미세 조직을 고려한 무산소동 다결정재의 변형 거동에 대한 미소역학적 연구

하상렬(Sangyul Ha),김기태(KiTae Kim) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.4

본 연구에서는 다결정재에서 발생하는 결정립 간의 불균일한 변형거동을 조사하기 위하여 조대한 주상 결정립을 가진 무산소동 구리 다결정재의 평면변형압축 실험을 수행하였다. 각 결정립의 결정학적 방위의 변화는 EBSD를 이용하여 측정하였으며, 압축 변형 후의 결정립 형상은 FE-SEM으로 관찰하였다. 또한, 샘플의 전체 형상 및 결정립의 형상 변형과 집합조직의 발전을 속도의존성 결정소성모델을 적용한 유한요소 해석결과와 비교하였다. In this study, plane strain compression experiments of an OFHC copper multicrystal which consists of large columnar grains were conducted to examine heterogeneous distribution of deformation patterns in each grain of the sample. The evolution of the crystallographic orientation in each grain was measured by the electron back scattered diffraction (EBSD) system. Also, the morphologies of the grains normal to TD were observed after compression by using a field emission scanning electron microscope (FE-SEM). Finally, finite element calculations based on the rate-dependent crystal plasticity model were compared with experimental results: the overall shape change of the compression sample, the deformed shapes of the individual grains, and the rotation of the crystallographic orientation.

The Effect of Relaxing on the Grain Refinement of Low Carbon High Strength Microalloyed Steel Produced by Compact Strip Production

G. Huang,K. M. Wu 대한금속·재료학회 2011 METALS AND MATERIALS International Vol.17 No.5

The microstructural characterization of a low carbon high strength microalloyed steel produced by compact strip production in conjunction with relaxation precipitation controlling the transformation technique was investigated. The microstructural observations were analyzed by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscattering diffraction. The microstructure of the investigated steel consisted of predominantly granular bainite and lots of acicular ferrite and polygonal ferrite. The average crystallographic grain size was approximately 4 mm. Relaxation before fast cooling facilitated the formation of dislocation cells and intragranular acicular ferrite grains. Lath-like or plate-like acicular ferrite partitioned the austenite grains into many smaller parts, and the transformation of granular bainite at lower temperatures was confined to the smaller zones, resulting in smaller grain sizes. The yield strength,elongation and low temperature (−60 oC) impact toughness of the steel plates were 614 MPa, 24.1 %, 116J, respectively. The excellent combination of mechanical properties was attributed to the formation of fine grains and sub-cellular structures.

Investigation of Fe-rich fragments in aluminum-steel friction stir welds <i>via</i> simultaneous Transmission Kikuchi Diffraction and EDS

Abbasi, Majid,Dehghani, Morteza,Guim, Hwan-Uk,Kim, Dong-Ik Elsevier 2016 Acta materialia Vol.117 No.-

<P>An attempt has been made to investigate Fe-rich fragments and surrounding intermetallic compounds in aluminum-steel friction stir welds using conventional EBSD + EDS as well as advanced Transmission Kikuchi Diffraction (TKD). Results indicate that the fragments are covered by a layer of orthorhombic Al5Fe2. This layer is surrounded by a mixed region composed of sub-micrometer Al3Fe crystals in a matrix of Al nano grains. Employing high beam current TKD on electron-transparent samples enables investigation of sub-micrometer grains in the mixed region with a higher spatial resolution than conventional EBSD. As a result, nano-twinning in Al in the mixed region has been observed. Microstructural and texture results imply that Fe-rich fragments travel through the Al matrix under extreme shear forces, severely refining Al grains, and inducing nano-twinning. Due to extreme shear forces and Fe outward diffusion, fine steel fragments dissolve in the surroundings leading to the nucleation of nano Al3Fe crystals particularly along Al grain boundaries. Al3Fe crystals consume the Al matrix as they grow in size and fraction. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

Grain-Structure Evolution of Brazing-Treated A4343/A3003/A4343 Aluminum Brazing Sheets Rolled with Different Reductions

김수현,강주희,어광준,김형욱 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.2

The grain-structure evolution of three-layered A4343(clad)/A3003(core)/A4343(clad) aluminum brazing sheetswas studied by means of electron backscatter diffraction. Three different samples were prepared by cold rollingwith 17%, 22%, and 44% reduction, and the brazing treatment was carried out in form of differential scanningcalorimetry. Orientation maps of the clad and the core suggested that the grains in the melted and re-solidifiedclad did not grow epitaxially from the core in case of the 17% and 22% cold-rolled samples. In contrast,the grains in the 44% cold-rolled sample seemed to have been formed by epitaxial growth from the core. Kernel average misorientation (KAM) analysis revealed that the samples cold rolled with 17% and 22%reduction retained a deformed microstructure in the core even after the brazing treatment, but the core ofthe 44% cold-rolled sample was composed of coarse elongated grains, free from any substructure. The dissolutiondepth corresponding to the thickness of the core affected by the brazing treatment was proportional tothe average KAM of the core.

Effect of Rolling Asymmetry on Selected Properties of Grade 2 Titanium Sheet

M. Wroński,K. Wierzbanowski,M. Wróbel,S. Wroński,S. Wroński 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.5

Asymmetric rolling can be used in order to modify material properties and to reduce forces and torques applied during deformation. This geometry of deformation is relatively easy to implement on existing industrial rolling mills and it can provide large volumes of a material. The study of microstructure, crystallographic texture and residual stress in asymmetrically rolled titanium (grade 2) is presented in this work. The above characteristics were examined using the EBSD technique and X-ray diffraction. The rolling asymmetry was realized using two identical rolls, driven by independent motors, rotating with different angular velocities. It was found that asymmetric rolling leads to microstructure modification and refinement. At low deformations one observes a process of grain size decrease caused by the asymmetry of rolling process. In contrast, at the medium range of deformations the microstructure refinement consists mainly in subgrain formation and grain fragmentation. Another observation is that for low to intermediate rolling reductions (≤40%) the predominant mechanisms are slip and twinning, while for higher deformation (>40%) the main mechanism is slip. It was found that grain refinement effect, caused by the rolling asymmetry, persists also after recrystallization annealing. And finally, texture homogenization and reduction of residual stress were confirmed for asymmetrically rolled samples.

Role of Deformation Twins in Static Recrystallization Kinetics of High-Purity Alpha Titanium

원종우,이태경,홍성구,Yongmoon Lee,이정훈,Chong Soo Lee 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.6

The importance of deformation twins in static recrystallization kinetics of high-purity alpha titanium was investigatedby carrying out thermal annealing tests of deformed materials in combination with electron-backscatterdiffraction-based microstructural analysis. Prior to thermal annealing, the material was compressed to a truestrain of 0.22 along three directions to introduce different twinning characteristics. Our results showed thatdeformation twins substantially promoted the static recrystallization process by deepening the microstructuralinhomogeneity induced by the formation of twin boundaries and twinning-induced crystallographic latticereorientation. Twin morphology was also observed to be important because it influenced the extent of microstructuralinhomogeneity. Intersecting twin morphology, caused by the activation of multiple twin variants, was moreeffective than parallel twin morphology, caused by the activation of a single twin variant (or a twin variantpair), because it gave rise to more twin boundaries, more twin boundary junctions (intersections, triplejunctions, etc.), and greater in-grain crystallographic orientation spread.

AZ80 마그네슘 합금 압출제의 압축 성형조건에 따른 방위특성 분석

윤종헌(J. H. Yoon),이상익(S. I. Lee),이정환(J. H. Lee),박성혁(S. H. Park) 한국소성가공학회 2012 한국소성가공학회 학술대회 논문집 Vol.2012 No.5

With an increasing demand for light-weight materials to reduce the fuel consumption especially in transportation industry, researches on magnesium alloys have been extensively conducted. The intrinsic poor formability and ductility at ambient temperature due to the hexagonal close-packed (HCP) crystal structure and the associated insufficient independent slip systems restricts their practical usage. Hot working of magnesium alloys during a forging and extrusion process enables net-shape manufacturing with enhanced formability and ductility since there are several non-basal slip systems operated in addition to the basal slip plane leading to increase of the plastic workability. In this research, thermomechanical properties of AZ80 Mg alloy are obtained through conducting compression tests at the various temperatures and strain rates in which microstructural behaviors such as distribution of misorientation and dynamic recrystallization are investigated to correlate it with hardening and softening behavior of Mg alloys using optical microscopy and EBSD. Experimental data with physical explanations suggest the optimal condition for net-shape forging under hot or warm temperature by controlling the grain refinement and working condition.

Characterization of Phase Fractions and Misorientations on Tempered Bainitic/Martensitic Ni-Cr-Mo Low Alloy RPV Steel with Various Ni Content

Sang-Gyu Park,Ki-Hyoung Lee,Ki-Deuk Min,Min-Chul Kim,Bong-Sang Lee 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.1

The aim of this paper to quantitatively characterize the microstructure of tempered martensitic/bainitic Ni-Cr-Mo steels with different Ni contents. Though it is difficult to characterize the phase factions in the Ni-Cr-Mo low alloy steel due to the microstructural similarity of tempered lower bainite and tempered martensite, the phase fractions could determined with the as-quenched specimens by using three successive measurements of ferrite scope, scanning electron microscopy images and electron back-scattered diffraction. The martensite fraction is increased as the Ni content increases, and it causes the reduction effective grain size. When compare the boundary distributions of both as-quenched and tempered specimens, larger amounts of Σ11, Σ13, and Σ39 CSL boundaries were observed only in as-quenched model alloys.These boundaries were diminished by the tempering process. However, the peak of the Σ3 boundary was observed in both the as-quenched and tempered states. The frequency of Σ3 boundaries in the tempered samples is significantly increased as the Ni content increases, and it is closely related with the martensite fraction.

Nb 첨가량에 따른 436 페라이트계 스테인리스강 용접부의 미세조직 변화 및 공식 저항성에 관한 연구

김종민 ( Jong Min Kim ),신용택 ( Yong Taek Shin ),이상화 ( Sang Hwa Lee ),이준희 ( Jun Hee Lee ),이성룡 ( Sung Riong Lee ),김순호 ( Soon Ho Kim ),이해우 ( Hae Woo Lee ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.12

This paper identified the effects of Nb on microstructure. Also, it has studied on uniform and pitting corrosion resistance in a ferritic stainless steel weld metal of the automobile exhaust system. We fabricated 3 flux cored wires designed with 0-1.0 wt% Nb and performed Flux Cored Arc Welding. We observed the microstructure with the SEM/EDS and EBSD. To evaluate the uniform and pitting corrosion resistance, we performed a potentiodynamic polarization test in 0.2 M H2SO4 and 0.1, 0.3, 0.5 M NaCl. As a result of the tests, we found that as the amount of addition of Nb rose, the amount of Cr-carbide fell. The microstructure became more refined. The specimen with 1.0%Nb added had the highest uniform and pitting corrosion resistance. After the pitting corrosion test, a pit was formed at the grain boundary that has no addition of Nb. In addition, in the specimen with added Nb, pits were formed at the inclusions.

AZ80 마그네슘 합금 압출재의 압축 성형조건에 따른 방위특성 분석

윤종헌(J. H. Yoon),이상익(S. I. Lee),이정환(J. H. Lee),박성혁(S. H. Park),조재형(J. H. Cho) 한국소성·가공학회 2012 소성가공 : 한국소성가공학회지 Vol.21 No.4