Diffuse scatterings appearing in electron diffraction patterns of Ti-(50 À x) Ni-xFe (x ¼ 6, 7, 8, 10, in at.%) alloys were investigated. In the alloys, martensitic (R-phase) transformation is suppressed down to 4.2 K, but the electrical resistivity exhibits a local minimum at T min (210 K, 195 K, 180 K and 140 K for x ¼ 6, 7, 8, 10, respectively). The following results were obtained for all the alloys. Diffuse scattering appears below T min and its intensity maximum is located at an incommensurate position of g þ h0i*, where g is a reciprocal lattice vector of the B2-phase. The value at T min is significantly smaller than 1/3 and increases with decreasing temperature; it decreases with increasing Fe content. The value of at T min agrees with the length of nesting vector obtained by a band calculation, suggesting that the diffuse scattering is caused by the nesting effect of the Fermi surface in the B2-type structure.

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Abstract. We have investigated electron diffraction patterns of Ti-44Ni-6Fe alloy exhibiting a negative temperature dependence in electrical resistivity below T min = 210K. The electron diffraction patterns taken below T min show diffuse satellites at g B2 + 0> * when the zone axis is [111] B2 and [100] B2 . For both beam directions, the value of is slightly smaller than 1/3 and shows the same temperature dependence. On the other hand, the satellites are missing when the zone axis is [110] B2 . This means that the incommensurate phase has a modulated structure with a propagation vector 0> * ( ~ 1/3) and displacement direction is one of <110> which is vertical to the propagation vector. This modulation is obviously related to the softening of TA 2 phonon branch with the propagation vector near 1/3 0> * . In addition to the satellites at g B2 +1/3 0> * , satellites appear at g B2 + 0> * with = 0.5 when the zone axis is [100] B2 and rod-like steaks appear in 2 * direction when the zone axis is [110] B2 . However, these satellites and rod-like streaks do not show clear temperature dependence, suggesting that they are not directly related to the instability of the B2-type structure.

We have studied diffuse scatterings appearing in Ti-(50-x)Ni-xFe (x = 6, 7, 8, 10) alloy, which shows negative temperature coefficient in electrical resistivity below a temperature Tmin. Electron diffraction pattern of each alloy exhibits diffuse scatterings below Tmin on cooling. The intensity maximum of the diffuse scatterings is located at an incommensurate position of g+<ζζ0>, where g represents reciprocal lattice points of the B2-phase. The value ζ is slightly smaller than 1/3 and decreases with increasing Fe content. The diffuse scatterings of the present alloys are well explained by the nesting effect of the Fermi surface in the B2-type structure, i.e., ζ of each alloy agrees with the length of nesting vector in the [110] direction, which is evaluated by a first-principle calculation.

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