Experimental characterization of voids and surrounding microstructures developed under tension of Mg, Mg–3Zn, and Ti: A statistical study

TLDR: In this article , the formation of voids is driven by local microstructural heterogeneities, which are predominantly twins for Mg and predominantly grain fragmentations including shear bands for Ti.

Abstract: This paper presents the main findings acquired in an experimental investigation into where voids form in microstructures of pure Mg, Mg–3Zn alloy, and pure Ti after tension to near fracture. To facilitate a statistical study, 52 voids in Mg and 62 voids in Ti along with local microstructures surrounding them were observed using a high-resolution electron backscattered diffraction (EBSD) mapping. Distributions capturing shape and orientation of voids with respect to loading direction were created as these geometric features give rise to local stress concentrations. Microstructural evolution and, in particular, the formation of deformation twins as local heterogeneities in the structures was observed to significantly influence the formation of voids in both Mg and Ti since these metals accommodate plastic strains by profuse twinning and grain fragmentations in addition to slip. To this end, distributions featuring the number of twin families neighboring each void, specific twin family neighboring each void, and voids taking shape of prior twins were formed. The observations and distributions reveal that formation of voids is driven by local microstructural heterogeneities, which are predominantly twins for Mg and predominantly grain fragmentations including shear bands for Ti. While substantial number of voids neighbor at least one twin family in both Mg and Ti, voids are predominantly twin-like i.e., lamellar in Mg but predominantly spherical in Ti. While the lamellar voids in Mg are elongated in the direction of twins, which is typically greater than ±30° from a pulling direction, a minor content of elongated voids in Ti align with the pulling direction. Such statistical distributions along with qualitative observations are presented and discusses highlighting the comparisons between the two metals exhibiting substantial differences in ductility and fracture behavior.