Phase formation in iron-containing titanium aluminide during two-step heat treatment

Processes occurring in a γ-base TiAl alloy containing 2 at.% Fe during two-step thermal treatment were investigated. Samples held at 1300 °C for 90 min were transferred to an intermediate temperature (in the range 700-1070 °C), aged for 2 h and water quenched. The decomposition of the high temperature B2 FeTi structure containing 42% Al was in the center of the research. This decomposition occurs by two successive reactions: depletion in Al followed by decomposition of the depleted phase into stable iron-alloyed α₂-Ti₃Al (Ni₃Sn type) and the ternary τ₂-Al₂TiFe (Mn₂₃Th₆ type). The diffusion required for the depletion reaction is governed by large values of the concentration gradient of the M_Fe-Al and M_Ti-Al diffusion potentials. It is shown that the diffusion is sufficiently fast to permit the depletion reaction at all the intermediate temperatures used. The driving forces were found for both reactions using the quasi-chemical model for the Gibbs energy. The rate of the τ₂ + α₂ nucleation was estimated; it has a maximum in the vicinity of 800 °C. Residual FeTi(Al) depleted in Al was found after aging at all intermediate temperatures, except at 800 °C. At this temperature the depleted phase decomposes completely to a very fine mixture of τ₂ and α₂.