| 摘要: | 本研究藉由顯微結構觀察、機械性質與電化學分析試驗等，探討微量銅 (0,03wt%、 0.21 wt%)與不同時效熱處理(T6, T76, T73, T77)對Al-4.9Zn-1.7Mg合金機械性質與抗應力腐蝕性質的影響。主要目標是獲得具有更好耐腐蝕性和機械性質的合金。結果顯示，合金中的Cu，會促進η’-MgZn2析出強化相的析出量，因而增強了合金強度，且微量銅並不會在固溶處理後，產生S-Al2CuMg相與Al7Cu2Fe相，對於機械性質不會造成額外的損害。
相較於T6處理，過時效(over aging, 如T76與T73)與T77(RRA)等熱處理，能使合金晶界粗大析出相(GBPs, grain boundary precipitates)呈不連續析出，降低了腐蝕環境下的陽極溶解，提升了合金之抗應力腐蝕性(SCC)，其中以T73態提升最為明顯。但經T76與T73處理之合金，因晶粒內析出強化相的粗化，導致合金強度的下降。而T77態也能使合金晶界粗大析出相呈不連續析出，隨回歸(retrogression)溫度的提升抗蝕性能有更進一步的提升，且經由再時效能使T77保留相當於T6狀態的強度。T77為兼顧抗蝕性與強度的組合。
由上述結果可知，含Cu (0.21 wt%)的合金比未另外添加Cu之合金具有更高的強度，綜合實驗結果顯示，Al-4.9Zn-1.7Mg含Cu (0.21 wt%)合金並施以T77-200熱處理，為能獲得最佳耐腐蝕性與機械性質組合。
;By means of microstructural observation, mechanical property tests, electrochemical analysis, this study is aimed at exploring effects of trace copper (0.03 wt%, 0.21 wt%) and various heat treatments (T6, T76, T73, T77) on the mechanical properties and stress corrosion resistance of Al-4.9Zn-1.7Mg alloys. The main objective is to obtain alloys with better corrosion resistance and mechanical properties. The results revealed that the addition of copper promoted the precipitation of the strengthening phase η’-MgZn2, thereby enhancing the strength of the alloys. Trace copper did not lead to the formation of S-Al2CuMg or Al7Cu2Fe phases after solution treatment, nor did it cause additional damage to mechanical properties.
Compared with T6 treatment, over-aging treatments (such as T76 and T73) and T77 (RRA) induced discontinuous precipitation of grain boundary precipitates (GBPs), which reduced anodic dissolution in corrosive environments and enhanced stress corrosion resistance (SCC). Especially with T73, it showed the most significant improvement. However, the strength of alloys treated with T76 and T73 decreased owing to the coarsening of the precipitation strengthening phase within the grains. The T77 state induces discontinuous precipitation of grain boundary precipitates (GBPs) and increasing the retrogression temperature further improves corrosion resistance. Additionally, re-aging allows T77 to retain strength comparable to the T6 state. T77 represents a combination that balances both corrosion resistance and strength.
Based on the findings, the alloy with 0.21 wt% Cu showed higher strength than alloys without added Cu. Overall, the best combination of corrosion resistance and mechanical properties was achieved with the Al-4.9Zn-1.7Mg alloy containing 0.21 wt% Cu, and by T77-200 heat treatment. |
