On the slip behavior and surface film effects in B2 -ordered nickel aluminide single crystals.

Abstract

Two single crystal orientations, (123) and (001), were chosen for the study of slip behavior and surface film effects. (123) NiAl deforms by the $\langle 001\rangle$ slip. The dislocation substructures, the change of slip traces with temperature, and the agreement of experimental results with theoretical analyses of dislocation stability and mobility indicate that deformation of (123) NiAl at low temperatures (77-298 K) is governed by the high Peierls stress of $\langle 001\rangle$ mixed dislocations. Single crystals of (001) NiAI, where $\langle 111\rangle$ slip is operative at low temperatures (77-298 K), exhibit complicated slip behavior above 298 K. At intermediate temperatures (298-770 K), $\langle 111\rangle$ slip operates and at the same time kinking occurs in (001) NiAl. Kinking occurs by the operation of either $\langle 001\rangle\{110\}$ or $\langle 001\rangle\{010\}$ depending on the orientations. Kinking tendency depends on the deformation temperature, the composition and the orientation of the NiAl. These results suggest that deformation of (001) NiAl at intermediate temperatures must be understood in terms of a balance between motion of $\langle 111\rangle$ and $\langle 001\rangle$ dislocations. At high temperatures (T $>$ 770 K), (001) NiAl exhibits a transition in slip direction from $\langle 111\rangle$ to non-$\langle 111\rangle$ directions. Single crystals of (001) NiAl were deformed at 298 K to produce $\langle 111\rangle$ dislocations and were subsequently annealed at various temperatures in order to see the effect of temperature on the stability of the $\langle 111\rangle$ dislocation. $\langle 111\rangle$ dislocations are unstable at high temperatures and the instability of $\langle 111\rangle$ dislocations is responsible for the slip transition from $\langle 111\rangle$ to non-$\langle 111\rangle$. It is also observed that $\langle 111\rangle$ dislocations can be dissociated into $\langle 110\rangle$ + $\langle 001\rangle$ dislocations during deformation in the vicinity of the transition temperature of approximately 770 K. Slip directions at high temperatures can be either $\langle 110\rangle$ or $\langle 001\rangle$. $\langle 110\rangle$ is the primary slip direction. In NiAl, surface film soffening occurs primarily in single crystals coated with adherent oxide films. Little or no effect is observed with non-adherent films which crack and subsequently spall during deformation. The surface film softening in (123) NiAl at 298 K is believed to be due to excess mobile dislocations supplied by film-substrate interface dislocation sources. The micro-lurking mechanism by $\langle 001\rangle$ dislocations is suggested to be responsible for the surface film softening in (001) NiAl. Such dislocations are observed in surface film softened (001) specimens.