Superlattice phase precipitation in Ti-Fe-Mo-Al alloys

Ti-Fe-Al and Ti-Fe-Mo alloys containing B2 superlattice precipitates within an A2 matrix have attracted interest for structural applications. However, few studies have considered alloys derived from the quaternary Ti-Fe-Mo-Al system. In this work, six Ti-Fe-Mo-Al compositions with 70 at.% Ti have been aged for 1000 h at 600, 800, and 1000 °C to assess their equilibrium phases. A2+B2 phase microstructures formed in two of the alloys considered: Ti-20Fe-5Mo-5Al at both 800 and 1000 °C, and Ti-15Fe-10Mo-5Al at 800 °C. A2+B2 microstructures were not observed in the other alloys investigated. At high temperatures, most alloys were single-phase A2, while at lower temperatures A3 and D0 19 phases were widely observed. Studying continuous cooling of Ti-20Fe-5Mo-5Al from the solid solution showed a change in precipitation behaviour with cooling rate. Intermediate cooling rates permitted discontinuous B2 precipitation. The slowest cooling rate led to continuous B2 precipitation and the observation of the D8 a G-phase. This transition in precipitation mechanism was attributed to the relative rates of bulk and grain boundary diffusion at varying temperatures. • Phase equilibria (A2-B2-D 0 19 -A3 phases) in the Ti-Fe-Mo-Al were characterised. • Linear cooling led to competing continuous and discontinuous B2 phase precipitation. • Cooling rate and Mo content were found to be key in precipitation mode competition. • Fe supersaturation, Mo diffusion, and time at temperature govern precipitation mode. • The D 8 a G-phase was found at lower than expected Al content after slow cooling.