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

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