Tailoring fracture resistance of a metastable Fe42Mn28Co10Cr15Si5 high entropy alloy by intrinsic toughening

Metastable high entropy alloys (HEAs) provide an exceptional combination of strength and ductility by the synergistic operation of slip, twinning, and transformation; however, their fracture behaviour remains unexplored. In the present investigation, tensile and elastic-plastic fracture toughness tests with a 2D digital image correlation setup were carried out for different microstructural states of Fe₄₂Mn₂₈Co₁₀Cr₁₅Si₅ HEA. Finite element analysis (FEA) coupled with combinatorial site-specific electron backscatter diffraction helps in developing a meso and micro scale mechanistic understanding of the extrinsic and intrinsic toughening processes. The calculated J-integral and plastic zone size using FEA simulations were corroborated with experimental results. The crack growth resistance (J-R) curve was evaluated across three distinct processing conditions: hot rolled (HR), 1 h annealed at 1173 K (AN1173), and 4 h annealed at 1373 K (AN1373). The HR material exhibited higher strength (yield strength = 630 ± 8 MPa), while the AN1373 demonstrated highest ductility (0.74 ± 0.04). The mode I plane strain fracture toughness was highest for the AN1373 (125.4 ± 15.8 MPa.m^0.5) and lowest for the AN1173 (46.3 ± 7.4 MPa.m^0.5). The Cr-rich sigma phase at grain boundaries in the HR and AN1173 led to pronounced intergranular fracture, resulting in lower fracture toughness and plasticity. The multiple variants of martensite in the AN1373 microstructural state, results in refined microstructure by interactions of transformation variants and dislocations that enhance the strength, ductility, and crack tip plasticity. The findings underscore the significant impact of intrinsic toughening on the fracture and deformation behaviour of the Fe₄₂Mn₂₈Co₁₀Cr₁₅Si₅ HEA.