Size- and strain rate-dependence of nickel and Ni-Co micropillars with varying stacking fault energy

The effect of the specimen size on the mechanical characteristics of material, a phenomenon commonly referred to as the "size effect", becomes increasingly important at micron and sub-micron dimensions of the specimen. However, the majority of size effect studies on face-centered cubic (fcc) metals have focused on pure elements, where deformation is accomplished by dislocation glide. In this work, single crystalline Ni - x at. % Co (with x = 0, 30, 50) solid solution micropillars were studied by mechanical testing and transmission electron microscopy (TEM) to elucidate the role of stacking fault energy on the size effect in Ni as a function of Co-addition. In situ strain rate jump (SRJ) microcompression testing allowed the evaluation of strain rate sensitivity and activation volumes for all pillar sizes at different strain rates, spanning over three orders of magnitude. TEM analysis showed that for an increase in Co content up to 50%, the SFE decreased from similar to 85 to similar to 28 mJ/m(2). We observed a l l Ni - x at. % Co micropillars to exhibit consistent size effect trends (n = (similar to)-0.7), independent of composition and yield criterion, and similar trends between activation volume and different sizes: activation volume increases with grain size/pillar size.