Microstructural Studies of a Ni–Al–Mo Directionally Solidified Eutectic Composite

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Abstract
Transmission electron microscopy and selected area electron diffraction have been used to investigate the microstructures of the directionally solidified eutectic composite of a Ni-12.7 at%Al-21.6 at%Mo alloy. The as-grown composite consists of Mo fibers of square or rectangular cross-sections surrounded by an ordered γ′ phase. Between the γ′ phase regions a multi-phase region is formed. A selected area electron diffraction analysis of the multi-phase region suggested that the region consists of Ni3Mo(tetragonal D022), Ni2Mo(body-centered orthorhombic) and γ(face-centered cubic) matrix phases. Heating the composite above 1490 K followed by quenching results in the structure consisting of Mo fibers surrounded by the γ matrix dispersed with the ordered γ′ phase. The characteristic microstructure of the as-grown composite is ascribed to the eutectic solidification, subsequent peritectoid reaction and precipitation of coherent phases in the γ matrix during cooling from the eutectic temperature.