INTERGRANULAR FRACTURE OF GAMMA-TITANIUM ALUMINIDES UNDER HOT-WORKINGCONDITIONS

A comparative study of the hot workability of a near gamma titanium al uminide 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 b oth fracture/peak stress and ductility with temperature and strain rat e. 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 t he main restorative mechanism to accommodate grain boundary sliding an d thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grai n size-based stress intensity factor given by the product of the peak/ fracture stress and the square root of grain size. A processing map fo r the near gamma titanium aluminides was constructed based on the comp arative analysis of the hot tension and compression test results.