Microstructural Evolution and Corrosion Behavior of Cu-Al-Be Shape Memory Alloys
Date Issued
2006
Date
2006
Author(s)
Kuo, Ho-Hung
DOI
en-US
Abstract
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.
Subjects
形狀記憶合金
銅合金
鋁合金
鈹合金
麻田散體
顯微結構
電化學
腐蝕
Shape Memory Alloy
Cu-based Alloy
Al-based Alloy
Be-based Alloy
Martensite
Microstructure
Electrochemistry
Corrosion
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-95-D89542009-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):1cc7a5b5c316283945e96eb5b96b8c7a