Effect of grain size on fracture behavior of pure magnesium sheet: An in-situ study combined with grain-scale DIC analysis

Abstract

In this work, a series of in-situ experiments were conducted to study the effect of grain size on fracture behavior of commercial-purity Mg polycrystalline aggregates. For this purpose, pure Mg rolled at 225 °C was annealed at 325 °C and 525 °C, each for 1 h, to acquire different grain structures. Micro specimens of the as-rolled and anneal-treated sheets were subjected to tension in the rolling direction using a micro-tensile loading stage integrated with an optical microscope to discover micro-mechanisms triggering failure at room temperature. Moreover, grain-scale DIC analysis was applied to the optical images taken during tension to quantify strain heterogeneity and plastic anisotropy coefficient in the specimens. It was found that the dominant failure mechanism changes by grain size. In the fine grain sample with an average grain size (AGS) of 13.3 μm, shear bands developed during tension played an important role in the damage. In the sample with intermediate grain size (AGS = 36.1 μm), intergranular fracture took place along surface steps that stemmed from slip-induced grain boundary sliding. In the sample with the largest grains (AGS = 137.2 μm), twin cracking as well as grain boundary decohesion were the primary mechanisms by which failure proceeded. Micro DIC analysis revealed that the formation of steps and twins was accompanied by heavy strain localizations in the microstructure, making them susceptible sites to fracture.