Mechanically enhanced grain boundary structural phase transformation in Cu

Abstract It has been previously proposed that grain boundary can be treated as a two-dimensional phase in materials with possible transformations between its multiple metastable states under external stimuli such as high temperature. In this work, the influences of another type of common external stimuli, i.e. mechanical load, on grain boundary structural phase transformation (GBSPT) are studied by molecular dynamics simulations. Two types of high angle symmetrical grain boundaries in Cu are used as model systems to investigate their structural transformation under both constant and cyclic mechanical loading. While in general GBSPT can be enhanced or weakened under a constant tensile or compressive stress as compared to that caused by temperature only, the influences of tension and compression are not the same. For this reason, cyclic loading consisting of symmetric tensile/compressive half-cycles can significantly influence the overall GBSPT. Furthermore, it is found that the macroscopic strain in the material caused by GBSPT under compression/tension can be well described by Coble creep, which implies that GBSPT could be a grain boundary diffusional process during the creep of polycrystalline materials that has been overlooked before. Additionally, it is shown that together with shear coupled grain boundary motion and sources of free volume such as voids or cracks, cyclic shear loading can also cause GBSPT between its metastable structures of different atomic density, which can be possibly utilized to heal damage or microcracks in engineering materials.