著者

牧 正志 古原 忠 辻 伸泰 森戸 茂一 宮本 吾郎 柴田 曉伸

出版者

一般社団法人 日本鉄鋼協会

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.100, no.9, pp.1062-1075, 2014 (Released:2014-08-31)

参考文献数

72

被引用文献数

24 30

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After ausforming appeared as the first thermomechanical processing of steels in the first half of the 1960 s, various thermomechanical processings have been developed for the improvement of mechanical properties over the last fifty years. Their application was mainly to martensitic steels in the 1960 s such as ausforming and TRIP, and moved to ferrite (+ pearlite) structures by the development of controlled rolling and accelerated cooling of HSLA steels in the 1970~1980 s. However, recently, interest has returned to martensite (and also bainite) because of the demand for higher strength, and the ausforming and TRIP have been revived and successfully applied to commercial practice. Very recently, severe plastic deformation (SPD) is the focus of attention as a new method of producing a very fine-grained structure with grain size of less than 1 μm. By the application of SPD, dynamic phenomena such as dynamic recrystallization and dynamic ferrite transformation occur in the process. We need more systematic studies on such phenomena for the development of new type of thermomechanical processing in steels.

著者

吉田 周平 池内 琢人 BHATTACHARJEE Tilak BAI Yu 柴田 曉伸 辻 伸泰

出版者

一般社団法人 粉体粉末冶金協会

雑誌

粉体および粉末冶金 (ISSN:05328799)

巻号頁・発行日

vol.67, no.3, pp.113-120, 2020-03-15 (Released:2020-03-15)

参考文献数

63

被引用文献数

1

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High entropy alloys (HEAs) and medium entropy alloys (MEAs) are new classes of materials, defined as alloys composed of five or more and four or fewer kinds, respectively, of alloying elements with (near-)equiatomic concentrations. In the present article, we reviewed our recent works on ultra-grain refinement of HEAs and MEAs. CoCrFeMnNi HEA and its sub-system MEAs were highly deformed by high-pressure torsion and subsequently annealed under various conditions to obtain fully-recrystallized microstructures with FCC single phase having different mean grain sizes. It was found that ultrafine-grained (UFG) microstructures could be easily obtained by simple thermomechanical processes. Grain size and chemical composition dependence on mechanical properties of the HEA and MEAs were evaluated by tensile tests at room temperature. UFG HEAs and MEAs exhibited characteristic phenomena, such as discontinuous yielding and extra-hardening, similar to other UFG metals. In addition, the UFG HEAs and MEAs showed better strength-ductility balance compared with conventional UFG metals. Friction stresses of HEAs and MEAs were determined from Hall-Petch relationships and found to be much higher than those of pure metals and dilute alloys having FCC structure. Analysis based on theoretical models suggested that the high friction stress reflected atomic-scale heterogeneity in HEAs and MEAs.