PSI - Issue 13

V. Di Cocco et al. / Procedia Structural Integrity 13 (2018) 204–209 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Large grain sizes Cu-Zn-Al SMAs show that crack paths follow preferentially the grains boundaries (intergranular cracks), sometimes with a propagation that follows a transgranular paths. This is due to the local chemical dis homogeneities (Natali et al. (2017)). Furthermore in the investigation on the thermodynamic properties of a Cu-Zn-Al SMA, Gomidželović et al. (2015) observed the presence of Zn and Al in the intermetallic phases in polygonal shape grains. In other words, a fine control of the production processes of Cu based alloys could allow to obtain Cu-Zn-Al alloys for different components, ranging from a traditional component obtained from fusion to a complex component made on hybrid composite SMA (Lo Conte et al. (2016). Modern constitutive models are available in the scientific literature (Cisse et al. 2016), covering also microscopic effects as the reorientation of martensite, in order to better describe the behaviour of SMA, not only in terms of stress-strain behaviour, but also in terms of microstructure transformations. In order to improve both the mechanical behavior and the memory performances of Cu based SMA alloys, a grains refinement could be an interesting way. Yang et al. (2016) in Cu-Al-Mn SMA observed an improvement of tensile performances and the damping capability at low temperature and low frequencies by a refinement from 105  m to 37  m. In this work, a Cu-Zn-Al alloy was investigated in order to observe the effects of both the grain sizes on fatigue crack growth propagation. Fracture surfaces were investigated by means of a Scanning Electron Microscope (SEM) in order to identify the main fatigue crack propagation micromechanisms.

2. Investigated alloy and experimental procedure

In this work, the Cu-Zn-Al pseudo-elastic alloy has been investigated considering two different grain sizes and focusing on the fatigue crack propagation resistance. The SMA alloy characterized by a higher value of grain size (Alloy A – chemical composition in Tab. 1) was produced in the laboratory, by using a controlled atmosphere furnace , and machined in “as cast” conditions.

Table 1: Chemical composition of investigated Cu-Zn-Al alloy (wt%). Cu Zn Al Other 73.00 21.80 5.04 Bal.

The second investigated alloy (Alloy B) is produced from Alloy A optimizing with the addition of Cerium in order to reduce the grain size. Grain size has been evaluated by means of the LOM (light optical microscope) observations, using Jeffiers method (standard ASTM E883-11) (2017). In order to evaluate the structure modifications, a customized testing machine equipped with a removable loading frame was used to perform X-Ray analyses at fixed values of applied load and/or deformations. XRD measurements were carried out by using a Philips X-PERT PRO diffractometer equipped with vertical Bragg-Brentano powder goniometer. A step- scan mode was used in the 2θ range from 40° to 45° with a step width of 0.02° and a counting time of 3 s per step (receiving slit 0.02 mm). The employed radiation was monochromated Cu Kα (40kV – 40 mA). The diffractions analyses were performed on a uniaxial specimen both in unloaded and at imposed deformation (  =10%). Fatigue crack propagation tests were performed by using the standard CT (Compact Type) specimens obtained by machining the cast ingots. A traditional hydraulic testing machine was used in order to investigate the fatigue crack propagation according to ASTM E 647 (2015). Fatigue cracks propagation tests were performed according to the following conditions.   P = constant  Stress ratio, R = Pmin/Pmax= 0.1  Loading waveform: Sinusoidal wave.  Load frequency = 30Hz.  Testing environment: Lab conditions. Tests were repeated three times for each investigated condition. Results were characterized by a very high repeatability. Finally, fracture surfaces analyses were performed by means of a Scanning Electron Microscope (SEM) in order to evaluate the influence of grain size, load ratio and applied  K on fatigue crack growth micromechanisms.