DISORDER-DRIVEN PRETRANSITIONAL TWEED PATTERN IN MARTENSITIC TRANSFORMATIONS

Defying the conventional wisdom regarding first-order transitions, sol id-solid displacive transformations are often accompanied by pronounce d pretransitional phenomena. Generally, these phenomena are indicative of some mesoscopic lattice deformation that ''anticipates'' the upcom ing phase transition. Among these precursive effects is the observatio n of the so-called ''tweed'' pattern in transmission electron microsco py in a wide variety of materials. We have investigated the tweed defo rmation in a two-dimensional model system, and found that it arises be cause the compositional disorder intrinsic to any alloy conspires with the natural geometric constraints of the lattice to produce a frustra ted, glassy phase. The predicted phase diagram and glassy behavior hav e been verified by numerical simulations, and diffraction patterns of simulated systems are found to compare well with experimental data. An alytically comparing to alternative models of strain-disorder coupling , we show that the present model best accounts for experimental observ ations.