| Summary: | Ti-6Al-4V titanium alloy specimens fabricated through hot isostatic pressing (HIP) were made by multi-pass thermal deformation. The temperature was 950 °C, 850 °C and 900 °C for one pass, two passes and three passes, respectively, and each pass corresponded to three strain rate (i.e. 0.01 s ^−1 , 0.1 s ^−1 and 1 s ^−1 ), in addition, the total reduction in height was 70%. When the true strain was greater than 0.51 and the strain rate was 1 s ^−1 , the stress-strain curves showed the phenomenon of flow softening. X-ray diffraction (XRD) patterns indicated that the diffraction peak of α lattice plane of (0002) increased first and then declined with the strain under the strain rate of 0.1 s ^−1 . Besides, the Vickers hardness increased with the strain at the same strain rate and was not sensitive to strain rate. Moreover, when the strain rate was 0.1 s ^−1 , the fraction of LAGBs decreased from two passes to three passes and the microstructure results showed that the fraction of equiaxed α phases increased with the passes increasing. This is due to the strain energy increased as the increasing passes which could provide driving force for dynamic recrystallization (DRX). At the same amount of deformation, increasing the strain rate, the slender lamellar α bended and the equiaxed α grains gradually took place of the discontinuous lamellar α . Additionally, the spheroidization mechanism of lamellar α phase was mainly through grain boundaries bulging. After completing three passes deformation, the average grain size was approximately 12 μ m.
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