Cast and aged β-NiAl-β′-Ni2AlTi-γ′-Ni3Al-α-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 investigations of both as-cast and aged (140 h at 850 and 1100°C) samples. Attention is focussed on the transformation of L2₁ type β′ (nominally Ni₂AlTi) dendrites. In particular, the influence of the formation of intradendritic L1₂ type γ′ (nominally Ni₃(Al,Ti)) on the stability of the β′-phase is examined. Morphological changes and phase transformations within a β′-α-Cr eutectic are discussed. Various modes of second-phase precipitation are considered. The microstructures of selected samples are correlated with mechanical properties determined by tensile testing and fractography. [ABSTRACT FROM AUTHOR]