Add time:08/06/2019         Source:sciencedirect.com

Size induced strengthening, i.e. increase in strength with decreasing sample size, has been reported for a variety of single crystal body centered cubic (bcc) materials. However, studies on bcc lath martensite are very limited, in part due to its complex microstructure and the difficulty of small-scale experimentation. In this work, we demonstrate for the first time a size induced strengthening followed by a softening in bcc lath martensite pillars by reducing pillar diameter from 8 µm to 250 nm. These martensitic pillars are tested under uniaxial compression conducted in-situ inside of a scanning electron microscope. High-resolution microstructural characterization by transmission electron microscopy is used for post-mortem examination of the compressed pillars in order to study the deformation mechanisms governing their plasticity. We find that the size-induced strengthening during compression can be rationalized in terms of the competition between dislocation slip and boundary sliding, while softening at the finest scales is attributed to the boundary sliding mechanisms. Our work provides direct observations of block/lath boundaries behavior during compression as well as new insights into the deformation mechanisms in lath martensite.

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