Effects of Hf on the Microstructure and Mechanical Properties of TiAl Base Alloys

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The effect of Hf additions to single- and two-phase titanium aluminide alloys was investigated by X-ray diffraction, differential scanning calorimetry, metallographic characterization by scanning electron microscopy, and mechanical testing. Hf was chosen as an alloying element because it exhibits a significant solubility in γ(TiAl) and α2 (Ti 3 Al) and substitutes for Ti. There is a relatively large size misfit between Hf and Ti atoms, which provides a good potential for solid solution hardening. Hf additions in the amount of 2 and 5 at.% significantly strengthen α 2 +γ alloys, whereas the effect in single-phase y alloys is almost negligible. This observation, together with structural characterization, gives rise to the assumption that Hf additions do not lead to significant solid solution hardening. Thus, the strengthening of (α 2 +γ) alloys by Hf additions was mainly attributed to structural changes, which are probable associated with a shift of the phase boundaries in the Ti-Al system. This interpretation is essentially consistent with thermodynamic glide parameters determined for the binary and ternary alloy system.