Origin of diffuse scattering appearing in iron-doped Ti-Ni shape memory alloys

Abstract

We have investigated diffuse scattering appearing in Ti-Ni based shape memory alloys to understand the origin of instability of the B2-type structure in the system. Transmission electron microscope observation and a first-principle electronic structure calculation have been made for Ti-(50-x)Ni-x Fe (x = 6, 7, 8, 10). They exhibit diffuse scattering in the electron diffraction pattern along 〈ξξ0 〉* direction on cooling with the intensity peak located at incommensurate positions of qdiffuse = 〈ζζ0 〉*. The value ζ increases with decreasing temperature for all alloys. The temperature at which the incommensurate scattering start to appear is close to the one at which the electrical resistivity exhibits a local minimum, Tmin, for all the alloys. The value of qdiffuse at Tmin is nearly the same as the nesting vector of Fermi surface at Tmin, which is obtained by calculating the generalized susceptibility of the B2-type structure along the 〈ξξ0 〉* direction. These results strongly suggest that the nesting effect of Fermi surface is responsible for the instability of the B2-type structure in Ti-Ni based shape memory alloys.