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

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 mu m, 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 (tau(i)) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of tau(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 similar to 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.

Automated summary

To study the influence of dislocation substructures on the size-dependent strength of micron-sized metals, single-crystal cylindrical micropillars with different diameters ranging from 1 to 10 μm were fabricated on fully annealed and cold-rolled high-purity aluminum samples. The annealed micropillars showed a size dependence of the resolved shear stress, while the correlation was weakened in the cold-rolled specimens. The presence of fine dislocation substructures introduced by cold-rolling contributed to the reduction in size-dependent strength, which can be explained using a stochastic model of dislocation source length and the assumption of homogeneously distributed dislocations in the experimental micropillars.