A Monte Carlo simulation of the omega phase transformation

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Abstract
A model of the β to ω phase transformation in Ti, Zr and Hf alloys is proposed whereby the three-dimensional displacive transition is decomposed into one-dimensional and two-dimensional components. The one-dimensional component is postulated to be a linear displacive defect along 〈111〉 closepacked rows of the b.c.c. structure, the two-dimensional component then consists of the cooperative ordering of neighbouring parallel linear defects in the (111) planes normal to the linear component. That the two-dimensional cooperative ordering of displaced row segments can lead to a sharp phase transition has been demonstrated by a two-dimensional Monte Carlo simulation involving three types of symbols with nearest-neighbour interactions. Computer simulation exhibits characteristic two-domain ω structures which, along with the linear defect structure, appear to explain, at least qualitatively, observed electron and neutron diffraction patterns. It is tentatively suggested that the postulated linear defects play an important role in internal friction and in anomalous diffusion in certain b.c.c. metals.