Reactive interdiffusion in the binary system Ni-Si:: Morphology of the Ni3Si2 phase

In binary multiphase diffusion, it is generally admitted that interfaces between phases are necessarily plane. However, a few cases exist, as the binary diffusion couples Ni-Si, Mo-Si, and Fe-Al, for which an intermediate phase of each system grows with an irregular needlelike morphology. To characterize the nonplanar growth of Ni3Si2 in bulk samples, the authors studied the behavior of intermetallic compound formation by optical microscopy and x-ray microtomography, for different annealing times. They show that both the average height and the tip radius of curvature grow as the square root of time with two diffusion coefficients separated by orders of magnitude. Moreover, x-ray diffraction indicates that the needles are aligned along the crystallographic c-axis. These results could be consistently explained by an anisotropic diffusion model.