New chemical layered structure in Ti-Cr alloys

The new chemical layered structure, referred to as a chemical stripe structure, was found as a metastable state in the (bcc-->hcp + C15) metastable phase separation in the Ti-Cr alloys. Note that a metastable separation is here defined as a separation characterized by a positive value of the second derivative of Gibbs free energy with respect to chemical composition. The experimental data showed that the layered structure had periodicity of 4 x d(002), about 0.72 nm, and chemical ordering of Ti : Cr = 3 : 1, and that a lot of antiphase boundaries with phase shifts of pi/2 and pi were involved in the structure. The coherent length of the layered structure was estimated to be about 15 nm along the modulated direction. In addition to these features, it was found that the layered structure was formed from the local bct state as another metastable state, not directly from the zone structure consisting of one Cr layer. On the basis of these data, the physical origin of the formation of the chemical layered structure is also discussed.