Formation of Periodic Layers in Ce–Mg–Zn Diffusion Couples

Periodic-layered structures form in some ternary diffusion couples, but their formation mechanism is still debated. Three mechanisms are proposed: unequal diffusion rates, chemical potential instability, and diffusion-induced stress. In the Ce–Mg–Zn system, periodic layers were reported at 300 °C without a mechanism. This study examines which mechanism the morphology supports. Two diffusion couples paired pure Zn with a binary Ce–Mg alloy. The alloy contained 24.54 at.% Ce in the first couple and 71.76 at.% Ce in the second. Both couples were annealed at 300 °C for 21 days. Phases were assigned from WDS compositions. Repeat distances were measured from grey value line profiles. Mg–Zn-rich and Ce-bearing layers alternated in both couples. The first couple formed Mg2Zn11, MgZn2, Ce2Zn17 and ternary τ phases across seven reaction zones. The second formed Mg2Zn11, MgZn2 and Ce–Zn-rich phases across eight. The coefficient of variation of the repeat distance ranged from 1.6% to 17.6%. In the first couple, spacing increased over the first three periodic zones and then levelled off. No cracks or voids were found at the layer boundaries. The initial Ce/Mg ratio strongly affected the phase sequence. The data could not separate the three mechanisms.