Broken Dynamic Symmetry and Phase Transition Precursor
Yongmei Jin (Michigan Tech), Yu Wang (Michigan Tech), Yang Ren (Argonne National Lab)
Prager Medal Symposium in honor of George Weng: Micromechanics, Composites and Multifunctional Materials
Wed 10:45 - 12:15
MacMillan 117
Phase transition precursor is a ubiquitous phenomenon that generally occurs in metals, alloys and ceramics. Various anomalies have been observed in precursor and remained long outstanding critical issues of phase transformations in crystalline solids. Using the unique capability of Advanced Photon Source at Argonne National Laboratory, we carry out systematic study of martensitic precursor in magnetic shape memory alloy Ni-Mn-Ga. In particular, we use high-energy high-flux synchrotron X-ray diffraction to perform three-dimensional (3D) diffuse scattering of bulk single crystals under in-situ conditions. Both Bragg reflection and diffuse scattering are simultaneously measured to obtain complementary information of the crystal lattice and lattice dynamics. Computational methods are used for quantitative data analysis. The observed in-situ 3D diffuse scattering reveals heterogeneous nano-domains in high-symmetry austenite phase in a wide temperature range above martensitic transformation temperature, which exhibit highly anisotropic, hysteresis-free, reversible lattice responses to externally applied stress. The experimental findings suggest a new concept of dynamic symmetry breaking of lattice vibrations due to incomplete phonon softening prior to phase transformation, whose natural consequences manifest as various precursor “anomalies” that have been difficult to understand. The dynamic symmetry breaking of lattice vibrations naturally precedes the static symmetry breaking of crystal lattice in phase transition and provides a natural mechanism and physical understanding of the ubiquitous phase transition precursor phenomena.