Abstract
7050 鋁合金是改進以往廣泛使用的 7075 鋁合金的缺點所設計的航空結構材料。 本研究利用滾壓作為熱機處理的變形方式，並以 T76、T73、RRA 等熱處理作為析出處理方法，來改變原有 7050 鋁合金的顯微結構， 希望能使 7050 鋁合金具有更優越的高強度及抗腐蝕能力。 原材料以 350 ℃做製程退火的冷滾壓，得到的晶粒大小為 dS =25μ m、dT=180 μ m、dL =220 μ m；而以 220 ℃熱滾壓，其晶粒大小為 dS =10μ m、dT =25 μ m、dL = 30 μ m。 再經過 T73、T76、RRA 熱處理的材料皆較 T7451- 原材料有較高的強度和伸長率。 在 3.5 wt% NaCl 水溶液的動態極化測試結果顯示， Initial Aging- 大晶粒的腐蝕電位為-1180 mVSCE，抗腐蝕能力最差； 而 RRA- 小晶粒的腐蝕電位為 -1050mVSCE， 抗腐蝕效果最好。 在剝蝕測試中，RRA及 T73 的小晶粒試片抗剝蝕能力是最好的， Initial Aging- 大晶粒試片的剝蝕情形最為嚴重，且所有的剝蝕都是沿晶腐蝕。 在 3.5 wt% NaCl 溶液中控制初始應變速率在 1.8x10^-6 /sec 做拉伸測試，其結果顯示， 強度和伸長率皆較空氣中為低且斷裂面有應力腐蝕的現象，從 SEM 觀察其斷裂面發現，所有細晶粒的材料和 Initial Aging- 大晶粒的試片應力腐蝕是沿晶斷裂，但對原材料和 RRA- 大晶粒的試片而言， 裂縫的成長則是穿晶斷裂，且應力腐蝕的破裂面上有許多相互平行的裂口停止條紋， 此意味著裂縫成長的機構並不是連續的，而是一步一步成長的過程。7050 aluminum alloy is an airframe structure material usedwidely by AIDC and developed with the goal of improving thedisadvantages of 7075 aluminum alloy. In this work, tworolling procedures and four types of heat treatments, namelyInitial Aging, T76, T73, and RRA, are used as deformingmethod and precipitation treatments for 7050 aluminum alloy.The grain sizes of specimens cold rolled and process annealedat 350 ℃ are dS =25 μ m, dT =180 μ m, dL =220 μ m and thosehot rolled at 220 ℃ are dS =10 μ m, dT =25 μ m, dL =30 μm. For all the T73, T76 and RRA treatments used in thisstudy, the material after thermomechanical treatmentscan have both higher strength and longer elongation thanthe as received material. From the result of thepotentiodynamic polarization curves of 7050 Al alloys in 3.5%NaCl aqueous solution, the corrosion potential of the InitialAging treated large grain specimen (-1180 mVSCE) is morenegative than the RRA treated small grain specimen (-1050mVSCE). It indicates that Initial Aging-large grain specimenhas the worst corrosion resistance, while the RRA-smallgrain specimen is the best. It is consistent with the resultof the exfoliation test. Intergranular corrosion was observedin the exfoliation test specimens. When tension tested in 3.5%NaCl solution at an initial strain rate of 1.8x10^-6/sec,both the strength and elongation of all materials werereduced. A region of stress corrosion cracking wasobserved on the fracture surfaces. The stress corrosioncracking were intergranular for all small grain materialsand Initial Aging treated large grain material. For the asreceived material and RRA treated large grain material, thecracks were transgranular. The observation of parallel crackstop marks in the as received material and RRA treatedlarge grain material tested in 3.5% NaCl solution impliesa stepwise growth mechanism of the crack.