Phase separation and coarsening of NiAl (β′) intermetallic in quench-aged Fe–Ni–Al alloys

The coarsening behavior of four quench-aged Fe–Ni–Al alloys with different Ni and Al contents is studied. The alloys were solubilized at 1100 °C for 24 h, water-quenched and aged at 850 °C for periods ranging from 1 to 100 h to promote phase separation and coarsening. The two alloys with the highest combined Ni and Al contents (≥ 35 at.%) showed signs of spinodal decomposition, while the alloys with lower Ni and Al contents followed a classical nucleation process. Higher Ni and Al contents promoted an increase in NiAl (β′) phase fraction in concordance with phase equilibrium diagrams. The alloy with the highest β′ fraction presented a completely interconnected structure. The coarsening of β′ particles in the other three alloys followed the predictions of the Lifshitz–Slyozov–Wagner (LSW) theory. The general trend was an increase in the coarsening kinetics with higher Ni and Al contents. The highest hardness was found in the alloys with higher β′ fraction albeit these compositions are less resistant to coarsening and have been reported to present a low room-temperature ductility.