Abstract
Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.
Highlights
Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes
Our results indicate that the activation takes place in the very early stage of plastic deformation like near the macroscopic yield point, and the grain size plays a major role in deformation mode alternation; deformation twin nucleation occurred at the grain boundaries in under-1 μm austenitic grains, while the normal in-grain slip was mostly observed in over-1 μm grains
Theses deformation twinning mechanisms based on the arrangement of highly coordinated Shockley partial dislocations on {111} slip planes[20,21,22,23,24] appear to be insufficient to explain the grain boundary mediated deformation twinning in the UFG TWIP steels because the deformation twinning in UFG TWIP steels appears to be highly correlated with grain boundary structure
Summary
Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Our results indicate that the activation takes place in the very early stage of plastic deformation like near the macroscopic yield point, and the grain size plays a major role in deformation mode alternation; deformation twin nucleation occurred at the grain boundaries in under-1 μm austenitic grains, while the normal in-grain slip was mostly observed in over-1 μm grains. This grain size dependence could be attributed to the lack of initial mobile dislocations and inactive in-grain dislocation sources; both are characteristics of the UFG microstructure. This leads an open question, that is, what mechanism would promote the deformation twin nucleation at a Σ3{111} twin boundary, which is the dominant boundary type in the UFG TWIP steels, when the localized stress concentration field cannot be generated by dislocation pile-up, or localized stress concentration field is no longer necessary for the deformation twin nucleation in the UFG TWIP steels
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