Aging Effects in Copper-Based Shape Memory Alloys
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I.
Table I.
INTRODUCTION
THE shape memory effect occurs in alloys having a metastable structure of martensite, retained parent phase, or a mixture of the two. In each case the metastable phase or phases will transform by diffusional processes at a temperature dependent rate to a more stable structure with concomitant degradation of the shape memory capacity. For applications of shape memory alloys that involve thermal cycling, it is important that transformation during thermal excursions does not reduce the capacity of a device to function satisfactorily during service. Alternatively, the gradual cumulative degradation which occurs inevitably during cycling is an important factor in determining the life expectancy of a device. This kind of limitation on the exploitation of the shape memory effect seems to have escaped detailed scrutiny, although some work on aging effects in copper-based shape memory and similar alloys has been reported. Some of these studies ~-7 detailed structural changes that occurred during thermal treatment, while others 8-13 described some accompanying changes in properties. Of the papers concerned specifically with shape memory alloys, 2'6'8'1~ the changes in properties related to the degradation of the shape memory effect have received very little attention. Consequently, the purpose of the present paper is to provide an account of the influence of aging on the behavior and shape memory capacity of three copperbased alloys. A companion paper describing the detailed microstructural changes resulting from the aging treatments will be published later.
II.
EXPERIMENTAL PROCEDURES AND RESULTS
Three copper-based alloys with nominal compositions given in Table I were prepared by induction melting under a protective cover of graphite powder. Small coarse grained specimens of the alloys were produced by homogenizing in the/3 phase field at 900 ~ and water quenching. The specimens were then aged for times
N.F. KENNON, D.P. DUNNE, and L. MIDDLETON are all with The University of Wollongong. Department of Metallurgy, P. O. Box 1144, Wollongong, N. S. W, 2500, Australia. Manuscript submitted June 5, 1981. METALLURGICAL TRANSACTIONS A
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Alloy 1 2 3
Cu 81.8 82.9 72.8
Compositions of Alloys (Wt Pct)
Zn
21.2
AI 15.1 14.2 6.0
Ni 3.1 2.9
M, -11 ~ 146 ~ 62.5 ~
Martensite y', ( 2 H ) /31 (18R) 131( 9 R )
up to approximately 10 6 seconds at temperatures between 200 ~ and 450 ~ Changes (at ambient temperature) in the ordered bcc/31 phase were studied using alloy 1 ( M , = - l l ~ while changes in the faulted/31 martensite were studied in alloys 2 and 3 (Ms, Mr=ambient). The following measurements were made as a function of aging time at the various aging temperatures: (i) Vickers hardness number (20 kg load) of the 131 phase (alloy 1, Figure l(a)) and the /3'1 phase (alloys 2, 3; Figures 2(a), 3(a)), (ii) shape recovery capacity by direct observation of thin specimens aged, then bent 2 3 0 deg before heating above the AI temperature (Figures l(b), 2(b), and 3(b)), (iii) martensitic transformation
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