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

本研究旨在探討Mg-2、5、8(wt. %)Sn合金，透過等通道轉角擠型(ECAE)後，其顯微組織與高溫機械性質的變化。實驗結果顯示，隨錫含量的增加，合金中Mg2Sn相的量亦隨之增加，且可在as-cast Mg-5(wt. %)Sn及Mg-8(wt. %)Sn合金的晶界上發現高連續性的α-Mg + Mg2Sn共晶相。除該共晶相外，極細小的Mg2Sn顆粒亦可在三種合金的基地中同時發現。經ECAE四道次擠製後，合金晶粒細化的效果非常明顯，以Mg-5(wt. %)Sn合金為例，其平均粒徑可由鑄態之147 μm細化至28 μm且高連續性的α-Mg +Mg2Sn共晶相已被ECAE所破斷。 高溫拉伸性質以Mg-5(wt. %)Sn合金最佳，該合金經ECAE四道次擠製後，其高溫(200 °C)抗拉強度可由鑄態的67 MPa提升至179 MPa，而高溫(200 °C)的伸長量亦可由10.6 %提升至18.9 %。 本研究再進一步將常溫及高溫機械性質最佳之Mg-5(wt. %)Sn合金施以等徑通道轉角擠型進行六道次擠製並調查該合金之超塑性成型能力。實驗結果顯示，Mg-5(wt. %)Sn合金經ECAE六道次擠製後，其平均粒徑可由鑄態之147 μm加以細化至10 μm。而連續性極高之α-Mg + Mg2Sn的析出相更被完全破斷且均勻分佈在鎂基地中。 而Mg-5(wt. %)Sn合金於350 °C 及應變速率 1 × 10-3 s-1的條件下，可獲得550 %的巨大變形量，同樣溫度下再將應變速率提升至1 × 10-2 s-1時，亦可獲得238 %的變形量，此亦說明Mg-5(wt. %)Sn合金除具有優良的高溫抗拉強度外亦同時具備超塑性的成型能力。 最後，本研究更針對Mg-5(wt. %)Sn合金先後進行固溶熱處理及ECAE四道次擠製，實驗結果顯示，Mg-5(wt. %)Sn合金分別經固溶熱處理及ECAE製程後， Mg-5(wt. %)Sn合金其平均粒徑將由鑄態的147 μm細化至16 μm，晶粒細化的幅度超過未施以固溶熱處理之Mg-5(wt. %)Sn合金，而細小的Mg2Sn顆粒更於EACE擠型過程中以動態析出的方式均勻的分佈於基地之中。 再就高溫拉伸性質而言，其高溫(200 °C)的抗拉強度則由67 MPa進一步提升至209 MPa，而高溫(200 °C)的伸長量亦由10.6 %提升至22.1 %。顯示合金預先施以固溶熱處理後再進行ECAE製程，確實對Mg-5(wt. %)Sn合金之高溫拉伸性質有所助益。

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.