Anomalous size-dependent strength in micropillar compression deformation of commercial-purity aluminum single-crystals

An anomalously reduced size-dependent strength of commercial-purity aluminum (Al) single-crystal micropillars with diameters ranging approximately from 1 to 10 mu m is reported. High-purity Al (99.99%) single-crystal micropillars exhibited an obvious size dependence of the resolved shear stress for slip. The measured shear stress resolved onto a primary slip system (tau(i)) scaled by the shear modulus (G) and the pillar diameter (d) scaled by the Burgers vector (b) showed the following correlation: tau(i)/G = 0.33(d/b)(-0.63), which agreed well with previous works. However, the commercial-purity Al samples exhibited a lower power-law exponent (0.19) for their size-dependent strength, resulting in (tau(i)/G) = 0.006(d/b)(-0.19). TEM characterization revealed the local presence of Al-Fe intermetallic precipitates surrounded by relatively high-density dislocations in annealed commercial-purity Al samples. These results indicate the relatively high-density dislocations could be responsible for the reduced size-dependent strength, which was confirmed by the remarkably reduced size dependence of their resolved shear stress by prior cold rolling. The reduced size-dependent strength can be rationalized using the stochastic model of the dislocation source length. Thus, this study provides a new insight to allow the application of the micropillar compression test to commercially produced Al alloys.