Microstructural evolution and phase transformation in the liquid-solid Al/Ni diffusion couple

The liquid-solid Al/Ni diffusion couple was successfully fabricated by annealing at 1373 K for 48 h followed by water-quenching. Cross-sectional scanning and transmission electron microscopic analyses show that the multilayered diffusion zones comprise the following sequence of layers: gamma-Ni(Al) | gamma '-Ni3Al | beta '-NiAl | Ni-rich beta-NiAl | beta-NiAl. The Ni-rich beta-NiAl upon quenching undergoes a martensitic transformation from beta (B2) to beta ' (L1(0)). The beta ' martensite is found to be internally twinned on the {111}system. The volume changes and strains due to martensitic phase transformation, the precipitation of gamma '-Ni3Al from gamma-Ni(Al) and lattice mismatch between Ni-rich beta-NiAl and beta-NiAl in the Al/Ni diffusion couple are quantitatively determined. The cuboidal gamma ' phase coherently precipitates cube-on-cube in gamma-Ni(Al). Composition fluctuations existing in the supersaturated solid solution gamma-Ni(Al), provide sufficient driving force for the precipitation and facilitate nucleation and growth of the gamma ' phase under isothermal annealing.