Influence of natural oxide layers at Ni/NiAl interfaces on Ni3Al phase formation

The influence of natural oxide layers at interfaces in diffusion couples is studied. Concentration distributions at (Ni)/NiAl interfaces and the formation of the Ni3Al phase at 650 degrees C are characterized using scanning electron microscopy (SEM) and aberration corrected scanning transmission electron microscopy (STEM) with EnergyDispersive X-ray spectroscopy (EDX) with unprecedented spatial resolution. Employing the common practice of chemical etching with 10 vol% HCl does not remove the oxide layers irrevocably - they are detected at the Kirkendall plane after diffusion annealing of (Ni)/NiAl diffusion couples, if high resolution microscopy is employed. The oxide layer at the Kirkendall plane is of a thickness of similar to 5 nm, comparable to that of the combined natural oxide layers on the (Ni) and NiAl surfaces. The oxide acts as a diffusion barrier, retarding or locally suppressing growth of Ni3Al into the (Ni) phase. For comparison, oxide-free Kirkendall planes were obtained by subjecting the joining surfaces of (Ni) and NiAl to gentle grinding with low contact pressure in a glove-box in purified Ar atmosphere before assembling the diffusion couple. After diffusion annealing, the Ni3Al layers are distinctly thicker and of a more homogeneous layer thickness as compared to samples with residual oxides. It is shown that evaluating interdiffusion coefficients in oxide-free diffusion couples yields higher values as compared to the conventionally assembled counterparts.

Automated summary

The study demonstrates the significant influence of natural oxide layers on the formation of Ni3Al phase at (Ni)/NiAl interfaces. These oxide layers act as diffusion barriers, retarding or locally suppressing growth of Ni3Al into the (Ni) phase. Removing the oxide layers leads to higher interdiffusion coefficients in diffusion couples.