Dislocation arrangement in small crystal volumes determines power-law size dependence of yield strength

Abstract

It is by now well-known that micron-sized metallic crystals exhibit a smaller-being-stronger size effect: the yield strength σ varies with specimen size D approximately as a power-law σ∼D−m, and the exponent m has been found to vary within a range of ∼0.3–1.0 for different metals. However, little is known about why such a power-law comes into play, and what determines the actual value of the exponent m involved. This work shows that if the yield strength is determined by the Taylor interaction mechanism within the initial dislocation network, then for the size dependence of strength to be of the power-law relation observed, it is necessary for the mesh lengths L of the dislocation network to be power-law distributed, i.e. p(L)∼L−q. In such a case, the exponent m of the size effect is predicted to be inversely proportional to the sum of q the exponent of the mesh-length distribution and n the exponent of the dislocation velocity vs. stress law. To verify these predictions, compression experiments on aluminum micro-pillars with different pre-strains from 0% to 15% were carried out. The different pre-strains led to different initial dislocation networks, as well as different exponent m in the size dependence of strength. Box-counting analyses of transmission electron micrographs of the initial dislocation networks showed that the 2-D projected dislocation patterns were approximate fractals. On increasing pre-strain, the exponent m for the size dependence of strength was found to decrease while the fractal dimension of the initial dislocation patterns increased, thus verifying the inverse relationship between the two quantities. These findings show that the commonly observed power-law scaling of strength with size is due to an approximate power-law distribution of the initial dislocation mesh lengths, which also appears to be a robust feature in deformed metals. Furthermore, for a given metal, it is the exponent q of the initial mesh-length distribution which determines the value of the exponent m in the size dependence of strength.