07-06-2009, 09:51 AM
In metallic alloys at room temperature, atomic diffusion is typically very sluggish, and equilibrium or metastable phases do not undergo transformations. However, during plastic deformation, significant atomic mobility is possible. In addition, deformation may lead to mixing, possibly favoring the formation of nonequilibrium phases. In this talk, two examples will be presented, in which the rate of transformation to equilibrium is enhanced by mechanical deformation. In an unstable, nanocrystalline Fe50Cu50 solid solution, ball milling leads to decomposition into the equilibrium, nearly elemental, phases. Non-monotonic effects are observed, which are attributed to the evolving mechanical properties. In amorphous Al90Fe5Gd5, bending at room temperature leads to the precipitation of Al-rich nanocrystals at shear bands. The mechanisms of this transformation are investigated using high-resolution transmission electron microscopy, nanodindentation and bending experiments. Nanovoids are observed in shear bands created under tension, when nanocrystals do not form. It is concluded that the nucleation and growth rate is significantly enhanced by the excess free volume generated in the shear bands, unless the free volume coalesces into nanovoids. The kinetics of free-volume annihilation are analyzed by observing the effect of strain rate on the resulting nanocrystallite-size distribution.
