On the use of nanometer-scale multilayers to determine interdiffusion coefficients: Comprehensive characterization of interdiffusion at low temperature in the Ni–Cr system

Nanoscale multilayers offer a convenient way to determine interdiffusion coefficients at low temperatures. However, knowledge regarding the impact of the microstructure on measurements is limited. In the present study, we measure the interdiffusion coefficient in the face-centered cubic (fcc) solid solution of the Ni–Cr system at 440 °C using multilayers composed of alternating layers of pure Ni and Ni78Cr22 (at.%), with a nominal wavelength of 4.5 nm. Three techniques were used to characterize the evolution of the multilayers with annealing time: atom probe tomography (APT), energy-dispersive X-ray spectroscopy (STEM-EDX) and X-ray reflectivity (XRR). Each technique allowed to determine an interdiffusion coefficient. The results evidence that the interdiffusion coefficient is dependent from the technique used to measure it. The primary cause is a very complex microstructure resulting from the elaboration method used to obtain the fine concentration modulation. The analysis of atom probe tomography volumes reveals a high density of columnar grain boundaries (GB) with extended chemical widths. The segregation at GB was measured and a model was derived, allowing to dissociate the contribution of lattice interdiffusion between layers from that of diffusion along and perpendicularly to GBs. The present results could serve as guides for future diffusion investigations involving multilayers.