  The mechanical properties of the stoichiometric B2 $\beta$-phase of NiAl are
well established, however the effect of off-stoichiometric composition on the
fracture toughness has not yet been systematically studied over the entire
composition range of 40-50% 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 mechanical properties. The fracture toughness is
maximal for the stoichiometric composition. It decreases with increasing
Ni-content in the Ni-rich composition range, where plastic deformation is
observed to accompany the fracture process. In contrast, no plasticity is
observed in Al-rich NiAl, which shows a nearly concentration-independent, low
fracture toughness. The theoretical fracture toughness according to Griffith,
however, shows only a very weak composition dependence in both, the Ni- and
Al-rich composition range. The differences in fracture toughness could
furthermore not be explained solely based on the different hardening
contributions of Ni-antisites in the Ni-rich and structural vacancies in the
Al-rich crystals. Atomistic fracture simulations show that crack propagation in
NiAl takes place by the nucleation and migration of kinks on the crack front.
The low fracture toughness of Al-rich NiAl can thus be understood by the dual
effect of structural vacancies as strong obstacles to dislocation motion and as
source of crack front kinks.
