  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.
