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

TL;DR

This study investigates how varying aluminum concentration affects the fracture toughness of NiAl, revealing maximum toughness at stoichiometry and explaining the behavior through microcantilever tests and atomistic simulations.

Contribution

It systematically examines the influence of Al concentration on NiAl's fracture toughness across the entire composition range using experimental and simulation methods.

Findings

Maximum fracture toughness at stoichiometric composition.

Decreased toughness with Ni-rich compositions, with plastic deformation.

Al-rich NiAl shows low, concentration-independent toughness.

Abstract

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.