Identification of microstructural mechanisms during densification of a TiAl alloy by spark plasma sintering

This work aims at identifying, by coupled scanning and transmission electron microscopy (SEM and TEM) observations, the densification mechanisms occurring when an atomized Ti-47Al-1W-1Re-0.2Si powder is densified by spark plasma sintering (SPS). For this purpose, interruptions of the SPS cycle have been performed to follow the evolution of the microstructure step by step. The powder particles exhibit a classical dendritic microstructure containing a large amount of out-of-equilibrium. phase. During heating-up, the microstructure undergoes successive transformations. At T = 525-875 degrees C the alpha phase transforms into gamma. The gamma phase formed is supersaturated in W and Re. It de-saturates for T above 875 degrees C by discontinuous precipitation of W and Re-rich B2 phase. Densification takes place for T between 900 degrees C and 1150 degrees C by plastic deformation of the powder particles. TEM observations show that the repartition of the plastic deformation is correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occur. Microstructural phenomena directly resulting from the high currents involved in the SPS process have not been observed. (C) 2011 Elsevier B.V. All rights reserved.