Research on oxidation and embrittlement of intermetallic compounds in the US

Research on the oxidation behavior of intermetallic compounds has been conducted in the U.S. for many years However, until about ten years ago, this work focusses on the compounds which are important in Ni-base superalloys and their coatings: mainly Ni3Al and NiAl. More recent work has been directed at systems which may be used in monolithic form or as the base for composites. Work has concentrated on three types of systems: Ni- and Fe-aluminides, refractory metal compounds, and titanium aluminides Work on the Ni- and Fe-aluminides has concentrated mainly on adherence problems and some anomolous behavior. Work on the refractory metal compounds, particularly MoSi2 and NbAl3, has dealt with the problem of selectively oxidizing Al or Si from a refractory metal base and various intermediate-temperature forms of degradation, such as ''pesting''. It has become increasingly more clear that, for a number of reasons, the titanium aluminides will be the first ''new'' metallic materials introduced into commercial high-temperature applications (probably aircraft gas turbines and automobiles engines) in many years. As a result a very large amount of work is being done on the oxidation behaviour of these compounds. Initial work dealt with oxidation mechanisms at temperatures on the order of 1000 degrees C. However, both oxidation and mechanical property considerations dictate that the alloys will not be used at temperatures much above 750 degrees C. Therefore, current work is being focussed on oxidation mechanisms at lower temperature and on what may be the ''Achilles heel'' of these materials, environmental embrittlement. This paper summarizes the work being done in the U.S. and highlights work on what the author believes are the more important problems.