Microstructural mechanisms during hot working of commercial grade Ti–6Al–4V with lamellar starting structure

Abstract

The hot deformation behavior of commercial grade Ti–6Al–4V with a lamellar starting microstructure is studied in the temperature range 750–1100°C and strain rate range 3×10−4–10 s−1 with a view to model the microstructural evolution. On the basis of flow stress data obtained as a function of temperature and strain rate in compression, a processing map for hot working has been developed. In the ranges 800–975°C and 3×10−4–10−2 s−1, globularization of lamellae occurs for which an apparent activation energy of 455 kJ mol−1 has been estimated using the kinetic rate equation. Stress-dependent thermal activation analyses proposed by Schöck and Cocks et al. have shown that the apparent activation energies are in the range 160–245 kJ mol−1 and the normalized activation volumes are in the range 20–80, which suggest that cross-slip is the rate controlling process during globularization. The variation of primary α grain size with Zener–Hollomon parameter (Z) in the globularization region exhibited a linear relationship on a log–log scale. At strain rates slower than 10−1 s−1 and temperatures below 900°C, cracking at the prior β grain boundaries/triple junctions occurs, which sets the lower limits for globularization. At strain rates higher than 10−1 s−1 in the α+β range, the material exhibited flow instabilities manifested as adiabatic shear bands. These bands are intense below 800°C and above 1 s−1 and caused cracking along the bands. In the β phase field, dynamic recrystallization (DRX) occurs at about 1100°C and in the strain rate range 10−3–10−1 s−1. The apparent activation energy for DRX of β is about 172 kJ mol−1 which is close to that for self-diffusion in β phase (153 kJ mol−1). The application of these results in the design of bulk metalworking processes for achieving microstructural control is discussed.