Issue 18

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 18 (2011) 45-53 ; DOI: 10.3221/IGF-ESIS.18.05 47 M ATERIAL AND METHODS n this work a commercial pseudo-elastic NiTi alloy (Type S, Memory metalle, Germany), with nominal chemical composition of 50.8at.% Ni - 49.2 at.% Ti, was used to investigate the evolution of microstructure during loading- unloading cycle. In Fig. 2 light micrographs of the initial austenitic microstructure of the alloy is illustrated at different magnification, which shows the presence of inclusions and subgrains. This is an expected results as the inclusions play a significant role in the stress-induced phase transformation mechanisms and, consequently, in the macroscopic pseudo-elastic response of the alloy. a) b) Figure 2 : Initial austenitic microstructure of the investigated NiTi alloy: a) at low magnification showing the presence of inclusions, b) high magnification with presence of subgrains. The engineering stress-strain curve of the material is illustrated in Fig 3, which were obtained from isothermal tensile tests, carried out at room temperature (T=298 K) by using a servo hydraulic universal testing machine, equipped with an electrical extensometer with a gauge length of 10 mm to measure the engineering deformations of standard dog-bone shaped specimen. In particular, Fig. 3.a shows the stress-strain curve obtained from a monotonic tensile test to fracture, while Fig. 3.b illustrates the marked hysteretic behavior of the material obtained from a loading-unloading cycle up to a maximum deformation of about 6.1 %. Furthermore, Fig. 3.b also illustrates the values of the main thermomechanical parameters of the alloys, i.e. the Young’s moduli of austenite (E A ) and martensite (E M ), the transformation stresses from austenite to martensite (  s AM and  f AM ), the stresses for reverse transformation from martensite to austenite (  s MA and  f MA ), the uniaxial transformation strain (  L ) and the Clausis-Clapeyron constant of the material (C=d  /dT). The evolution of the microstructure during uniaxial deformation was analyzed by a miniature testing machine which allows in-situ scanning electron microscopic (SEM) observations as well as X-Ray micro-diffraction analyses. In particular, the testing machine is equipped with a simple and removable loading frame, which allows SEM and X-Ray analyses at fixed values of applied load and/or deformations. The machine is powered by a stepping motor, which applies the mechanical deformation to the specimen through a calibrated screw, with pitch of 0.8mm, and a control electronic allows simultaneous measurement and/or control of applied load and stroke of the specimen head. The stroke is measured by a Linear Variable Differential Transformer (LVDT) while the load is measured by two miniaturized load cells with maximum capacity of 10 kN. Miniature dog bone shaped specimens with dimension showed in Fig. 4 were machined from NiTi sheets, by wire electro discharge machining, due to the poor workability of this class of materials by conventional machining processes as well as to reduce the formation of thermo-mechanical affected zone. Step by step isothermal tensile tests were carried out, at room temperature, at increasing values of the specimen elongation. In particular, three levels of elongation have been applied,  =0.8, 1.6 and 2.4 mm, which can be expressed as gross engineering strain (  g ) with respect to the gauge length L 0 =16 mm, i.e.  g =5, 10 and 15%. In particular, for each loading step the loading frame containing the specimen was removed from the testing machine, at fixed values of deformation, and analyzed by means of a Philips diffractometer in order to evaluate XRD spectra. XRD measurements were made with a Philips X-PERT diffractometer equipped with a vertical Bragg–Brentano powder goniometer. A step–scan mode was used in the 2θ range from 30° to 90° with a step width of 0.02° and a counting time of 2 s per step. The employed radiation was monochromated CuKα (40 kV – 40 mA). The calculation of theoretical diffractograms and the generation of structure models were performed using the PowderCell software [20]. I