Deformation Behavior of Mg-Al Single Crystals

Abstract

Due to the high specific strength, stiffness, low density and so on, magnesium alloys have been attracted by many industry sectors, like automobile, 3C products. However, it expresses a poor ductility and formability owing to the hexagonal close packed crystal structure (HCP) which results in the limited independent slip systems at room temperature. This limits the further application and development. Therefore, how to improve mechanical property of Mg alloy has a great significance. In this paper, the pure Mg and Mg-Al alloys obtained by the directional solidification technology are tested to eliminate the mutual influence of the grain orientation and twinning behavior on the deformation mechanism in a single crystal. Besides, the activity of slip/twinning and strain hardening behaviors are simulated by VPSC so as to provide more theoretical basis of plastic deformation for Mg-Al alloy.

In this experiment, pure Mg and Mg-Al alloy single crystal are obtained by Bridgeman method. The microstructures are measured by means of optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), electron backscattered diffraction (EBSD). The mechanical properties are tested by standard tensile test. The activity of deformation mode and strain hardening are simulated by the visco-plastic self-consistent simulation (VPSC). The results can be summarized as follows:

At the optimal condition of 780oC, 1.3mm/h, a large size of Mg-Al (0, 0.2%, 0.6% and 1.0%) single crystals can be achieved. For the orientation A (loading direction is parallel to [-40 20 20 37]) tensile test, the flow stress and the yield strength are very low which are not sensitive to temperature from 298K to 698K. The yield strength increases with the Al concentration increases. The slip traces inclined by 45° can be seen in all tensile specimens over the whole temperatures. This indicates that basal <a> slip play an important role during the deformation.

For the orientation B (loading direction is parallel to [0 -1 1 0]) tensile and compressive test, multiple deformation modes (unfavorable slip and twin mode) are contributed to the plastic deformation. The yield strength decreases sharply with temperature increasing. The prismatic slip was also observed by the potential analysis at high temperatures (693 K) with [0 -1 10 0] crystal orientation. At the same temperature, the volume fraction of tensile twins increases as Al concentration increasing which indicates that the tensile twin can be activated by Al concentration.

The CRSS value of the major slip and twinning modes for Mg and Mg-Al single crystals undergoing tensile and compressive deformation have been systematically analyzed. In Mg-Al alloys with 0, 0.2, 0.6 and 1.0 at% Al, it is expected that the Non-basal/Basal CRSS ratio is significant reduced by strengthening of the CRSS for basal slip , which is the relevant quantity for assessing constraint stress development. For binary Mg-Al alloys with 0, 0.2, 0.6 and 1.0 at% Al, thus reducing Non-basal/Basal CRSS ratio which could active the non-basal slip easily significantly.

Keywords: Mg-Al alloy, Single Crystal, Mechanical Properties, Microstructure, Texture, VPSC Simulation

Student Number: 2016-29123