Composition dependence of aging kinetics in some Cu-Zn-Al shape memory alloys
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I.
INTRODUCTION
A M O N G the various shape memory alloys, the Cu-base shape memory alloys have been of particular interest compared to Ni-Ti alloys because of their lower cost and easier formability.II] However, at elevated temperatures, the metastable /31 (B2 o r D03) and fl'l (M9R or M18R) phases are transformed to more stable structures of bainitic al, complex cubic 72, or fcc a phases by thermally activated processes, which leads to a concomitant degradation of the shape memory capacity and a change in martensitic transformation temperatures, t2~ This susceptibility to deleterious aging effects in the Cu-base shape memory alloys reduces their potential commercial applications. Aging effects in the Cu-base shape memory alloys have been reported by several investigators. ~2-s~ However, the compositional dependence of aging kinetics has received very little attention, although aging kinetics are very sensitive to Zn or A1 content in the Cu-Zn-AI alloys. This work is concerned with the dependence of aging kinetics on alloy composition in the Cu-Zn-A1 alloys. The Zn content varied from 13 to 28 wt pet, while the AI content varied from 3 to 9 wt pct in the five different Cu-Zn-A1 alloys. The isothermal aging kinetics of the five Cu-Zn-A1 alloys have been studied by monitoring the variation of electrical resistivity with time. On the other hand, differential scanning calorimetry (DSC) has been used to investigate the nonisothermal aging kinetics of the five Cu-Zn-AI alloys. Microstructural changes resuiting from the aging treatments have also been studied. II.
EXPERIMENTAL
PROCEDURES
Five Cu-Zn-A1 shape memory alloys with the nominal compositions given in Table I were melted in a graphite
EON-SIK LEE, Graduate Student, and YOUNG G. KIM, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea. Manuscript submitted May 30, 1989. METALLURGICAL TRANSACTIONS A
crucible under a reducing atmosphere using a highfrequency induction furnace. The five 4-kg melts were poured into a chilled mold 5 • 5 • 18 cm 3 in size. The ingots were homogenized at 800 ~ for 24 hours in an inert atmosphere. The homogenized ingots were hot forged and then hot rolled after holding at 800 ~ for 2 hours, resulting in a final thickness of 1 mm. All specimens were solution treated in an inert atmosphere at 800 ~ for 5 minutes, followed by quenching into room-temperature water. The quenched specimens were annealed immediately at 140 ~ for 50 minutes in order to eliminate the inhomogeneities such as the excess vacancy and the quenched-in disorder. The solvus line temperatures (Ts) for the a or 72 phases were determined by means of metallographic examinations after annealing and quenching. The isothermal aging kinetics were measured by monitoring the variation of electrical resistivity with time during isothermal holding at temperatures from 180 ~ to 340 ~ The resistivity specimen was 120-mm long and 1 • 3 mm 2 in cross section. Four coppe
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