Processing math: 100%
  • Title/Summary/Keyword: Grain refinement

검색결과 297건 처리시간 0.022초

Effects of Carbon and Sulfur Content on Mechanical Properties of High Purity Steel (고순도강의 기계적 성질에 미치는 탄소 및 황 함량의 영향)

  • Yoon, Jeong-Bong;Kim, Sung-Il;Kim, In-Bea
    • Korean Journal of Metals and Materials
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    • 제47권6호
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    • pp.331-337
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    • 2009
  • To lower the annealing temperature and the deviation of the mechanical properties of bake hardening steels, high purity steels were investigated. The steels were characterized by treating at low recrystallization temperature. It was confirmed that the strengthening originated from the solid solution of carbon and the ferrite grain refinement by fine MnS precipitates as carbon and sulfur contents increased in high purity steels. However, it was observed that there was no more increase of strength in steels containing over 40 ppm of carbon. It was considered that the excess carbon formed either the carbon cluster or the low temperature unstable carbides which had the negligible effect on the strengthening because they were reported to be highly coherent with the matrix. The carbon cluster and unstable carbides could be transformed to the stable cementite during bake hardening treatment. MnS was not observed in the high purity steel containing 5 ppm S, resulting in very coarse recrystallized grains and good ductility. As sulfur content increased, the recrystallized grain size decreased due to the formation of the fine MnS precipitates.

Cold Compaction Behavior of Nano and Micro Aluminum Powder under High Pressure

  • Kim, Dasom;Park, Kwangjae;Kim, Kyungju;Cho, Seungchan;Hirayama, Yusuke;Takagi, Kenta;Kwon, Hansang
    • Composites Research
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    • 제32권3호
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    • pp.141-147
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    • 2019
  • In this study, micro-sized and nano-sized pure aluminum (Al) powders were compressed by unidirectional pressure at room temperature. Although neither type of Al bulk was heated, they had a high relative density and improved mechanical properties. The microstructural analysis showed a difference in the process of densification according to particle size, and the mechanical properties were measured by the Vickers hardness test and the nano indentation test. The Vickers hardness of micro Al and nano Al fabricated in this study was five to eight times that of ordinary Al. The grain refinement effect was considered to be one of the strengthening factors, and the Hall-Petch equation was introduced to analyze the improved hardness caused by grain size reduction. In addition, the effect of particle size and dispersion of aluminum oxide in the bulk were additionally considered. Based on these results, the present study facilitates the examination of the effect of particle size on the mechanical properties of compacted bulk fabricated by the powder metallurgy method and suggests the possible way to improve the mechanical properties of nano-crystalline powders.

Microstructure and Mechanical Properties of Pure Titanium Processed using Friction Stir Welding (순수 타이타늄의 기계적 특성에 미치는 마찰 교반 용접 공정 조건의 영향)

  • Lee, Y.J.;Choi, A.;Lee, S.J.;Fujii, Hidetoshi;Shin, S.E.;Lee, D.G.
    • Journal of the Korean Society for Heat Treatment
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    • 제32권3호
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    • pp.124-130
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    • 2019
  • Friction stir welding is one of the interesting welding methods for titanium and its alloy which proceeds with plastic flow due to thermo-mechanical stirring and friction heat. Solid-state welding can solve severe problems such as high-temperature oxidation, interstitial oxygen diffusion and grain coarsening by liquid-state welding. Dynamic recrystallization and grain refinement can vary significantly with the plunging load and rotational speed of tool during friction stir welding, and suitable process conditions must be optimized to obtain microstructure and better mechanical characteristics. Suitable FSW conditions were 1000 kg of plunging load and 200 rpm of rotational speed and it showed YS 270 MPa, UTS 332.1 MPa, and El 17.3%, which were very similar to those of wrought titanium sheet.

Fabrication and Structural Observation of a Soft Magnetic Composite Powders by Mechanical Alloying (기계적합금화법에 의한 연자성 복합분말의 제조 및 구조관찰)

  • Lee, Chung-Hyo
    • Korean Journal of Materials Research
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    • 제29권4호
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    • pp.252-257
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    • 2019
  • Fabrication of soft magnetic composite powders for the Fe2O3Ca system by mechanical alloying(MA) has been investigated at room temperature. It is found that soft magnetic composite powders in which CaO is dispersed in αFe matrix are obtained by MA of Fe2O3 with Ca for 5 hours. Changes in magnetization and coercivity also reflect the details of the solidstate reduction process of hematite by pure metal of Ca during MA. The saturation magnetization of MA powders increases with increasing MA time and reaches a maximum value of 65 emu/g after 7 hours of MA. The average grain size of αFe in MA powders, estimated by diffraction line-width, gradually decreases with increasing MA time and reaches 52 nm after 5 hours of MA. It can also be seen that the coercivity of the 5-hour MA sample is fairly high at 190 Oe, suggesting that the grain refinement of already-produced αFe tends to clearly occur during MA.

Evolution on Microstructures and Tensile Properties of 10Cr-1Mo ODS Steel with Different Lengths of Mechanical Alloying Process Times (10Cr-1Mo 산화물 분산강화 강의 미세조직과 인장특성에 미치는 기계적 합금화 공정시간의 영향)

  • Noh, Sanghoon;Kim, Tae Kyu
    • Journal of Powder Materials
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    • 제28권5호
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    • pp.375-380
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    • 2021
  • In this study, we investigate the effect of the duration of mechanical alloying on the microstructures and mechanical properties of ODS ferritic/martensitic steel. The Fe(bal.)-10Cr-1Mo pre-alloyed powder and Y2O3 powder are mechanically alloyed for the different mechanical alloying duration (0 to 40 h) and then constantly fabricated using a uniaxial hot pressing process. Upon increasing the mechanical alloying time, the average powder diameter and crystallite size increased dramatically. In the initial stages within 5 h of mechanical alloying, inhomogeneous grain morphology is observed along with coarsened carbide and oxide distributions; thus, precipitate phases are temporarily observed between the two powders because of insufficient collision energy to get fragmented. After 40 h of the MA process, however, fine martensitic grains and uniformly distributed oxide particles are observed. This led to a favorable tensile strength and elongation at room temperature and 650℃.

Improvement of R-value in Al-Mg-Si-Cu Alloy Sheets by Cross Rolling (크로스 압연에 의한 Al-Mg-Si-Cu 합금 판재의 소성변형비의 향상)

  • Lee, Kwang-jin;Jeon, Jae-yeol;Woo, Kee-do
    • Korean Journal of Metals and Materials
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    • 제49권6호
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    • pp.488-492
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    • 2011
  • Heat-treatable Al-Mg-Si-Cu alloy sheets, which are expected to have a growing demand, were fabricated by Cross rolling to improve their formability. The mechanical properties and texture of the sheets after the final annealing process were investigated by a tensile test, EBSD and XRD analysis. The grain size of the cross-rolled sheets was remarkably decreased compared to conventional rolled sheets, and the R-value of the cross-rolled sheets was notably increased by about one and a half times that of the conventional rolled sheet. Cube{001}<100> and cubic system orientations were strongly developed in conventional rolled sheets. However, randomized textures were formed in the cross-rolled sheets without specific texture. It is thought that much shear deformation was induced during the cross rolling. The results show that the cross rolling method is effective for improving the R-value of aluminum alloys sheets and their grain refinement. As a result, it is considered that cross rolling is effective for improving formability.

Ductility Enhancement in Sn-40Bi-X Alloys by Minor Additions of Alloying Elements (합금원소 첨가에 의한 Sn-40Bi-X 합금의 연성 향상)

  • Kim, Ju-Hyung;Lee, Jong-Hyun
    • Korean Journal of Metals and Materials
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    • 제49권3호
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    • pp.211-220
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    • 2011
  • To improve the low ductility and high strain-rate sensitivity in Sn-Bi based solder alloys, the influences of the minor additions of alloying elements (Ag, Mn, In) were investigated. The strain-stress curves of various Sn-40Bi(-X) alloys, including a pre-suggested Sn-40Bi-0.1Cu composition were measured using a tensile testing machine. As a result, the elongation and ultimate tensile strength (UTS) values were compared. The small addition (0.5 wt.%) of Ag significantly enhanced the ductility and high strain-rate sensitivity of the alloys at strain rates of 104 to 102s1 mainly due to the increase and refinement of eutectic lamellar structures. The microstructure change increased the area of grain boundaries, thus ameliorating the grain boundary sliding mode. It was also found that Mn is an effective element in enhancing the ductility, especially at the strain rates of 103 to 102s1 The enhancement is likely attributed to the fine and homogeneous microstructure in the alloys containing Mn.

Fabrication and Estimation of an Ultrafine Grained Complex Aluminum Alloy Sheet by the ARB Process Using Dissimilar Aluminum Alloys (이종 알루미늄의 ARB공정에 의한 초미세립 복합알루미늄합금판재의 제조 및 평가)

  • Lee, Seong-Hee;Kang, Chang-Seog
    • Korean Journal of Metals and Materials
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    • 제49권11호
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    • pp.893-899
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    • 2011
  • Fabrication of a complex aluminum alloy by the ARB process using dissimilar aluminum alloys has been carried out. Two-layer stack ARB was performed for up to six cycles at ambient temperature without a lubricant according to the conventional procedure. Dissimilar aluminum sheets of AA1050 and AA5052 with thickness of 1 mm were degreased and wire-brushed for the ARB process. The sheets were then stacked together and rolled to 50% reduction such that the thickness became 1 mm again. The sheet was then cut into two pieces of identical length and the same procedure was repeated for up to six cycles. A sound complex aluminum alloy sheet was successfully fabricated by the ARB process. The tensile strength increased as the number of ARB cycles was increased, reaching 298 MPa after 5 cycles, which is about 2.2 times that of the initial material. The average grain size was 24μm after 1 cycle, and became 1.8μm after 6 cycles.

Cryogenic milling for the fabrication of high Jc MgB2 bulk superconductors

  • Kim, D.N.;Kang, M.O.;Jun, B.H.;Kim, C.J.;Park, H.W.
    • Progress in Superconductivity and Cryogenics
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    • 제19권2호
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    • pp.19-24
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    • 2017
  • Cryogenic milling which is a combined process of low-temperature treatment and mechanical milling was applied to fabricate high critical current density (Jc)MgB2 bulk superconductors. Liquid nitrogen was used as a coolant, and no solvent or lubricant was used. Spherical Mg (612μm, 99.9 % purity) and plate-like B powder (1μm, 97 % purity) were milled simultaneously for various time periods (0, 2, 4, 6 h) at a rotating speed of 500 rpm using ZrO2 balls. The (Mg+2B) powders milled were pressed into pellets and heat-treated at 700C for 1 h in flowing argon. The use of cryomilled powders as raw materials promoted the formation reaction of superconducting MgB2, reduced the grain size of MgB2, and suppressed the formation of impurity MgO. The superconducting critical temperature (Tc) of MgB2 was not influenced as the milling time (t) increased up to 6 h. Meanwhile, the critical current density (Jc) of MgB2 increased significantly when t increased to 4 h. When t increased further to 6 h, however, Jc decreased. The Jc enhancement of MgB2 by cryogenic milling is attributed to the formation of the fine grain MgB2 and a suppression of the MgO formation.

Evaluation of the Microstructures and Mechanical Properties on Friction Welded A6063 Alloy (마찰접합 된 A6063 합금의 미세조직과 기계적 특성 평가)

  • Kim, Eun-Hye;Cho, Hyung-Hwan;Song, Kuk-Hyun
    • Korean Journal of Materials Research
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    • 제27권5호
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    • pp.276-280
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    • 2017
  • This study was carried out to evaluate the developed microstructures and mechanical properties of friction welded A6063 alloy. For this work, specimens were prepared at a size of 12 mm \O×80mm, and friction welding was carried out at a rotation speed of 2,000 RPM, friction pressure of 12kgf/cm2 and upset pressure of 25kgf/cm2. To perform an analysis of the grain boundary characteristic distributions, such as the grain size, orientation and misorientation angle distributions, the electron back-scattering diffraction method was used. In addition, in order to identify the dispersed intermetallic compounds of the base and welded materials, transmission electron microscopy was used. The experimental results found that the application of friction welding on A6063 led to significant grain refinement of the welded zone relative to that of the base material. Besides this, intermetallic compounds such as AlMnSi and Al2Cu were found to be dispersed with more refined size relative to that of the base material. This formation retains the mechanical properties of the welds, which results in the fracture aspect at the base material zone. Therefore, based on the developed microstructures and mechanical properties, the application of friction welding on A6063 could be used to obtain a sound weld zone.