Issue34

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 34 (2015) 415-421; DOI: 10.3221/IGF-ESIS.34.46 416 In the last years [1], many alloys characterized by memory property have been studied. Often the composition was characterized by the presence of rare metals and poor mechanical properties and effects that did not allow an industrial development on a large scale. Cu-Zn-Al alloys are characterized by good shape memory properties due to a β-phase (bcc disordered structure, sometimes named austenite) that is stable at high temperature. A cooling process induces the transformation of the β- phase into a B2 structure (sometimes named as martensite) and a further cooling implies the transformation of the B2 phase into a DO 3 phase (martensitic phase). Martensitic phases can be either thermally-induced or stress-induced. In recent years, improved mechanical properties of many SMAs allowed their application in many specific industrial applications [2-5]. Cu-based shape memory alloys are characterized by the precipitation of many different intermetallic phases and this can negatively affects their mechanical properties. Cu based alloys with low Al content are characterized by the precipitation of α-phase: this implies a strong degradation of shape recovery [6]. Anyway, Cu-Zn-Al SMAs characterized by aluminum contents less than 5% are characterized by a good cold workability, with a cost that is lower than traditional NiTi SMAs. In this work, a Cu-Zn-Al SMA in “as cast” conditions has been microstructurally and metallographically characterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. The investigated alloy is characterized by the stress-strain curve shown in Fig. 1 [7]. Fatigue crack propagation resistance and crack propagation micromechanisms have been investigated considering three different load ratios (R = 0.10, 0.50 and 0.75). 0 50 100 150 200 250 300 350 0 0.02 0.04 0.06 0.08 0.1 σ[ MPa] ε Unloading M f T=18°C A f M s A s Loading Figure 1 : Cu-Zn-Al SMA stress-strain curve with hysteresis [7]. M ATERIAL AND METHODS he investigated Cu-Zn-Al SMA (chemical composition is shown in Tab. 1) was obtained by means of an atmosphere controlled furnace by using nitrogen gas, in order to reduce the presence of oxides and other precipitates (e.g, non-metallic inclusions are often due to oxidation process of evaporated Zn at high temperatures). Obtained castings were characterized by a pseudo-elastic behavior. Mini ingots were obtained by means of centrifugal furnace and casted into mold with a CT specimen shape. Solidification and cooling process were performed in lab conditions. Cu Zn Al Other 72.20 21.71 5.77 0.32 Table 1 : Chemical composition of Cu-Zn-Al investigated alloy. T