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Recently, advanced oxide dispersion strengthened (ODS) copper alloys have been developed using mechanical alloying process as a fusion material. In this study, to develop a superior ODS copper alloy containing 0.5wt% Y2O3, the effect of cryogenic rolling on microstructure development and tensile properties was studied using high resolution EBSD, TEM and tensile tests. During cryogenic deformation of ODS copper, grain structure remains in submicron size scale as a combinatorial result of geometrically effects, nanotwin bundle deformation, interaction of dislocations with fine oxide particles and some diffusional processes including static recovery and recrystallization. Clear microstructural characterizations confirmed nucleation of fine new oriented recrystallized grains mainly on the HABs of 80%cryogenic rolled ODS copper. Quantitative analyses indicated grain boundary migration at room temperature following cryogenic deformation originated from high driving force induced by grain boundary bulging and high mobility induced by vacancies. The tensile properties of cryogenic deformed samples showed superior tensile strength than room temperature deformation leading to UTS: 624 MPa, elt: 5.5%, while saturation of strength between 60%-80% reduction, approved occurrence of softening by diffusional processes., source:S.M.S. Aghamiri, S.H. Zhang, S. Ukai, N. Oono, R. Kasada, H. Noto, Y. Hishinuma, T. Muroga, Microstructure development in cryogenically rolled oxide dispersion strengthened copper, Materialia, Volume 9, 2020, 100520, ISSN 2589-1529, https://doi.org/10.1016/j.mtla.2019.100520., source:https://doi.org/10.1016/j.mtla.2019.100520

Thermomechanical deformation is one of the most efficient and facile routes to tailor microstructure in structural materials for mechanical property enhancement. Herein, the Ce‐modified SAF2507 super duplex stainless steel (Ce‐SAF2507) is deformed at different levels from 30% to 90% at a cryogenic temperature (–196 °C) to achieve superior mechanical performances. Cryogenic rolling increase fiber texture and induce ultra‐fine grain refinement which brings grains to ≈10 nm in the selected steel. The high‐density dislocations and deformation twins in the cryogenically rolled Ce‐SAF2507 lead to the nucleation and growth of martensite. Increases in the martensite volume fraction and nanoscale grain refinement occur at higher deformation levels. Cryogenically rolled deformation results in the overall increase in the Ce‐SAF2507 hardness. A higher hardness increment of austenite–martensite dual‐phase compared to that of ferrite is attributed to the austenite–martensite‘s higher work hardening ability. Furthermore, the ultimate tensile strength and yield strength increase with the deformation level, but the elongation decrease. Observed microstructural evolutions induced by cryogenic rolling enunciate the superiority of the present method over conventional ones to promote steel’ mechanical properties.

A high-resolution electron-back-scatter-diffraction (EBSD) technique was applied to quantify grain-structure development and texture evolution during/after cryogenic rolling of pure copper. Microstructure evolution was found to be a complex process involving mainly geometrical effects associated with strain and discontinuous recrystallization but also including limited twinning and grain subdivision. Recrystallization was deduced to be static in nature and probably occurred during static storage of the material at room temperature after cryogenic rolling. The texture contained a pronounced {110}<112> brass component; this observation was interpreted in terms of the suppression of cross slip at cryogenic temperatures. In general, cryogenic rolling was found to be ineffective for the formation of a nanocrystalline (NC) structure in pure copper., The original document contains color images. All DTIC reproductions will be in black and white. Prepared in collaboration with Tohoku University, Sendai, Japan.

研究科: 千葉大学大学院自然科学研究科, 千大院自博甲第工335号, 修了年: 2003