When 10:30 AM - 11:30 AM Mar 25, 2016
Where 1670 Beyster Building
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Lattice Instability and Its Effects on Deformation Defects of Advanced Alloys

Liang Qi
Department of Materials Science and Engineering, University of Michigan

Advanced structural alloys, such as Mg, Ti, Mo and W alloys, have special mechanical properties (lightweight, refractory, etc.) but suffer from the disadvantages like low strength/ductility/toughness or mechanical performance degradation due to environmental effects. These mechanical problems are largely due to the intrinsic properties of interatomic interactions experienced in their lattice structures. For example, alloys with bcc or hcp lattice usually lack easy-access plastic deformation systems compared with those with close-packed fcc lattice, resulting in brittle deformation and poor formability. Lattice instability, which describes how the lattice structure under the variation of external conditions (applied stress, etc.) loses its stability and transforms into other structures due to small disturbance, is critical to determine these intrinsic properties. First principles calculations, atomistic simulations and continuum models will be applied to investigate lattice instability and its effects on the properties of deformation defects in these advanced alloys. Two examples will be investigated: the elastic and phonon instability in bcc alloys and their effects on dislocation core structures and mobility; the elastic instability and ideal strength in hcp alloys and its effects on the nucleation of deformation twinning. The results will provide physical insights on how lattice instability and its dependence on chemical compositions, which can be directly investigated by first-principles calculations, determine the individual deformation event at atomistic scale and macroscopic mechanical properties.

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