• Title/Summary/Keyword: Boundary Diffusion Creep

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A Boundary Diffusion Creep Model for the Plastic Deformation of Grain Boundary Phase of Nanocrystalline Materials (나노재료 입계상의 소성변형에 대한 입계확산크립 모델)

  • 김형섭;오승탁;이재성
    • Transactions of Materials Processing
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    • v.10 no.5
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    • pp.383-388
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    • 2001
  • In describing the plastic deformation behaviour of ultrafine-grained materials, a phase mixture model in which a polycrystalline material is regarded as a mixture of a crystalline phase and a grain boundary phase has been successful. The deformation mechanism for the grain boundary phase, which is necessary for applying the phase mixture model to polycrystalline materials, is modelled as a diffusional flow of matter along the grain boundary. A constitutive equation for the boundary diffusion creep of the boundary phase was proposed, in which the strain rate is proportional to (stress/grain siz $e^{2}$). The upper limit of the stress of the boundary phase was set to equal to the strength to the amorphous phase. The proposed model can explain the strain rate and grain size dependence of the strength of the grain boundary phase. Successful applications of the model compared with published experimental data are described.

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Deformation Mechanism Map for Creep and Superplastic Deformation in $YBa_2Cu_3O_{7-x}$ Ceramic Superconductors ($YBa_2Cu_3O_{7-x}$ 세라믹 초전도체의 크리프와 초소성변형에 대한 변형기관도)

  • 윤존도;초우예
    • Journal of the Korean Ceramic Society
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    • v.33 no.6
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    • pp.718-724
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    • 1996
  • Deformation mechanism map of Langdon-Mohammed type for YBa2Cu3O7-x superconducting ceramic was constructed by considering mechanisms of Nabarro-Herring Coble and powder-law creep and grain boundary sliding (GBS) with an accommodation by grain boundary diffusion. The map was found consistent with experi-mental results not only of the creep the also of the superplastic deformation. It showed the transition from interface reaction-controlled to the grain boundary diffusion-controlled GBS mechanism at about 1 ${\mu}{\textrm}{m}$ grain size and 100 MPa flow stress in agreement with the experimental results.

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A Boundary diffusion creep model of grain boundary phase of materials (재료결정립계상의 입계확산크립 모델)

  • 김형섭
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • 2000.04a
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    • pp.192-195
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    • 2000
  • In describing the plastic deformation behaviour of fine grained materials a phase mixture model in which a polycrystalline material is regarded as a mixture of a crystalline phase and a grain boundary phase has been successful. The deformation mechanism for the grain boundary phase which is necessary for applying the phase mixture model is modelled as a diffusional flow of matter though the grain boundary. The proposed model can explain the strain rate and grain size dependence of the strength of the grain boundary phase.

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Analysis of Hot Isostatic Pressing of Powder Compacts Considering Diffusion and Power-Law Creep (확산과 Power- law 크립을 고려한 압분체 열간정수압압축 공정의 해석)

  • Seo M. H.;Kim H. S.
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • 2000.10a
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    • pp.66-69
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    • 2000
  • In order to analyze the densification behaviour of stainless steel powder compacts during hot isostatic pressing (HIP) at elevated temperatures, a power-law creep constitutive model based on the plastic deformation theory for porous materials was applied to the densification. Various densification mechanisms including interparticle boundary diffusion, grain boundary diffusion and lattice diffusion mechanisms were incorporated in the constitutive model, as well. The power-law creep model in conjunction with various diffusion models was applied to the HIP process of 316L stainless steel powder compacts under 50 and 100 MPa at 1125 $!`\acute{\dot{E}}$. The results of the calculations were verified using literature data It could be found that the contribution of the diffusional mechanisms is not significant under the current process conditions.

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Modelling the Densification Behaviour of Powders Considering Diffusion and Power-Law Creep Mechanisms during Hot Isostatic Pressing (열간정수압압축 시 확산기구 및 Power-law크립기구를 고려한 분말 치밀화거동의 모델링)

  • 김형섭
    • Journal of Powder Materials
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    • v.7 no.3
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    • pp.137-142
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    • 2000
  • In order to analyze the densification behaviour of stainless steel powder compacts during hot isostatic pressing (HIP) at elevated temperatures, a power-law creep constitutive model based on the plastic deformation theory for porous materials was applied to the densification. Various densification mechanisms including interparticle boundary diffusion, grain boundary diffusion and lattice diffusion mechanisms were incorporated in the constitutive model, as well. The power-law creep model in conjunction with various diffusion models was applied to the HIP process of 316L stainless steel powder compacts under 50 and 100 MPa at $1125^{\circ}C$. The results of the calculations were verified using literature data. It could be found that the contribution of the diffusional mechanisms is not significant under the current process conditions.

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Analysis of Creep Crack Growth at High-Temperature Components by Diffusive Growth Model of Grain Boundary Cavities (I)-Effect of Grain Boundary Cavitation on Stress Field and Crack Growth Rate- (입계기공의 확산성장 모델을 이용한 고온기기의 크립균열전파해석(1)-응력장 및 균열전파속도에 미치는 입계기공의 영향-)

  • Jeon, Jae-Young
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.20 no.4
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    • pp.1177-1185
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    • 1996
  • The crack growth under creep condition is one of the major damage mechanisms which determines remaining life of the component operating at high temperatures. In this paper, the creep crack growth by grain boundary cavitation is studied, which is frequently observed failure mechanism for creep brittle materials. As a result of diffusive growth of creep cavities, it is shown that the crack-tip stress field is modified from the original stress distribution by the amount of singularity attenuation parameter which is function of crack growth rate and material properties. Also, the stress relaxation at crack-tip results in the extension of cavitating area by the load dump effect to meet the macroscopic force equilibrium conditdion.

Creep Mechanisms of Calcium-Silicate-Hydrate: An Overview of Recent Advances and Challenges

  • Ye, Hailong
    • International Journal of Concrete Structures and Materials
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    • v.9 no.4
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    • pp.453-462
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    • 2015
  • A critical review on existing creep theories in calcium-silicate-hydrate (C-S-H) is presented with an emphasis on several fundamental questions (e.g. the roles of water, relative humidity, temperature, atomic ordering of C-S-H). A consensus on the rearrangement of nanostructures of C-S-H as a main consequence of creep, has almost been achieved. However, main disagreement still exists on two basic aspects regarding creep mechanisms: (1) at which site the creep occurs, like at interlayer, intergranular, or regions where C-S-H has a relatively higher solubility; (2) how the structural rearrangement evolutes, like in a manner of interlayer sliding, intra-transfer of water at various scales, recrystallization of gelled-like particles, or dissolution-diffusion-reprecipitation at inter-particle boundary. The further understanding of creep behavior of C-S-H relies heavily on the appropriate characterization of its nanostructure.

Effect of Minor Additives on the MgO Creep (MgO의 고온 Creep에 미치는 미량 첨가물의 영향)

  • Kim, Hwan
    • Journal of the Korean Ceramic Society
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    • v.14 no.3
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    • pp.182-186
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    • 1977
  • Compression creep of polycrystalline magnesia at about 1$600^{\circ}C$ under 5-40kg/$\textrm{cm}^2$ was examined, and also the effects on it of minor additives such as B2O3, CaO and SiO2 were examined. The high temperature creep of high purity magnesia was primarily controlled by the Nabarro-Herring type lattice diffusion of Mg in magnesia. B2O3 was included in the molten state and showed on increasing B2O3 contents. Some of the CaO and SiO2 were also included in the molten state, promoted the grain boundary sliding, so that creep rate was increased with an increasing content of them.

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Development of Heat- and Creep-resistant Fine-grained Rapidly Solidified P/M Aluminum Alloy

  • Kaji, Toshihiko;Tokuoka, Terukazu;Nishioka, Takao
    • Proceedings of the Korean Powder Metallurgy Institute Conference
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    • 2006.09b
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    • pp.720-721
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    • 2006
  • The new alloy$^{1)}$ is made from rapidly solidified Al-Ni-Zr-Ce aluminum alloy powder, and has the following unique mechanical characteristics:(1) The stress-strain curve shows a yield point; (2) The alloy shows high heat resistance; (3) Although the alloy is submicron particle diameter, it shows excellent creep resistance. We observed the micro structures of this new alloy, and it is thought that is based on the following reasons:(1) The dislocation strongly adheres to the alloy's many crystal boundaries;(2) The added alloying elements have a small diffusion coefficient in aluminum;(3) The tiny intermetallic compound particles crystallizing at the grain boundary.

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γ'-Precipitation Free Zone and γ' Rafting Related to Surface Oxidation in Creep Condition of Directionally Solidified CM247LC Superalloy (일방향 응고 CM247LC 초내열합금의 크리프 조건에서 표면 산화와 연계된 γ'-석출 고갈 지역 및 γ' 조대화)

  • Byung Hak Choe;Kwang Soo Choi;Sung Hee Han;Dae Hyun Kim;Jong Kee Ahn;Dong Su Kang;Seong-Moon Seo
    • Korean Journal of Materials Research
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    • v.33 no.10
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    • pp.406-413
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    • 2023
  • This study used optical and scanning electron microscopy to analyze the surface oxidation phenomenon that accompanies a γ'-precipitate free zone in a directional solidified CM247LC high temperature creep specimen. Surface oxidation occurs on nickel-based superalloy gas turbine blades due to high temperature during use. Among the superalloy components, Al and Cr are greatly affected by diffusion and movement, and Al is a major component of the surface oxidation products. This out-diffusion of Al was accompanied by γ' (Ni3Al) deficiency in the matrix, and formed a γ'-precipitate free zone at the boundary of the surface oxide layer. Among the components of CM247LC, Cr and Al related to surface oxidation consist of 8 % and 5.6 %, respectively. When Al, the main component of the γ' precipitation phase, diffused out to the surface, a high content of Cr was observed in these PFZs. This is because the PFZ is made of a high Cr γ phase. Surface oxidation of DS CM247LC was observed in high temperature creep specimens, and γ'-rafting occurred due to stress applied to the creep specimens. However, the stress states applied to the grip and gauge length of the creep specimen were different, and accordingly, different γ'-rafting patterns were observed. Such surface oxidation and PFZ and γ'-rafting are shown to affect CM247LC creep lifetime. Mapping the microstructure and composition of major components such as Al and Cr and their role in surface oxidation, revealed in this study, will be utilized in the development of alloys to improve creep life.