Symmetry and reversibility of martensitic transformations

Martensitic transformations are diffusionless solid-to-solid phase transformations characterized by a rapid change of crystal structure, observed in metals, alloys, ceramics, and proteins. They come in two widely different classes. In steels, the transformation microstructure induced by quenching remains essentially unchanged upon subsequent loading or heating; the transformation is not reversible. In shape-memory alloys, on the contrary, the microstructures formed on cooling are easily manipulated by loads and disappear upon reheating; the transformation is reversible. Here we show that these sharp differences are dictated by the symmetry of the energetic landscape. In weak transformations the symmetry groups of both phases are included in a common finite symmetry group, in reconstructive transformations they are not. We demonstrate that for reconstructive transformations the energy barrier to lattice-invariant shears (as in twinning or slip) is no higher than the barrier to the phase transition itself. A remarkable implication is that reconstructive transformations are accompanied by plastic deformation through dislocations and twinning in the parent phase, making these phase changes irreversible. In contrast, for weak transformations, the energy barrier to lattice-invariant shears is independent of that to the phase transition. Consequently, weak transformations can occur with virtually no plasticity and are potentially reversible. Martensitic transformations are at the basis of numerous technological applications. Most notable amongst these is in steel, where the transformation induced by quenching (fast cooling) is exploited for enhancing the alloy’s strength. Another is the fascinating shape-memory effect in alloys like Nitinol, used in medical and engineering devices. Martensitic phase changes are also exploited to toughen structural ceramics such as zirconia, and observed in biological systems such as the tail sheath of the T4 bacteriophage virus. Ideas