On the influence of Al-concentration on the fracture toughness of NiAl: Microcantilever fracture tests and atomistic simulations

While the mechanical properties of the stoichiometric B2 β-phase of NiAl are well established, the effect of off-stoichiometric composition on the fracture behavior has not yet been systematically studied over the entire composition range of 40-55% Al. Here we use microbending tests on notched cantilever beams FIB-milled from NiAl single crystals with an aluminized as well as an oxidation-induced composition gradient to determine the influence of the Al concentration on the fracture toughness. Small variations from the stoichiometric composition are shown to lead to drastic changes in fracture behavior. Ni-rich NiAl shows a gradual decrease of fracture toughness and plastic energy dissipation with Ni-concentration that follows the inverse proportionality to the yield stress postulated by the Ritchie, Knott and Rice (RKR) model. Al-rich NiAl, however, shows a concentration-independent low fracture toughness and no signs of accompanying plasticity - both which can not be described by the RKR model. Detailed atomistic simulations show that rather than affecting the theoretical fracture toughness according to the thermodynamic Griffith criterion, structural vacancies in Al-rich NiAl lower the kinetic barrier for crack propagation, explaining the observed different concentration dependence of the fracture toughness. Similar observations are also expected for fracture in other off-stoichiometric B2 aluminides.