PSI - Issue 69

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

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2. Results

3.1 Microstructure Microstructural investigations revealed valuable insight regarding the addition of Cu and Co to the original Ni 3 Ta intermetallic compound. Figure 3 shows optical micrographs (left column) and SEM-BSE images (right column) of the five alloy samples. From a general perspective, as one can appreciate, all as-cast samples show segregation of intermetallic secondary phases at grain boundaries, potentially causing intrinsic brittleness. Columnar grains of relevant length and width equal to several tenths of microns are revealed. The matrix is characterized by Ni/Ta ratio equal to 3:1, as expected for the binary alloy [18]. The binary alloy shows the presence of two secondary phases, one richer in Ni content than the matrix (Ni 8 Ta) and the second one richer in Ta content (Ni 2 Ta), as confirmed by the phase diagram of the system [24]. In ternary systems Cu is not present in the matrix, but only as Ta substitute in the grain boundary intermetallic phases. On the other hand, Co was detected both in the main matrix as Ni substitute and at the grain boundary in the intermetallic phases substituting either Ni or Ta. Both Cu and Co additions in the selected amount do not work as grain refiners as no difference in size could be measured by linear intercept method.

1

150 µ m

100 µ m

2

150 µ m

100 µ m

3

150 µ m

100 µ m

4

150 µ m

100 µ m

5

150 µ m

100 µ m

Figure 3 Representative micrographs of investigated alloys, etched microstructure on the left,