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The macroscopic mechanical response of polycrystals is the average manifestation of local behavior and states inside the grains. Such local behavior and states (i.e. stress distributions, dislocations, grain size, twin structures, etc) is usually very inhomogeneous. As a consequence, macroscopic models that rely on describing the mechanical response in terms of effective average magnitudes (i.e. average dislocation density) may be insufficient when the aforementioned inhomogeneity affects the macroscopic response, such as when strain-path-changes are involved.
In our DOE-BES Program on Materials of Hexagonal Structure we are developing a paradigm for linking length scales, consisting in introducing statistical characterization of micro-structure and stress distributions into grain-scale models. The latter are, in turn, implemented probabilistically into the polycrystal plasticity frameworks VPSC and E-VPSC.
We present three applications of such approach. In Example 1 twin nucleation in HCP is treated as a stochastic event driven by local stresses and atomistic states at grain boundaries. Such approach explains the proportion of active twin variants and texture evolution associated with mechanical deformation of Mg and Zr. In Example 2 we show how twin growth is affected by back stresses induced by the twin shear transformation and the reaction to it exerted by neighboring grains. Grain size and neighbor misorientation are treated stochastically in the model. In Example 3 we show that a model of intra-granular stress distributions inside grains explains the relaxation of internal stress associated with strain holds and Bauschinger effects associated with strain reversals. BIOGRAPHY: Tome earned a doctorate in physics from the National University of La Plata, Argentina, and joined the Laboratory in 1996. He pioneered the theoretical and numerical development of physically based modeling of the mechanical behavior of polycrystals. Tome has published more than 170 papers in international journals, with more than 12,000 citations. The Structural Materials Division of The Minerals, Metals & Materials Society (TMS) presented him the 2013 Distinguished Scientist/Engineer Award. In 2015 he received the Khan International Medal and he was just named a Laboratory Fellow.
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