PSI - Issue 69

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

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3. Results 3.1. Microstructural characterization

SEM images, using back scattered electrons (BSE) signal, of the alloys are shown in Figure 1, where columnar grains of relevant length and width equal to several tenths of microns are depicted. The matrix, present in the grains, is characterized by Ni/Ta ratio equal to 3:1, as reported in literature by Firstov et al. for the binary alloy [12]. At the border of the grains different secondary phases have been mainly detected. The binary alloy shows the presence of two secondary phases, one richer in Ni content than the matrix (Ni8Ta) and the second one richer in Ta content (Ni2Ta), as confirmed by the phase diagram of the system [22].

Fig. 1. SEM-BSE micrographs of NiTa (a), NiTaB-A1 (b) and NiTaB-A2 (c) in as cast condition.

This means that the cooling from the melting point was quite slowly permitting the generation of these secondary phases, as reported in the literature. On the other hand, the presence of these secondary phases looks to be negligible with respect to the matrix, according with XRD analysis not here reported. The B addition of 0.8% at in the place of Ta and Ni does not drastically change the microstructure of the alloys. In fact, in the NiTaB-A2 the two secondary phases are both present in proximity of the grain boundary while the presence of a unique secondary phase, Ni8Ta, has been detected in the NiTaB-A1. The amount of B was not detected, as it could be expected, because too low in comparison with the measurement error of the EDS analysis. Moreover, it could be affirmed that probably B is homogenously dispersed in the matrix, because it is not detected in the minority phases. Suzuki et al. neither observed the presence of B in the matrix, because its precipitation in TiB2 was detected in TiPdNi alloys [19]. These small precipitates, in the size of 1 µ m, were seen being distributed in the matrix. The B addition does not noticeable change the grain size, as it could be expected. Suzuki et al. showed that the B doping can limit the grain growth rate during annealing at high temperatures in the B doped NiPdTi system [19].

Fig. 2. SEM-BSE micrographs of NiTaB-A2 after TT1 (a) and TT2 (b).

After the homogenizing thermal treatment, the secondary phases are mainly broken and then dispersed in the matrix at increasing the treatment temperature. Figure 2 shows the evolution of the microstructure of NiTaB-A2 after the thermal treatment respectively at 1200°C and 1400°C. As it can be observed, the secondary phase (the white one) looks to be separated in small fractions in comparison with the as cast condition (see Figure