Martensitic Transformation Precursors: Phonon Theory and Critical Experiments

This paper addresses martensitic precursor phenomena from new theoretical and experimental perspectives. Driven by a phonon domain hypothesis, based on the premise of incomplete phonon softening, and employing Born's dynamical approach to statistical mechanics of anharmonic crystal lattices, we develop a Gruneisen-type phonon theory that predicts a pre-martensitic transition via "spinodal decomposition" of phonon populations and, without resort to static defects, explains the precursor "anomalies" on the same physical footing of thermal expansion, both being intrinsic properties of anharmonic crystal lattices. The theory reveals the nature of this pre-martensitic transition as phonon pseudo-Jahn-Teller instability transition, and predicts formation of elastic and phonon domains whose behaviors are primarily characterized by the broken dynamic symmetry of lattice vibrations rather than the broken static symmetry of crystal lattice and, as a particular case of Le Chatelier's principle, an internal relaxation in response to applied stress through phonon population redistribution in the domains. The predictions are critically tested by in-situ three-dimensional phonon diffuse scattering and Bragg reflection using high-energy synchrotron X-ray single-crystal diffraction, which observes exotic domain phenomenon in pre-martensitic austenite that is fundamentally different from the usual ferroelastic domain switching phenomenon in martensitic phase. The theory consistently explains the martensitic precursor "anomalies" from the viewpoint of phonon domains.