Dynamic Transformation of Two-Phase Titanium Alloys in Stable and Unstable States
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namic transformation plays a significant role in superplastic deformation and serves as the primary source of flow softening during the thermomechanical processing of titanium alloys. For example, Koike et al.[1] observed a considerable increase in volume fraction of the beta phase during tensile tests in the twophase region of Ti-Al-Fe alloy, and the transformation acted as an additional stress accommodation mechanism. In the work by Zhang et al.,[2] the excellent superplastic elongation of Ti-6Al-4V was partially ascribed to the dynamic transformation from alpha to beta phases during tensile testing at 700 C and 800 C. Wang et al.[3] carried out tensile tests on a near-alpha titanium alloy (TA15), and their results indicated that new beta phases were formed around the equiaxed alpha grains. Dynamic globularization was also activated
BAOQI GUO, YANG LIU, and JOHN J. JONAS are with the Department of Materials Engineering, McGill University, 3610 University St., Montreal, H3A 0C5, Canada. Contact e-mail: [email protected] Manuscript submitted May 20, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
during straining and was promoted by the increase in beta phase fraction. Recently Jonas et al.[4] reported the dynamic transformation in a near-alpha titanium alloy (IMI 834) using hot compression testing. The variation of beta phase fraction was measured at various temperatures and an approximate 20% increase in beta phase fraction was found at these elevated temperatures. They concluded that the dynamic transformation accounted for the flow softening. The above dynamic transformations were triggered in an initial equilibrium state with sufficient holding. For example, various isothermal holding times of 20 minutes,[1] 15 minutes,[5,6] 5 minutes[7] prior to deformation were used in these reports on dynamic transformation. In thermodynamically unstable states, the dynamic transformation during deformation exhibited a reverse trend compared with the aforementioned transformation. For instance, the research by He et al.[8] showed that alpha phase fraction in a Ti-6Al-4V alloy was noticeably increased by the deformation during slow cooling. In the experiment by Dehghan-Manshadi and Dippenaar,[9] hot deformation was performed on a twophase titanium alloy (Ti-6246) following rapid cooling to the deformation temperature, and they found that beta phases transformed into alpha phases during straining. The directions of dynamic transformations in various conditions are still unclear. The present paper illustrated this unusual phenomenon in both unstable and stable states using thermodynamic approaches. The roles of Gibbs energy and deformation in the transformation are different in the two cases. First, two methods associated with the calculation of Gibbs energy were developed here. In the unstable state such as cooling conditions, the Gibbs energy acted as the driving force for the transformation, and its variation DGb!a is described by DGb!a ¼ DHavg TDSavg
½1
where DSavg denotes the average entropy associated with pr
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