Effect of composition and ageing on damping characteristics of Cu–Al–Mn shape memory alloys
Introduction
In recent times, the need for materials with high damping capacity, in such fields as automotive, aerospace and structure, has spurred the research and development of high damping materials. Damping occurs in materials when there is a loss of energy in the process of converting one form of energy into another. Among the different means available to achieve damping, much importance has been attached to the use of shape memory alloys (SMAs) because of their unique microstructure, leading to good damping capacity.
Shape memory alloys exhibit high damping capacity or internal friction due to thermoelastic martensitic transformation, which is related to the hysteretic movement of interfaces, i.e. martensitic variant interfaces and twin boundaries [1]. The high density of mobile twins in the martensitic phase and mobile interfaces between the parent phase and martensite leads to internal friction [2]. Considerable amount of energy is absorbed due to internal friction, which gives rise to high damping in shape memory alloys. In the martensitic phase of the alloys, the martensite/martensite interfaces and twin boundaries are mobile due to self-accommodation of the martensitic plates, leading to higher damping than that of the parent austenitic phase [3]. Shape memory alloys exhibit maximum damping in their thermoelastic martensitic transformation temperature range [4]. The transient, phase transition and intrinsic components contribute to the overall internal friction peak of the shape memory alloys, i.e. IF (total) = IF (transient) + IF (phase transition) + IF (intrinsic) [5].
In the recent past, much of the research was devoted to a study of damping properties of Ni–Ti SMAs. Some of the disadvantages that these alloys suffer from are low transformation temperatures, difficulty in production and processing, and high cost. Copper based SMAs, on the other hand, are easier to produce and process, and are also less expensive. The copper based ternary alloys Cu–Al–Ni and Cu–Zn–Al have been studied extensively in recent years. But these copper based SMAs are brittle and cannot, therefore, be processed easily. But it has been found by recent studies that the Cu–Al–Mn SMAs exhibit good ductility since their parent phase has an L21 structure with a low degree of order [6]. These alloys exhibit good damping capacity, with their internal friction peak lying within the hysteresis loop/zone of the alloys [7]. To date no reports are available on the damping properties of these alloys and, therefore, an extensive study of these alloys for their damping characteristics is required before they can be used in service.
The shape memory characteristics of Cu–Al–Mn SMAs vary with the variation in the amount of aluminum and manganese [8]. In the present work, the effects of variation in the amount of aluminum and manganese on the damping capacity of some Cu–Al–Mn SMAs are, therefore, studied.
The Cu–Al–Mn SMAs are susceptible to ageing, which results in the formation of precipitates, and subsequent transformation of martensite to martensite [9]. It leads to a change in the shape memory characteristics of the alloys, especially their transformation temperatures. No reports are available till date on the effect of ageing on the damping capacity of the Cu–Al–Mn SMAs. In the present work, the effects of post quench ageing on the damping capacity of the Cu–Al–Mn SMAs are, therefore, studied. All alloys exhibit maximum damping capacity in the transformation region and that the damping capacity in the martensitic phase is higher than that of the austenitic phase of the alloys. The damping capacity of the alloys decreases with ageing of the alloys due to the pinning effect caused by the precipitate particles formed.
Section snippets
Alloy preparation and characterization
Cu–Al–Mn SMAs with 10–14.5 wt.% of aluminum and 0–10 wt.% of manganese were chosen for the present study, as the alloys exhibit β-phase at high temperatures and manifest shape memory effect on quenching to form martensite in this composition range [8]. The alloys were prepared in such a way that either the Al (or Cu/Al ratio) or the Mn (or Cu/Mn ratio) was maintained constant and either the amount of Mn (or Cu/Mn ratio) or Al (or Cu/Al ratio) was varied respectively. Small pieces of pure copper,
Transformation temperatures and microstructure
The chemical composition of the alloys and the corresponding transformation temperatures, are given in Table 1. Two series of the alloys were considered from these alloys for the analysis. In the first series, the alloys were subgrouped such that the amount of Al (or Cu/Al ratio) was more or less maintained constant and the amount of Mn (or Cu/Mn ratio) was varied. In the second series, the alloys were subgrouped such that the amount of Mn (or Cu/Mn ratio) was more or less maintained constant
Conclusions
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The transformation temperatures of the Cu–Al–Mn SMAs vary with variation in chemical composition of the alloys. The transformation temperatures decrease with an increase in either aluminum or manganese concentration of the alloys.
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The damping capacity or IF of the alloys increases with an increase in the aluminum content when the amount of manganese (or Cu/Mn ratio) was more or less maintained constant.
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The damping capacity or IF of the alloys decreases with an increase in the manganese content
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