Microstructural changes observed in a rapidly solidified Ni3Al alloy during isothermal treatments

A series of metastable conditions arises in Ni3Al alloys out of their rapid solidification, the most important one being the formation of a bimodal distribution of ordered domain sizes. The fine ordered domains are surrounded by elongated ones. Another characteristic is that the number of vacancies traps is higher than the equilibrium one. In this study, several tapes obtained by melt-spinning a Ni3Al alloy with 76.5 at% Ni were annealed at 500°C for several periods from 0.5 to 120 h. These tapes were studied by means of transmission electron microscopy and X-ray diffraction. The latter technique allowed us to detect that the degree of ordering decreases in the first stages of the treatment; this occurs due to the formation of prismatic dislocation loops as a result of vacancy precipitation, this behavior is also due to the initial high long range order. Thus, the beginning of atomic movement causes the “right” sub-lattice atoms to occupy “wrong” sub-lattice positions. The structure recovers at the end of the process, followed by an increase in the degree of ordering. The vacancy precipitation was found to be heterogeneous; the region with higher density of fine ordered domains provides more sites to prismatic defects than the one with coarse domains.