Low-density Fe40Mn19Ni15Al15Si10C1 high entropy steel processed by mechanical alloying and spark plasma sintering: Phase evolution, microstructure and mechanical properties

Abstract

The present study deals with the synthesis of a novel Fe40Mn19Ni15Al15Si10C1 low-density high entropy steel (HES) processed through mechanical alloying (MA) and spark plasma sintering (SPS). The elemental powder was milled for 35 h, leading to the formation of multiphase structures i.e., BCC phase (a = 0.286 nm; cI2), γ-brass type (a = 0.872 nm; cI52) and partially ordered B2-type (a = 0.290 nm; cP2) phases with the trace amount of Si. The milled powder was found to be thermally stable up to ∼500 °C, however, the formation of Fe5Si3-type phase was evident at ∼520 °C. The spark plasma sintered (SPSed) sample was able to retain the BCC phase (a = 0.286 nm), γ-brass type (a = 0.872 nm), B2 (a = 0.289 nm) along with the formation of Fe5Si3 type silicide phase (a = b = 0.667 nm, c = 0.468 nm; hP16). These SPSed HES sample was found to have low density (∼6.49 ± 0.3 g cm−3), high microhardness (∼7.8 ± 0.3 GPa) and good compressive strength (∼2046 ± 160 MPa) with an appreciable ductility of ∼19%. The enhanced mechanical properties of the SPSed sample can be attributed to dual-phase microstructure i.e., BCC and B2 along with finer nano-sized silicide precipitates leading to dominant strengthening mechanisms (i.e., grain-boundary and dislocation strengthening). The experimental findings on phase evolution, thermal stability and mechanical properties of non-equiatomic HES were correlated with the various thermodynamic parameters, CALPHAD modelling and strengthening mechanisms. The significance of the present study is to design a high entropy steel containing the dual phase microstructures i.e., BCC as major phase and B2 as secondary phase, for giving rise to high mechanical strength and high temperature stability.