Intrinsic grain-size gradients upon grain growth near a free surface

TL;DR

This study reveals that free surfaces induce a grain-size gradient in polycrystalline nickel during annealing, driven by elastic effects and shear-coupled grain boundary migration, affecting microstructural evolution.

Contribution

It demonstrates the influence of free surfaces on grain growth kinetics and the resulting intrinsic grain-size gradient due to elastic relaxation effects.

Findings

Grain size increases gradually from surface to interior.

The grain-size gradient extends several layers deep, beyond the surface.

Elastic relaxation at free surfaces significantly affects stress fields and grain growth.

Abstract

Grain growth fundamentally shapes the microstructure of crystalline materials upon annealing, affecting their overall mechanical and functional properties. Recently, it has been rationalized that grain growth in polycrystals does not result solely from weighted curvature flow, but elastic effects (intrinsic stress) arised from shear coupling also need to be taken into account. We characterize and examine the effect of free surfaces on grain growth kinetics of high-purity, bulk polycrystalline nickel. By analyzing the microstructural evolution on cross sections of 1 mm thick specimens from the surface to the interior, as well as through in-plane investigations on specimens with varying thickness (1 mm, 40 $\mu$m, and 10 $\mu$m), an intrinsic grain-size gradient was identified, characterized by a gradual increase in grain size towards the interior. Interestingly, this grading was not restricted to the very surface but continued to depths of five to ten layers of grains, where effects from thermal grooves are considered negligible. We demonstrate that this behavior is significantly affected by elastic relaxation at the free surface, which alters the internal stress fields generated by shear-coupled grain boundary migration. These findings emphasize the relevance of free surfaces to the microstructural evolution of polycrystal.