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Tsuru, Tomohito; Lobzenko, I.; Ogata, Shigenobu*; Han, W.-Z.*
Journal of Materials Research and Technology, 28, p.1013 - 1021, 2024/01
Times Cited Count：0 Percentile：0(Materials Science, Multidisciplinary)Some solute atoms induce hardening and embrittlement in body-centered-cubic refractory metals. Especially interstitial oxygen has a dramatic hardening effect in Nb, where the yield stress of oxygen-doped Nb alloys becomes more than twice as high as that of pure Nb. Conventional mechanisms cannot explain the oxygen-induced dramatic hardening since the interaction between dislocation and oxygen is relatively weak. Here, we focused on the three-body interaction of a screw dislocation with oxygen and vacancy. Our first-principles calculations revealed that the formation of vacancy-oxygen pair enhances the attractive interaction with a screw dislocation though the interaction between oxygen and dislocation is repulsive. Furthermore, this feature was found to be a unique nature of oxygen in Nb. The vacancy-oxygen pair increases the energy barrier for dislocation motion more significantly than an isolated vacancy and oxygen interstitial. We have discovered a new oxygen-induced mechanism: a unique octahedral-tetrahedral shuffling process of oxygen dominantly contributes to the dramatic hardening. Thus, the widely distributed vacancy-oxygen pairs behave as strong obstacles for dislocation motion that causes damage accumulation and successive hardening in oxygen-doped BCC alloys.
Mao, W.; Gao, S.*; Bai, Y.*; Park, M.-H.*; Shibata, Akinobu*; Tsuji, Nobuhiro*
Journal of Materials Research and Technology, 17, p.2690 - 2700, 2022/03
Times Cited Count：9 Percentile：81.6(Materials Science, Multidisciplinary)Metastable austenitic steels having ultrafine grained (UFG) microstructures can be fabricated by conventional cold rolling and annealing processes by utilizing the deformation-induced martensitic transformation during cold rolling and its reverse transformation to austenite upon annealing. However, such processes are not applicable when the austenite has high mechanical stability against deformation-induced martensitic transformation, since there is no sufficient amount of martensite formed during cold rolling. In the present study, a two-step cold rolling and annealing process was applied to an Fe-24Ni-0.3C metastable austenitic steel having high mechanical stability. Prior to the cold rolling, a repetitive subzero treatment and reverse annealing treatment were applied. Such a treatment dramatically decreased the mechanical stability of the austenite and greatly accelerated the formation of deformation-induced martensite during the following cold rolling processes. As a result, the grain refinement was significantly promoted, and a fully recrystallized specimen with a mean austenite grain size of 0.5 mm was successfully fabricated, which exhibited both high strength and high ductility.
Ukai, Shigeharu; Yamashita, Shinichiro
Journal of Materials Research and Technology, 16, p.891 - 898, 2022/01
Times Cited Count：10 Percentile：84.53(Materials Science, Multidisciplinary)The creep strain rate of FeCrAl oxide dispersion-strengthened alloys, as a promising accident-tolerant fuel (ATF) cladding of the light-water reactors, is accelerated in YAlO
dispersoids by two to three orders of magnitude compared with Y
ZrO
dispersoid at 1273K and even occurs at an applied stress less than threshold stress for dislocation detachment. Two approaches were carried out to interpret new findings and to clarify their mechanism. By optimizing the relaxation of the dislocation line energy at the dispersoid interface, numerical analyses proved the accelerated dislocation-climbing in the YAlO dispersoids. The other is a more atomistic approach. The climbing force on the dislocation induced by the stress field around the dispersoid was analyzed in terms of the Peach-Koehler relationship. The accelerated creep strain rate in YAlO dispersoids is attributed to a larger climbing force induced by larger lattice misfit with less coherency in YAlO dispersoid.
