Influence of Alloying and Processing on the Microstructure and Properties of Beta-NiAl

摘要

The overall objective of this program was to provide a fundamental understanding of the effects of alloying and processing on the microstructure- property-processing relationships of monolithic b-NiAl. The emphasis was to determine the factors that affect both the low and high temperature compound by studying single crystals, bicrystals and polycrystals of selected alloys. Modeling of the solidification behavior was performed with the goal of understanding better the role of processing parameters on contamination and its effects on mechanical properties. Furthermore, the effects of interstitials, microalloying additions, thermal and constitutional vacancies, deviations from stoichiometry, processing defects/inhomoganeities end precipitate phases on both the low and high temperature properties of NiAl and how these are affected by processing history were examined in some detail. It is shown that b-NiAl is a reasonably well-behaved material that is both soft (low resistance to dislocation motion) and brittle (low fracture strength). This combination of intrinsic properties makes it difficult to develop NiAl into a viable engineering material. Specifically, while it is possible to strengthen NiAl using traditional methods, the stronger alloys have reduced toughness. Conversely, approaches for toughening are even less obvious since the intrinsic reason for the low toughness is not related to a lack of dislocation mobility but rather to the lack of a sufficient number of operative slip systems needed to avoid the build-up of stress at stresses concentrations. Finally, the results indicate that the brittle to ductile transition in NiAl should be interpreted with caution.