13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Investigation of Void Linkage in Magnesium Using SEM and Micro Computed X-ray Tomography Michael J. Nemcko1,*, David S. Wilkinson1 1 Department of Materials Science and Engineering, McMaster University, Hamilton L8S 4L8, Canada * Corresponding author: nemckomj@mcmaster.ca Abstract Ductile fracture in metallic materials occurs by the nucleation, growth, and linkage of microvoids within the bulk of the material. As a result, two dimensional techniques must be complimented with three dimensional techniques in order to completely characterize the fracture process. In the present study, tensile testing coupled with: scanning electron microscope (SEM) imaging, electron backscattered diffraction (EBSD) patterning, and micro computed x-ray tomography (XCT), have been used to analyze void linkage in magnesium, which exhibits poor formability at room temperature. SEM imaging and EBSD patterning have been used to characterize the mechanisms responsible for void linkage and to determine the effects of void fraction and void orientation on the failure strain. Micro XCT has been used to examine the evolution of internal voids. It has been established that void fraction and void orientation have a weak influence on the failure strain due to the premature linkage of voids. EBSD analysis has shown that this premature void linkage is associated with the failure of twin and grain boundaries. The results suggest that (in contrast with more ductile fcc metals) the local microstructure has a significant impact on void linkage. Keywords Magnesium, Void Linkage, Micro computed tomography 1. Introduction Magnesium exhibits limited ductility when deformed at room temperature. The HCP crystal structure of the material does not provide an adequate amount of slip systems to satisfy the von Mises criterion. As a result, mechanical twins are activated to accommodate stress concentrations [1]. Void nucleation typically occurs within the bulk of the material. As a result, characterization techniques which can detect internal flaws are required to fully understand fracture. In the past, model materials were fabricated to better understand void growth and coalescence [2]. The 2D model materials consisted of thin sheets containing laser drilled holes through the thickness which simulated voids [3]. These model materials were pulled in tension within the SEM such that the growth and linkage of the holes could be analyzed in increments of deformation. FCC materials (aluminum and copper) were studied which are more isotropic than HCP materials. The results showed that void growth was uniform. Linkage of the voids occurred by the internal necking mechanism observed by Puttick [4]. Furthermore, experiments were carried out to determine the effects of the void fraction and void orientation on the failure strain. It was concluded that both parameters had a strong correlation with the failure strain. The experiments were then extended to 3D to determine if the 2D experiments were representative of what occurs in the bulk [5]. 3D model materials were fabricated by diffusion bonding multiple sheets containing laser drilled holes between hole free sheets. These were used to simulate a 3D distribution of internal voids. The 3D model materials were tested in situ. The voids in the material are resolved using x-ray tomography from a synchrotron source based on the difference in the attenuation coefficient between the metal and the void. The use of tomography in materials science has significantly increased in the past few years [6]. In the present work, tomography is coupled with EBSD and SEM to understand the fracture of magnesium materials which display mechanical anisotropy.
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