Change in Dislocation Mobility with Ni Content in Ferritic Steels and Its Effect on Brittle-to-Ductile Transition

Abstract

緒言In order to elucidate the mechanism behind the decrease in the brittle-to-ductile transition (BDT) temperature with the addition of Ni, impact tests and tensile tests were performed at various test temperatures from 130K to 320K with Ni added ultra-low carbon steels. The dependence of absorbed impact energy on temperature and the Ni content indicates that the BDT temperature was decreased with the increasing Ni content, which suggests that the dislocation mobility at low temperatures was increased with the Ni content. The yield stress which is also influenced by the dislocation mobility was decreased at low temperatures while it was increased at room temperature with the Ni content. The values of the activation volume and the effective stress were measured at several temperatures, and then the dependence of the activation energy for dislocation gliding on Ni content was obtained by extrapolating the relation between temperature and the multiplication of the activation volume and the effective stress to 0K. The activation energy was found to decrease with the increasing Ni content, which suggests that the dislocation mobility was increased with the addition of Ni. Discrete dislocation dynamics simulation was also performed in order to calculate the dependence of fracture toughness on temperature and the Ni content, and it was clarified that the BDT temperature is decreased by increasing dislocation mobility.

Decrease in the Brittle-to-ductile Transition Temperature in Cu Added Nickel-free Austenitic Stainless Steels

“…The activation volume decreases with temperature, which is the same as those in ferritic steels. 19,20) The activation volume for FeCrN linearly decreases with the decrease in temperature while that for FeCrN3.0Cu decreases with the decrease in temperature and saturates at the value of approximately 20 b 3 m 3 . The value of activation volume for FeCrN3.0Cu is always lower than that for FeCrN at a given temperature.…”

“…The decrease in the 0.2% proof stress with Cu is sort of solid solution softening. Solid solution softening is also reported in ferritic steels at low temperatures, 19,[23][24][25][26][27] where the frequency to form a double-kink pair controls dislocation mobility. The mechanism behind the solid solution softening is the decrease in the kink-pair nucleation energy.…”

Strain Rate Sensitivity of Flow Stress Measured by Micropillar Compression Test for Single Crystals of 18Cr Ferritic Stainless Steel

“…22,27) However, the strength of ferrous materials measured by micro-mechanical testing has been scarcely reported. 20) Furthermore, the strain rate sensitivity of flow stress of ferritic steels could significantly influence the fracture toughness controlling the room temperature ductility, [11][12][13] whereas there is no literature on the effect of strain rate on mechanical response of micro-scale steel specimens.…”

“…DBTT can be considered responsible for varied stress levels controlled by thermally activated process of dislocation glide. [11][12][13] In practical terms, 18Cr ferritic stainless steels…”

Suppression Mechanism of Strain-age-hardening in Carbon Steel Associated with Hydrogen Uptake

“…These fracture behaviors have been known to depend on temperature, 1,2) frequency, 3,4) member shape, 5,6) etc. Therefore, we need to understand the critical conditions causing these fractures.…”