Plastic deformation of single crystals of the equiatomic Cr-Fe-Co-Ni medium entropy alloy – A comparison with Cr-Mn-Fe-Co-Ni and Cr-Co-Ni alloys

Abstract

The plastic deformation behavior of single crystals of the equiatomic Cr-Fe-Co-Ni medium-entropy alloy was investigated in compression and tension from 9 K to 1373 K. The critical resolved shear stress (CRSS) for {111}<11¯0> slip is 44–45 MPa at room temperature and does not exhibit significant tension-compression asymmetry. It increases rapidly with decreasing temperature but exhibits a dulling of the temperature dependence below 77 K due to the inertia effect. The 0 K CRSS was determined to be 200 MPa by extrapolating the temperature dependence of CRSS from above 77 K to lower temperatures. This CRSS value is higher than that of the equiatomic Cr-Mn-Fe-Co-Ni high-entropy alloy, 174 MPa, but lower than that of the equiatomic Cr-Co-Ni medium-entropy alloy, 225 MPa. The rank of CRSS at 0 K of the three equiatomic alloys is consistent with the prediction of some solid solution strengthening models. The stacking fault energy (SFE) of the Cr-Fe-Co-Ni is about 20 mJ/m2, which falls between those of the Cr-Mn-Fe-Co-Ni and Cr-Co-Ni alloys. Deformation twinning in the Cr-Fe-Co-Ni MEA occurs at a shear stress of 249 MPa on conjugate (1¯1¯1) planes in the form of Lüders deformation at 77 K. The relationship between twinning stress and SFE is not monotonic as previously believed, but displays a concave with a minimum twinning stress at an intermediate SFE value.