ISSN 1061-3862, International Journal of Self-Propagating High-Temperature Synthesis, 2008, Vol. 17, No. 1, pp. 41–48. © Allerton Press, Inc., 2008.
SHS of Shape Memory CuZnAl Alloys M. Gueriounea, Y. Amioura, W. Bounoura, O. Guellatia, A. Benaldjiaa, A. Amaraa, N. E. Chakria, M. Ali-Rachedia, and D. Vrelb a Laboratoire
b LIMHP,
LEREC, Département de Physique, Université de Annaba, Annaba, Algérie CNRS, Université Paris XIII, Institut Galilée, 99 Av. J.-B. Clément, Villetaneuse, Paris 13, 93430 France Received November 13, 2007
Abstract—Aiming at preparation of shape memory alloys (SMAs), we explored the SHS of Cu1 – xZn1 – yAl1 – z alloys (0.29 < x < 0.30, 0.74 < y < 0.75, and 0.83 < z < 0.96). The most pronounced shape memory effect was exhibited by the alloys of the following compositions (wt %): (1) Cu(70.6)Zn(25.4)Al(4.0), (2) Cu(70.1)Zn(25.9)Al(4.0), and (3) Cu(69.9)Zn(26.1)Al(4.0). The effect of process parameters on the synthesis of CuZnAl alloys was studied by XRD, optical microscopy, and scanning electron microscopy (SEM). The grain size of CuZnAl was found to depend on the relative amount of the primary CuZn and AlZn phases. Changes in the transformation temperature and heat of transformation are discussed in terms of ignition intensity and compaction. Mechanism of the process depends on the level of the temperature attained relative to the melting point of components. At the melting point of AlZn, the process is controlled by the solid-state diffusion of AlZn into a product layer. The ignition temperature for this system depends on the temperature of the austenite-martensite transformation in CuZnAl alloys. The composition and structure of the products was found to markedly depend on process parameters. The SHS technique has been successfully used to prepare a variety of SMAs. Key words: shape memory alloys, SHS, stoichiometry, milling, phase composition. PACS numbers: 81.05.Je, 81.20.Ka DOI: 10.3103/S1061386208010044
1. INTRODUCTION
fatigue life makes CuZnAl a promising material in the elimination of vibration-induced fatigue failures in structures [8].
The shape memory alloys are known to exhibit unique thermomechanical properties, such as the shape memory effect, superelasticity, and pseudo-elasticity, which arise due to martensitic transformations (firstorder transitions) driven by external stress or temperature [1, 2]. It is a displacive, diffusionless, and reversible phase transformation [3, 4] that is the underlying mechanism for these properties. Two crystallographic structures attainable by shape memory alloys are the low-temperature/high stress martensite phase and the high-temperature/low stress austenite phase [5]. The austenitic phase (β-phase) is also referred to as a parent (or memory) phase of the alloy (Fig. 1).
In this work, we explored the SHS [9, 10, 11] of CuZnAl alloys of different stoichiometry. 2. THEORETICAL BACKGROUNDS OF SHAPE MEMORY EFFECT (SME) SMAs may exhibit two kinds of shape memory effect, one-way and two-way shape memory [12, 13]. The procedure is as follows: martensite is subjected to either reversible deformation for the one-way effect or severe irreversible deformation for the two-way effect, which is followed by sample heating and cooling it down again (Figs. 1, 2).
Numerous research efforts have been focused on Cu-based (CuZnAl, CuNiAl) SMAs because of their good memory properties and low production cost. A particular attention is set on the elaboration of methods regarding the stress-induced martensitic transformation (microstructure determination of the parent phase, different martensitic variants) and relevant theories and models. The stress-induced phase transition occurring in SMAs causes inelastic deformation and gives rise to an energy-absorbing capacity [6, 7]. Due to this, the inelastic deformation associated with SMA, CuZnAl alloys should be capable of having high fatigue lives. The energy-absorbing capacity and a possible high
In the one-way shape memory effect (OWSME), SMAs are deformed at a given temperature and then, upon heating or cooling, they return to their initial shape. Strains of up to 8% can be completely recovered. In OWSME, cooling down from high temperatures does not cause any macroscopic shape change. Deformation is needed to create the low-temperature shape. Upon heating, transformation starts at As (Fig. 1) and is completed at Af (typically 2–20°C or higher, depending on the type of alloy and loading conditions). The As 41
