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Evolution of heterogeneous nano-structure in heavily cold-rolled SUS316LN stainless steels was investigated in detail. Transmission electron microscopic observations from the transverse direction (TD) of the 92% rolled specimen revealed the formation of a typical hetero-nano structure composed of ultra-fine lamellar grains embedded with deformation twin domains. The twin domains had prolate ellipsoidal shape elongated parallel to TD. Two types of twin domains with different crystallographical orientations to matrices could be identified, i.e., i) < 211 > // rolling direction (RD) and < 110 > // TD or ii) < 110 > // RD and < 211 > // TD, although all the {111} twining planes of both twin domains were oriented nearly parallel to the rolling planes. The ultra-fine lamellar grains were elongated along < 100 > direction and nearly parallel to RD. Deformation twins with a few nano-meter spacing were also frequently observed to develop in the lamellar grains. Evolution sequence of the hetero-nano structure during cold rolling was also investigated. At an early stage of rolling, deformation twins were gradually formed in the whole grains. Then, the regions fragmented grains by twins were further subdivided by a numerous number of shear bands inclined at about 20-45° from the RD, resulting in the formation of "eye-shaped" twin domains surrounded by shear bands and their crystallographical rotation. Cold rolling up to 50% caused a considerable increase in strength and decrease in ductility. While the strength was raised more with increasing reduction up to 92%, both the strength and ductility eventually slightly decreased by further rolling.

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Synopsis :Severe plastic deformation has attracted interests as one of the breakthrough procedures to improve various properties of metals and alloys.Recently, it has been revealed that heavy cold rolling of some kinds of austenitic stainless steels can cause ultrafine-grained structure comparable with those achieved by severe plastic deformation. Coarse initial grains were fragmented by deformation induced microstructure to develop heterogeneous nanostructure. Tensile strength of heterogeneous-nanostructured stainless steel exceeds 2 GPa. It is considered that high strength of heterogeneous-nanostructured metals is attributed to such peculiar microstructure with dispersed "eye-shaped twin domains".In this study, microstructural mechanisms and factors which contribute to macroscopic strength of heterogeneous-nanostructured austenitic stainless steel were evaluated on the basis of multiscale crystal plasticity simulation. Microstructure of heavily cold-rolled SUS316LN austenitic stainless steel was investigated by transmission electron microscopy, and stress-strain curves were attained by tensile tests. It was observed that microstructure of SUS316LN manufactured by 92% cold rolling was composed of deformation nano-twins, shear bands, and lamella structure. Evaluation of mechanical properties of heterogeneous-nanostructured SUS316LN was conducted using crystal plasticity finite element simulation considering microstructural information, such as dislocation density, crystal orientation, shape of grains, and dislocation sources. Information of microstructure obtained by electron backscatter diffraction, e.g. geometry of heterogeneous nanostructures and crystal orientation, were introduced to computational models for multiscale crystal plasticity simulation. It was revealed that deformation behavior depends on the tensile direction and the strength increases with the increase of volume fraction of twin domains as well as nano-twin and lamellar inter-spacings.

SUS316LN austenitic stainless steel was simply and heavily cold-rolled up to 92％ reduction in thickness. The microstructure developed was composed of complicated heterogeneous nano-structure; ＂eye-shaped＂ twin domains, which were surrounded by shear bands, were embedded in low-angle lamellar boundaries. The cold-rolled austenitic steel exhibited marvelous high strength of 1.95 GPa when tensile tested normal to the rolling direction, while lower strength of 1.57 GPa along the rolling direction. Moderate ductility around 10 ％ was still retained in spite of the high strength. The superior mechanical properties of the heavily coldrolled austenitic stainless steel would be attributed to complicated heterogeneous nano-structures. These achieved strengths were comparable with those obtained by methods of severe plastic deformation.

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