Effects of ageing at 673 K on the compressive behaviour of <110> oriented ('soft') NiAl single crystals and polycrystals with and without Ti additions

Since the remarkable increase in room temperature (RT) ductility of <110> oriented NiAl single crystals by microalloying with Fe, Ga or Mo the effects of microalloying and of impurity gettering on the mechanical behaviour of NiAl are still under discussion. Thus the compressive behaviour of <110> oriented ('soft') NiAl single crystals and polycrystals with and without Ti (up to 0.1 at%), which has long been used as a getter in steel metallurgy, has been studied in detail. In particular, the effects of ageing at 673 K on RT mechanical properties, and static strain-ageing at 673 K, have been investigated. It was found that dislocation-impurity interactions play an important role in the mechanical behaviour of NiAl, and Ti additions may indeed serve as a getter for solute atoms. However, this effect does not lead to improved RT ductility of NiAl. The motion of <100> dislocations at RT was found to be restricted in single crystals with Ti compared with crystals without Ti. Ageing al 673 K decreases the RT ductility drastically and this is due to the formation of Ti-containing precipitates rather than pinning of mobile dislocations by interstitials. Likewise, the Ti-containing precipitates on dislocations lead to strain-ageing effects at 673 K.