Utilizing 4D-STEM to characterize diffusion-induced grain boundary migration in Ni-Cr alloys

Abstract Alloy degradation poses a significant challenge to the safe and reliable performance of alloys in advanced energy systems. Ni-Cr alloys are quite durable in these aggressive conditions but can become susceptible to intergranular oxidation and cracking when subjected to high temperature (300–400°C) oxidizing conditions and stress. Selective oxidation of Cr has been observed to coincide with accelerated grain boundary diffusion and associated diffusion-induced grain boundary migration (DIGM). In these systems, DIGM is characterized by localized Cr depletion and Ni enrichment in the wake of 10s to 100s of nm of lateral grain boundary migration that can extend several micrometers beneath the alloy surface or ahead of propagating crack tips. While it has been established this Cr depletion can impact further corrosion behavior, it is less clear how it affects local mechanical behavior. In this work we use 4-dimensional scanning transmission electron microscopy (4D-STEM) to analyze the impact of DIGM on the local microstructure changes around degraded grain boundaries. By integrating 4D-STEM with elemental analysis via energy dispersive X-ray spectroscopy (EDS), we establish correlations between structural and compositional changes within the DIGM-zones. At low Cr concentrations (5 at.% Cr), compositional changes are insufficient to generate significant coherency strain, whereas at high Cr concentrations (~ 30 at.% Cr), the resulting strain can not only be anisotropic but also exceed what compositional changes alone predict.