Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 454-461; DOI: 10.3221/IGF-ESIS.30.55 455 years, mechanical properties of many SMA have improved, allowing to introduce these alloys in specific field of industry. Main examples of these alloys are the NiTi, Cu-Zn-Al and Cu-Al-Ni which are used in many fields of engineering such as aerospace or mechanical systems [1]. Many scientific papers are published mainly on NiTi alloy (Nitinol), analyzing both the microstructure peculiarities and the thermo-mechanical properties [2]. However, this alloy is characterized by some processing difficulties that imply an increase of costs. The main difficulty is due to presence of titanium that makes it readily oxidisable and, because of its good mechanical properties, it must be hot worked. In the last decades, different shape memory alloys have been optimized, such as the copper-zinc-aluminum (ZnCuAl), copper-aluminum-nickel (CuAlNi), nickel-manganese-gallium (NiMnGa), nickel-titanium (NiTi), and other SMAs obtained alloying zinc, copper, gold, iron, etc.. However, the near equiatomic NiTi binary system shows the most interesting properties and it is currently used in an increasing number of applications in many fields of engineering, for the realization of smart sensors and actuators, joining devices, hydraulic and pneumatic valves, release/separation systems, consumer applications and commercial gadgets [1, 2]. Due to their good biocompatibility, another important field of SMA application is medicine, where the pseudo-elasticity is mainly exploited for the realization of several components such as cardiovascular stent, embolic protection filters, orthopedic components, orthodontic wires, micro surgical and endoscopic devices [3]. From the microstructural point of view, shape memory and pseudo-elastic effects are due to a reversible solid state microstructural transition from austenite to martensite, which can be activated by mechanical and/or thermal loads [4]. Copper-based shape-memory alloys are very sensitive to thermal effects, and it is possible that in thermal cycles its properties change (e.g., shape-recovery ratio, transformation temperatures, crystal structures, hysteresis and mechanical behavior). Cu-Zn-Al alloys are characterized by good shape memory properties due to a bcc disordered structure stable at high temperature called β-phase, which is able to change by means of a reversible transition to a B2 structure after appropriate cooling, and reversible transition from B2 secondary to DO3 order, under other types of cooling. In β-Cu-Zn-Al shape memory alloys, the martensitic transformation is not in equilibrium at room temperature. It is therefore often necessary to obtain the martensitic structure, using a thermal treatment at high temperature followed by quenching. The martensitic phases can be either thermally-induced spontaneous transformation, or stress-induced, or cooling, or stressing the β- phase. Direct quenching from high temperatures to the martensite phase is the most effective because of the non- diffusive character of the transformation. The martensite inherits the atomic order from the β-phase [5]. Precipitation of many kinds of intermetallic phases is the main problem of treatment on Cu-based shape memory alloy. For instance, a precipitation of α-phase occurs in many low aluminum copper based SMA alloy and presence of α-phase implies a strong degradation of shape recovery [6]. However, Cu-Zn-Al SMA alloys characterized by aluminum contents less than 5% cover a good cold machining and cost is lower than traditional NiTi SMA alloys. Other investigations carried out on CuZnAl alloys, showed a strain influence on the macroscopic behavior and on martensite morphology. Martensitic transformation occurs initially in deformed material and the manufact shape follows the transformation [7]. Larger grains dimensions allow an easier transformation process, allowing the growth of 18R martensite [8]. In this work a Cu-Zn-Al SMA alloy obtained in laboratory has been microstructurally and metallographically characterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. These analyses have performed under load conditions in order to identify the behavior of alloy. Furthermore fatigue crack propagation and fatigue crack paths were investigated by means of a scanning electron microscope (SEM). M ATERIAL AND PROCEDURES n this work, a CuZnAl pseudo-elastic alloy, made in laboratory by using controlled atmosphere furnace and characterized by chemical composition shown in Tab. 1, has been investigated focusing the fatigue crack propagation paths. Cu Zn Al Other 73.00 21.80 5.04 0.16 Table 1 : Chemical composition of Cu-Zn-Al investigated alloy . I