Notch fracture toughness of body-centered-cubic (TiZrNbTa)-Mo high-entropy alloys

The notch fracture toughness (K-Q) and its dependence on Mo concentration in as-cast body-centered-cubic (TiZrNbTa)(100-x)Mo-x high-entropy alloys have been measured at room temperature. It is shown that the increase of Mo concentration results in a significant reduction in fracture toughness, with the K-Q decreasing from 28.5 MPa root m for the Mo-free TiZrNbTa quaternary alloy to 18.7 MPaVin for the TiZrNbTaMo quinary alloy. The K-Q of these HEAs scales inversely with increasing (d + s) electrons per atom. The fracture mode under Mode I loading transists from monolithic intergranular fracture for Mo-free TiZrNbTa to completely transgranular cleavage for the TiZrNbTaMo alloy. The brittleness is consistent with the known effects of refractory solutes on increasing the brittle-to-ductile transition temperature in Nb-based solutions. The embrittlement effect with alloying (especially Mo) is also attributable to the elevation of the critical temperature (T-0), making the activation to overcome lattice resistance to dislocation motion increasingly difficult. The low ratio T/T-0 (T = 300 K in our case) can in fact be inferred from the very small activation volume (3b(3)) measured for TiZrNbTa and TiZrNbTaMo.