The effect of stress on soft modes for the phase transformation in a Ti-Ni alloy I. The stress-induced transformation and soft modes

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Abstract
The in situ transmission electron microscopy observation of stress-induced phase transformations in an approximately equiatomic Ti-Ni alloy and a related theoretical analysis have been made. The results show that the stress has strong effects on the lattice instability. A tensile stress promotes soft modes which relate to the nucleation of the R phase and the martensite phase so that the tensile stress helps the formation of the R and the martensite phases. The theoretical analysis shows that a compressive stress has negative effects (experimental confirmation will be given in Part II). The investigation shows that the effect of stress on the effective elastic constant in 〈111〉B2 directions, given by C eff〈111〉=⅓(C 11 + 2C 12 + 4C 44), is much larger than that on the shear elastic constant, given by C=½(C 11-C 12), which relates to the martensitic transformation. Therefore the tensile stress particularly enhances the R transformation which relates to the softening of C eff〈111〉. It has been identified that the diffuse streaks in the electron diffraction associated with the increase in the amount of R phase induced by the tensile stress arise from the softening of phonons with 〈111〉 polarization vectors which results from the softening of C eff〈111〉.

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