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

An investigation of microstructural development in three are-cast Ni-Al-Cr-Ti multiphase alloys derived from the B type beta-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-ll 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 degrees C. The formation of a eutectic between an L2(1)-type beta' phase (Heusler phase, with a nominal composition of Ni2AlTi) and elemental alpha-Cr is examined. The precipitation of alpha within beta((/)) and vice versa, in both the inter- and intradendritic regions, is considered. The formation of L1(2)-type gamma' (nominally Ni3Al) precipitates within the beta and beta'-phases is discussed, as is the transformation of beta to two-phase beta/beta' during ageing. (C) 1998 Chapman & Hall.