*2.1. Materials*

The materials used for bi-crystalline micro-pillars are Ni with very high purity (99.999%) and *α*-Brass (70%Cu-30%Zn, wt%) with impurities (Fe, Pb, P and As, etc.) less than 0.001%. The elastic stiffness constants of Ni are *C*<sup>11</sup> = 246.5 GPa, *C*<sup>12</sup> = 147.3 GPa, *C*<sup>44</sup> = 124.7 GPa [30] and those of *α*-Brass are *C*<sup>11</sup> = 139.21 GPa, *C*<sup>12</sup> = 104.51 GPa, *C*<sup>44</sup> = 71.3 GPa [31,32]. Thus, these two materials have an elastic anisotropy Zener ratio *A* = 2*C*44/ (*C*<sup>11</sup> − *C*12) of the order of 2.51 and 4.11, respectively. Furthermore, the stacking fault energies for *α*-Brass and Ni are about 14 mJ/m<sup>2</sup> [33] and 90 mJ/m2 [34], respectively. The stacking fault energy of *α*-Brass is low which promotes planar slip and thus facilitates the observation of localized slip. Although the stacking fault energy of Ni is higher, planar slip was still well observed in Ni bi-crystals during compression tests [35].
