Nanocrystalline NiAl-processing, characterization and mechanical properties

Nanocrystalline ordered NiAl (n-NiAl) was successfully synthesized by an electron beam gas condensation and compacted in-situ at various temperatures. As-compacted material exhibits grain sizes between 2 and 4 nm and densities between 78 and 94% of the theoretical density, increasing with increasing compaction temperature above all as a result of reduced porosity. The nanocrystalline structure of NiAl is stable up to about 1000 degrees C. Microhardness of as compacted n-NiAl increases with increasing density, above all as a result of reduced porosity. For the reasons not fully understood yet, microhardness of n-NiAl increases also with increasing grain size following annealing, a response different from that in the conventional, coarse-grained NiAl. The present material is significantly stronger than its conventional counterpart but not as strong as predicted by Hall-Fetch-type modeling. Also, in the nanocrystalline form, NiAl exhibits room temperature ductility, unlike its coarse-grained counterpart. The present study provides probably the first unequivocal experimental evidence of the room temperature ductility of a nanocrystalline intermetallic material. The mechanical behavior of n-NiAl can be rationalized assuming that diffusional, rather than dislocation, mechanisms control strength and ductility of n-materials. (C) 1998 Elsevier Science S.A. All rights reserved.