PSI - Issue 69

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

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1c), when the thermal treatment is performed at low temperature (1200°C). By increasing the temperature of the thermal treatment till 1400°C, the white phase appears to be spread in the matrix, probably due to diffusion phenomena at high temperature. Besides, the chemical composition of the matrix remains exactly the same. As shown in the thermal scans at high temperature of Figure 3, a unique melting point was detected for all the alloys during the heating; this means that the Ni3Ta matrix is greatly predominant and the contribute of the secondary phases is negligible, as confirmed by the SEM analysis, where their trace is just present. Moreover, the melting point looks to be not influenced by the addition of such as small percentage of B. During the cooling, on the contrary, secondary peaks can be seen for the ternary alloys; in fact, the alloy NiTaBA2 shows a

secondary peak, very wide, at about 1380°C in the cooling. The intensity of this secondary peak is increased in the alloy NiTaBA1 and it is also characterized by a significant decrease of its temperature (below 1250°C). Fig. 3. HTDSC scans of the alloys in as cast condition upon heating and cooling.

3.2. Evolution of the martensitic transformation Remarkable thermal stability of the MT by the 20th cycle of NiTaB-A1 after TT2 can be observed from the DSC scan of Figure 4. During heating a small shift on the temperature axis of the reverse MT peak can be observed while any variation can be seen during the direct MT. Moreover, in contrast with the XRD and HTDSC analyses, here multi peaks during both heating and cooling can be observed from Figure 5, due to the coexistence of different phases involved in the MT. The thermal stability during the MT of these alloys appears to be of great relevance and the transformation temperatures over 200°C can be reached. The heats exchanged and the corresponding characteristic temperatures, corresponding to the second thermal cycle, are reported in Table 1. The heats exchanged are quite low, in the range of 0.5-2 J/g, in comparison with the other SMAs [3]; their values decrease after the homogenizing thermal treatments for NiTa while the opposite trend has been detected for both the ternary alloys. It can be appreciated that after the thermal treatments, the heats exchanged are higher for the ternary alloys in comparison with the binary one. This is a benefit because a bigger amount of material is involved in the MT; this effect could be associated to the additional presence of B in the matrix, finely dispersed. On the contrary, the characteristic temperatures of the direct and reverse MT are above at least 200°C. Both the composition and the condition of the alloys look to influence the characteristic temperatures, even if a certain trend cannot be identified with clarity. Concerning the direct MT, the start (Ms) and finish (Mf) martensitic temperatures range in the interval 255-275°C and 25-255°C, respectively. Ms has been increased in NiTa after the thermal treatments while the addition of B produces the opposite effect; the increase of Mf has been detected in NiTaB-A2 after the thermal treatments while Mf shows a minimum point at TT1 for the other alloys. Besides, the characteristic temperatures of the reverse MT (As and Af) range in the interval 290-350°C and 310-400°C, respectively. In particular, As can be increased by thermal treatments for NiTaB-A1 while it can be decreased for the other alloys; Af is not changed by the thermal treatment for NiTa while it is raised up for the ternary alloys. The thermal hysteresis of