Measerement of Intrinsic Diffusion Confficients in Intevrmtallics using Analytical Electon Microscopy

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Diffusion experiments were conducted on NiAl (beta phase) coupled with Ni (gamma Phase). Analytical electron microscopy is used to examinc microstructural features, crystallographie orientation and cmpositional variation across the intcrface. The results are summarized as below.1.The Ni_3 Al-based intermetallic phase (gamma' Phase) formed at the Kirkendall interface and grew during diffusion annealing.The gamma' phase grew towards the gamma phase following a parabolic law and kept the same orientation with the gamma phase at annealing temperatures higher than 1273 K.The gamma/gamma' interface was smooth and straight. At annealing temperatures lower than 1273 K,the growth of the gamma' phase occurred towards the gamma phase and the beta phase with a 4th power of annealing time. The same orientation did not exist at the gamma/gamma' interface and the gamma'/beta interface. Both interfaces exhibited wavy and irregular configurations.3.Interdiffusion coefficients of the gamma, gamma'and beta phases were measured from the compositional variations across the diffusion-couple interface. The interdifhsion coefficients in the corresponding puases were consistent with the published values measucd from single-phase diffusion cmplcs and two-phase diffusion couples cxccpt when the couples were annealed at at temperatures lower than 1273 K.This exception arose from the effect of grain boundary diffusion.4.It was suggested that a single crystal of Ni_3 Al can be produced if single crystalline Ni is used as an end member of the couple.