Why do cracks occur in the weld joint of Ti-22Al-25Nb alloy during post-weld heat treatment?

Ultrasonic pulse frequency tungsten inert gas welding technology was adopted to join Ti-22Al-25Nb alloy. There were some cracks in the Ti-22Al-25Nb alloy weld joint after post-weld heat treatment. The hardness and Young's modulus of alpha(2), O, and beta/B2 phases in Ti-22Al-25Nb alloy were examined with an in situ nanoindentation technique. The phase transition stresses of three different phases in the weld joint of Ti-22Al-25Nb alloy were analyzed to explain why cracks occur in the weld joint of Ti-22Al-25Nb alloy during post- weld heat treatment. The results show that mean hardness is highest for the alpha(2) phase, second-highest for the O phase, and lowest for the beta/B2 phase; the mean Young's modulus has the same trend in Ti-22Al-25Nb alloy. Phase transition stress results in cracks in the weld joint of Ti-22Al-25Nb alloy during post- weld heat treatment. By improving post- weld heat treatment, the ultimate strength of the Ti-22Al- 25Nb alloy weld joints reaches 750 MPa, which is 72.5% that of the base material.