Crystallographic and Experimental Disproof of Pyramidal < c+a > Slip in Magnesium

Abstract

The activation of non-basal pyramidal < c+a > slip has been perceived as key to enhance the ductility of magnesium and its alloys. However, there has never been convincing evidence to show the physical existence of < c+a > dislocations and their involvement in deformation has been a core issue in magnesium research. In the present work, the impossibility of < c+a > slip is analyzed based on fundamental concepts of dislocation and atomic interactions. The atomic configurations and crystallographic features in association with < c+a > dislocations are unambiguously revealed for the first time, demonstrating that any possible < c+a > dislocation core structures would involve too many atoms on multiple lattice planes and are physically impossible. Experiments of magnesium single crystal compression along its c-axis were conducted at temperatures from 20°C to 500°C and the results showed no evidence of the involvement of < c+a > dislocations in any form as a mechanism of deformation during either plastic flow or fracture. von Mises criterion for compatible deformation, which drives the pursuit of pyramidal < c+a > slip, is critically discussed.

Keywords: Magnesium, pyramidal < c+a > slip, dislocation structure, compression test