Issue34

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 34 (2015) 415-421; DOI: 10.3221/IGF-ESIS.34.46 417 Alloy microstructure was investigated following the procedure described in ASTM E407-07 [8]. In order to evaluate the differences of diffraction spectra due to phases transformation, X-ray diffraction test were performed using minitensile specimens [9] considering both loaded and unloaded conditions. X-ray light was characterized by wave length of 1.53995 Å (Cu filament) and the analysis software PowderCell 2.3 [10] was used. Finally, CT specimens were obtained using the casted ingots according to ASTM E 647 [11]. Specimens lateral surfaces were metallographically prepared and chemically etched according to ASTM E407-07 [8]. This procedure allowed the specimens lateral surfaces observations by means of a light optical microscope (LOM). Fatigue crack propagation tests were performed by means of an hydraulic testing machine according to ASTM E 647 standard [11]. Fatigue crack propagation test were performed according the following conditions:  Stress ratio R = P min /P max =0.10, 0.50, 0.75:   P = constant  Sinusoidal waveform  Loading frequency=30Hz  Lab conditions Fracture surfaces were analyzed by means of a Scanning Electron Microscope (SEM) in order to evaluate the main crack micromechanisms corresponding to the different applied ΔK values. R ESULTS AND DISCUSSION Microstructural analysis and X-ray diffraction results OM observations show a grain diameter mean value of about 600μm. Microstructure is not completely homogeneous and grains are characterized by a needle like morphology as shown in Fig. 2a and b. a) b) Figure 2 : Etched surface of investigated material: a) grain, b) bulk needles. Needles structure covers the complete diameter of grains. No subgrains are observed. X-ray analyses are shown in Fig. 3, where, in unloaded condition, four main peaks at 2θ=79.79, 43.46, 41.73 and 70.23° are evident. Under loaded conditions, they sharply reduce and new peaks grow at 2θ=68.86 and at 2θ of 41.73°. The observed spectra are very close to spectra observed by authors in a previous work [3]. Difference between “unloaded” and “loaded” conditions are due to the microstructure modification due to the presence of induced martensite. Fatigue crack propagation results Fatigue crack propagation results are shown in Fig. 4. Considering R = 0.10 (Fig. 4a), five different stages can be observed [3]. It is worth to note the presence of a plateau in the stage III (with a crack growth rate of about 4 10 -8 m/cycle). Stages I and II approximately correspond to the threshold stage and to a sort of Paris stage with a high “m” value, whereas stages IV and V correspond to a sort of Paris stage (but L 50  m 500  m