Effect of Temperature and Composition on NiAl Precipitation and Morphology in Fe-Ni-Al Alloys

NiAl-strengthened ferritic alloys have been of particular interest because of their possibilities as a high-temperature material for power-generation purposes. In the present work, the effect of temperature and composition on the precipitation of the NiAl ( β ′) phase was studied using a diffusion couple made of alloys Fe 0.50 Ni 0.25 Al 0.25 and Fe 0.80 Ni 0.08 Al 0.12 . The composition gradient was obtained with a diffusion annealing treatment at 1373 K (1100°C), and the precipitation was promoted by aging at 1123 K, 1173 K and 1223 K (850 °C, 900 °C and 950 °C) for 5 hours. The formation of a supersaturated solid solution and the precipitation of the β ′ phase were obtained after the diffusion annealing and aging treatments, respectively. A gradual increase in the size and volumetric fraction of the precipitates was evident in compositions with higher NiAl. This promoted changes in the precipitate morphology that followed the sequence: rounded cuboids → plates → irregular (maze-like). Compositions with a low β ′ volumetric fraction followed the predicted size distributions of the Lifshitz-Slyozov-Wagner (LSW) theory. Finally, it was observed that the hardness tends to increase inversely to the Fe content and decreases only because of precipitate coarsening. The achieved results provide new information regarding the dependency of the morphology of β ′ precipitates with composition and temperature.