MODELING OF BRANCHED NEEDLE MICROSTRUCTURES AT THE EDGE OF A MARTENSITE LAMINATE

We present models of the microstructure of martensite that occurs when twinned layers of martensite meet a pure variant. This microstructure often occurs below the A(s) temperature, with loading that favors the passage from one pure variant of martensite to another. This microstr ucture gives a mechanism for variant rearrangement. In this microstruc ture, the layers are observed to pinch down to sharply pointed needles . If the laminate is sufficiently long, branching of the needles is ob served. In materials with a reasonably large transformation strain, th e needles exhibit a characteristic bent shape; the bending is in the o pposite sense as might be expected from the overall shear. Our analyti cal and numerical results imply that this microstructure forms as a re sult of energy minimization in the transition layer between the marten site laminate and the pure variant. The bending of the needles plays a key role in lowering the energy of this transition layer.