Tial3 – An Intermetallic Compound

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An intermetallic compound formed from Ti and Al atoms. It forms in a reaction-controlled process at both Ti/tial3 and Al/tial3 interfaces, as evidenced by X-ray diffraction and microscopic observations (SEM and OM).

Formation of tial3 is facilitated by its negative Gibbs energy – i.e., lower than that of a pure metal – which makes it easier to form than other high-aluminum phases, such as Ti2Al5. During the welding of the multi-laminated Ti/Al diffusion couple, Ti and Al atoms diffused across the intermetallic layer and reacted with each other to generate tial3. The reaction controlled nucleation and growth of tial3 exhibited by its atomic distribution along the intermetallic layer can be explained using an interdiffusion model.

Upon annealing, a tri-modal grain structure was observed in the intermetallic layer between the Ti and Al sheets. The grain structure consists of fine, nearly nano-sized tial3 nuclei/grains near the Ti/tial3 and Al/tial3 boundaries respectively. The grains at the centre portion of the intermetallic layer appear to be larger than those at the boundary areas, implying grain growth.

Given the faster diffusivity of Al compared to that of Ti, it is expected that huge mass transport of atoms occurs from the Al side to the Ti side during the diffusion process, which ensures a net growth of tial3 grains at the boundary areas, especially at the regions A and C indicated by red and blue dots in Fig. 6c respectively. This is confirmed by the arrows pointing towards these regions in Fig. 3b, which represents the shifting direction of the newly grown tial3 grains.

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