| The development and application of magnesium(Mg) alloy has become the important research field in the world currently. However, the plastic formability and press working formability of Mg alloy are poor, which limits its further application and development. Mg has limited independent slip systems at ambient temperature due to its hexagonal close packed structure. Therefore, it is of great significance to research the plastic deformation process to improve the mechanical property of Mg alloy. The pure Mg and Mg-Sn alloy obtained by the directional solidification technology are tested in this article, which can eliminate the mutual influence between the grains and research the grain orientation and twinning behavior in a single crystal during the stretching test. Also, the internal relationships between slip/twinning and the yield strength, elongation and strain hardening behavior will be discussed to provide some theoretical basis of plastic deformation for Mg alloy.In this experiment, the falling speed of directional solidification furnace was controlled at 0.2mm/min to obtain the pure Mg and Mg alloy ingot in order to research the microstructure, mechanical properties, grain orientation and so on. The investigation was carried out by the means of optical microscope, scanning electron microscope, X-ray diffraction, electron backscattered diffraction and mechanical property. Following research results were obtained:(1) The mechanical property difference between the Mg single crystal and Mg dual-crystal is small, but the twinning morphology is different after stretching. In Mg dual-crystal, the twins were gathered around the grain boundary like leaves due to the stress concentration and grain orientation difference. However, the twins observed in Mg single crystal were distributed randomly in the matrix of Mg. Also, it can be concluded that slip deformation was dominated in the early stage of plastic deformation, then twinning deformation was dominated in the late stage of deformation which can retard even improve the decline trend of work hardening rate the work-hardening rate of the material.(2) The strain accommodation factor(m') can be provided to test the twinning mode selection in Mg dual-crystal. When a specific twin variant was stimulated, the {10-12} twinning variants can be selected according to the m' value across the grain boundary. Also, the operating twin variants can be investigated by m' in a single crystal.(3) In the terms of mechanical properties of Mg-Sn alloy, the yield strength of Mg-0.1Sn is the highest and the elongation is the smallest among the three contents of Mg-Sn alloy. The yield strength in parallel and vertical direction of Mg-0.1Sn are 23.7MPa and 19.9MPa, respectively. The yield strength decreased and elongation rates increased when the content of Sn was obtained to 0.5%, but there has no distinct yield strength difference between the two directions(9.84 MPa and7.82MPa). Compared with Mg-0.5Sn, the yield strength of Mg-0.85 Sn increased a little bit and the elongation rate was close to Mg-0.5Sn. However, there has a distinct yield strength difference between parallel and vertical direction, which is 16.37 MPa and 23 MPa. In general, the mechanical property difference in parallel and vertical direction of Mg-0.1Sn is the smallest because the Schmid Factor(SF) is 0.176 and 0.288, respectively.(4) In the terms of grow direction, Mg-0.1Sn and Mg-0.85 Sn single crystal were formed almost paralleled with the C axis, and the external stress direction was close to the heat flow direction, which can be contributed to stimulate twins, as the SF of Mg-0.1Sn and Mg-0.85 Sn are 0.482 and 0.174 under the tensile stress along the Y axis. However, there had some deflection angle between the C axis when Mg-0.5Sn single crystal formed in the directional solidification process. As a results, the twins were influenced more by the compressive stress along the X axis or Z axis while the SF value along Y axis are negative. |
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