Neutron-scattering and electron-microscopy studies of the premartensitic phenomena inNixAl100−xalloys

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Abstract

Inelastic-neutron-scattering and electron-microscopy studies were performed on single crystals of the cubic phase of ${\mathrm{Ni}}_{\mathit{x}}$${\mathrm{Al}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ alloys for x=50, 58, 62.5, and 63.9 at. %. The [110]-${\mathrm{TA}}_{2}$ phonon branch, corresponding to atomic displacements along the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction, is shown to exhibit anomalous behavior in that the entire branch is very sensitive to composition and exhibits a ``kink'' whose position in q space is also sensitive to x. A peak in the elastic diffuse scattering is also present at the same q as the kink. Amplitude-contrast electron-microscope images display a ``tweed'' pattern characteristic of many martensitically transforming materials. The high-resolution (phase-contrast) electron image reveals microscopic deviations of the cubic structure and can be viewed as embryos of the low-temperature (5,2\ifmmode\bar\else\textasciimacron\fi{}) martensitic phase. The displacement fields associated with these distortions are shown to be the origin of the elastic central peak observed in the experiments. Temperature-dependence studies show a substantial softening of the phonon branch in the vicinity of the kink and the elastic diffuse scattering increases as T approaches ${\mathit{T}}_{\mathit{M}}$, the martensitic-transformation onset temperature. The mode never becomes completely soft. Furthermore, the softening is narrowly restricted to the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction, which is perpendicular to [110]. Studies of the phonon dispersion curve under uniaxial stress show a softening of the phonon mode near the q value of the kink, which can be interpreted in terms of Clapp's proposed localized-soft-mode theory of nucleation of materials undergoing martensitic transformation.

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