TEM characterization was conducted on the ABH-973-48 sample to further identify the Al-rich precipitates inside the grains. Fig. 6a shows large submicron-scale precipitates with a dark shell dispersed inside the grain, with an average size of ~600 nm. Numerous small particles within the large precipitate, marked by a dashed rectangle (regions R6 and R7), were observed with an average grain size of ~50 nm. The enlarged TEM images of regions R6 and R7 are shown in Figs. 6b and c, respectively, revealing that the tiny precipitates consist of two types of semi-spherical particles. Chemical qualitative analysis results (Figs. 6d-i) confirm that elements Co, Cr, and Ni are depleted in the precipitates. The large precipitate is rich in Al while the tiny ones are rich in Ti. O appears in some Al-rich particles with an average chemical composition determined to be Al55.83O44.14.
According to FFT patterns corresponding to region R5 (Fig. 6j), Al-rich particles can be identified as a κ-Al2O3 intermediate phase with a P63mc crystallographic space group resulting from air trapped between powders during laser melting causing oxidation of more reactive elements such as aluminum [48,49]. Corresponding FFT patterns of regions R8 and R9 displayed in Figs. 6k and l suggest that nanoparticles were h-Al2Ti and Al5Ti3 phases, respectively; their occurrence at low temperatures is attributed to TiAl decomposition [48,49]. Although these particles have small volume fractions, they imply that Al and Ti may segregate inside grains or at grain boundaries acting as nucleation sites for other nanocrystals.
To verify this speculation further TEM was conducted on the ABH-973-48 sample shown in Fig. 7.
