Effects of rapid heating on non-equilibrium microstructure evolution and strengthening mechanisms of titanium alloy

In this paper, the effects of heating parameters, including temperature ranging from 900 degrees C to 1000 degrees C, heating rates ranging from 2 degrees C & BULL;s  1 to 100 degrees C & BULL;s  1, and 120 s soaking on the non-equilibrium microstructure evolution of Ti-6Al-4V alloy were studied. Microstructures after heating were characterized to reveal the mechanism of nonequilibrium phase transformation. Uniaxial tensile tests at room temperature were carried out to evaluate the effects of non-equilibrium microstructure on the mechanical properties. Results show that higher heating rate and lower temperature lead to lower & beta; phase volume fraction and finer & beta; grains. A transition region with element gradient forms in the & alpha;p grain near the & alpha;p/& beta; phase boundary and transfers into & beta; phase gradually during the heating. Rapid heating could confine the movement of the transition region, and therefore reduce the & alpha;& RARR;& beta; transition and growth of the & beta; phase. When the Ti-6Al-4V alloy was heated to 1000 degrees C at a rate of 50 degrees C/s and then quenched immediately, the tensile strength was improved by 19.5% and reached up to 1263.0 MPa with the elongation only decreasing from 13.6% to 9.6% compared with the initial material. The main strengthening mechanism is that the rapid heating in the single-phase region avoids the rapid growth of the & beta; phase, which leads to fully fine martensite formation after water quenching.