Issue 21

C. Maletta et alii, Frattura ed Integrità Strutturale, 21 (2012) 5-12; DOI: 10.3221/IGF-ESIS.21.01 7 DSC thermogram of the raw material which was obtained at a heating/cooling rate of 1.6 Ks-1 in the temperature range - 100°C to 100°C. This analysis revealed the presence of a two-stage phase transformation (B2 R B19’) during cooling, with the presence of R-phase (Rhombohedral phase), while a single-stage phase transformation (B2 B19’) was observed during heating, as is usual in Ni-rich NiTi alloys. The figure also gives the values of the transformation temperatures (M s , M f , A s , A f , R s and R f ), which have been estimated by the tangent line method; the alloy shows an austenite finish temperature A f = 13,7°C, which indicates a fully austenitic structure at room temperature, i.e. the alloy exhibits a pseudoelastic response. Fig. 1.b presents a stress-strain curve of the alloy obtained from an isothermal ( T = 298 K) displacement controlled loading-unloading cycle up to a maximum deformation of 6.2% which corresponds to the maximum deformation of the stress-strain transformation plateau. The figure also shows the values of the main mechanical parameters of the alloy, Young’s moduli ( E A and E M ), transformation stresses   , , , s f s f AM AM MA MA     and transformation strain   L  , together with the Clausius-Clapeyron constants   / , / A MA M AM C d dT C d dT     , which have been obtained from isothermal tests carried out at different temperatures. (a) (b) Figure 1 : Thermo-mechanical properties of the alloy investigated: a) DSC thermograph with transformation temperatures and b) Loading-unloading isothermal stress-strain cycle (298 K). Indentation tests The indentation response of the alloy investigated has been determined by using a NanoTest 600 (Micro Materials Ltd, United Kingdom) nanoindenter. Rectangular shaped samples (20 mm x 10 mm), were cut from the as-received sheet and prepared, prior to indentation tests, by grinding with progressively finer silicon carbide papers (#800-#4000), and polishing with 1  m diamond compound; finally, the specimens were cleaned with acetone and dried in air. After the mechanical polishing procedure the specimens were analyzed by a 3-D optical profilometer (Infinite Focus, Alicona, Austria) to ensure that the surface finish was within acceptable limits for micro indentation measurements. Indentation tests were carried out at room temperature, using a spherical indenter (R=25  m), as a sharp tip indenter (such as Berkovich, Vickers, etc.), causes high strain gradients immediately beneath the indenter which promote plastic deformation, which inhibits the subsequent reverse transformation from martensite to the parent phase. In fact, previous research [11] has demonstrated that there is no evidence of superelastic recovery upon unloading when a Berkovich indenter is used, while a large recovery is observed when using a spherical tip indenter. Preliminary indentation tests were performed to identify optimum test parameters, such as maximum load range, loading/unloading rate and dwell time, in order to reduce measurement errors and avoid creep effects on the P-  curve. Several indents were made at increasing values of maximum load (50, 150, 300 and 450 mN), with a loading/unloading rate of 2.5 mNs -1 and a holding time of 60 seconds at the maximum load; furthermore, a set of 20 indentations were carried out for each value of the maximum load, so as to capture the average response of the material, i.e. to analyze different grains of the polycrystalline structure.