HIERARCHICAL MICROSTRUCTURE IN STRUCTURAL PHASE TRANSITIONS. A. Saxena, S. R. Shenoy^*, A. R. Bishop, Y. Wu^+, T. Lookman⁺, Theoretical Division, Los Alamos National Lab., Los Alamos, NM 87545, USA, ^*School of Physics, University of Hyderabad, Hyderabad, 500134, India, International Center for Theoretical Physics, P.O. Box 586, Trieste 34100, Italy, ⁺Department of Applied Mathematics, University of Western Ontario, London, Ontario, N6A 5B7 Canada

We consider a model in the context of shape memory materials in which hierarchical twinning near the habit plane (austenite-martensite interface) is a new and crucial ingredient. The model includes (1) a triple-well potential in local shear strain, (2) strain gradient terms up to second order in strain and fourth order in gradient, and (3) all symmetry allowed compositional fluctuation-induced strain gradient terms. The last term favors branching of domain walls which enables communication between macroscopic and microscopic regions essential for shape memory. Below the transition temperature (T₀) we obtain the conditions under which branching of twins is energetically favorable. Above T₀ a hierarchy of branched domain walls also stabilizes tweed formation (criss-cross patterns of twins). External stress or pressure modulates ( "patterns" ) the spacing of domain walls. Therefore the "pattern" is encoded in the modulated hierarchical variation of the depth and width of the twins. We attempt to explain the four processes of the complete shape memory cycle--write, record, erase and recall--within this model. Preliminary results based on 2D molecular dynamics are shown for twins, tweed and hierarchy formation.