銅鋁鈹形狀記憶合金之顯微結構變化及腐蝕行為

Abstract

本論文主要是探討Cu-Al-Be形狀記憶合金之顯微結構變化及腐蝕行為。顯微結構觀察的重點有二：一是有關合金固溶處理材內部的自發調適及穩定化麻田散體(self-accommodation and stabilized martensites)的界面結構及內部缺陷；其二是時效處理材內部析出相的結構變化。此外，Be含量及時效處理對於合金腐蝕行為的影響亦是本文討論的重點之一。 顯微結構分析採用的實驗方法有光學顯微鏡(OM)、X-ray繞射儀(XRD)、穿透式電子顯微鏡(TEM)、應變場分析、場發射掃瞄式電子顯微鏡附加能量散射光譜儀(FESEM with EDS)及電子探測微分析儀(EPMA)。三種電化學檢測法，包括陽極極化(AP)、循環伏安(CV)和交流阻抗(AC impedance)等，則是用來研究合金的腐蝕行為。實驗所得的結果如下簡述： 觀察共析Cu-11.4wt.%Al-0.47wt.%Be形狀記憶合金之固溶處理材內的A/C型自發調適及A/D型非自發調適M18R麻田散體兄弟晶對(variant pairs)後發現，此兩組兄弟晶對皆具有第一類型雙晶關係(Type-I twin relation)。A/C型及A/D型兄弟晶對分別具有晶癖面(habit plane)(1 )M18R及(2020)M18R，且其晶癖面對應於各自晶對的雙晶面。從這兩組兄弟晶對的高解析像可看出，整合且筆直的A/C界面是由規則的Shockley部分差排(Shockley partial dislocations)所構成的；而鋸齒狀的A/D界面則是由不規則的部分差排所構成的。根據原子剪移(atomic shuffling)狀態及界面結構，此兩組兄弟晶對的形貌亦可被描繪出來。應變場分析結果顯示A/C型兄弟晶對的應變場主要是集中在C兄弟晶的內部疊差結構。 當加熱形狀記憶合金時，其內部自發調適麻田散體可以變態成母相；相對地，當麻田散體無法順利變態成母相時，則此類麻田散體稱為穩定化麻田散體。導致麻田散體穩定化因素可分化學因素(chemical contributions)及機械因素(mechanical contributions)。其中後者可分為麻田散體界面穩定(pinning of martensite interface)及麻田散體內部結構的改變(change in the nature of martensite structure)。 在Cu-10wt.%Al-0.55wt.%Be合金之固溶處理材內所觀察的具有內部缺陷之A/C型穩定化麻田散體界面，不若A/C型自發調適麻田散體界面是由部分差排所構成的，反而是由高密度的全差排所構成。因此，在此類穩定化麻田散體兄弟晶對中，應變場不僅分佈在麻田散體內部，且在兄弟晶界面上。經由觀察結果可推論，在逆向麻田散體變態過程中，發生在穩定化麻田散體中的界面穩定化是因界面差排(interfacial dislocations)所導致的。 在某些穩定化麻田散體中可發現兩種不同的內部缺陷，即kink band和jog。一般來說，發生在基底面(0018)M18R上的原子剪移是M18R麻田散體變態至母相的關鍵因素。但是，經由顯微結構觀察發現，這兩種內部缺陷是皆延著垂直於原子剪移的方向所形成的。針對具有kink band的麻田散體進行應變場分析，結果顯示在此麻田散體中，kink band的內部及周圍具有最大的應變場。所以，這些內部缺陷可以改變麻田散體結構的本質，進而可能導致麻田散體的穩定化發生。 顯微結構變化的第二個研究重點是觀察經由200 oC不同時效時間(20~160小時)的Cu-10wt.%Al-0.8wt.%Be合金內部的二種析出相，α1-plate和γ2相的顯微結構。α1-plate的長程堆疊序化(LPSO)結構導致此相內部具有高密度的疊差。根據原子剪移狀態，α1-plate的晶體結構模型得以建立。雖然α1-plate與M18R麻田散體的晶體結構特性非常相似，但在高解析像分析中可發現兩者的結構模型的差異在於彼此的原子剪移狀態不同。在200 oC經160小時的時效處理材中發現，α1-plate的Al濃度些微地低於基底。此一結果應證亞共析Cu-Al-Be合金的α1-plate析出相之成長是與原子或成分擴散有關。另外，在此時效處理材中發現均勻析出的奈米尺寸顆粒(nano-particle)，經鑑定為γ2相。在200 oC時效160小時內，這些奈米γ2相的成長速率並沒有明顯地增加。 有關Cu-Al及Cu-Al-Be(0.55~1.0wt.%)合金在25℃的0.5莫耳硫酸溶液中之腐蝕行為研究是利用三種電化學檢測法—陽極極化(AP)、循環伏安(CV)及交流阻抗(AC impedance)來進行的。陽極極化及循環伏安的測試結果顯示，Al和Be含量的增加可些微地降低合金在陽極的解離速率及表面氧化還原反應。Cu-Al合金固溶處理材經由偏壓0.6V交流阻抗測試後發現，其內部晶界受到優先腐蝕而形成嚴重的粒間腐蝕(intergranular corrosion)；但是，此粒間腐蝕現象可經由添加Be來加以抑制。對於Be添加得以提高Cu-Al合金粒間腐蝕的阻抗之機制可推論為，晶界周圍的Be原子於淬火過程中，即隨著過剩空孔遷移至晶界，進而降低晶界能，使得晶界不會被優先腐蝕。 Cu-10wt.%Al-0.8wt.%Be合金在經200 oC不同時效時間(1.5~160小時)處理後，亦於0.5莫耳溶液中進行偏壓0.6V交流阻抗測試，並配合FESEM、EDS及EPMA的分析以探討時效處理對Cu-Al-Be合金之腐蝕行為的影響。實驗結果顯示，合金粒間腐蝕的程度會隨著時效時間而加劇。此外，由FESEM顯微觀察結果發現，時效1.5小時的試片內部已因交流阻抗測試而有粒間腐蝕產生。EDS和EPMA的結果指出，時效處理材的粒間腐蝕是因時效導致晶界析出二種不同形貌且具有不同Al含量的析出相。這些晶界析出相的數量會隨著時效時間而增加，進而降低合金晶界的腐蝕阻抗，導致粒間腐蝕加劇。

Study on microstructural evolution of Cu-Al-Be shape-memory alloys (SMAs) is focused on the investigation of interfacial structures and internal defects of the self-accommodation and stabilized martensites in the solution-treated SMAs and precipitates in the aged SMAs. The effect of beryllium concentration and aging treatment on corrosion behavior of the solution-treated and aged SMAs is also discussed. The results obtained by optical microscopy (OM), X-ray diffractometer (XRD), high resolution transmission electron microscopy (HRTEM), strain-field analysis, field-emission scanning electron microscopy with energy dispersive X-ray spectroscopy (FESEM with EDS), electron probe microanalyzer (EPMA), and electrochemical tests including anodic polarization (AP), cyclic voltammetry (CV), and alternative current (AC) impedance are given as follows. Self-accommodation and non-self-accommodation M18R martensite combinations, namely A/C and A/D variant pairs, in the solution-treated eutectoid Cu-11.4wt.%Al-0.47wt.%Be SMA have a type I twin relation with respect to the habit planes (1 )M18R and (2020)M18R, respectively. HRTEM-micrographs show that the straight A/C interface consists of regular Shockley partial dislocations; while the zigzag A/D interface contains the irregular ones. The illustrations of the two variant pairs are also proposed based on the atomic shuffling and interface structure. Strain-field analysis shows the strain fields of A/C variant pair are concentrated in the stacking faults of variant C. When SMAs are heated, the self-accommodation martensites can transform easily to parent phase; contrarily, the martensites which cannot transform successfully to parent phase are termed as stabilized martensites. “Mechanical contributions”, including pinning of martensite interface and change in the nature of martensite structure, lead to the martensitic stabilization. Stabilized martensites with internal defects were investigated in the solution-treated hypoeutectoid Cu-10wt.%Al-0.55wt.%Be SMA. Unlike self-accommodation A/C martensite interfaces consisting of Shockley partial dislocations, the stabilized A/C martensite interfaces are composed of high density of complete dislocations. Consequently, the strain fields of stabilized martensite combinations are concentrated not only in the martensites but also in the interface. Owing to the interfacial dislocations, the pinning of interface occurs in the stabilized martensites during the reverse martensitic transformation. Two kinds of internal defects, kink band and jog, were also investigated in the stabilized martensites of Cu-10wt.%Al-0.55wt.%Be SMA. It is well know that the mobility of atomic shuffling on the basal (0018)M18R of M18R martensite is the key for the transformation from martensite to parent phase. However, both of the defects are formed in the direction perpendicular to the gliding direction of atomic shuffling. And, according to the strain-field analysis, the kink band involves highest strain field in the martensite. The results imply that these internal defects change the nature of martensite structure and lead to the martensitic stabilization. Two major types of precipitates, α1-plate and γ2 phases, formed in the hypoeutectoid Cu-10wt.%Al-0.8wt.%Be specimen aged at 200 oC for different periods of time (20~160 h) were investigated. The α1-plates contain a high density of stacking faults due to the 18R long period stacking order (LPSO) structure, and an atomic model of the structure was developed based on the atomic shuffling of the regular stacking faults. Owing to different atomic shuffling, the atomic model of 18R α1-plate is distinct from that of M18R martensite; even though these two phases have quite similar crystallographic features. Since the aluminum concentration is found to be lowered, the growth of α1-plates is suggested to be dominated by atomic diffusion process in the hypoeutectoid Cu-Al-Be SMAs. The nano-particle observed to distribute homogeneously in the as-quenched and aged SMAs was identified to be γ2 phase. The growth rate of nano-particle γ2 phase seems not be increased with increasing aging time when aged at 200 oC for less than 160 h. The corrosion behavior of Cu-Al and Cu-Al-Be(0.55~1.0wt.%) SMAs in 0.5 M H2SO4 solution at 25 ℃ was studied by electrochemical tests including AP, CV, and AC impedance. The results of AC and CV tests show that anodic dissolution rates and surface redox of alloys decreased slightly with increasing the concentrations of aluminum or beryllium. Severe intergranular corrosion of Cu-Al alloy was observed after AC impedance test performed at the anodic potential of 0.6 V. However, the addition of a small amount of beryllium was effective to prevent the intergranular corrosion. The effect of beryllium addition on the prevention of intergranular corrosion is possibly attributed to the diffusion of beryllium atoms into grain boundaries, which in turn deactivates the grain boundaries. The effect of aging treatment at 200 oC for different periods of time on the corrosion behavior of Cu-10wt.%Al-0.8wt.%Be SMAs in the 0.5 M H2SO4 solution is also studied by AC impedance test at 0.6 V, FESEM with EDS, and EPMA. According to the Nyquist diagrams and FESEM-micrographs, the degree of intergranular corrosion increases with increasing aging time. In addition, FESEM-micrograph shows that the intergranular corrosion occurred in the specimen aged for 1.5 h, which is not revealed in the Nyquist diagram. The results of EDS and EPMA show that the intergranular corrosion is due to the segregation of low aluminum-containing precipitates formed along grain boundaries. The amount of these precipitates along the grain boundaries increases with increasing aging time, by which the resistance to intergranular corrosion decreases.