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Material Heterogeneity and Emergent Length Scale Due to Non-equilibrium Phase Transition



Most processes in both material fabrication (e.g., solidification in casting of alloys, diffusion and displacive phase transition, high speed metal forming and cutting, etc) and service (e.g., field-induced phase transitions in structural/functional materials) are intrinsically thermodynamic non-equilibrium. These processes involve, at one stage or another, different versions of material nonlinearity, instability and multi-field coupling at different length and time scales, and constitute the main physical origins of material heterogeneity observed in various microstructure and pattern evolutions under non-equilibrium conditions [see 1-3]. In this talk I will present recent experiment and modeling on domain patterns and emergent length scales during stress-induced phase transition process, emphasizing the roles of loading time scale and thermo-mechanical coupling. Their effects on the stress-strain curves and domain patterns observed in NiTi strips in the strain rate range of 10-4/s ~ 102/s are reported (see Fig. 1). We found that, with the increase in loading rate, the deformation mode changed from the localized-propagation mode with a few domains to the heterogeneous multiple-nucleation mode with fine domains and eventually to homogeneous deformation. The domain spacing, as the emergent length scale of the structure, decreased with the applied stretching rate and follows a power-law strain rate dependency which originates from the strong coupling among the material’s strong nonlinear property, temperature dependence of transition stress and the heat transfer. The physical origin of material heterogeneity is explored and the important competition of heat transfer time and loading time in imposing the emerging length scales of the material’s heterogeneity is demonstrated. Scaling laws for the rate dependent domain spacing is developed which agree well with the experimental data over the strain rate range of 105 / s ~ 102 / s


Emergent length scale, non-equilibrium phase transition, loading time scale, thermo-mechanical couplingText

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