Fatigue and microstructural properties of quenched Ti- 6Al- 4V
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INTRODUCTION
TITANIUM alloys such as the Ti-6A1-4V can have numerous microstructures, and it has been demonstrated that the microstructure of titanium alloys can have a significant effect on fatigue properties. 1-14 In the alloy Ti-6AI-4V, the vanadium stabilizes the/3 (bcc) phase of titanium and the aluminum stabilizes the a (hcp) phase so that at temperatures below the/3 transus it is possible to have in equilibrium a vanadium rich /3 phase and an aluminum rich a phase. At temperatures above the /3 transus titanium alloys are entirely/3 phase. Rapid cooling from above the /3 transus results in the formation of an hcp martensitic phase a '. Solution treated and quenched carbon steels are generally brittle immediately after a quench, and tempering of the steel is necessary. Thus, metallurgists generally avoid the as-quenched metallurgical condition; however, there are some observations in the literature that indicate as-quenched microstructures of titanium alloys might have some interesting properties. Stubbington and Ballett have observed that fatigue failure of electron beam welded Ti-6A1-4V occurred in the base metal and not in the weld. 14Electron beam welds are rapidly cooled in a way similar to quenching. Crossley and Lewis observed that solution treatment at 1065 ~ (1950 ~ and water quenching of Ti-6A1-4V resulted in slower crack propagation than in equiaxed a-/3 microstructures. 15 Sherman and Kessler observed that solution treating at 843 ~ (1550 ~ and water quenching resulted in a metal alloy with low yield strength, good machinability, and excellent low temperature formability. 16 Thus, it is obvious that solution treatment and water quenching of the alloy Ti-6A1-4V does not necessarily produce a brittle metal, and it is possible that some useful properties may result from solution treated and quenched titanium alloys. However, very little work has been conducted on as-quenched titanium alloys. M. A. IMAM, Adjunct Assistant Professor, and C. M. GILMORE, Professor of Engineering and Applied Science and Chairman of Civil, Mechanical, and Environmental Engineering, are both with George Washington University, Washington, DC 20052. Manuscript submitted March 12, 1980. METALLURGICALTRANSACTIONS A
The phase transformations, microstructures, and physical properties of the alloy Ti-6AI-4V have been investigated by several authors. One of the studies of solution treated Ti6AI-4V was conducted by Fopiano et al. 17 It was observed that the/3 phase when water quenched from solution temperatures of 800 ~ (1472 ~ or higher always formed the a ' phase. Ti-6A1-4V solution treated at temperatures between 800 ~ and the/3 transus and water quenched formed a mixture of primary a (that existed before the water quench) and ~' formed by martensitic transformation of the /3 phase. They also observed that solution treating at 750 ~ (1382 ~ resulted in primary a and/3; the/3 phase appeared to be stable during water quenching from 750 ~ (1382 ~ An interesting observation by Fopiano was that increasing solution treatment tempe
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