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 studies 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 describe 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 boundary 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.