Multiscale composition modulated Ti–Al composite processed by severe plastic deformation

Abstract

Using severe plastic deformation processes to consolidate and co-deform powder mixtures to make ultrafine grain composites is a very attractive approach because it offers an almost non-limited room for combinations of phases and composite structures. The aim of this work was to investigate the mechanisms operating at different length scales and leading to multiscale structures, namely co-deformation, fragmentation and mechanical mixing. A Ti–Al composite was processed from a Ti–Al powder mixture prepared by ball milling and subsequently deformed by equal channel angular pressing. Microstructures were characterized at all length scales, down to the nanometre, using optical microscopy, scanning electron microscopy and transmission electron microscopy. It was found that the final structure exhibits unique features at various length scales. Chemical heterogeneities at the micron scale are the result of co-deformation, while at the sub-micron scale they result from the fragmentation and necking of the Ti hard phase. Then, at the nanometer scale, intermixing occurred and nanoscaled intermetallic particles were discovered. This work highlights the possibilities offered by all these mechanisms to design ultrafine grain composite structures for optimized properties.