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

The notch fracture toughness (KQ) and its dependence on Mo concentration in as-cast body-centered-cubic (TiZrNbTa)100-xMox 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 KQ decreasing from 28.5 MPa√m for the Mo-free TiZrNbTa quaternary alloy to 18.7 MPa√m for the TiZrNbTaMo quinary alloy. The KQ 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 (T0), making the activation to overcome lattice resistance to dislocation motion increasingly difficult. The low ratio T/T0 (T = 300 K in our case) can in fact be inferred from the very small activation volume (3b3) measured for TiZrNbTa and TiZrNbTaMo.