Progress in The Development and Understanding of Ti 3 Al Based Intermetallic Materials
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PROGRESS IN THE DEVELOPMENT AND UNDERSTANDING OF Ti 3AI BASED INTERMETALLIC MATERIALS HARRY A. LIPSITT Department of Mechanical and Materials Engineering, Wright State University, Dayton, Ohio 45435 ABSTRACT Recent results on the development of Ti 3AI base materials, including orthorhombic alloys, are reviewed. Included in this discussion are new data and ideas concerning alloy development, phase equilibria, processing for improved toughness and ductility, environmental degradation, oxidation and corrosion resistance, and coating development. Several new families of alloys have been developed in recent years and the microstructure/processing/property relationships in these alloys are reviewed. It is shown that errors in processing this class of alloy can lead to microstructural instabilities under service conditions. Finally, some immediate needs and some directions for further development are suggested. INTRODUCTION Over the period of the past two decades, military and industrial interest in TiAI (a2) base materials has generally lagged behind the interest in TiAI (y) base materials. This has been because of the apparent "potential" for the broader application of the lighter, stiffer, more oxidation resistant y base alloys. This attitude assumes that the ease of solving the various developmental problems is nearly equivalent in the two cases. However, the technical literature shows that this assumption is not valid. Perhaps this is because of the many similarities between the a 2 alloys and conventional titanium alloys. Nevertheless, the appearance of a (nearly) useful a 2 alloy occurred about 10 years before the same event in the y base systems. However, when one attempts to understand alloy behavior, it becomes clear that the development of the alloys had significantly preceded the real understanding of their metallurgy. Today, studies to provide new understanding and studies to provide new alloy compositions proceed together. As a result, there is more information to review than can be digested in a single article. Fortunately, there have been several recent reviews of the literature of Ti2 AI [1-31 base materials, so it is not necessary for this review to begin at the beginning. Even more fortunately, Dr. Dipankar Banerjee, Assistant Director, Defence Metallurgical Research Laboratory, Hyderabad, India, has recently completed a most extensive review of our basic understanding of Ti 3AI base materials [4] and it se'emed unnecessary to repeat what he had done so well. Therefore, using that article as a point of departure, this review will attempt to define the current state of understanding of the nearly useful materials that have been developed. Thus, the present review will concentrate on material properties and behavior in a service environment, on property goals and deficiencies, and on research necessary to validate the use of Ti 3AI base materials for aircraft turbine engine service. ALLOY DEVELOPMENT The a2 base alloys which have been studied most are the Ti-24AI-1 1Nb composition reported by Blackburn, R
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