Microstructural development of indirect-extruded TiAl–Mn–Mo–C intermetallic alloys during aging

The effect of aging at 800 °C on the microstructure in indirect-extruded TiAl–Mn–Mo–C alloys has been studied by transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), energy dispersive X-ray spectrometry (EDS) techniques and microhardness testing. Indirect extrusion processing, as a means of consolidation, effectively refined the microstructure of elemental powder metallurgy (EPM) TiAl–Mn–Mo–C alloys. The alloys showed a sensitive response to aging heat treatment at 800 °C, the hardness peaking at the 0.3 at.% C-alloy. The morphology of α2+γ plate colonies was related to the various nucleation conditions for the α→α2+γ transformations at the prior α grain boundaries. Carbides, P-phase as well as H-phase, formed mainly at the α2/γ interfaces and less frequently within the γ grain or prior α grain boundaries. The presence of ledges between the P-phase carbides and α2 phase interfaces indicated that the nucleation stage of the P-phase carbides involved an interface-controlled process.