Oxidation and its influence on the mechanical behavior of nickel(3)-aluminide.

Oxidation of Ni$\\sb3$Al at 1000$\\sp\\circ$C and two potential stress-oxidation interactions were examined. Nickel-rich Ni$\\sb3$Al containing 0.5 at% Hf was isothermally oxidized in air or 500 ppm O$\\sb2$ in argon. The kinetics of oxidation approached a parabolic time dependence after 30 to 40 hrs. The parabolic growth constants had a positive dependence on P$\\sb{\\rm O\\sb2}$, thus indicating p-type semiconducting behavior for the $\\alpha$-Al$\\sb2$O$\\sb3$ film. Prior to parabolic kinetics the primary oxide was a transition Al$\\sb2$O$\\sb3$ in the form of whiskers which transformed to $\\alpha$-Al$\\sb2$O$\\sb3$ with concomitant surface smoothing. Oxidation also produced an Al depleted zone of $\\gamma$-Ni containing 12 to 14 at% Al. The depleted zone depth was calculated based on weight gain data and compared to measured values. One of the potential stress-oxidation interactions examined was the flow stress reduction of Ni$\\sb3$Al at elevated temperatures from the introduction of mobile edge dislocations. Thin oxide films or sample prestrains have been found to significantly reduce the flow stress of BCC metals and NiAl. In this study, the influence of both thermal oxide films and $-$196$\\sp\\circ$C prestrains on 550 to 700$\\sp\\circ$C flow behavior was examined. Low temperature prestrains produced an initial flow stress reduction of up to 50% in $\\langle$123$\\rangle$ oriented Ni$\\sb3$Al single crystals tested in compression at 550$\\sp\\circ$C. TEM dislocation analysis showed that the prestrain introduced a r and om array of dislocations which was subsequently exhausted into the screw orientation with further deformation at 550$\\sp\\circ$C. The initial 550$\\sp\\circ$C flow stress was controlled by the edge component of the existing dislocation structure. However, the surface oxide films examined had no effect on the deformation behavior of Ni$\\sb3$Al at 550$\\sp\\circ$C. Another stress-oxidation interaction evaluated in this study was the influence of oxygen on the low cycle fatigue behavior of polycrystalline Ni$\\sb3$Al at 600$\\sp\\circ$C. Recently, oxygen was found to cause severe embrittlement of polycrystalline Ni$\\sb3$Al during tensile experiments at elevated temperatures. In this investigation Ni$\\sb3$Al alloyed with Hf and B was tested in plastic-strain-controlled low cycle fatigue at 600$\\sp\\circ$C in either air or vacuum (10$\\sp{-3}$ Pa). The accumulated plastic strain to failure was reduced by nearly two orders of magnitude by the exposure to air, with a concomitant change in failure mode from transgranular in vacuum to intergranular in vacuum. An increase in strain rate from 0.001/s to 0.01/s doubled the fatigue life in air and a thin oxide coating ($\\sim$350 nm) quadrupled fatigue life in air. The cyclic deformation produced compressive peak stresses that were 10% greater than the tensile peak stress for the duration of the experiment. Evidence of strain localizations similar to persistent slip b and s was found on the sample surface after cyclic saturation. However, the dislocation substructure appeared homogeneous.PhDMaterials scienceUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/162181/1/8920527.pd