Cast and aged beta-NiAl-beta '-Ni2AlTi-gamma '-Ni3Al-alpha-Cr alloys: a microstructural and mechanical properties investigation

Microstructural development and stability in cast Ni-30 at % Al-21 at % Cr- 4 at % Ti, Ni-28 at % Al-18 at % Cr-6 at % Ti and Ni-26 at % Al-19 at % Cr- 8 at % Ti alloys are examined in this paper. The paper considers the extent to which microstructural control and stability is improved in these alloys , when compared with earlier work by the authors on similar materials with lower aluminum (25 or 26 at %) and higher titanium (11 to 15 at %) contents . The paper discusses detailed transmission electron microscopy investigati ons of both as-cast and aged (140 h at 850 and 1100 degrees C) samples. Att ention is focussed on the transformation of L2(1) type beta' (nominally Ni2 AlTi) dendrites. In particular, the influence of the formation of intradend ritic L1(2) type gamma' (nominally Ni-3(Al,Ti)) on the stability of the bet a'-phase is examined. Morphological changes and phase transformations withi n a beta'-alpha-Cr eutectic are discussed. Various modes of second-phase pr ecipitation are considered. The microstructures of selected samples are cor related with mechanical properties determined by tensile testing and fracto graphy. (C) 1999 Kluwer Academic Publishers.