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Kinetics of a Fast Moving Twin Boundary

K.T. Ramesh (Johns Hopkins University), Neha Dixit (Johns Hopkins University), Nitin Daphalapurkar (Johns Hopkins University)

From Atomistics to Reality: Spanning Scales in Simulations and Experiments Symposium B

Mon 4:20 - 5:40

CIT 227

Twinning is an important deformation mechanism in materials for structural applications, such as FCC metals (e.g. nanocrystalline Cu, Ni) and HCP metals (e.g. Mg, Ti). A twin can accommodate a considerable amount of plastic deformation as it grows in size, and the growth is in turn accomplished by the motion of a twin boundary (TB). The velocities with which a TB can move will affect the net rate of plastic deformation and perhaps the rate-sensitivity of the strength. Characterizing the kinetics (stress-velocity relations) of TBs and understanding the corresponding atomistic mechanisms are important steps towards developing the ability to tune the twin boundary mobility. We approach these objectives using both experimental and computational approaches. In the experimental approach, we use normal plate impact recovery experiments with microsecond pulse duration to measure twin boundary velocities under known stress states. These experiments are performed first on pure polycrystalline Mg specimens, and subsequently on single crystal samples. Estimates of average TB velocity under the controlled impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities of several m/s are several orders of magnitude higher than those reported in the literature for grain boundaries. Molecular Dynamics simulations are used to gain further understanding of the kinetics of a TB. A steadily moving TB is simulated in Ni by allowing a controlled frequency of twinning partial dislocations to glide over the TB under an applied shear stress. The molecular dynamics simulations suggest that the TB velocity depends on the local shear stress and the frequency of the available twinning partial dislocations, that very high TB velocities can be attained, and that the TB velocity saturates at a value significantly below the shear wave speed.