Neutron scattering study of premartensitic behavior in Ni-Al alloys

Phonon dispersion curves for the [110] 〈1¯10〉 ${\mathrm{TA}}_2$ branch in ${\mathrm{Ni}}_{\mathrm{x}}$ ${\mathrm{Al}}_{100\mathrm{-}\mathrm{x}}$ (50<x<65) reveal a pronounced minimum whose position in q space is composition dependent but whose frequency does not change appreciably with temperature. In the same q range, substantial elastic scattering also appears which is associated with localized embryos of a low-temperature phase. The diffuse scattering from the elastic displacement field about these embryos is highly anisotropic, giving rise to the striated strain contrast (tweed) in the electron microscope image. A common electronic origin of these effects is proposed.