Elastic diffuse scattering of neutrons in FeNi Invar alloys

Elastic diffuse scattering of neutrons was found around various Bragg-peak positions in FeNi Invar alloys. The diffuse scattering intensities depend on the temperature and Ni concentration. The intensities increase with decreasing temperature and decrease with increasing Ni concentration. The distribution of diffuse scattering intensity changes from peak to peak and is well explained by the formation of clusters with a lattice deformation consisting of a shear wave propagating along the < 1 1 0 > direction and with the < 1 - 1 0 > polarization vector. The ranges of temperature and Ni concentration, for which diffuse scattering is observed, coincide with those for which the Invar anomalies are observable. The origin of the clusters together with the lattice deformation and their role with regard to the Invar effects are discussed as well as the possibility of a precursor for the fcc-bcc martensitic transformation observed in FeNi alloys.