| 摘要: | A201鋁合金(Al-4.6Cu-0.3Mg-0.6Ag)具有高強度與熱穩定性，已廣泛應用於航太與軍事工業。A201鋁合金含銀量介於0.4 wt.%至1 wt.%之間，除原有之θ′相之外，可析出具熱穩定性之強化相Ω，使合金具有極為優良的常溫與高溫(200℃以下)強度，此外在銅含量固定下鎂含量可影響析出相的種類與分布情形(Ω、S′與θ′相)，故銀與鎂含量皆為影響Ω析出的重要合金元素。應用於航空設備之高強度Al-Cu鋁合金易受應力腐蝕而破壞，因此A201鋁合金之應力腐蝕敏感性極需重視，而合金的熱處理狀態則會影響應力腐蝕的性質，此外航空器材常須承受高速與空氣摩擦所引發的高溫，故合金於高溫下之熱穩定性為合金相當重要的性質。 基於合金元素與熱處理對於A201鋁合金微結構與性質上的影響，因此本研究將探討合金元素與熱處理對於合金微結構、機械強度、應力腐蝕與熱穩定性之變化與效應，以完整呈現此一高強度鋁合金之特性。 由於銀為刺激Ω相析出的重要合金元素，於本論文中首先將藉由不同銀含量(0、0.3、0.6與0.9 wt.% Ag)之合金，施以頂時效(T6)、過時效(T7)以及回復與再時效(retrogression and reaging，RRA)等不同熱處理製程來探討銀含量對於A201鑄鋁合金強化析出相、機械性質與應力腐蝕敏感性的影響；爾後，再藉由不同銅/鎂比(分別為29、19、8與4)的熱擠製Al-4.6Cu-XMg-0.6Ag合金，探討合金於107、135與155℃下1000小時之熱穩定性。 本論文透過金相、導電度(Electrical Conductivity, %IACS)、微差熱掃瞄(Differential Scanning Calorimetry)與穿透式電子顯微鏡(Transmission Electron Microscopy)等微結構之觀察與分析，配合硬度(HRB)、拉伸與應力腐蝕等合金性質上的試驗，獲得以下結論：Ω相析出量與機械強度隨銀含量增加而增加，但是銀含量高達0.9重量百分比時卻出現抑制θ′相之情形；銀含量不會影響晶界析出物，但高銀含量合金會增加富銅之強化相（Ω與θ′相）整體析出效果，導致應力腐蝕敏感性增加。相較於過時效(T7)熱處理之合金，經頂時效(T6)處理後合金無析出帶較窄，造成明顯的面積效應，導致應力腐蝕性提高，而回復與再時效(RRA)熱處理無法明顯改變晶界析出物型態或降低面積效應，故未能降低鑄造A201鋁合金應力腐蝕敏感性；高鎂含量之合金(即低銅/鎂比)，產生大量之銀-鎂(Ag-Mg co-cluster)聚集，引發Ω相大量析出，故此合金擁有高強度與高熱穩定性。 A201 aluminum alloy (Al-4.6Cu-0.3Mg-0.6Ag) has been widely applied in the aviation and military industries due to its very high mechanical strength and thermal stability. The silver containing, between 0.4 to 1 wt.%, is unequal characteristic of A201 alloy that change the precipitated sequence and lead to trigger precipitation of the thermal stable Ω phase along with θ′ phase of Al-Cu alloy. Due to the precipitated of Ω phase, A201 alloy possess execllent mechanical strength under 200℃. The concentration of magnesium affect the relative distribution of Ω, S′ and θ′ phases at constant copper containing that influence the thermal stability of alloy. Therefore, the concentration of silver and magnesium are both the important factor for precipitation of Ω phase. High strength Al-Cu alloy is known to be damanged by stress corrosion cracking. The heat treatment condition affect the stress corrosion cracking susceptibility of alloy. Moreover, the skin of a super sonic bear aero dynamic heating, thus, the thermal stsbility treat as an serious properties of A201 alloy. Due to the alloying elements and heat treatment significantly affect the microstructure and properties of A201 alloy, hence, present works attempt to discover the effect of alloying elements (silver and magnesium) and heat treatment on the stress corrosion cracking and thermal stability of A201 aluminum alloy. For the purpose of reveal the bottom of the effect on of silver and magnesium contain on precipitation of A201 alloy, present work start with made various quantity of silver (0 to 0.9 wt.%) containing alloy, subsequencely heat treated to T6, T7 and RRA condition. Second step of this investigation was performed on the different quantity of magnesium (0.15 to 1.1 wt. %) containing hot extrusion alloys. The alloys were long-term exposed at 105, 135 and 155℃ up to 1000 hours. Microstructural features were elucidated by optical microscopy, electron probe X-ray microanalysis, measurement of electrical conductivity and differential scanning calorimetry. The microstructure was correlated with Rockwell hardness and tensile testing. The stress corrosion cracking susceptibility of alloy was assessed by performing the slow strain rate test in air and salt solution. The results of present works revealed that the addition of Ag at a concentration of under 0.6 mass% promoted the precipitation of the Ω and θ' phases and the augmentation of hardness of T7 tempered alloys. 0.9 mass% Ag caused the extensive precipitation of the Ω phase, but only mildly suppressed the precipitation of the θ' phase, slightly increasing the hardness. The high density of the precipitates of the Ω phase is responsible for the excellent thermal stability under 185℃ exposure for 100 hours and mechanical properties. The Ω precipitates out before the θ' phase, and does so more quickly, during the aging process. The continuous grain boundary precipitations and the high Ag concentration alloy that exist high density of the Ω and θ' phases caused susceptibility to high stress corrosion cracking. The presence of a wide precipitation-free zone and discontinuous large particles in the grain boundary precipitates caused the T7 alloy to have a low susceptibility to stress corrosion cracking. The alloys tempered to the retrogression and reaging condition cannot reduce susceptibility of the Al-Cu-Mg-Ag alloy to stress corrosion cracking. Decreasing the Cu/Mg ratio of alloy by furthermore addition of Mg leads Ω to become major strengthening phase after ageing treatment. Therefore, the hot extrusion process did not bother the precipitated of Ω phase. However, the 1.1 wt. % Mg contain alloy was too hard and brittle lad to serious cracking occurred following hot extrusion. |
