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Ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn-0.35N austenitic steels containing Ni and Cu was investigated by means of Charpy impact test and fractographic analysis. The commonly observed fracture mode of the specimens tested at -196 oC was transgranular cleavage-like brittle with flat facets occurring along {111} crystallographic planes, thereby leading to the occurrence of ductile-to-brittle transition. For all the steels investigated in the present study, the ductile-to-brittle transition temperature (DBTT) measured from Charpy impact tests was much higher by 90 to 135 oC than that predicted by empirical equation strongly depending on N content. The combined addition of Ni and Cu enabled the 18Cr-10Mn-0.35N steels to have the lowest DBTT, which could be explained by relatively high austenite stability and favorable effect of Cu as well as the absence of delta-ferrite.

High deformability has been considered as a critical factor of ultra-high strength steel plates subjected to compressive, tensile, and bending deformation induced by large ground movements. In this paper, various dual phase microstructures consisting of soft ferrite and strong low-temperature transformation phases without deformation in the (austenite + ferrite) two-phase temperature region after controlled rolling were introduced and then the mechanical properties were discussed with emphasis on deformability such as yield ratio and uniform elongation. Ultra-high strength steel plates fabricated by a modified thermo-mechanical control process showed lower yield ratio of under 0.75 and higher uniform elongation of 5% as a minimum, as compared to commercial API X100 and X120 grade pipeline steels, without much sacrifice of Charpy impact properties because of an appropriate formation of soft ferrite and strong low-temperature transformation phases.

Dynamic deformation and fracture behavior of Zr-based bulk metallic glass (BMG) and BMG composite containing dendritic β phases was investigated in this study. Dynamic compressive test results indicated that both maximum compressive stress and total strain of the BMG and BMG composite decreased with increasing test temperature because shear bands could propagate rapidly as the adiabatic heating effect was added at high temperatures. Above the glass transition temperature, total strain decreased more abruptly due to crystallization of amorphous phases. Maximum compressive stress and total strain of the BMG composite were higher than those of the BMG because β phases played a role in forming multiple shear bands. The BMG composite having more excellent dynamic properties than the BMG can be more reliably applied to the structures or parts requiring dynamic properties.

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