The Effect of Texture Evolution on Flow Softening During Hot Working of Ti-6Al-4V with a Colony Microstructure
摘要
The effect of texture changes on flow softening of Ti-6Al-4V with an initial colony-alpha microstructure and strong alpha-phase basal texture was investigated. For this purpose, nominally isothermal hot-compression and hot-tension tests were performed at temperatures of 816 °C, 899 °C, and 954 °C and strain rates of 0.001 or 0.1 s−1. Electron-backscatter diffraction (EBSD) was used to quantify the alpha-phase texture of the starting (undeformed) material and the entire cross-sections of deformed samples. Local strains associated with the various textures were estimated via finite element analysis (compression samples), local diameter measurements (tension samples), and flow-localization calculations (tension samples). The measured textures, (observed) axisymmetric deformation-increment matrix, and postulated alpha-phase critical resolved shear stresses (CRSSs) for the various assumed slip systems were then inserted into EBSD software to quantify the evolution of the Taylor factor (M) as a function of strain. For both compression and tension modes of deformation, M was found to decrease with increasing strain. The magnitude of such texture softening was determined to represent a relatively small fraction (~20 pct. for the compression tests and ~ 16 pct. for the tension tests) of the overall flow-softening response. Coupled with calculations which demonstrated that deformation heating accounted for ~ 10 pct. of the flow softening for the higher imposed strain rate, it was thus concluded that the evolution of substructure (and microstructure) is the principal source of flow softening observed in flow curves for alpha/beta titanium alloys with a colony-alpha microstructure and thus must be considered in developing future, physics-based constitutive equations.