The microstructures of multiphase Ni-20Al-30Fe and its constituent phases

The microstructures of the multiphase (β′ + γ/γ′) alloy Ni-20Al-30Fe (in at.%) and alloys similar to its constituent β′ (Ni-30Al-20Fe) and γ/γ′ (Ni-12Al-40Fe) components, processed through a casting and hot-extrusion route, were studied after water quenching following annealing at 700K, 900K, and 1100K. The microstructures at 700K and 900K were similar to the room temperature microstructure, consisting of a β′ pro-eutectic phase and a eutectic of β′ and γ/γ′ phases (0.5μm lamellae width). However, at 1100K, the γ′ dissolved. The nominally Ni-12Al-40Fe alloy was determined, using energy dispersive x-ray spectroscopy, to be of composition Ni-15Al-37Fe. The alloy exhibited a γ/γ′ microstructure at all test temperatures that was consistent with the phase diagram for the latter composition; the γ′ precipitates were observed to coarsen with increasing temperatures. The β′ alloy, Ni-30Al-20Fe, exhibited an equiaxed microstructure at room temperature with a thin (~ 10nm), discontinuous iron-rich grain boundary film. At 900K, the microstructure was three-phase, β′ + γ/γ′, instead of two-phase, β′ + γ, as predicted by the isotherms. At 1100K, however, the microstructure was two-phase β′ + γ, which was consistent with the isotherms.