Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 454-461; DOI: 10.3221/IGF-ESIS.30.55 457 4.E‐03 5.E‐03 6.E‐03 7.E‐03 8.E‐03 9.E‐03 1.E‐02 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Current density [A/cm 2 ] Time [s] a) b) Figure 2 : Electrochemical behavior and microstructure etched of material: a) polarization curve, b) microstructure of boundary grain. “Acicular like” zones near grains boundaries are shown in Fig. 2b. Evidence of microstructure transitions are in Fig. 3, where two diffractograms show respectively the undeformed and the deformed at  eng = 5% specimen. 0 200 400 600 800 1000 1200 1400 40 50 60 70 80 90 Intensity 2θ [°] Serie1 Eng. Epsilon 5% ε eng = 0% ε eng = 5% Figure 3 : Diffraction spectra. The undeformed specimen spectrum shows four peaks corresponding to 42.35°, 43.71°, 70.39°, 80.23° that correspond to two different phases [10]: - Cubic B 2 phase (like CsCl lattice), sometimes named as β 2 that take place at 550°C, but measured peaks are not sufficient to evaluate the cell parameter; - Cubic L 21 phase (like Cu 2 MnAl), sometimes named as β 3 that take place at 320°C, characterized by cell parameter about a=5.8707 Ǻ. The  eng = 5% deformed specimen shows also four peaks but corresponding to different diffraction angles (42.27°, 43.43°, 43.85° and 85.71°). Presence of these peaks are compatible with presence of two phases [11, 12]: - Austenite β 3 (L 21 structure not stress induced transformed), characterized by cell parameter about a=5.8707 Ǻ as in the undeformed state.; - Martensite M18R structure, characterized by a=4.4189 Ǻ, b=5.332 Ǻ, c=38.8 Ǻ and angle β=89.7°. Peaks modifications (considering both angles and intensity) show the mechanical deformation influence on the microstructure modifications. Fatigue crack propagation results (da/dN-  K) at R=0.1 and R=0.5 are shown in Fig. 4a.