Processing Maps for Hot Working Behavior of a PM TiAl Alloy

The hot deformation behavior of a Ti-47Al-2Cr-2Nb-0.2W-0.15B (at.%) titanium aluminide alloy fabricated by pre-alloyed powder metallurgy has been investigated by using the hot compression tests in the temperature range from 950 degrees C to 1300 degrees C and at the strain rates between 10(-3) s(-1) and 10 s(-1). The processing maps have been established to evaluate the optimum hot processing conditions and reveal the instability regions. It is found that the flow stress of the investigated alloy is a strong function of the temperature and the strain rate. The investigated alloy has the optimum hot-working condition at 950 degrees C and 10(-3) s(-1), since the material undergoes dynamic recrystallization to produce a fine-grained microstructure. At 1250 degrees C and 10(-3) s(-1), the alloy exhibits superplastic deformation. At 1300 degrees C and 10(-1) s(-1), the cyclic dynamic recrystallization with high temperature grain coarsening takes place. The material undergoes flow instabilities at lower temperatures and higher strain rates, as predicted by the instability criterion. The processing maps demonstrate that the strain significantly affected the instability regions. The manifestations of the instabilities have been observed in the form of microvoids, wedge cracks, and surface fractures.