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  • 學位論文

超高強度Al-Zn-Mg-Cu鋁合金的時效析出、機械性質與應力腐蝕特性研究

Precipitation Characteristics, Mechanical Properties and Stress Corrosion Cracking of High-Strength 7XXX Al-Zn-Mg-Cu Alloys

指導教授 : 王文雄
共同指導教授 : 徐永富

摘要


本論文研究主要針對Cu/Mg比對Al-Zn-Mg-Cu合金之析出行為影響, Al-Zn-Mg-Cu合金析出物之高解析顯微結構分析,熱處理製程對於Al-Zn-Mg-Cu材料機械性能及應力腐蝕之影響來進行探討。 由實驗結果得知,兩個具有不同Cu/Mg比的合金(M9F及M9G,Cu/Mg:M9F < M9G)在室溫下均產生明顯的自然時效,並在120℃下具有很好的熱穩定性。GPI zones和η’相在120℃/6h時效初期均可被觀察到,而GPI zones和η’相亦為120℃/24h之尖峰時效主要的強化相。GPII zones並未自實驗過程中被觀察到。因此,η’相之形成可自GPI zone轉變而來或直接經由基地中析出。我們也觀察到Cu/Mg比並不會造成M9F及M9G合金圓形析出物形貌分布上明顯差異,但對於長形析出物具有很大的影響。長形析出物在{111}Al上成長,在具有高Cu/Mg比的M9G合金中具有較高的長寬比,由<110>Al觀察,呈現盤狀(plate-like)形貌;而在較低Cu/Mg比的M9F合金中,長形析出物的長寬比較低,呈現板狀(lath-like)形貌。高Cu/Mg比合金由於在析出過程中會發生較大的晶格應變能,因此析出物以形成薄板狀的方式來釋放更多應變能,這或許可以解釋M9F及M9G在長形析出物上形貌的差異。 合金經120℃/24h尖峰時效處理,細小且高密度的η’自基地析出,提供合金的高強度,其最大強度板材可達670Mpa,擠製材可達800MPa。合金在120℃呈現極佳的熱穩定性。經各種不同時效處理後,除T7外,板材抗拉強度均可達到600MPa以上,且破壞的方式由原本熱軋韌性破壞轉變為劈裂破壞;T6及two stage前進行自然時效處理,並不能使合金強度進一步提升,而自然時效對T6強度並沒有明顯助益。 由於Al-Zn-Mg(-Cu)合金析出物的晶體結構及成分一直備受爭議,本論文首次使用先進的高角度環形暗場(High-Angle Annular Dark Field, HAADF)技術來分析Al-Zn-Mg-Cu合金中析出物的化學與結構的訊息並和一般傳統高解析顯微鏡(High-Resolution electron microscope, HRTEM)的影像進行比較。由HAADF影像我們可以直接觀察到盤狀析出物η’為5層富Mg及富Zn的原子層交替相疊並有1層富Zn的原子層偏析於上下兩盤面,因此η’為7層原子層的盤狀結構,然而由HAADF的原子序對比,本論文並不支持前人所提的η’晶體結構。最小的η相為11層原子層結構,並在中心層處產生界面差排舒緩厚化產生的應變能。然而並未發現8-10層或是小於7層的盤狀結構。傳統的HRTEM影像對欠焦值敏感,可能會觀察到許多不同形貌的析出物使得對析出物的結構上造成誤判。Cu由於原子效應的關係,穩定了細小的溶質叢,使其可以穩定的成長,產生細小緻密的析出物,促成高強度。 Al-Zn-Mg(-Cu)合金的抗應力腐蝕性也一直是航太工業上很重要的議題。本論文採用定負荷腐蝕測試及電化學極化曲線的量測來探討Al-Zn-Mg-Cu合金應力腐蝕行為,並更進一步施加冷加工於試片上,探討應力應變對Al-Zn-Mg(-Cu)合金電化學腐蝕行為的影響。T7,RRA及Two stage抗應力腐蝕明顯較T6佳,然而在極化曲線的量測上,T7,RRA及Two stage卻具有較大的腐蝕電流。腐蝕電位對熱處理並不敏感,但第二崩潰電位(E b2) 對熱處理製程敏感並會隨著時效時間的增長而消退,因此推測E b2和晶界的組態有很密切的關係。T6具有最大的Eb2電流反轉,雖然沒有T7大的腐蝕電流,但由於其溶解主要集中於晶界處,晶界析出物的連續溶解及氫原子吸附晶界可能造成晶界弱化,是造成T6具有最大的應力敏感性的可能主因。T6處理後的試片對冷加工更為敏感,腐蝕電流明顯的上升,加速了晶界處析出物或基地的溶解,然而冷加工對T7試片並無明顯影響,這或許也可以間接說明T6的應力腐蝕敏感性。

並列摘要


This thesis primarily study the precipitation behavior of Al-Zn-Mg-Cu alloys with different Cu/Mg ratio, high resolution characterization of the precipitation behavior of an Al-Zn-Mg-Cu alloy, and the influences of heat treatments on the mechanical properties and corrosion behavior of a high-strength Al-Zn-Mg-Cu alloy. From our experimental results, both M9F and M9G alloys, Cu/Mg: M9F < M9G, exhibit obvious precipitation hardening during natural aging, and possess good thermal stability during 120℃ artificial aging. GPI zones and η’ phases could be found at the early stage of 120℃/6h artificial aging and these precipitates were also the main strengthening particles at the peak aging condition (120℃/24h). While GPII was not found in our study. Therefore, the formation of η’ phase could be transformed from GPI or directly formed from the matrix. The Cu/Mg ratio shows less influence on the size distribution of the round precipitates presented in both alloys than the elongated ones. The elongated precipitates grow on the {111}Al, and they have greater length-to-thickness ratio in M9G than in M9F, and plate-like in M9G while lath-like in M9F. For the higher Cu/Mg would introduce greater lattice strain field during precipitation, the precipitates would prefer to grow in the form of plate-like to minimize the strain energy. High density of fine η’ phases precipitate from the matrix after 120℃/24h peak aging contributing to the high strength. The strength of the alloy can be greatly elevated to 670MPa for plate specimen and 800 MPa for the extruded specimens after T6 heat treatment. In addition, the studied Al-Zn-Mg-Cu alloy shows excellent thermal stability when aged at 120℃. Except T7 temper, when heat treated after various tempers, all of the plate specimens can achieve UTS greater than 600Mpa, and change the fracture surface from ductile to brittle. However, the strength of the Al-Zn-Mg-Cu alloy cannot be enhanced by implementing natural aging before T6 and two stage aging. The crystal structure and the composition of the Al-Zn-Mg(-Cu) alloys are widely studied but still quite disputing over the past 50 years. The metastable particles in an Al-Zn-Mg-Cu alloy were firstly examined by high-angle annular dark field (HAADF) in this study and the results are compared with the conventional high-resolution electron transmission microscope (HRTEM) images. From our HAADF images, the η’ thin platelets are composed of 7 atom layers with 5 Mg- and Zn-rich atom layers alternate in structure and an extra Zn-rich plane segregates on each facet side. However, the Z contrasts of the HAADF images do not support the η’ crystal structures proposed by previous authors. The smallest η phase is an 11-layered structure and an extra atomic plane forms in the layer center to reduce the strain energy due to the thickening. Almost no particles less than 7 layers and between 7 and 11 layers were found in our study. The conventional HRTEM images are sensitive to the defocus value which could introduce various morphologies of precipitates. The inconsistent results of the HRTEM images would lead to a misunderstanding of the structure of the precipitates. Cu stabilizes the clustering process and metastable phases can be attributed to the size effect. Stress corrosion cracking (SCC) of high-strength 7XXX-series aluminum posses a continuing issue for the aerospace industry. This research utilized SCC tensile testing and potentiodynamic polarization technique to study the corrosion behavior of the Al-Zn-Mg-Cu alloy under various tempers. To further clarify the SCC, deformation was also implemented on the specimens to investigate the influence of dislocations on the SCC behavior of Al-Zn-Mg-Cu alloy. T7,RRA and two stage exhibit better SCC resistant than T6. From the polarization testing, we found that E b2 reduces with aging time and is very sensitive to the tempers. It seems that E b2 is highly associated to the susceptibility of SCC of Al-Zn-Mg-Cu alloy but not the E corr. Eb2 is highly related to the grain boundary configuration. Although T6 exhibited lower I corr than T7, it shows greatest current inversion at E b2. This means continues dissolution at the grain boundaries and H absorption on the grain boundary precipitates are the more crucial factors that contribute to the high SCC sensitivity of T6. Plastic deformation seems to increase the corrosion current density and promote inter granular corrosion of T6 specimen, while it did not show great influence on T7 specimen. This could indirectly explain the SCC sensitivity of T6 specimens.

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被引用紀錄


吳灝展(2013)。6系列與7系列鍛造用鋁合金之機械性質及微結構特性之探討〔碩士論文,國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342%2fNTU.2013.10140
徐維駿(2014)。多道次等通道彎角擠製製程與T6處理 對7075鋁鋅鎂合金機械性質與顯微組織之影響〔碩士論文,國立虎尾科技大學〕。華藝線上圖書館。https://www.airitilibrary.com/Article/Detail?DocID=U0028-0812201416131900

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