Evolution of microstructure and microhardness of the weld simulated heat-affected zone of Ti-22Al-25Nb (at.%) alloy with continuous cooling rate

The influences of continuous cooling rate on microstructure and microhardness of the weld simulated heat-affected zone (HAZ) of Ti-22Al-25Nb (at.%) alloy were investigated in this study. The continuous cooling process of the weld HAZ of Ti-22Al-25Nb alloy was simulated using a Gleeble3500 thermo-mechanical simulator. The phase constituent investigated by X-ray diffraction showed that the weld simulated HAZs were all composed of lath-shaped O phase, equiaxed α₂ particle and β/B2 matrix at different continuous cooling rates. Microstructural observations by scanning electron microscopy indicated that the size of O phase significantly decreased and the shape of α₂ particle did not obviously change with the increase of continuous cooling rate. Quantitative analysis presented that with increasing continuous cooling rate the volume fraction of O phase decreased, while that of α₂ and β/B2 phases increased. Phase transformation temperatures were determined by using dilatometric analysis and the weld simulated HAZ continuous cooling transformation diagram was determined. The microhardness of the weld simulated HAZ increased with the increase of continuous cooling rate.