Initiation and stagnation of room temperature grain coarsening in cyclically strained gold films

TL;DR

This study investigates how cyclic strain induces grain coarsening in gold films at room temperature, revealing that a thermodynamic model explains initiation and stagnation, with grains of lower yield stress coarsening preferentially.

Contribution

It provides a systematic analysis of room temperature grain coarsening under cyclic strain and introduces a thermodynamic model explaining the phenomena across different conditions.

Findings

Grain coarsening depends on film thickness and strain amplitude.

A thermodynamic model explains both initiation and stagnation of coarsening.

Lower yield stress grains tend to coarsen preferentially.

Abstract

Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening.