Effects of microstructure on the fracture toughness of Ti3Al-based titanium aluminides

The influence of microstructure on the fracture toughness of Ti-23A1-9Nb-2Mo-1Zr-1.2Si (at. pct) and Ti-23A1-11Nb-0.9Si (at. pct) Ti₃Al-based alloys has been investigated. Basket-weave microstructures comprising different volume fractions of α ₂ and retained β phases were produced by systematic heat treatments. Besides the volume fraction of the retained β phase, the average size of the β laths has also been used to characterize these microstructures. The toughness of both alloys was examined at room temperature, and the brittle transgranular fracture modes were found to be controlled by microstructure. However, the toughness is not determined solely by the volume fraction of the retained β phase, and a linear relationship has been obtained between the fracture toughness and the average size of the retained β laths. It appears therefore that the toughness of Ti₃Al-based alloys at room temperature is controlled primarily by the width of retained β laths rather than by the retained β volume fraction.