PSI - Issue 69

Alberto Coda et al. / Procedia Structural Integrity 69 (2025) 26–34

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untreated on the right.

Heat treatments had very little effects on the microstructural features of the alloys as shown in selected representative micrographs in Figure 4a and 4b. On the other hand, thermal treatments at higher temperature or for longer time are more effective in the partial dissolution of secondary phase at the grain boundaries. As an example, in Figure 4 the microstructural evolution of Alloy 5 with Co addition was reported. It can be observed that the secondary phases begin to form with a small globular morphology by increasing the temperature of the thermal treatment. At 1350°C, the segregations at the grain boundaries appear to be partially spread in the matrix, due to diffusion phenomena occurring at high temperature and for longer times. The chemical composition of the matrix remains the same.

a)

b)

c)

(Ni, Co)Ta

(Ni, Co) 3 Ta

(Ni, Co)Ta

(Ni, Co) 3 Ta

(Ni, Co) 3 Ta

Ni 8 (Ta, Co)

(Ni, Co)Ta

10 µ m

10 µ m

10 µ m

As-cast

1200°C x 4h

1350°C x 4h

Figure 4 Representative micrographs of alloy 5: a) as cast; b) after annealing at 1200°C for 4h; c) after annealing at 1350°C for 4h.

3.2 Microhardness Figure 5 summarizes microhardness results for as-cast and thermally treated samples. Looking at values and trends, it can be seen that additions of Cu and Co were not effective in improving the plastic deformation behavior at room temperature of the intermetallic compound Ni 3 Ta. Even thermal treatments at high temperature had a very little influence on microhardness values that all are in the range of 500 - 600 HV or above in few cases. These microhardness values are significantly higher than other HT-SMA systems, like NiTiHf or NiTiZr (above 300 HV) in almost the same homogenized conditions [ 9] , showing intrinsic brittleness and several plastic deformation issues under conventional processing. However, for the ternary alloys prepared in this work still limited information about the mechanical behavior are available and more tests are necessary to clarify the effect of Cu and Co on the softening and deformation conditions.