Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)
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xamined methods of optimizing the strength and fracture-toughness performance of ␣- titanium alloys through variations in solution-treatment temperature, cooling rate, and aging temperatures/times. Most recently, Hunter et al.[1,2] have shown that an increased solution-treatment temperature and increased cooling rate both have a beneficial influence on the fracture-toughness behavior of the ␣- alloy IMI550 (Ti4Al-4Mo-2Sn-0.5Si). These observations are in substantial agreement with the earlier studies of Vaughan et al.,[3] where an increase in fracture toughness without a significance loss in strength was observed through the use of a higher solutiontreatment temperature. Additionally, Flower et al.[4] reported that a substantial increase in the hardening response could be achieved in IMI 550 by increasing the cooling rate from the solution-treatment temperature prior to aging. IMI550 has, additionally, been found to exhibit enhanced superplastic forming (SPF) capabilities when compared to Ti-6Al-4V.[5] Other studies[6] have indicated, however, that a post-SPF solution treatment is necessary in order to achieve acceptable strength levels, as direct aging of superplastically formed and cooled components leads to lower mechanical performance. All of these studies suggest that a fundamental understanding of the phase transformations occurring during the solution-treatment stage of thermal processing is required to achieve optimal mechanical performance. The objective of this investigation was to provide this understanding through an examination of the effect of solution-treatment temperature on the phase transformations observed in IMI550 following rapid quenching. II. EXPERIMENTAL PROCEDURE This investigation utilized an 87.5-mm-thick plate of IMI550 (Table I) that had been equilibrated for 2 hours at K.K. KHARIA, Graduate Research Assistant, and H.J. RACK, Professor, are with the Department of Ceramic and Materials Engineering, Clemson University, Clemson, SC 29634. Manuscript submitted June 12, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
1253 K, cooled at a rate of 10 K/hr to 1023 K, held for 120 hours, and water quenched. Optical examination of samples that had been mechanically polished and chemically etched (5 pct HNO3 ⫹ 5 pct HF, the balance H2O) indicated that this treatment resulted in a microstructure that contained primary ␣, both equiaxed and acicular, the latter lying within a  matrix (Figure 1). Thermal analysis was carried out to determine the beta transus (T) temperature.[7] This transformation temperature was determined under a dynamic argon atmosphere (⬍1 ppb O2), at cooling rates of 10, 15, 17, and 20 K/min. Both the heat flow (J ), normalized per unit mass (mJ⭈s⫺1⭈mg⫺1), and its first derivative with respect to temperature (dJ/dT ) were recorded. The T temperature for each cooling rate was defined where J and J ⬘ diverged from the baseline, the equilibrium transformation temperature being established through the extrapolation method suggested by Zhu and Devletian.[8] The beta transus temper
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