The stabilization of martensite in Cu-Zn-AI shape memory alloys
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I.
INTRODUCTION
CU-based shape memory alloys are susceptible to low temperature aging effects which can radically alter their transformation behavior, and may limit their reliability as temperature sensitive components in devices such as thermostats. Both the parent and the martensitic phase are susceptible to aging effects. As has been recently reviewed, ~ changes in M, which can be reversible (e.g., References 2 to 5) result either from aging treatments above Ms or from subjecting the alloy to different quench temperatures above M,. In Cu-Zn-A1 these effects, which are accelerated by a high concentration of quenched-in vacancies, can be modeled on changes in the configurational order of the parent phase altering the relative free energies of the parent and martensite. ~-~ Here we are concerned with the 'stabilizing' effect of aging Cu-Zn-A1 in the martensitic state. 1.3 Such treatments, which are again accelerated by a high vacancy concentration, can raise A, sufficiently to alter the normal transformation sequence to ordered cubic ~31. 4'9 If, however, the parent phase is formed, subsequent thermal cycles show normal transformation temperatures unless the alloy is again aged in the martensitic state. 9'1~ An apparently related effect has been commercially applied for the convenient storage of SME pipeline couplings made of Cu-Zn-Si. ~2 This alloy will remain martensitic if heated slowly to a temperature Tc above the 'normal' As, but if it is then cooled and rapidly heated to above To, it transforms to parent phase at a temperature near Tc. Various mechanisms have been proposed for martensite stabilization. These are: (i) martensite boundary pinning by vacancies or vacancy clusters;~~ (ii) the lowering of the martensite free energy by changes in the configurational order of the martensite, assisted by a high quenched-in vacancy population; 9 and (iii) the inhibition of the normal transformation either by changes in the nature of the fault structure of the martensite, or by the formation of localized regions of the stable phase. 13,14 G. SCARSBROOK and W.M. STOBBS are with the Department of Metallurgy and Materials Science, University of Cambridge, England. J.M. COOK, formerly with the Department of Metallurgy and Materials Science, University of Cambridge, England, is now with Schlumberger Cambridge Research, Cambridge, England. Manuscript submitted January 23, 1984. METALLURGICALTRANSACTIONS A
Here we describe the results of further experiments designed to characterize the stabilization effect in a Cu-Zn-A1 alloy (Section III). We compare the structure and order of the stabilized and unstabilized martensite (Section IV), give evidence of a high temperature martensitic transformation in stabilized material (Section V), and finally discuss the extent to which our results clarify the mechanism(s) of the stabilization process.
II.
E X P E R I M E N T A L DETAILS
Three alloys, chosen to demonstrate different aspects of the stabilization phenomena, were supplied by Delta Memory Metal Ltd. (Ipswich, England) with
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