Effect of Crystal Orientation on Nanoindentation Behavior in Magnesium
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TRODUCTION
SINCE magnesium has a significant difference in slip activation stress between the basal hai planes, the non-basal prismatic hci and pyramidal hc+ai planes, prismatic and pyramidal slip are difficult to activate at room temperature. As a result, twinning can become active as an additional mode of deformation and to compensate for the lack of a hci component in slip. Many types of twinning, e.g., {1011}-, {1012}-, {10 13}-twinning, and {1011}-{1012} double twinning, have been observed in magnesium.[1–3] The crystallographic relation of the twinning lattice is fixed, and the three single twin variants are rotated around the h1120i direction by 56, 86, and 64 deg, respectively. In addition, {10 11}- or {1012}-twinning occurs when compressive or tensile stress is applied along the hci-axis; thus, these are called hci-axis compression or tension twins, respectively.[4] Furthermore, {1012} twinning is most commonly found to occur since it has the lowest critical resolved shear stress (CRSS) among such twins. This type of twinning is well known to be significantly anisotropic, and gives rise to large mechanical asymmetry wherein the mechanical properties are influenced by the crystal orientation, i.e., basal plane orientation
HIDETOSHI SOMEKAWA, Senior Researcher, is with the Research Center for Strategic Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan. Contact e-mail: [email protected] CHRISTOPHER A. SCHUH, Professor and Department Head, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. Contact e-mail: [email protected] Manuscript submitted June 13, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
distribution. For instance, wrought-processed polycrystalline magnesium and its alloys, which tend to favor basal plane alignment parallel to the processing direction, show much lower yield strength in compression compared to that in tension due to the easy activation of this twinning system in compression.[4] The formation of twins under contact loads in magnesium is a more complex problem, but one that is relevant to indentation testing as well as many mechanical loading situations common in machine components. Crystal orientation is likely to play a significant role in contact loading much as it does in simpler uniaxial loading. Indeed, for other metals, many papers have reported the effect of crystal orientation on measured indentation responses, including mechanical properties such as hardness and modulus, as well as incipient plasticity; this is especially true for FCC and BCC metals studied in single crystal form (e.g., Cu,[5–8] Au,[9–11] Pt,[12] Al,[6,10,13] and Ag[14]). Significantly different morphologies in the indentation pile-up pattern are observed with orientation change, and these are related to the strong crystallographic anisotropy of out-of-plane displacements around indentations. The number of available slip planes also tends to affect the inde
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