the matrix, thereby providing fewer number of active high energy sites for electrochemical attack, resulting in decreased corrosion [4].

References

[1] Z. Li, C. C. Tasan, K. G. Pradeep, and D. Raabe, Acta Mater., 2017, Vol. 131, pp. 323–335.

[2] Z. Li, D. Raabe, JOM, 2017, Vol. 69, pp. 2099-2106

[3] Z. Li, C. C. Tasan, H. Springer, B. Gault, and D. Raabe, Sci. Rep., 2017, Vol. 7, pp. 1–7.

[4] G. Wang, Y. Zhang, C. Gao, G. T. Xu, and M. H. Zhao, Anti-Corrosion Methods Mater., 2020, Vol. 67, pp. 357–366.