Grain growth and secondary recrystallization in mechanically alloyed NiAl

Abstract

The ordered B{sub 2} intermetallic NiAl has potential for high temperature structural applications because of its low density, high melting point and high thermal conductivity. However, single phase structures suffer from poor room temperature ductility and low creep strength. Improvements in high temperature strength have been observed in composite or dispersion strengthened NiAl produced by mechanical alloying (MA). The structures developed by the MA process are typically fine grained with a grain size less than 1 {mu}m, and contain a fine distribution of dispersoids in the range 10 to 100 nm. Improvements in the creep resistance of dispersion strengthened Ni-base superalloys are observed to result from increases in grain size and grain aspect ratio. The effect is attributed to a reduction of grain boundary sliding and cavity growth. Consequently the production of a coarse grain structure, in combination with a fine dispersoid morphology represents an attractive design philosophy to improve the creep strength of MA NiAl. However, grain growth and dispersoid coarsening are generally concurrent and thus make normal grain growth treatments unsuitable. Secondary recrystallization (SRx) represents a mechanism that can give a pronounced increase in grain size and, because it develops rapidly, can give coarse grain structures without concurrentmore » dispersoid coarsening. The current work is focused on an exploration of this approach and involves metallographic and microhardness studies of the microstructural changes that accompany isothermal annealing and thermomechanical treatment of MA NiAl.« less