The Effect of Surface Oxide Films on the Mechanical Behavior of NiAl

Abstract

ABSTRACTThin surface films have been shown to enhance the ductility and decrease the flow stress of several body-centered cubic metals at temperatures T < 0.2 Tm. The origin of this effect lies in the large difference in the intrinsic mobilities of edge and screw dislocations in body-centered cubic crystals. B2 ordered intermetallic alloys, although simple cubic in structure, are based on the body-centered cubic structure and have dislocation core structures, dislocation mobilities and temperature and orientation dependent deformation qualitatively similar to that of bcc metals. This investigation was initiated to examine possible effects of surface films on the mechanical behavior of B2 ordered intermetallic alloys, using oxidized NiAl as the initial material for investigation. Experiments were performed on an impure non-stoichiometric (47.1 at.% Al) single crystal material with an axial orientation near [123]. Surface film softening was observed at room temperature in compression at a strain rate of 2 x 10-4 s-1. Flow stresses of the oxide coated crystals were as much as 20% lower than those of identically prepared uncoated crystals. The strains to fracture of coated specimens were larger than those of uncoated specimens and in a few instances the ductility enhancement was as much as four times. Of the several oxides examined, the largest softening effects were found for a thermally deposited delta-A1203 film formed at 1000 °C for 1 hour. The current results for NiAl are compared to results previously obtained for bcc metals. Experiments which could further enhance the film softening effects observed in B2 ordered intermetallic alloys are suggested.