X-Ray Diffraction Investigation of β′-NiAl Alloys

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Abstract
An x-ray diffraction investigation of the structure of β′-NiAl alloys was conducted to define the state of order, identify the lattice sites occupied by compositional vacancies, and measure both static and vibrational components of atomic displacements, all as functions of temperature and composition. Long-range order was found to be virtually complete up to 1000 °C and remained substantial up to the melting points of the alloys. Compositional vacancies present in Al-rich β′ alloys were found to occupy Ni lattice sites only, at least to 1000 °C. Static atomic displacements were minimal at stoichiometry, increased with initial Ni or Al additions, and then decreased with further Ni or Al additions, with the maximum displacement about midway between stoichiometry and the β′ phase boundaries. Vibrational atomic displacements were also minimal at stoichiometry, but in contrast to static displacements, increased continually as either Ni or Al was added. Atomic displacements were correlated with known mechanical properties of β′-NiAl alloys.

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