Al-Cu系合金為典型的熱處理型鋁合金，常被使用於需要高強度的結構材料及航空工業，應用方面Al-Cu系合金延伸率至少須達10%以上。本實驗利用過熱固溶處理來控制2218(Al-4Cu)合金的拉伸延性及強度；過熱固溶處理的第一階段先施以高於傳統固溶化(T4)溫度，藉由改變此階段的溫度及時間，來控制2218合金中金屬間化合物的形貌，並造成熔融產生孔洞，使其延性提升，其條件為490°C ~560°C持溫5小時及535°C 持溫1~15小時；接著第二階段施以峰值時效熱處理，使其強度獲得提升。對經過熱固溶處理後的2218合金進行室溫拉伸試驗，並針對微觀組織變化和裂紋傳播機制做進一步的分析。
根據經490°C ~560°C持溫5小時過熱固溶處理的實驗結果推測，2218合金經550°C以上的過熱固溶處理後，由於產生過大的熔融孔洞，導致其拉伸強度相對於其他過熱固溶溫度的拉伸強度開始有顯著的下降。當過熱固溶溫度低於535°C時，Al7Cu4Ni分佈密集進而導致應力集中，使裂紋容易串聯傳播，導致延性下降；535°C時，由於Al7Cu4Ni因粗大化而使顆粒總數減少，顆粒間的平均距離增加並沿著晶胞界分佈，增加了裂紋傳播距離，此外，孔洞的產生可以緩和裂紋尖端的應力集中進而抑制裂紋的成長，延性因此提升；溫度高於535°C時，Al7Cu4Ni於晶胞界處呈連續性分佈，大量熔融產生巨大的孔洞，使得裂紋的傳播發生沿晶破壞使延性明顯下降。
2218合金在535°C持溫1~15小時，隨著持溫時間的增加，拉伸強度並不會產生顯著的變化。由於Ostwald Ripening的效應使Al7Cu4Ni逐漸粗大化而顆粒數減少，顆粒間之平均距離增加導致裂紋傳播距離增加，使得延性上升；當持溫5小時，由於適量大小的熔融孔洞緩和裂紋尖端的應力集中，使延性獲得最佳的提升；超過5小時的持溫時間，仍然是由粗大的Al7Cu4Ni主導裂紋傳播，少數過大的熔融孔洞並不會導致延性明顯下降。

Al-Cu alloys are heat treatable Al alloys and are often applied as the structural component and in the aircraft industries. With regard to the structural component, the tensile ductility of Al-Cu alloy must be more than 10%. Overheated solution treatment were carried out in this study. At the first stage, 2218 alloy was subjected in the overheated solution treatment to modify the intermetallic compounds and to induce voids to the matrix such that the tensile ductility of 2218 alloy could be improved. The selected temperatures of overheated solution treatment were higher than the solution treatment temperature (T4). Following the first stage, peak aging was conducted to the 2218 alloy for increasing the tensile stress of 2218 alloy. After overheated solution treatment ranging from 490°C to 560°C and peak aging, specimens were tested in uniaxial tensile tests. The tensile properties of specimens after overheated solution treatment which is 535°C for 1 to 15 hours were also discussed in this study. During tensile deformation, the microstructure evolution was observed to realize the mechanism of crack growth in 2218 alloy.
The specimen treated with 550°C overheated solution treatment had an obvious decrease in the tensile stress in comparison with specimens treated at other temperatures, which could be attributed to the coarsening of induced voids. When overheated solution treatment at less than 535°C, the clustering of Al7Cu4Ni particles made the connection between cracks more easy, which caused the decrease in tensile elongation of 2218 alloy. In 2218 alloy treated at 535°C, Al7Cu4Ni particles distributed on the cell boundaries and the amount of these particles decreased due to particles coarsening; therefore the average distance between particles increased. The distance of crack propagation was raised with increasing distance between particles. Besides, the stress concentration at the tip of the crack was released when the crack meet the void, which could inhibit the growth of the crack and raise the tensile ductility of 2218 alloy. With overheated solution treatment at more than 535°C, Al7Cu4Ni particles distributed along cell boundaries and the melting of second phases induced large voids in 2218 alloy, which caused intergranular fracturing, and therefore the tensile ductility of 2218 alloy declined.
After peak aging, the tensile stress of 2218 alloy remained with increasing the duration time of overheated solution treatment at 535°C. Al7Cu4Ni particles coarsened with increasing duration time as a result of Ostwald ripening, and subsequently the amount of particles was also reduced. With increasing the distance between particles, it was inhibited that cracks propagated from one particle to another one, and this phenomenon could raise the tensile ductility of 2218 alloy. With overheated solution treatment at 535°C for 5 hours, the morphology Al7Cu4Ni particles and distribution of void in 2218 alloy was suitable for tensile ductility improvement.

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