Precipitation of Cr-rich phases in rapidly solidified Ni-20Al-8Cr (at.%) powders

The evolution of the microstructure in rapidly solidified Ni-20.9Al-8Cr-0.49B (at.%) powders after different continuous and isothermal heat treatments at temperatures up to 1100 °C has been studied by electron microscopy and microanalysis. Powders in the rapidly solidified condition have a dendritic microstructure consisting of Ni₃Al dendrites and a NiAl phase in the interdendritic regions. Chromium is in solid solution in both phases. This microstructure is stable when heating at 10 K min^-1 up to 750 °C. When the powders are heated up to 950 °C, partial dissolution of the NiAl phase and the precipitation of very small chromium-rich particles take place. The microstructure of the powders after annealing at temperatures between 750 and 1100 °C for different times is characterised by the dissolution of the β-NiAl phase and the simultaneous precipitation of various Cr-rich phases. α-Chromium, the metastable X-phase, and dark polygonal Cr₅B₃ precipitates have been identified. The segregation of chromium and boron in the form of borides removes these elements from the intermetallic matrix, so the content of both elements should be optimised to preserve their beneficial influence on the ductility of the γ′-Ni₃Al phase.