• Title/Summary/Keyword: Hall-Petch equation

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Modeling the Hall-Petch Relation of Ni-Base Polycrystalline Superalloys Using Strain-Gradient Crystal Plasticity Finite Element Method (변형구배 결정소성 유한요소해석법을 이용한 니켈기 다결정 합금의 Hall-Petch 관계 모델링)

  • Choi, Yoon Suk;Cho, Kyung-Mox;Nam, Dae-Geun;Choi, Il-Dong
    • Korean Journal of Materials Research
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    • v.25 no.2
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    • pp.81-89
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    • 2015
  • A strain-gradient crystal plasticity constitutive model was developed in order to predict the Hall-Petch behavior of a Ni-base polycrystalline superalloy. The constitutive model involves statistically stored dislocation and geometrically necessary dislocation densities, which were incorporated into the Bailey-Hirsch type flow stress equation with six strength interaction coefficients. A strain-gradient term (called slip-system lattice incompatibility) developed by Acharya was used to calculate the geometrically necessary dislocation density. The description of Kocks-Argon-Ashby type thermally activated strain rate was also used to represent the shear rate of an individual slip system. The constitutive model was implemented in a user material subroutine for crystal plasticity finite element method simulations. The grain size dependence of the flow stress (viz., the Hall-Petch behavior) was predicted for a Ni-base polycrystalline superalloy NIMONIC PE16. Simulation results showed that the present constitutive model fairly reasonably predicts 0.2%-offset yield stresses in a limited range of the grain size.

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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    • v.32 no.3
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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.

Microstructures of Friction Stir Lap Weld in A5052-H112 Alloy (A5052-H112 합금의 겹치기 마찰교반접합 조직 특성)

  • Ko, Young-Bong;Lee, Joong-Hun;Park, Kyeung-Chae
    • Journal of Welding and Joining
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    • v.27 no.6
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    • pp.17-24
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    • 2009
  • The Friction Stir Welding(FSW) has mainly been used for making butt joints in Al alloys. Development of Friction Stir Lap Welding(FSLW) would expand the number of applications. Microstructure of FSLW in A5052-H112 alloy was investigated under varying rotation and welding speed. As the rotation speed was increased and the welding speed was decreased, a amount of heat was increased. As a result, bead interval was narrower, bead width are larger, and experimental bead interval was almost similar to theoretical bead interval. Typical microstructures of FSLW A5052-H112 alloy consist of three zones, including Stir Zone(SZ), Thermo-Mechanically Affected Zone(TMAZ) and Heat Affected Zone(HAZ). As a amount of heat was increased, average grain size was larger in three zones. Nevertheless, the aspect ratio was almost fixed for FSLW conditions. The misorientation of SZ, HAZ and TMAZ was examined. A large number of low angle grain boundaries, which were formed by severe plastic deformation, were showed in TMAZ as comparison with SZ and HAZ. Microhardness distribution was high in order of BM, SZ, TMAZ, and HAZ. The Micro-hardness distribution in HAZ, TMAZ of upper plate were lager than lower plate. Relationship between average grain size and microhardness was almost corresponded to Hall-Petch equation.

Theoretical Study on the Consolidation Behavior and Mechanical Property for Molybdenum Powders (몰리브데늄 분말의 치밀화 거동 및 기계적 물성의 이론적 연구)

  • Kim, Young-Moo
    • Journal of Powder Materials
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    • v.15 no.3
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    • pp.214-220
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    • 2008
  • In this study, consolidation behavior and hardness of commercially available molybdenum powder were investigated. In order to analyze compaction response of the powders, the elastoplastic constitutive equation based on the yield function by Shima and Oyane was applied to predict the compact density under uniaxial pressure from 100MPa to 700MPa. The compacts were sintered at $1400-1600^{\circ}C$ for 20-60 min. The sintered density and grain size of molybdenum were increased with increasing the compacting pressure and processing temperature and time. The constitutive equation, proposed by Kwon and Kim, was applied to simulate the creep densification rate and grain growth of molybdenum powder compacts. The calculated results were compared with experimental data for the powders. The effects of the porosity and grain size on the hardness of the specimens were explained based on the modified plasticity theory of porous material and Hall-Petch type equation.

Effects of rolling condition on recrystalized structure and strength in over aged 7075 AI alloy (과시효처리된 7075 AI합금에 있어서 압연조건이 재결정조직과 강도에 미치는 영향)

  • Kim, Chang-Ju;Kim, Hyeong-Uk
    • Korean Journal of Materials Research
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    • v.4 no.2
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    • pp.241-249
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    • 1994
  • We studied on the effects of hot-worm rolling on recrystalized structures and tensile strength in over-aged 7075 A1 alloy, to develop the process for improving properties. It showed more clear effect of the grain refinement with over-aging before plastic deformation. That means, the coarse precipitates from over-aging play a roll as nucleation sites in the course of recrystallization. And on this study, the relations between yield strength and grain size was not satisfied with Hall-Petch equation because of the elongated structure, but the yield strength is proportional to aspect ratio of grains. In TMT process for improving strength and toughness, the worm working is available for increase of those properties than cold working.

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Effect of Welding Parameters on the Friction Stir Weldability of 5052 Al alloy (5052 알루미늄 합금 마찰교반접합부 특성에 미치는 접합인자의 영향)

  • 이원배;김상원;이창용;연윤모;장웅성;서창제;정승부
    • Journal of Welding and Joining
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    • v.22 no.3
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    • pp.69-76
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    • 2004
  • Effects of friction stir welding parameters such as tool rotation speed and welding speed on the joints properties of 5052 Al alloys were studied in this study. A wide range of friction stir welding conditions could be applied to join 5052 AA alloy without defects in the weld zone except for certain welding conditions with a lower heat input. Microstructures near the weld zone showed general weld structures such as stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Each zone showed the dynamically recrystallized grain, transient grain and structure similar to base metal's, respectively. Hardness distribution near the weld zone represented a similar value of the base metal under wide welding conditions. However, in case of 800 rpm of tool rotation speed, hardness of the stir zone had a higher value due to the fine grain with lots of dislocation tangle, a higher angle grain boundary and some of Al3Fe particles. Except joints with weld defects, tensile strength and elongation of the joints had values similar to the base metal values and fracture always occurred in the regions approximately 5mm away from the weld center.

Effect of Annealing Conditions on Microstructure and Damping Capacity in AZ61 Magnesium Alloy (열처리조건에 따른 AZ61 마그네슘 합금의 미세조직과 감쇠능에 미치는 영향)

  • Ahn, Jae-Hyeon;Kim, Kwon-Hoo
    • Journal of the Korean Society for Heat Treatment
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    • v.31 no.2
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    • pp.56-62
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    • 2018
  • Many researchers have studied on the precipitation control after solution treatment to improve the damping capacity without decreasing the strength. However, studies on the damping capacity and microstructure changes after deformation in the solid solution strengthening alloys were inadequate, such as the Al-Zn series magnesium alloys. Therefore, in order to investigate the effect of annealing condition on microstructure change and damping a capacity of AZ61 magnesium alloy. In this study, it was confirmed that the microstructure changes affect the damping capacity and hardness when annealed AZ61 alloy. AZ61 magnesium alloy was rolled at $400^{\circ}C$ with rolling reduction of 30%. These specimens were annealed at $350^{\circ}C$ to $450^{\circ}C$ for 30-180 minutes. After annealing, microstructure was observed by using optical microscopy, and damping capacity was measured by using internal friction measurement machine. Hardness was measured by Vickers hardness tester under a condition of 0.3 N. In this study, static recrystallization was observed regardless of the annealing conditions. In addition, uniform equiaxed grain structure was developed by annealing treatment. Hardness is decreased with increasing grain size. This is associated with Hall-Petch equation and static recrystallization. In case of damping capacity, bigger grain size show the larger damping capacity.