The effects of interstitial content, heat treatment, and prestrain on the tensile properties of NiAl

The tensile stress-strain response of polycrystalline NiAl was studied as a function of purity and pretest treatment (annealing and/or prestrain). After annealing at 1100 K for 7200 s (i.e. 2 h) followed by furnace cooling, high-purity and nitrogen-doped alloys exhibited continuous yielding, while conventional-purity or carbon-doped alloys exhibited a distinct yield point and Lüders strain. Prestrain by hydrostatic pressurization removed the yield point, but it could be reintroduced by further annealing treatments. Yield points could be reintroduced more rapidly if the specimens were prestrained uniaxially rather than hydrostatically, owing to the arrangement of dislocations into cell structures during uniaxial deformation. The time dependence of the strain aging events followed a t23 relationship. In total, these results suggest that the yield points observed in polycrystalline NiAl result from the pinning of mobile dislocations by interstitials, specifically carbon, i.e. classic static strain aging.