Mechanical properties of high-temperature titanium intermetallic compounds
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I.
INTRODUCTION
IN recent years, intermetallic compounds have been the subjects of intensive study, [1,2,3] spurred by aircraft and aerospace demands for materials that are usefully strong at increasingly elevated temperatures. Intermetallics have, in many cases, greater stiffnesses than the metals from which they are formed t4] and frequently decrease less in strength with increasing temperature than do disordered alloys. [5,6] In many cases, low specific gravities give them high specific stiffness and specific strength, two quantities that are prized for use in aircraft or rotating parts. This paper reports results on titanium-containing intermetallics that are part of a search for future hightemperature materials. The opportunity that exists derives in part from the fact that there are hundreds of high-temperature intermetallics, tT] most of which are untested mechanically. The overriding difficulty in the use of intermetallics, however, comes from their tendency toward brittleness. It follows that any systematic screening of intermetallics should include a measure of the ease of crack formation. Titanium and its alloys have been of special interest because of their low density and high strength. This work received its impetus from an Air Force-sponsored study designed to explore the fundamental temperature limits of titanium-base materials for airframes and engines. ]81 That work and extensions of it have led to the study of the series of titanium-containing intermetallic compounds reported here. II.
MATERIAL SELECTION
Approaches are needed to decide in an orderly way a logical priority as to which compounds are tested first. The method of selection suggested and followed here t7,9] emphasizes two structure-insensitive properties, melting temperature, Tm, and specific gravity, p. Strength, elastic moduli, creep resistance, and dimensional stability tend to rise with T,~; and low p is an advantage, as was noted in the first paragraph. In Figure 1, such data for titanium intermetallics are displayed graphically, along R.L. FLEISCHER, Physicist, and R.J. ZABALA, Metallurgical Specialist, are with General Electric Corporate Research and Development, Schenectady, NY 12301. Manuscript submitted September 25, 1989. METALLURGICAL TRANSACTIONS A
with a line that gives a rough limit on high-Tin and lowp values of the collected data for all identified hightemperature intermetallics, t71The starting compounds for this study were chosen to be (1) as close to the line as possible and (2) at the same time include a wide variety of crystal structures. In principle, the elastic moduli are other structureinsensitive properties that would be useful in materials selection. Unfortunately, such measurements are rare for intermetallic compounds of high Tin. As of 1987, measurements of Young's modulus, E, were available for only 24 of approximately 290 intermetallics that melt at or above 1500 ~ These included only Bel2Ti among the Ti intermetallics that are shown in Figure 1. Since that search, we have measured moduli in a good
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