Fig. 1. The computed isothermal section of Mg-Al-Zn at 633 K according to the previous thermodynamic description of Mg-Al-Zn system using Thermo-Calc software [14,30]. The composition of two end-members, i.e. Mg and τ phase, of the diffusion couples are shown as red dots. The red dashed line is the hypothetical diffusion path that goes across ? phase.

Fig. 2. XRD pattern of currently prepared ternary τ phase alloy annealed at 450 °C for 12 h.

Fig. 3. SEM images of ? ternary phase at the Mg-τ interface after annealing at 360 °C for (a) 4 h, (b) 16 h, (c) 49 h and (d) 100 h.

Fig. 4. EMPA result of element distribution in each phase region along diffusion direction after annealing at 360 °C for (a) 4 h, (b) 16 h, (c) 49 h and (d) 100 h. The open symbols are experimental data, while the solid lines are simulation results using the composition-dependent interdiffusion coefficients estimated in this work.

Fig. 5. Diffusion path of 100 h heated Mg-τ diffusion couple along with the isothermal section of Mg-Al-Zn system at 360 °C: (a) whole composition range, (b) enlargement of the Mg-rich corner, (c) enlargement of the ? phase region and (d) enlargement of the τ phase region.

Fig. 6. In-situ observation of the diffusion growth of ? phase at 360 °C using HTLSCM. $W_0^\phi$ is the initial layer width of ? phase at the starting point of observation, i.e. t0 = 3089 s. $\Delta W_{\text{r}}^\phi$ and $\Delta W_{\text{l}}^\phi$ are the increment of IMC layer thickness at the Mg/? and ?/τ interfaces, respectively.

Fig. 7. In-situ observation data (open circle) of the layer thickness depending on the square root of annealing time together with the SEM data (open square). The dashed line is fitting result to the SEM data, while the solid line is the simulation result using current interdiffusion coefficients.

Fig. 8. Interdiffusion flux of Mg-τ diffusion couples annealed at 360 °C for 4 h, 16 h, 49 h and 100 h in each phase region computed by numerical inverse methods (solid lines) along with the current experimental data (open symbols).

Fig. 9. 3-D plot of composition-dependent interdiffusion coefficients: (a) along the Mg-τ diffusion path; (b) in Mg solid-solution phase; (c) in ? phase; (d) in τ phase.