Effect of Ti additions on phase transitions, lattice misfit, coarsening, and hardening mechanisms in a Fe2AlV-strengthened ferritic alloy

Ferritic Fe-Al-V alloys strengthened with L21-Fe2AlV precipitates are candidate materials to meet the steam temperature/pressure requirements of super- or ultra-supercritical thermal power plants. Precipitate features, such as the size, morphology, volume fraction, coherency and modulus misfit, temperature of dissolution and coarsening rate, are important parameters to determine their mechanical properties at elevated temperatures. In this study, the effects of the addition of Ti on the formation and microstructural evolution of precipitation in the recently proposed Fe76Al12V12 alloy have been systematically investigated, focusing specifically on Fe76Al12V(12-X)TiX (at.%) where x = 0, 0.5, 1, 1.5, 2, 3. For the Ti-modified alloys, vanadium sites of the nanometric L21-Fe2AlV precipitates are the main places where Ti is located. The maximum service temperature of the alloy is slightly increased with Ti substitution. Volume fraction, coarsening rate and coherency misfit of precipitates for the alloys aged at 700 °C increases not linearly with Ti addition. The features observed in the coarsening rate and in the lattice misfit are consistent with an elastic contribution to the interfacial energy between precipitate and matrix. It is shown that the residual stress, generated when the precipitate/matrix interface is formed, plays a decisive role to slow down the coarsening rate increment with the substitution of Ti, even when there is a strong increase of the misfit strain. As Ti increases, the room temperature strength of the alloy aged for short time increases gradually even when the average precipitate radius grows slightly. Based on the aforementioned microstructural and mechanical effects, it is discussed whether substituting V for Ti could be beneficial to improve the creep resistance of precipitate strengthened Fe76Al12V12 ferritic alloy.