Intergranular fracture of gamma titanium aluminides under hot working conditions

A comparative study of the hot workability of a near gamma titanium aluminide alloy Ti-49.5Al-2.5Nb-1.1Mn in the cast and wrought conditions was performed. Tension tests conducted on coarse grain, cast material, and fine grain wrought material revealed a pronounced variation in both fracture/peak stress and ductility with temperature and strain rate. Brittle, intergranular fracture occurring at high strain rates was found to be controlled by wedge crack nucleation, whereas the ductile fracture observed at low strain rates was controlled by the growth of wedge cracks and cavities. Dynamic recrystallization was shown to be the main restorative mechanism to accommodate grain boundary sliding and thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grain size-based stress intensity factor given by the product of the peak/fracture stress and the square root of grain size. A processing map for the near gamma titanium aluminides was constructed based on the comparative analysis of the hot tension and compression test results