The effect of strain rate on mechanical properties and microstructure of a metastable FeMnCoCr high entropy alloy

Abstract The relationship between mechanical properties and microstructures of a metastable dual-phase high entropy alloy Fe50Mn30Co10Cr10 under uniaxial tensile testing at different strain rates (10-3 s-1∼103 s-1) has been studied systematically. As the strain rate increases, yield strength, ultimate tensile strength and uniform elongation decrease first and then increase. Namely, when the strain rate is 10-3 s-1, yield strength and ultimate tensile strength are 280 MPa and 720 MPa, respectively, with the uniform elongation of 64.2%. When the strain rate is increased to 1 s-1, yield strength and ultimate tensile strength are 300 MPa and 672 MPa, respectively, and uniform elongation decreases to 49.2%. When the strain rate reaches 103 s-1, yield strength and ultimate tensile strength increase to 380 MPa and 810 MPa, respectively, while uniform elongation is elevated to 67%. As dynamic deformation is affected by the adiabatic heating, the stacking fault energy of the alloy is increased by ∼13 mJ m-2 at a strain rate of 103 s-1 compared with that in the quasi-static condition. Under quasi-static loading, martensitic transformation is the dominant deformation mechanism. Under dynamic loading, when the strain is low the deformation induced phase transformation dominates, whereas as the loading proceeds mechanical twinning becomes the dominant deformation mode. At the same time, the adiabatic temperature rise under dynamic tests also causes a reverse transformation from e-martensite to austenite. Accordingly, the release of internal stress and the formation of soft and ductile austenite jointly contribute to the elevated uniform elongation of the material. Both mechanical twinning and martensitic reverse transformation promote the microstructure to be dynamically refined, so that the alloy shows the good plasticity while maintaining the high ultimate tensile strength at dynamic strain rates.