Hall-Petch Strengthening by Grain Boundaries and Annealing Twin Boundaries in Non-Equiatomic Ni2FeCr Medium-Entropy Alloy

Abstract

A novel Co-free non-equiatomic Ni2FeCr medium-entropy alloy (MEA) was designed, and the Hall–Petch strengthening by grain boundaries and annealing twin boundaries was investigated. For this purpose, the alloy was prepared by cold rolling and recrystallization at 873–1323 K for 40 min–6 h. Annealing at different temperatures revealed that Ni2CrFe alloy forms a stable face-centered cubic (FCC) solid solution. Mean grain sizes (excluding annealing twin boundaries) and mean crystallite sizes (including both grain and annealing twin boundaries) were determined using the linear intercept method and the equivalent circle diameter in electron back-scattered diffraction (EBSD) soft. Tensile tests at 293 K indicated that the Hall-Petch slopes of grain sizes and crystallite sizes are 673 and 544 MPa μm1/2, respectively, and this contribution was then subtracted from the overall strength to calculate the intrinsic uniaxial lattice strength (90 MPa). Additionally, tensile tests, performed between 293 K and 873 K, revealed that the Ni2CrFe MEA has a stronger resistance to softening at high temperatures. Transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at annealing twin boundaries, indicating that it is also an obstacle to dislocation slip. Furthermore, the thickening of the annealing twin boundary after deformation was observed and illustrated by the interaction between different dislocations and annealing twin boundaries.