Thermal stability of Ti-44Al-4Nb-4Hf-0.2Si-1B alloy

A γ-TiAl based fully lamellar alloy Ti-44Al-4Nb-4Hf-0.2Si-1B was isothermally forged and exposed at 700 °C for up to 10,000 h. It has been found that the combined addition of Nb and Hf rather than sole Nb has a positive effect on stabilisation of α2 lamellae. Compared to alloy Ti-44Al-8Nb-1B, less amount of α2 dissolved and less amount of β(B2 + ω) formed in alloy 4Nb-4Hf after the same forging and exposure scheme. As a result, the exposure-induced embrittlement is less severe in alloy 4Nb-4Hf. The ductility after 10,000-h exposure is reduced by 1/3, compared to the reduction by 2/3 in alloy 8Nb. The tensile strengths remain essentially unchanged while fatigue limit increased significantly for alloy 4Nb-4Hf. The mechanism behind the reduced α2 dissolution and β formation is discussed in relation to a changed partitioning behaviour and a slower diffusivity due to the combined addition of Nb and Hf.