Microscale plastic anisotropy of basal and pyramidal I slip in pure magnesium tested in shear

An optimised micro-shear testing protocol was adopted to measure the critical resolved shear stresses for basal and pyramidal I slip systems in pure magnesium. The micro-shear samples are carefully aligned for basal and pyramidal I slip by electron backscatter diffraction and fabricated by focussed ion beam milling. In situ scanning electron microscopy based shear testing identified that the two different sample orientations lead to activation of basal or 〈c+a〉pyramidal I slip, respectively. The critical resolved shear stress for basal slip was found to be 57 ± 19 MPa, and 371 ± 81 MPa for pyramidal I slip, albeit for slightly different geometric dimensions. Accounting for sample size-dependent flow stress for basal slip, we found that the plastic anisotropy with respect to pyramidal I slip is substantially reduced to a factor of 3 at the microscale compared to nearly a factor of 100 in the bulk. Multiple slip systems are therefore expected to operate in ultra-fine grain sized magnesium offering a pathway for improving ductility.