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Antonio H. Y. Ngan - One of the best experts on this subject based on the ideXlab platform.

  • Size effects of nanoindentation creep
    Journal of Materials Research, 2004
    Co-Authors: H Li, Antonio H. Y. Ngan
    Abstract:

    The size effects on indentation creep were studied on single-crystal Ni 3 Al, polycrystalline pure Al, and fused quartz samples at room temperature. The stress exponents were measured by monitoring the displacement during constant indentation loads after correction for thermal drift effects. The stress exponents were found to exhibit a very strong size effect. In the two metals Al and Ni 3 Al, the stress exponent for very small indents is very small, and for Al, this even approaches unity, suggesting that linear Diffusional Flow may be the controlling mechanism. The stress exponents in these two metals rise rapidly to over 100 as the indent size gets larger, indicating a rapid change of the dominating mechanism to climb-controlled to eventually glide-controlled events. In fused quartz, the stress exponent also exhibits a sharply rising trend as the indent size increases. The stress exponent is also close to unity at the smallest indents studied, and it rises rapidly to a few tens as the indent size gets larger.

  • Size effects of nanoindentation creep
    Journal of Materials Research, 2004
    Co-Authors: Antonio H. Y. Ngan
    Abstract:

    The size effects on indentation creep were studied on single-crystal Ni3Al, polycrystalline pure Al, and fused quartz samples at room temperature. The stress exponents were measured by monitoring the displacement during constant indentation loads after correction for thermal drift effects. The stress exponents were found to exhibit a very strong size effect. In the two metals Al and Ni3Al, the stress exponent for very small indents is very small, and for Al, this even approaches unity, suggesting that linear Diffusional Flow may be the controlling mechanism. The stress exponents in these two metals rise rapidly to over 100 as the indent size gets larger, indicating a rapid change of the dominating mechanism to climb-controlled to eventually glide-controlled events. In fused quartz, the stress exponent also exhibits a sharply rising trend as the indent size increases. The stress exponent is also close to unity at the smallest indents studied, and it rises rapidly to a few tens as the indent size gets larger.published_or_final_versio

Robert M Mcmeeking - One of the best experts on this subject based on the ideXlab platform.

  • the effect of interface diffusion and slip on the creep resistance of particulate composite materials
    Mechanics of Materials, 1994
    Co-Authors: Petros Athanasios Sofronis, Robert M Mcmeeking
    Abstract:

    Abstract Reinforcements are known to increase the creep resistance of metal and intermetallic matrix composite materials. Experimental measurements at modest temperatures indicate that under a given applied strain rate the composite strength is higher than that of the matrix alone. However, at temperatures higher than approximately half of the melting temperature of the matrix, the composite strength is limited and in some cases the strengthening imparted by the reinforcements is completely lost. Diffusional relaxation and slip on the reinforcement-matrix interface are suggested as mechanisms responsible for the loss of strengthening. According to the proposed model, stress gradients caused by plastic constraint induce Diffusional Flow along the interface accompanied by slip of the matrix over the reinforcement. As a result the constraint tends to be relaxed and strengthening can be eliminated. The composite behavior is investigated by coupling the diffusion and slip along the interface with deformation of the matrix in the power law creep regime. A unit cell model is used in axial symmetry and the relevant boundary value problem is solved by the finite element method. Numerical results indicate that either Diffusional relaxation or slip may knock down the creep resistance of the composite to levels even below the matrix strength.

  • the effect of interface diffusion and slip on the creep resistance of particulate composite materials
    Mechanics of Materials, 1994
    Co-Authors: Petros Athanasios Sofronis, Robert M Mcmeeking
    Abstract:

    Abstract Reinforcements are known to increase the creep resistance of metal and intermetallic matrix composite materials. Experimental measurements at modest temperatures indicate that under a given applied strain rate the composite strength is higher than that of the matrix alone. However, at temperatures higher than approximately half of the melting temperature of the matrix, the composite strength is limited and in some cases the strengthening imparted by the reinforcements is completely lost. Diffusional relaxation and slip on the reinforcement-matrix interface are suggested as mechanisms responsible for the loss of strengthening. According to the proposed model, stress gradients caused by plastic constraint induce Diffusional Flow along the interface accompanied by slip of the matrix over the reinforcement. As a result the constraint tends to be relaxed and strengthening can be eliminated. The composite behavior is investigated by coupling the diffusion and slip along the interface with deformation of the matrix in the power law creep regime. A unit cell model is used in axial symmetry and the relevant boundary value problem is solved by the finite element method. Numerical results indicate that either Diffusional relaxation or slip may knock down the creep resistance of the composite to levels even below the matrix strength.

Petros Athanasios Sofronis - One of the best experts on this subject based on the ideXlab platform.

  • the effect of interface diffusion and slip on the creep resistance of particulate composite materials
    Mechanics of Materials, 1994
    Co-Authors: Petros Athanasios Sofronis, Robert M Mcmeeking
    Abstract:

    Abstract Reinforcements are known to increase the creep resistance of metal and intermetallic matrix composite materials. Experimental measurements at modest temperatures indicate that under a given applied strain rate the composite strength is higher than that of the matrix alone. However, at temperatures higher than approximately half of the melting temperature of the matrix, the composite strength is limited and in some cases the strengthening imparted by the reinforcements is completely lost. Diffusional relaxation and slip on the reinforcement-matrix interface are suggested as mechanisms responsible for the loss of strengthening. According to the proposed model, stress gradients caused by plastic constraint induce Diffusional Flow along the interface accompanied by slip of the matrix over the reinforcement. As a result the constraint tends to be relaxed and strengthening can be eliminated. The composite behavior is investigated by coupling the diffusion and slip along the interface with deformation of the matrix in the power law creep regime. A unit cell model is used in axial symmetry and the relevant boundary value problem is solved by the finite element method. Numerical results indicate that either Diffusional relaxation or slip may knock down the creep resistance of the composite to levels even below the matrix strength.

  • the effect of interface diffusion and slip on the creep resistance of particulate composite materials
    Mechanics of Materials, 1994
    Co-Authors: Petros Athanasios Sofronis, Robert M Mcmeeking
    Abstract:

    Abstract Reinforcements are known to increase the creep resistance of metal and intermetallic matrix composite materials. Experimental measurements at modest temperatures indicate that under a given applied strain rate the composite strength is higher than that of the matrix alone. However, at temperatures higher than approximately half of the melting temperature of the matrix, the composite strength is limited and in some cases the strengthening imparted by the reinforcements is completely lost. Diffusional relaxation and slip on the reinforcement-matrix interface are suggested as mechanisms responsible for the loss of strengthening. According to the proposed model, stress gradients caused by plastic constraint induce Diffusional Flow along the interface accompanied by slip of the matrix over the reinforcement. As a result the constraint tends to be relaxed and strengthening can be eliminated. The composite behavior is investigated by coupling the diffusion and slip along the interface with deformation of the matrix in the power law creep regime. A unit cell model is used in axial symmetry and the relevant boundary value problem is solved by the finite element method. Numerical results indicate that either Diffusional relaxation or slip may knock down the creep resistance of the composite to levels even below the matrix strength.

Wei Zhang - One of the best experts on this subject based on the ideXlab platform.

  • type iv failure in weldment of creep resistant ferritic alloys i micromechanical origin of creep strain localization in the heat affected zone
    Journal of The Mechanics and Physics of Solids, 2020
    Co-Authors: Wei Zhang, Xue Wang, Yiyu Wang, Yanfei Gao, Zhili Feng
    Abstract:

    Abstract Creep strength enhanced ferritic (CSEF) steels containing 9-12wt% chromium have been extensively used in fossil-fuel-fired power plants. Despite their excellent creep resistance at high temperatures, premature failures (especially Type IV cracking) are often found in the fine-grained heat affected zone (HAZ) or intercritical HAZ of the welded components. This failure mode is preceded by the strain localization in the HAZ, as measured by the Digital Image Correlation (DIC) technique. The present work aims to develop a finite-element based computational method to determine the micromechanical and microstructural origin of the strain localization phenomenon. We construct a two-dimensional digital microstructure based on the actual microstructure of ferritic steel weldments by using the Voronoi-tessellation method, to account for the effects of its large grain-size gradients. A mechanism-based finite element method is developed for modeling the high temperature deformation resulting from a synergy of thermally activated dislocation movements, Diffusional Flow and grain boundary sliding. The numerical results agree well with the strain measurements by our DIC technique, particularly revealing the effect of pre-welding tempering on the evolution of strain localization in HAZ of creep resistant steel weldments. It is found that the Diffusional creep with dependence on grain sizes, dislocation creep with dependence on material strength, and more importantly, grain boundary sliding, contribute synergistically to the creep strain accumulation in the HAZ, and their relative degree of significance is quantified. The creep rupture life will be investigated in the companion paper.

Zhili Feng - One of the best experts on this subject based on the ideXlab platform.

  • type iv failure in weldment of creep resistant ferritic alloys i micromechanical origin of creep strain localization in the heat affected zone
    Journal of The Mechanics and Physics of Solids, 2020
    Co-Authors: Wei Zhang, Xue Wang, Yiyu Wang, Yanfei Gao, Zhili Feng
    Abstract:

    Abstract Creep strength enhanced ferritic (CSEF) steels containing 9-12wt% chromium have been extensively used in fossil-fuel-fired power plants. Despite their excellent creep resistance at high temperatures, premature failures (especially Type IV cracking) are often found in the fine-grained heat affected zone (HAZ) or intercritical HAZ of the welded components. This failure mode is preceded by the strain localization in the HAZ, as measured by the Digital Image Correlation (DIC) technique. The present work aims to develop a finite-element based computational method to determine the micromechanical and microstructural origin of the strain localization phenomenon. We construct a two-dimensional digital microstructure based on the actual microstructure of ferritic steel weldments by using the Voronoi-tessellation method, to account for the effects of its large grain-size gradients. A mechanism-based finite element method is developed for modeling the high temperature deformation resulting from a synergy of thermally activated dislocation movements, Diffusional Flow and grain boundary sliding. The numerical results agree well with the strain measurements by our DIC technique, particularly revealing the effect of pre-welding tempering on the evolution of strain localization in HAZ of creep resistant steel weldments. It is found that the Diffusional creep with dependence on grain sizes, dislocation creep with dependence on material strength, and more importantly, grain boundary sliding, contribute synergistically to the creep strain accumulation in the HAZ, and their relative degree of significance is quantified. The creep rupture life will be investigated in the companion paper.