Issue 18

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 18 (2011) 45-53 ; DOI: 10.3221/IGF-ESIS.18.05 50 austenitic phases as illustrated in Fig. 2 at different magnifications. Using Bragg’s low, the cell parameter evaluated for the two picks is about 3.012 Å. Figure 7 : Relation between gross and net engineering strain obtained from FE simulations. Figure 8 : X-Ray spectra of the investigated NiTi alloy in stress free condition. The evolution of the microstructure during mechanical loading and subsequent unloading was analyzed by XRD investigations carried out at fixed values of applied deformations. In particular, when loading up to the gross engineering strain  g =5%, corresponding to the effective engineering strain  e =4.5% and to a stress  =400 MPa (Fig. 7), a different spectra was obtained with respect to the stress-free condition, as illustrated in Fig. 9; in fact three new pecks at different angles (42.99, 78.17 and 80.29°) are observed and, as expected, these new pecks reveal the presence of another phase which is obtained on stress plateau of the  curve. In addition, the simultaneous presence of two phases indicate an incomplete transformation, i.e. a partial transformation from austenite to a martensite, characterized by different cell parameter and different lattice. Evidence of phase transition is confirmed when increasing the gross deformation to  g =10%, corresponding to the effective engineering strain  e =7.9% and to a stress of about  =800 MPa (Fig. 7); this loading condition corresponds to the fully transformed martensitic structure as illustrated in Fig. 7. In this case the new phase is completely developed and his spectra is illustrated in Fig. 10, where five picks are observed corresponding to a monoclinic phase characterized by three cell parameters of about a=b=3.800 Å, c=2.600 Å and α=80°. The Miller indexes of the three peaks are: 35 45 55 65 75 85 95 Counts [u. a.] 2 · θ [°] A ‐ [011] A ‐ [022]