High temperature deformation of B2 nickel-aluminum alloys with and without dispersoid additions.

Abstract

Polycrystalline NiAl has an ordered B2 crystal structure and is a material of considerable interest in high temperature applications for turbine engines. In this study, addition of ceramic phase dispersoids to NiAl has been examined for improving the creep resistance of this material. Three alloys of nickel aluminide--NiAl, NiAl with TiB$\sb2$ dispersoids and NiAl with HfC dispersoids--were prepared by extruding rapidly solidified powders. It is seen from microstructural analysis that these extruded materials consist of fine recrystallized grains and the alloys containing dispersoid particles (TiB$\sb2$ and HfC) show noticeable grain refinement over NiAl. Compression tests under constant strain rate and creep tests under constant load conditions were conducted in the temperature range of 1200-1477 K. Although, these materials showed steady-state flow stress behavior in most test conditions, strain hardening behavior due to dynamic grain growth was also observed at low strain rates (below 10$\sp{-5}$s$\sp{-1}$). Microstructural changes occurring during the high temperature tests controlled the deformation behavior of the NiAl alloys. The fine grains (5 $\mu$m) of extruded NiAl + 4%HfC alloy converted to coarse grains (200 $\mu$m) by secondary recrystallization. On the other hand, the NiAl + 2%TiB$\sb2$ alloy retained a fine grain microstructure (2.5 $\mu$m) during the compression tests. As a result of these microstructural changes, the strengthening was only achieved in the HfC dispersoid containing alloy. Processability of these alloys was studied by performing hot rolling experiments to produce thin gauge foils. NiAl rolled easily at 1223 K, and showed good ductility during the hot rolling. Thin foils, up to 0.2 mm in thickness, were obtained by pack rolling operation. Rolling of dispersoid-containing alloys was difficult due to edge cracking, which is resulting from the high flow stress associated with the rolling operation. Crystallographic textures were determined in the thermo-mechanically processed materials by measuring X-Ray pole figures. While the rolling texture of NiAl was invariably $\{111\}\langle 112\rangle$, axisymmetrical compression (forging) produced $\langle 111\rangle$ and/or $\langle 110\rangle$ fiber textures. The relative amounts of the $\langle 111\rangle$ and $\langle 110\rangle$ components in forged materials were found to depend on the forming rate and temperature.