Thermal ridges-Formation of hillock-like structures in deformed bulk nickel

The effect of surface topography on grain boundary migration has been the focus of numerous stud-ies due to its importance for microstructure evolution in thin films. In this work we have studied grain boundaries in samples of pure bulk Ni after mild cold deformation and a subsequent anneal. The surface topography in the vicinity of grain boundaries featured large protrusions formed during the anneal with a striking resemblance to downscaled terrestrial mountains. We coined the term 'thermal ridges' to de-scribe these protrusions. Their size decreases with increasing annealing time and degree of deformation. We developed a kinetic model of ridges based on the original Mullins' model of mobile grain bound-ary grooves modified to account for high surface anisotropy. The combination of our experimental and modeling results sheds new light on the phenomenon of hillock formation in thin polycrystalline films. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The effect of surface topography on grain boundary migration in thin films has been investigated in this study. In pure bulk Ni samples, large protrusions resembling downscaled terrestrial mountains were observed near grain boundaries after mild cold deformation and subsequent annealing. These protrusions, called 'thermal ridges', decrease in size with increasing annealing time and degree of deformation. A kinetic model of ridges based on the modified Mullins' model was developed to explain this phenomenon. The combination of experimental and modeling results provides new insights into the formation of hillocks in thin polycrystalline films.