Some Effects of Parent Phase Aging on the Martensitic Transformation in a Cu- AI- Ni Shape Memory Alloy
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I.
INTRODUCTION
SHAPE memory alloys Cu-A1-Ni have been of particular interest for over a decade, and practical applications of them have been attempted for various uses. However, Cu-A1-Ni alloys, as with most other Cu-based shape memory alloys, show considerable aging effects 1'2 which have constrained potential applications of them. To overcome difficulties resulting from aging effects and to achieve a long lifetime of shape memory capacity it is important first to clarify details of the structural changes which occur in the alloy during aging, In the present work a Cu-14 pct A1-4 pct Ni (wt pct) alloy was aged at 673 K in an electron microscope to observe microstructural changes, and it is reported that not only large cuboidal precipitates of the 35 phase but also many domains of a highly ordered phase are formed in the DO3 matrix. These precipitates and domains exert an influence on the subsequent martensitic transformation.
II.
specimens were aged at 673 K as long as 7.2 to 9.0 ks, cooled to room temperature, and then reheated to 673 K. The heating stage could be tilted to +--60 deg about a single axis. III.
EXPERIMENTAL RESULTS
Figure 1 shows a typical bright-field (BF) image of an as-quenched specimen of the present alloy. Large spear-like plates of martensite are seen. The spear-like martensite is
EXPERIMENTAL
A single crystal of the Cu-14A1-4Ni alloy was prepared using the Bridgmann technique. A slab was cut from the crystal with a [001] surface orientation, and after mechanical and chemical polishing to a thickness of 0.3 mm, discs 3 mm in diameter were punched out. The discs were sealed in an evacuated quartz capsule, heated to 1073 K and waterquenched to room temperatures, thus obtaining the 9 0 3 state. The as-quenched discs were electropolished at room temperature using a conventional jet method and an electrolyte containing phosphoric acid and water. The Me temperature was measured for both bulk and thin foil specimens and found to be 240 to 250 K. It was confirmed initially that products such as a "2H-type phase ''3 and an "co-like phase ''4 had not formed in thin foils of the as-quenched specimens. Heat treatments and in situ observations were carried out for the quenched specimens using the heating stage of a Hitachi H500 electron microscope operated at 125 kV. The N. KUWANO is with the Department of Materials Science and Technology, Kyushu University, Fukuoka 816, Japan. C. M. WAYMAN is with the Department of Metallurgy and Mining Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted May 13, 1983. METALLURGICALTRANSACTIONS A
Fig. 1--Typical BF image and diffraction patterns taken at room temperature for an as-quenched specimen. (a) BE (b) Diffraction pattern from martensite. Indices are referred to the 2H structure. (c) Diffraction pattern from the DO3 matrix. VOLUME 15A, APRIL 1984--621
characteristic of Cu-A1-Ni alloys and has been investigated in detail by others. 5 Since the M~ temperature of the present all
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