退火溫度對Mg-9wt.%Li-3wt.%Al-1wt.%Zn合金軋延板材微觀組織與機械性質影響之研究

Abstract

In this study, the effect of annealing temperature on microstructure and mechanical properties of as-rolled Mg-9wt.%Li-3wt.%Al-1wt.%Zn (LAZ931) alloy sheet was studied. As-extruded LAZ931 alloy plates was rolled from 3mm to 1mm in thickness(67% of reduction ratio), and then annealed at the temperature in the range of 100℃ to 350℃ with increasing steps of 50℃ for the 1 minute to 2 hours. The mechanical properties of the LAZ931 alloy sheet were examined by tensile and Vickers hardness tests. The crystalline structure and microstructures were examined by X-ray diffractometer (XRD) and field-emission scanning electron microscopy (FESEM), respectively. The elongation of the as-rolled LAZ931 alloy sheet increased from room temperature to 150℃, and then decreased at temperature higher than 200℃. In particular, the specimens had remarkably high tensile strength and extremely low elongation (only about 0.5%) at temperature higher than 300℃. The results of vickers hardness testing indicate that hardness of β phase (bcc Li) increases by quenching after isothermal holding for 2 hours at a temperature higher than 200℃, and show similar trend with tensile strength. This means that β phases is the key phases after annealing treatment. According to the XRD and the microstructural observations, the growth of brittle AlLi phase lead to the decreasing of elongation at temperature higher than 200℃. In addition, the AlLi phase dissolved in bcc Li matrix at temperature higher than 300℃. The dissolution of the AlLi phase into the β phase (bcc Li) matrix maybe was the reason lead to the increasing of tensile strength at temperature higher than 300℃. The phenomenon of annealing embrittlement (annealing temperature higher than 300℃) could be related to the formation of low ductility α phase (hcp Mg) and brittle AlLi phase in the grain boundary of bcc Li phase.