The optimal temperature for enhanced low-temperature superplasticity in fine-grained Ti–15V–3Cr–3Sn–3Al alloy fabricated by friction stir processing

Abstract

The rolled Ti–15V–3Cr–3Sn–3Al (Ti-15-3) alloy plates were processed by friction stir processing (FSP) using an ultra-low heat input parameter of 100 rpm- 100 mm/min and 1.6 μm grain-sized microstructure was obtained. Subsequently, tensile tests were conducted in this microstructure within the temperature range of 625–725 °C with an interval of 25 °C at three different strain rates of 1 × 10−4 s−1, 3 × 10−4 s−1 and 1 × 10−3 s−1. The largest elongation of 842% was achieved at 675 °C and 3 × 10−4 s−1 and the present superplastic deformation is dominantly controlled by grain boundary sliding (GBS) accompanied by phase transformation and continuous dynamic recrystallization (CDRX). To the best of our knowledge, the elongation of 842% is the largest one for Ti-15-3 alloy at the temperature as low as 675 °C. The increasing temperature supplied more energy for GBS, which was beneficial to the achievement of excellent superplasticity. However, when the temperature increased over 675 °C, the α phase volume fraction severely decreased which led to the significant β grains growth, the high angle grain boundaries (HAGBs) remarkably reduced, all of which impeded the GBS hence the achievement of excellent superplasticity. Therefore, the largest elongation, most stable and uniform superplastic flow and lowest peak stress were achieved at the temperature of 675 °C for all the three testing strain rates of 1 × 10−4 s−1, 3 × 10−4 s−1 and 1 × 10−3 s−1.