Microstructural Degradation During Creep Loading and Improvement Methods for a Directional Lamellar TiAl Alloy Prepared by Conventional Casting

Microstructure differences between the polysynthetically twinned crystal (PST) and the directional lamellar structure prepared by conventional casting for TiAl based alloy were analyzed, in order to discuss their influence on the microstructural degradation during creep loading. And micro-alloying methods were proposed to decrease the adverse effect. The results show that microstructural differences mainly include colony boundary, Al segregation and excess alpha 2 phase. In addition, equiaxed gamma grain precipitation and lamellar coarsening during hot isostatic pressing are also different. These differences aggravate the microstructure degradation of directional lamellar structure prepared by conventional casting during creep loading. Al segregation and excess alpha 2 phase promote the microstructural degradation for as-cast sample, and the equiaxed gamma grains precipitated during hot isostatic pressing aggravate the microstructural degradation. They cause significant reduction of creep rupture life. The effect of Al segregation is less than that of excess alpha 2 phase. The colony boundary has a little effect on the microstructural degradation, Besides, the idea that minor addition of Zr, Si, C improve the creep rupture properties by inhibiting microstructural degradation was also discussed.