Effects of chromium on the morphology and mechanical properties of Fe₂B intermetallic in Fe-3.0B alloy

In this work, the effects of chromium addition on the morphology and mechanical properties of Fe₂B in Fe-3.0B alloy have been systematically investigated by a combination of experimental observations and the first-principles calculations. The results indicate that, with chromium addition in Fe-3.0B alloy ranging from 0 to 2.5 wt%, the average grain size of the boride firstly decreases and then increases slightly, mainly attributed to the volume fraction of coarse block boride. The nanoindentation hardness H of the boride remains almost constant while the elastic modulus E_r firstly drops and then rises. Accordingly, the H/E_r of the boride achieves a peak value at the chromium addition of 2.0 wt%, where the highest toughness is obtained. High-resolution transmission electron microscopy (HRTEM) observation demonstrates that the lattice of borides evolves from tetragonal to orthorhombic ((Fe, Cr)₂B) after chromium addition, in good accordance with the calculation results. During the lattice evolution, a shrinkage of B–B bond along [002] direction is simultaneously revealed. The inherently weak B–B bond can be strengthened, which improves fracture toughness of Fe₂B.