Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 454-461; DOI: 10.3221/IGF-ESIS.30.55 456 Prior to evaluate mechanical properties, crono-amperometric tests have been performed in order to evaluate the homogeneity of material and its electrochemical behaviours, using a 0.1 mol NaCl solution at -50mV/SCE for 1800s [9]. In order to evaluate the structure transformation, 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 25° to 90° with a step width of 0.02° and a counting time of 3 s per step, and receiving slit 0.02 mm. The employed radiation was monochromated Cu Kα (40kV – 40 mA). Fatigue crack propagation tests were performed by using CT specimens obtained by machining the cast ingots according to ASTM E 647 prescriptions. Both lateral surfaces have been metallographically prepared to Scanning Electron Microscope (SEM) observations. A servohydraulic testing machine has been used in order to evaluate fatigue crack propagation using the ASTM E 647 testing conditions. Fatigue cracks propagation tests are performed by using the following conditions: 1)  P = const. 2) R = P min /P max = 0.1 and 0.5. 3) Load: Sinusoidal waveform. 4) Load frequency = 30Hz. 5) Lab conditions. Finally, fracture paths have been analysed by means of a SEM in order to evaluate the loading conditions influence. R ESULTS AND COMMENTS OM observations show a traditional structure whose grain diameter mean value is about 700  m. Alloy microstructure is not completely homogeneous and grains are characterized by a needle like morphology as shown in Fig. 1. Figure 1: Etched surface of the investigated material. Needles microstructure is oriented: needles only partially cover the diameter of the prior austenitic grains. No subgrains are observed. The alloy inhomogeneities have been highlighted by crono-amperometric tests which the results are shown in Fig. 2a [9]. The electrochemical behaviour is characterized by a decrease of current due to presence of corrosion products on the etched surface and to the consequent lower passive property. Corrosion products are not homogenous due to the microstructure differences between grains bulk and boundary. L