鎂-2、5、8(wt.%)錫合金經等通道轉角擠型後 其顯微組織暨室溫與高溫機械性質之研究

Abstract

This study focused on the microstructures and mechanical properties of Mg-2, 5, 8(wt. %)Sn alloys at high temperatures after Equal Channel Angular Extrusion (ECAE).The results showed that the Mg2Sn phase increased gradually with the addition of tin. The grain boundary of the Mg-2, 5, 8(wt. %)Sn alloys was seen to contain continuous eutectic α-Mg + Mg2Sn precipitates in the Mg-5(wt. %)Sn and Mg-8(wt. %)Sn as-cast alloys. Besides the grain boundary particles, ultra-fine Mg2Sn particles were observed within the matrix of Mg-2, 5, 8(wt. %)Sn alloys. After ECAE four passes, the effect of grain refining was very obvious. For example, the average grain size of Mg-5(wt. %)Sn alloy could refined from 147 μm to 28 μm after ECAE four passes. The continuous eutectic α-Mg + Mg2Sn were broken by ECAE four passes. Optimum tensile properties were attained by adding 5(wt. %)Sn. The UTS of as-cast increased from 67 MPa to 179 MPa at 200 °C, as well as the elongation increased from 10.6 % to 18.9 % at 200 °C after ECAE four passes. The Mg-5(wt. %)Sn alloy was extruded six passes by using ECAE process. The superplastic ability of Mg-5(wt. %)Sn alloy has been investigated. The result showed that average grain size was refined from 147 μm (as-cast) to 10 μm (N = 6); the continuous eutectic α-Mg + Mg2Sn precipitates were broken and distributed more uniformly in the matrix after ECAE six passes. The Mg-5(wt. %)Sn alloy demonstrated that an elongation of 550 % has been obtained at 350 °C with a strain rate of 1 × 10-3 s-1. Further, an elongation of 238 % has been obtained with a high strain rate of 1 × 10-2 s-1 at the same temperature. Those results mean that the Mg-5(wt. %)Sn alloy not only has the best UTS at high temperatures, but also has ability of superplasticity. Moreover, this study tried to use in succession Solution Heat Treatment (SHT) and ECAE four passes extrusion on the Mg-5(wt. %)Sn alloy. The results exhibited that the average grain size was significantly refined from 147 μm (as-cast) to 16 μm by SHT + ECAE four passes. The degree of grain refinement was larger than that of none SHT process of Mg-5(wt. %)Sn alloy. Furthermore, the fine Mg2Sn particles were uniformly distributed in the matrix by dynamic precipitation during ECAE process. The UTS of as-cast increased from 67 MPa to 209 MPa at 200 °C, as well as the elongation increased from 10.6 % to 22.1 % at 200 °C after SHT + ECAE four passes. Those results showed that SHT + ECAE process was useful to enhance the high temperatures mechanical properties of Mg-5(wt. %)Sn alloy.