等通道轉角擠型製程對ZA85鎂合金之顯微組織與機械性質改善之研究

Abstract

本研究係以重力澆鑄法製成ZA85 (Mg–8 wt.% Zn–5 wt.% Al) 鎂合金鑄錠，接著分別將此鑄造材及經過固溶熱處理(solution heat treatment, SHT)的ZA85鎂合金經由等通道轉角擠型(equal–channel angular extrusion, ECAE)以改善合金的顯微組織與機械性質。研究結果顯示，高溫下對ZA85鎂合金進行ECAE，其晶粒細化的機制為動態再結晶，ZA85鎂合金鑄造材的初始晶粒可從150 μm 大幅細化至 4 μm。在晶界上的Mg32(Al,Zn)49 (τ–phase)不連續晶出相尺寸亦從100 μm 被剪切至 1 μm，且此細小的τ–phase顆粒均勻的分布在動態再結晶的晶界上。在室溫機械性質的部分，試片在經過ECAE後，其最大拉伸強度(ultimate tensile strength, UTS)及降伏強度(yield strength, YS)可分別從鑄造材的175及131 MPa提升至402及281 MPa；在200 °C高溫機械性質的部分，經過ECAE製程的試片其UTS及YS亦分別從鑄造材的105及74 MPa提升至249及162 MPa。此機械性質的顯著提升歸因於大幅細化的晶粒以及均勻分布在動態再結晶晶界上的細小τ–phase顆粒。 另一方面，經過SHT的ZA85鎂合金其顯微組織顯示幾乎所有的不連續晶出相τ–phase皆溶回鎂基地內，晶粒大小相較於鑄造材些微長大至170 μm。經過兩階段的ECAE製程後，平均晶粒大小可大幅細化至4 μm，未完全固溶回鎂基地的τ–phase被剪切至1 μm且均勻的分布在動態再結晶的晶界上，此外，亦可發現有許多平均尺寸約為100 nm的細小析出物τ–phase均勻的分布在鎂基地內，此細小的析出物是在ECAE製程中發生動態析出所產生。從拉伸試驗結果可發現，藉由SHT + ECAE製程可進一步提升ZA85鎂合金的機械性質。在室溫以及200 °C的環境下，ZA85鎂合金的UTS及YS可分別提升至415 MPa/284 MPa及261 MPa/173 MPa。此強化的結果歸因於晶粒細化、析出強化以及細小且均勻分布的高溫穩定相τ–phase。 經過ECAE製程的ZA85鎂合金除了可以大幅提升強度外，延性亦能獲得大幅的改善，在適當的溫度及應變速率範圍內，本研究結果發現經過ECAE的ZA85鎂合金具由低溫超塑性(low temperature superplasticity, LTSP)以及高應變速率超塑性(high strain rate superplasticity, HSRSP)。LTSP的部分，在300 °C，應變速率為1.0 × 10-3 s-1以及 1.0 × 10-4 s-1的測試條件下，ZA85鎂合金的伸長量分別為147%及400%；在250 °C，應變速率為1.0 × 10-4 s-1的測試條件下，伸長量可達205%。HSRSP的部分，在400 °C，應變速率為1.0 × 10-2 s-1的測試條件下，伸長量可達113%。進一步探討材料的變形機制，ZA85鎂合金在300及350 °C，應變速率為1.0 × 10-3 s-1以及 1.0 × 10-4 s-1的測試條件下，其變形機制為晶界擴散控制的晶界滑動，在更高溫的400 °C，其變形機制轉換成差排潛變。

In this study, the as–cast and solution–heat–treated Mg–8 wt.% Zn–5 wt.% Al (ZA85) alloys were subjected to the equal–channel angular extrusion (ECAE). The microstructural evolutions and tensile properties of the experimental alloys were investigated. In the as–cast ZA85 alloy, the initial grain size and precipitate size of 150 and 100 μm were greatly reduced to 4 and 1 μm, respectively, after the ECAE process. The grain–refinement mechanism of the experimental alloy fabricated by the ECAE process is dynamic recrystallization. At room temperature (RT), the ultimate tensile strength (UTS) and yield strength (YS) of the ECAE processed specimens were 402 and 281 MPa, respectively, compared with 175 (UTS) and 131 MPa (YS) for the as–cast specimens. At 200 °C, the UTS and YS of the ECAE processed specimens improved to 249 and 162 MPa, respectively, compared with 105 MPa (UTS) and 74 MPa (YS) for the as–cast specimens. This improvement in tensile properties of the ZA85 alloy was attributed to the refined grains and the well–distributed fine Mg32(Al,Zn)49 (τ–phase) precipitates. In order to further improve the mechanical properties of the ZA85 alloy, the as–cast ZA85 alloy was subjected to solution heat treatment (SHT). Dynamic precipitation was then induced using two–step ECAE process. After the SHT process, almost all the non–continuous τ–phase dissolved into the α–Mg matrix and the average grain size slightly increased to 170 μm. After six ECAE passes, the average grain size was greatly reduced to 4 μm, and fine τ–phase particles with ~100 nm in size were uniformly distributed in the α–Mg matrix by dynamic precipitation. The combination of SHT + ECAE process was demonstrated to greatly improve the tensile properties of the experimental alloy. By testing over a range of temperatures, the maximum ultimate tensile strength and the yield strength of 415 MPa/284 MPa and 261 MPa/173 MPa were obtained at RT and 200 °C, respectively. The strengthening factors for the SHT + ECAE alloy are the grain refinement, precipitation hardening, and presence of fine and well–distributed τ–phase particles. It was also demonstrated that ECAE processing produces superplasticity. By testing over a range of temperatures and strain rates, the ECAE processed ZA85 alloy exhibits both low temperature superplasticity (elongations of 147% and 400% at 300 °C with initial strain rates of 1.0 × 10-3 s-1 and 1.0 × 10-4 s-1, respectively; an elongation of 205% at 250 °C with the initial strain rate of 1.0 × 10-4 s-1) and high strain rate superplasticity (an elongation of 113% at 400 °C with the initial strain rate of 1.0 × 10-2 s-1). The dominant deformation mechanism for the specimens tested at 300 and 350 °C with the initial strain rates ranging from 1.0 × 10-4 s-1 to 1.0 × 10-3 s-1 is GBS controlled by grain boundary diffusion. At the higher testing temperature of 400 °C, the deformation mechanism for the experimental alloy is dislocation creep.