Solid State Chemical Interaction Between Ti and Al-Si Alloys

Diffusion couples of Ti and biphasic alloy Al-7wt%Si were produced and heat-treated at 535 °C for between 1 and 200 hours. The nature and thickness of the phases present in the reaction layer sequence were characterized using conventional techniques such as microprobe analysis and transmission electron microscopy. For short interaction times the interfacial reaction layer consists mainly of silicides, whereas aluminides are formed after times longer than 40h. This variation in the spatial reaction layer sequence over time is explained by the biphasic nature of one of the diffusion couple end-members, with high activity for both elements, and by the huge difference between the composition of the first phase formed at the initial interface and the average composition of the biphasic end-member. This situation, which can be observed in other systems, illustrates the difficulty of confirming that a true diffusion path has been attained in a diffusion couple. Most of the criteria that were first proposed by Kirkaldy and Brown, can apparently be fulfilled, but the system could continue shifting over time towards a more stable configuration. In some systems, as in the case of Al-Si-Ti, the slow kinetics can be problematic, making it impossible to predict a radical change in material properties.