The effect of rapid heat treatment on the high-temperature tensile behavior of superplastic Ti-6Al-4V
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NTRODUCTION
SEVERAL studies[1–12] on the effect of rapid heat treatment in titanium alloys have reported that increasing the heating rate affects the (␣ ⫹  ) →  transformation, the scale and morphology of transformed  structures, and the service properties of the heat-treated alloys. Rapid heat treatment does not allow sufficient time for equilibrium to be approached, so alloying elements remain inhomogeneously distributed even after treatment at temperatures entirely within the  -phase region. It also enables nonequilibrium primary ␣ phase to persist to temperatures significantly higher than the equilibrium  -transus temperature.[5–8,10] The microstructure after a short exposure to high temperature remains much finer than that after a typical furnace heat treatment in the  -phase field, not only because the time for grain growth is restricted, but also because grain growth in the  -phase field is constrained by chemical inhomogeneity.[3,10] However, for rapid heat treatment in the (␣ ⫹  )phase region, the influence of rapid heating in inhibiting grain growth appears to be weaker, since the coexistence of the ␣ and  phases is rather more effective in inhibiting grain growth than chemical inhomogeneity.[3,10] Significant improvement in the mechanical properties of titanium alloys has been found after rapid heating. Ivasishin and Lutjering[11] and Ivasishin et al.[12] showed that rapid heating could be used to improve the relatively poor ductility in fully transformed microstructures by reducing the prior- grain size and refining the intragranular morphology. Fatigue properties were also improved to the level obtained with bimodal structures with an equivalent lamellar dimension, while the mechanical properties of fully transformed structures are more isotropic than those of bimodal structures, since the latter are sensitive to the crystallographic texture of the retained primary ␣ phase.
In the present work, the effect of rapid treatment on material behavior during deformation at high temperatures was investigated. This was a part of a program investigating the potential of using localized heat treatment prior to superplastic forming to control the thickness distribution (the microstructural gradient technique).[13,14] More details of this will be given in a subsequent publication. In order to use that method, the effect of rapid heat treatment on the microstructure and mechanical behavior during hot deformation to relatively high plastic strains needs to be determined. II. EXPERIMENTAL PROCEDURE A. Material and Heat Treatments Experiments were conducted on 0.77-mm-thick commercial, mill-annealed grade Ti-6Al-4V sheets with the chemical composition (wt pct) given in Table I. A series of heattreatment types, detailed as follows, was carried out to investigate the effect of temperature and heating rate on microstructural development. HT1: Direct resistance heating to temperatures between 950 ⬚C and 1160 ⬚C at 30 ⬚C intervals, with a constant heating rate of 100 ⬚C s⫺1 and without isothermal holding, followed
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