Creep behavior and effects of heat treatment on creep resistance of Ti-22Al-24Nb-0.5Mo alloy

Microstructure and creep behavior of Ti-22Al-24Nb-0.5Mo alloy were investigated in a stress range from 0.4σ0.2 to 0.6σ0.2 and a temperature range from 750 to 850 ℃. The creep parameters, such as steady-state creep rates, stress exponent n, and creep activation energy Q, have been determined and the microstructural observations of dislocations, microstructure evolution and crack propagation of the crept specimens have been characterized. The results show that the dislocation-controlled creep mechanism is indicated by the values n of 3-3.3 and Q of 336 kJ/mol. During the creep process, the orthorhombic O phase was coarsened and the B2 phase partially transformed into O phase, which were both accelerated by the creep deformation. Besides, the abnormal B2/O coarse-laths existing in the as-received Ti-22Al-24Nb-0.5Mo alloy are found to be favored by crack propagation. After solution-aging, the creep resistance of this alloy is dramatically improved through the elimination of the abnormal B2/O coarse-laths.