Issue 18

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 18 (2011) 45-53 ; DOI: 10.3221/IGF-ESIS.18.05 46 alloying zinc, copper, gold, iron, etc. Within this class of materials the near equiatomic NiTi binary system shows the most exploitable characteristics and it is currently used in an increasing number of applications in many fields of engineering [1], for the realization of smart sensors and actuators, joining devices, hydraulic and pneumatic valves, release/separation systems, consumer applications and commercial gadgets. However, due to their good biocompatibility the most important applications of NiTi alloys are in the field of 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. From the microstructural point of view shape memory and pseudo-elastic effects are due to a reversible solid state microstructural transitions from austenite to martensite, which can be activated by mechanical and/or thermal loads [2]. The phase transition of near equiatomic NiTi systems is illustrated in the phase diagram of Fig. 1, where at T<900°C complete temperature transformations are not well specified. In the last years a triple transitions has been accepted, from an austenitic B2 phase for slowly cooling a B19 orthorhomic phase transformation occurs, but for long time at 500°C (about 120 hours) an monoclinic B19’ phase is obtained. But not all the transformation are possible when changing the Ni content; in particular when increasing the Ni content last transformation (B19’) can take place only under environmental temperature or under the zero absolute [3-11]. Figure 1 : NiTi alloy phase diagram. The near equiatomic NiTi system is capable of two successive martensitic phase transformations during cooling from its high temperature austenitic phase. In Ti rich NiTi SMAs, the first phase transformation during cooling is observed just above room temperature and results in the R-phase, the second one occurs around room temperature and results in M- phase (monoclinic structure), often with a fine lath morphology. These transformations give rise to thermo-elasticity and twin deformations in NiTi alloy facilitating shape memory effect (SME) [12-14]. Elastic strain energy provides the reversible nonchemical contribution to the overall free energy of the system. The method of elastic strain energy generation depends upon whether or not external stress is applied. In the absence of external stress, the martensite transformations in NiTi produce a self-accommodating arrangement of martensite correspondent variant pairs (CVPs) that minimizes elastic strain energy. As the volume fraction of self-accommodating groups (SAGs) of martensite CVPs grows, the concomitant increases in interfacial energy and elastic strain raise the stored elastic strain energy of the system. The evolution of microstructure and the formation of microcracks during cyclic loading has been observed and crack propagation under cyclic loading conditions has been monitored in many works, but behavior of cracks under static loading conditions in martensitic, pseudo-plastic and austenitic pseudo-elastic NiTi microstructures is not yet clear [8, 15-19]. In this work the mechanical properties of a commercial NiTi shape memory alloy have been investigated by tensile tests of miniaturized dog bone shaped specimens carried out by using a mini testing machine. In addition, in situ XRD analyses were performed during mechanical tests, in order to understand the influence of microstructure and crystallographic parameter on the pseudo-elastic effect of the alloy, as well as on the related hysteresis in the stress-strain response. 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 0 10 20 30 40 50 60 70 80 90 100 Temperature [°C] Ni [at%] Ti 2 Ni TiNi 3 TiNi (  Ti)  (  Ti) 1670°C 1310°C 1380°C 1455°C (Ni) 1118°C 1304°C 942°C 675°C 882°C 984°C 630°C