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 the alloys with Ti-(50-x)Ni-xFe (x = 6, 7, 8, 10). Obvious first order R-phase transformation is suppressed in the examined alloys. Still, they exhibit diffuse scattering in the electron diffraction pattern at incommensulate positions of <ζζ0> on cooling. The value ζ increases with decreasing temperature for all the alloys. It locks into the exact 1/3 for the alloy with x = 6, while does not for the alloys with x = 7 and more down to about 20 K (the lowest limit of the present study). The temperature at which the incommensulate scattering start to appear, T₁, is close to the one at which the electrical resistivity exhibits a local minimum for all the alloys and the value ζ at T₁ decreases with increasing Fe content. For all the alloys, the value of ζ at T₁ is nearly the same as the length of the Fermi surface nesting vector, which is obtained by calculating generalized susceptibility of the B2-type structure along the [110] direction. From these results, it is most likely that Fermi surface nesting is responsible for the instability of B2-type structure in Ti-Ni based shape memory alloys.

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