• 한국재료학회지
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• 제25권2호
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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.

• 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.

• Journal of Welding and Joining
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• 제27권6호
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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.

• 한국분말재료학회지
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• 제15권3호
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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.

• 한국재료학회지
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• 제4권2호
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• pp.241-249
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• 1994

항공기용 고력 AI합금인 7075 AI합금의 물성개선을 위한 가공열처리 방안으로서 과시효처리후 온간 및 냉간 압연에 의한 소성가공이 재결정조직과 강도에 미치는 영향에 대하여 연구하였다. 소송가공후 재결정처리에 의해 결정립을 미세화하는 공정에서 소성가공 전 과시효를 함으로써 재결정립의 미세화 정도는 더욱 현저하였다. 이는 과시효에 의한 조대한 석출물이 재결정처리시 핵생성 site로서 작용하였음을 의미한다. 본 연구에서 항복강도와 재결정립의 크기와는 Hall-petch식을 만족하지 않았으며 이는 등축이 아닌 연신된 결정조직에 기인된 결과로 보이며, 항복강도가 재결정립의 종횡비에 직선적으로 비례하였다. 또한 결정립 미세화를 통한 강도, 인성향상을 목적으로 하는 TMT(Thermo mechanical Treatment)공정에서는 심한 냉간가공에 의해서보다는 온간에서의 소성가공이 바람직함을 알 수 있었다.

• Journal of Welding and Joining
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• 제22권3호
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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.

• 열처리공학회지
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• 제31권2호
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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.