Role of microstructural heterogeneities in damage formation and fracture of oligocrystalline Mg under tensile loading

Abstract

This paper describes the main results from in-situ and ex-situ testing and characterization studies to reveal the effects of microstructural heterogeneities that develop during plastic deformation of coarse-grained Mg on damage formation and fracture of the material. Experiments are carried out using a micro-test tensile stage in and outside a scanning electron microscope in combination with electron backscatter diffraction (EBSD), micro X-ray computed tomography (μXCT), and optical microscopy. It is seen that the micro-specimens exhibit significant macroscopic shape changes upon interrupting the tests. Such shape changes result from microstructural changes and underlying relaxation of inter-granular back-stress accumulated during loading. Twins with low Schmid factor (SF) for basal slip in grains with low SF for basal slip are observed to de-twin. Upon de-twinning, dislocations are found to remain in the structure as is evident by increase in the kernel average misorientation (KAM). Interestingly, twins nucleate in these regions again with continuous forward straining. In contrast, twinned domains favorably oriented for basal slip do not de-twin but thicken with plastic straining. These twins soften the material leading to localization of deformation and damage nucleation. At the interface between twinned domains and parent grains voids are observed. Voids nucleate due to lack of slip transfer causing pile-ups, as is evident from high KAM. Additionally, twins are observed to induce a substantial surface roughness. Large twins accommodating significant shear strain form surface ridges. Such significant heterogeneities also result in nucleation of voids from the surface. In addition to twinning induced voids, damage accumulates and voids form and propagate along high-angle grain boundaries between grains with low SF or those with disparate SF for basal slip. μXCT reveals voids away from the fractured surface highlighting the extent of heterogeneous deformation and fracture behavior of Mg. Fracture surfaces exhibit characteristics of mixed ductile-brittle fracture with signs of trans-granular cleavage and fluted facets with minor content of inter-granular fracture.