Microstructural development in cast alloys based on the β-NiAl–β′-Ni2AlTi–γ′-Ni3Al–α-Cr system

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An investigation of microstructural development in three arc-cast Ni–Al–Cr–Ti multiphase alloys derived from the B2 type β-NiAl phase is presented. Detailed microstructural characterization of Ni–25 at % Al–20 at % Cr–15 at % Ti, Ni–26 at % Al–21 at % Cr–11 at % Ti and Ni–25 at % Al–24 at % Cr–15 at % Ti materials by transmission electron microscopy (TEM), is described. Microstructural development is examined in both the as-cast condition and after 140 h homogenization treatments at both 850 and 1100 °C. The formation of a eutectic between an L21-type β′ phase (Heusler phase, with a nominal composition of Ni2AlTi) and elemental α-Cr is examined. The precipitation of α within β(′) and vice versa, in both the inter- and intradendritic regions, is considered. The formation of L12-type γ′ (nominally Ni3Al) precipitates within the β and β′-phases is discussed, as is the transformation of β to two-phase β/β′ during ageing.

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