Topological dislocation modes in three-dimensional acoustic topological insulators

Dislocations are ubiquitous in three-dimensional solid-state materials. The interplay of such real space topology with the emergent band topology defined in reciprocal space gives rise to gapless helical modes bound to the line defects. This is known as bulk-dislocation correspondence, in contrast to the conventional bulk-boundary correspondence featuring topological states at boundaries. However, to date rare compelling experimental evidences have been presented for this intriguing topological observable in solid-state systems, owing to the huge challenges in creating controllable dislocations and conclusively identifying topological signals. Here, using a three-dimensional acoustic topological insulator with precisely controllable dislocations, we report an unambiguous experimental evidence for the long-desired bulk-dislocation correspondence, through directly measuring the gapless dispersion of the one-dimensional topological dislocation modes. Remarkably, as revealed in our further experiments, the pseudospin-locked dislocation modes can be unidirectionally guided in an arbitrarily-shaped dislocation path. The peculiar topological dislocation transport, expected in a variety of classical wave systems, can provide unprecedented control over wave propagations.

Relation Between Microstructures and Constitutive Behavior of Advanced Tempered Martensitic Steels

ABSTRACTIn an effort to better understand the plasticity of the tempered martensitic steels, the strain-hardening of two 7-9Cr steels is examined in terms of dislocation mechanics. It is shown that, over the temperature range investigated (173K-523K), the strain-hardening as a function of stress can be described by an equation taking account the dislocation storage and annihilation. The model of strain-hardening used in this study is based on the original Kocks description of the dislocation density evolution with plastic strain but the heterogeneous distribution of dislocations resulting from the lath boundaries and prior austenite grain boundaries is taking into account. The effect of stress and temperature on the storage and annihilation is discussed. The relation between the determined mean dislocation path associated to the heterogeneous lath dislocation structure and the transmission electron microscope observations performed on the two different steels is outlined.