Microstructural studies of a Cu-Zn-Al shape-memory alloy with manganese and zirconium addition
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I.
INTRODUCTION
Cu-Zn-Al is an important Cu-based shape-memory alloy (SMA) which possesses a good shape-memory effect (SME) and has the advantage of a lower price than the TiNi SMA. However, it suffers from martensitic stabilization[1] and intergranular cracking during processing and service because of the large shape-memory anisotropy and large grain size when in the b parent-phase condition.[2] The addition of suitable alloying elements can improve the mechanical properties and stabilize the martensitic transformations and the SME of a Cu-based SMA. For example, with the increase of manganese concentration, the thermoelastic and pseudoelastic behaviors of Cu-Al-Ni-Mn-B SMAs were enhanced and the ductility of the alloy was improved.[3,4] On the other hand, the elements B, V, Zr, and Ti can be added to refine the grain size of the parent phase in Cu-based SMAs.[5,6,7] As a modification of Cu-Zn-Al SMAs, Zr and Mn are added to the Cu-Zn-Al alloy to refine the grain size of the alloy, to increase the ductility of the matrix, and to suppress the stabilization of the martensite. Compared to a Cu-ZnAl SMA, a significant grain refinement effect was found in the Cu-Zn-Al-Zr and Cu-Zn-Al-Mn-Zr alloys, and the characteristics of martensitic transformation in the Cu-Zn-AlMn-Zr alloy are better than those of the Cu-Zn-Al SMA.[8] Zr-rich precipitates formed in a Zr-added Cu-Zn-Al SMA. These precipitates were initially observed in the Cu-Zn-AlZr alloy and at the grain boundaries.[5] However, in a Cu19.0Zn-13.1Al-1.1Mn-0.3Zr (at. pct) SMA, Zr-rich precipitates appeared not only in the vicinity of grain boundaries W.H. ZOU, Postdoctoral Fellow, C.W.H. LAM, Lecturer, C.Y. CHUNG, Assistant Professor, and J.K.L. LAI, Chair Professor of Materials Science, are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong. Manuscript submitted June 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
but also in the matrix, as revealed by optical microscopy.[8] The grain-refinement mechanism of Zr addition in a CuZn-Al SMA is still unknown. In the present work, the microstructure of a Cu-Zn-AlMn-Zr alloy and the detailed structure of Zr-rich precipitates in the alloy are studied using energy-dispersive X-ray spectroscopy (EDXS) and electron diffraction analysis in a transmission electron microscope (TEM). The effect of Zrrich precipitates formation on the characteristics of Cu-ZnAl-Mn-Zr SMA will be discussed. II.
EXPERIMENTAL
Cu-19.0Zn-13.1Al-1.1Mn-0.3Zr (at. pct) alloys were prepared by melting elements with industrial purity in an induction furnace. The ingots were hot rolled to 1-mm sheets. The specimens were solid-solution treated at 1093 K for 5 minutes, step quenched into the 373 K boiling water, and then cooled in water at room temperature. The TEM foils were prepared by double jet electropolishing 3-mm-diameter discs of 0.1 mm in thickness using HNO3-CH3OH solution (volume ratio 1:2). The TEM observations were carried out using a PHILIPS* CM-20 TEM *PHILIPS is a trademark of Philip
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