Microstructure and toughness of nitrogen-doped TiAl alloys

The effect of nitrogen doping of TiAl in the nitrogen levels up to 1.0 at.% N (0.3, 0.5,and 1.0%) on microstructure, compressive yield strength and fracture toughness has been examined. The mean colony size decreases with increasing nitrogen content in the alloy. The lamellar morphology is preserved in the 0.3 at.% N alloy and a uniform, fine lamellar spacing is recognized. There was no evidence of nitrides being present. In the 0.5 at.% N alloy, the lamellar spacing is relatively coarser, there are clear indications of the destabilization of the lamellar morphology both within the colonies and at colony boundaries, and there are more or less continuous films of nitride precipitates along certain lamellar interfaces. In addition, fine nitride precipitates are present often on dislocations within the equiaxed gamma grains that result from the destabilization of the lamellar structure adjacent to colony boundaries. These effects are significantly more pronounced in the 1.0 at.% N alloy, and in addition, coarse nitride precipitates occur frequently at colony boundaries. The effects of these microstructural modifications on ambient temperature compressive yield stress and notched-bend fracture toughness have been examined and the observations are discussed.