PSI - Issue 69

Carlo Alberto Biffi et al. / Procedia Structural Integrity 69 (2025) 61 – 68

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literature, dealing with the microstructural and thermo-mechanical characterizations of HT SMAs. Among these, NiTiX alloys (with X = Zr, Hf, Pd, Pt) have shown promising functional properties with transformation temperatures in the range of 100-250°C [3-7]. The literature proposes also some works on other less common systems, such as Cu based alloys with a decent shape memory effect but low thermal stability [8], RuNb [9] with very high HT martensitic and NiMnGa, in the form of bulk or thin films, able to offer an additional ferromagnetic transformation [10]. Other investigated metallic SMAs for high temperature applications can be Ni free [11] or Ti free [12-16], Firstov et al. [12] proposed Ni3Ta system as innovative HT SMA with interesting high temperatures and good thermal stability, even if its mechanical and workability properties seem to be very poor. Rudajevova et al. [13] studied in a first work the mechanical behaviour of this alloy in both non-deformed and pre-deformed states. Following, in a second work, the investigated the thermal properties of the binary alloy in both single crystal and polycrystalline forms [14]. Literature suggests the B addition in few quantities for improving the mechanical properties; an evident benefit was found on the ductility and strength of some brittle systems, such as Ni3Al [17-18] and NiPdTi [19], thanks to B addition. Microalloying with B addition, occupying interstitial sites in Ni3Al alloy, improves the ductility and strongly reduces the intergranular fracture at room temperature [18]. The B addition was also tested in NiTi based SMAs. Limited amount of B in TiPdNi and NiTi can improve the high temperature mechanical properties, thanks to its precipitation in micro size TiB2 [19] and the damping performances in a wide temperature range [20]. Due to the benefits observed in other alloys, Biffi et al. explored the effect of B addition in Ni3Ta system in different contents on the MT [21]; the phase transformation was not suppressed for limited B contents. The present work is focused on the effect of B addition in Ni3Ta SMA in the place of both Ni and Ta, respectively, on the correlation among the MT, the microstructure and the mechanical properties. 2. Experimental High purity Ni, Ta and B elements were melted by means of a vacuum arc furnace (Leybold mod. LK 6/45) with a non-consumable tungsten electrode. The melting of the alloys was performed in a water-cooled copper crucible under inert atmosphere (constant Ar flow) in order to avoid the contamination of the liquid pool. The composition of the produced alloys is here reported: Ni 75 Ta 25 , Ni 75 Ta 24.2 B 0.8 and Ni 74.2 Ta 25 B 0.8 [at %]. From here, the ternary alloys are indicated for sakeness respectively as NiTaB-A1 for Ni 75 Ta 25 , Ni 75 Ta 24.2 B 0.8 and NiTaB-A2 for Ni 74.2 Ta 25 B 0.8 . The obtained ingots are six times remelted in order to increase the chemical homogeneity degree. Moreover, annealing at 1200°C (namely TT1) and at 1400°C (namely TT2) for 4 hours were performed in argon with following water quench. A Seiko model 520 differential scanning calorimetry (DSC) was used to study the martensitic transformation as function of the different samples. During the DSC measurements, the temperature of the thermal cycle was in the range from 50 ºC up to 400ºC using a heating/cooling rate of 10°C/min. Besides, high temperature calorimetric analysis (HTDSC) was also performed on as cast alloys with 1°C/min heating rate till 1450ºC for the evaluation of the critical points. The microstructure and the corresponding chemical composition of the alloys were analyzed by means of a scanning electron microscope (SEM), equipped with an energy dispersive spectroscopic (EDS) microanalysis probe. Mechanical properties have been evaluated by means of compression tests at 30°C (below Mf temperature). Cylindrical samples (diameter=5 mm; height= 7 mm) were tested up to a maximum strain of 1.5%., using a MTS universal testing machine (0.2 mm/min rate), equipped with thermostatic chamber and with 25 mm gauge length extensometer.