Influence of Dislocation Substructure on Size-Dependent Strength of High-Purity Aluminum Single-Crystal Micropillars

Abstract

In order to understand the influence of dislocation substructures on the size-dependent strength (smaller is stronger) of micron-sized metals, we have fabricated single-crystal cylindrical micropillars with various diameters approximately ranging from 1 to 10 µm, which were prepared on the surface of the fully annealed sample and the subsequently cold-rolled samples of high-purity aluminum (Al). The annealed micropillars exhibited a size dependence of the resolved shear stress required for slip. The shear stress (τ_i) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of τ_i/G = 0.33(d/b)^−0.63. The size-dependent strength was reduced by cold-rolling, resulting in lower power-law exponents (0.26∼0.31) for the correlation in the cold-rolled specimens. The fine dislocation substructures introduced by the cold-rolling could be associated with the reduced size-dependent strength, which can be rationalized using the stochastic model of the dislocation source length in an assumption of homogenously distributed dislocations existing in the experimental micropillars. The inhomogeneous dislocation substructure with various dislocation cell sizes would contribute to a variation in the measured strength depending on the location, likely due to the probability of exiting the dislocation cell walls (local variation in dislocation density) in the micropillars fabricated on the cold-rolled Al samples.

Fig. 9 (a) Experimental resolved shear stress required for initial slip (τ_i) as a function of the micropillar diameter (d), for the 15% cold-rolled micropillars, compared with the calculated values using different dislocation densities (ρ). (b) Schematic illustrations showing single-crystal micropillars with different diameters (d) fabricated on the grains with the dislocation substructure.