Heterogeneous Nano-structure and its Evolution in Heavily Cold-rolled SUS316LN Stainless Steels

Abstract

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.

Comparisons of Microstructures and Mechanical Properties of Heterogeneous Nano-Structure Induced by Heavy Cold Rolling and Ultrafine-Grained Structure by Multi-Directional Forging of Cu–Al Alloy

“…11) In fact, rather good elongation by TWIP is confirmed in SUS316LN stainless steel with heterogeneous nano-structure. 10) From the above results and discussion, it is concluded that UFGed structure can be easily achieved by simple heavy cold-rolling and comparable or more superior mechanical properties than those obtained by SPDs can be attained when grain fragmentation takes place dominantly by dense mechanical twinning. Nevertheless, the experimental results about heterogeneous nano-structure and the properties induced by heavy cold rolling are still not sufficient.…”

“…In this way, sharp evolution of ©001ª{110} texture is effectively suppressed. 10,11) It is expected from the above observations of heterogeneous nano-structure, therefore, bendability and ductility would be improved. 12) This will be discussed in detail in the section of 3.3.…”

“…Although the twinning planes appear at a glance parallel from each other in the matrix lamellae which aligned to form ©001ª{110} texture, they have rather large misorientation distribution up to 10 degrees. 10) This misorientation distribution among the ultrafine twins in the low-angle lamellae would be understood from continuous formation, crystal rotation of matrices and shear banding during cold rolling. The formation of different R.D.…”

Effect of hetero- and homo-nanostructure on the hydrogen embrittlement resistance in heavily deformed 316LN austenitic stainless steel

“…1, the twin domains surrounded by the shear bands were further embedded in the low-angle lamellae. The twin domains and lamellae were mainly composed of low-energy boundaries of Σ3 coherent twin and low-angle ones, respectively [20], which would provide high restriction to the flow of hydrogen atoms. Therefore, the released hydrogen atoms from the trap sites face high obstacles to preferentially diffuse and accumulate at the stress concentration region (the shear bands located at the tips of twin domains).…”

Microstructure and Mechanical Properties of the Heterogeneous-Nano Structured Cu–Be System Alloys

“…The subsequent cold-rolling process led to subdivision and rotation of the twinned region by shear bands, and eventually to the development of HN. 8) Therefore, it can be stated that the deformation twinning that occurred within the initial grains plays the most essential role in the subsequent formation and variant selection of the twin domains. Generally, the deformation twinning is strongly affected by the SFE, 15,16) where deformation twinning becomes easier with decreasing SFE.…”