Thermal equilibria and mechanical stability of Ti 3 Al phase in Ti-Mo-Al alloys
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study of the phase equilibria and mechanical properties in the titanium-rich corner of the Ti-Mo-A1 system was carried out recently. 1'2 It was shown that in alloys having a molybdenum content as low as 7 at. pct the deleterious omega phase could be entirely suppressed by increasing the aluminum content and that at 16 at. pct aluminum a high-temperature, two-phase region existed, consisting of/3 + Ti3AI , which exhibited v e r y promising mechanical properties. 2 Since the metallographic observations indicated that this twophase region may extend to higher temperatures with increase of aluminum content, it appeared to be worth while to determine the extent of the two-phase field, keeping the molybdenum content constant, and to investigate the mechanical stability of these alloys. Since the strength and stability of the alloys seemed to be closely associated with the ordered DO~9 s t r u c ture of the Ti3Al particles, 2 in analogy to the 7' of the nickel base superalloys, additional studies correlating the m i c r o s t r u c t u r e s to the mechanical properties appeared to be warranted. This r e p o r t concerns itself with such studies, focussing special attention on the role of the TidAl phase in fl alloys with r e g a r d to the mechanical and structural stability.
EXPERIMENTAL PROCEDURE The binary alloy of nominal composition Ti-7 Mo (in atomic percent), as well as six t e r n a r y alloys of the Ti-7 Mo-A1 system with increasing aluminum content, were investigated. The alloys were prepared in sheet or rod form by the RMI Titanium Company, Niles, Ohio, and the chemical composition and initial heat treatments are given in Table I. The details of temperatures and times for the various final heat treatments are given in the results. All specimens were sealed in evacuated quartz tubes and after the various heat treatments were quenched to room t e m perature by cracking the tubes in water.
T. HAMAJIMA is with University of Bochum, Germany. S. WEISSMANN is a Professor of the Department of Mechanics and Materials Science, College of Engineering, Rutgers University, New Brunswick, N.J. 08903, where T. Hamajima was formerly a Graduate Student. This paper is based on a thesis submitted by T. Hamajima in partial fulfillment of the requirements for a Ph.D. degree tu Rutgers University. Manuscript submitted May 16, 1974. METALLURGICAL TRANSACTIONS A
The phase determinations were carried out mainly by light m i c r o s c o p y (LM) and by electron diffraction and transmission m i c r o s c o p y (TEM). For the latter studies the specimens were electrolytically thinned and polished at below - 50~ following the procedure described by Blackburn and Williams. 3 The operating voltage of the electron microscope was 200 kV. Dilatometric measurements were carried out to detect the exact/3 transus and other phase transformations. For this purpose the specimens were swaged and machined into rod shapes of 6 mm in diam and about 15 mm in length. The specimens were heated i n v a c u o by high-frequency induction heating during
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