Diffusion studies were carried out on the Fe-Ni-Al system at 1000C, using solid/solid diffusion couples comprising β(B2), β’(bcc) and γ(fcc) single-phase alloys, for the development of diffusion structures, diffusion paths and the determination of interdiffusion and intrinsic diffusion coefficients. The diffusion structures were examined by using optical and scanning electron microscopy, and the concentration profiles were determined by means of electron microprobe analysis. The diffusion couples included several series of β/γ and β′/γ couples, characterized by a common terminal alloy bonded to several terminal alloys having various compositions. The development of planar and non-planar interfaces, as well as 2-phase layers, as observed in various couples, were related to the diffusion paths. The interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the diffusional interactions among components were examined in the light of zero-flux planes and flux

reversals, which were identified in several couples. Ternary interdiffusion coefficients Di,jFe>i,j (i, j = Al, Ni), with Fe considered to be the dependent concentration variable, were evaluated at composition points of the intersection of diffusion paths of single-phase couples and of multiphase couples that developed planar interfaces. The interdiffusion coefficients were largest for the β′ alloys; especially near to the β/β′ miscibility gap, and decreased for the β and γ alloys. In the β and γ phases, the main interdiffusion coefficient for Al was larger than those for Ni and Fe. Also, Fe interdiffused faster than Ni in the Fe-rich β and β′ phases. The cross-interdiffusion coefficients (DAlNiFe, DNiAlFe) were negative in all 3 phases. In general, the DAlNiFe coefficients were larger in magnitude than the DNiAlFe coefficients. However, the magnitude of DNiAlFe was greater than that of DAlNiFe near to the β/(β + γ) phase boundary of the ternary isotherm. In the β-phase, the magnitude of Di,jFe (i, j = Al, Ni) coefficients increased by over 1 to 2 orders of magnitude with a decrease in the Al concentration and increase in the Fe/Ni concentration ratio. The interdiffusion coefficients, extrapolated from the ternary coefficients for binary alloys, were consistent with published values. The intrinsic diffusion coefficients were also determined at selected compositions. In addition, tracer diffusion coefficients were estimated for the binary Fe-Al and Ni-Al alloys at selected compositions, from an extrapolation of ternary interdiffusion coefficients.

Diffusion Studies in the β(B2), β’(BCC), and γ(FCC) Fe-Ni-Al Alloys at 1000C. Y.H.Sohn, M.A.Dayananda: Metallurgical and Materials Transactions A, 2002, 33[12], 3375-92