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Farghalli A Mohamed - One of the best experts on this subject based on the ideXlab platform.

  • correlation between the Minimum Grain Size obtainable by ball milling and lattice strain
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2019
    Co-Authors: Farghalli A Mohamed
    Abstract:

    Abstract Primary among the top-down processing techniques that can be used to syntheSize nanocrystalline materials is ball milling. During milling, the gain Size decreases with milling time, reaching a Minimum Grain Size, d min . Minimum Grain Size attainable is a characteristic of each material. Ball milling introduces considerable amount of lattice strain, e. Such a strain increases rapidly with time, reaching a maximum value that coincides with d min . By applying a recently developed dislocation model, which relates, d min , to several physical parameters (such as the stacking fault energy and hardness), and by adopting several fundamental concepts, which are related to deformation behavior in materials, it is shown that d min / b depends on ( 1 / e ) 0.94 , where b is the Burgers vector . The validity of this functional dependence is examined by analyzing experimental data on several nanocrystalline materials.

  • on the Minimum Grain Size obtainable by equal channel angular pressing
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2013
    Co-Authors: Farghalli A Mohamed, Shehreen S Dheda
    Abstract:

    Abstract Recently, a dislocation model that relates Minimum Grain Size, d min , obtainable in metals by ball milling to several physical parameters was developed and also applied to high-pressure torsion. In this paper, the model is extended to d min obtainable by equal channel angular pressing (ECAP), another severe plastic deformation (SPD) process.

  • on the Minimum Grain Size obtainable by high pressure torsion
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2012
    Co-Authors: Farghalli A Mohamed, Shehreen S Dheda
    Abstract:

    Abstract Recently, a dislocation model that quantitatively relates the Minimum Grain Size obtainable by ball milling, dmin, to several physical parameters, such as the activation energy for self-diffusion and the stacking fault energy, in a nanocrystalline (nc) material was developed. In this paper, it is shown that the predictions of the model are consistent with the characteristics of the Minimum Grain Size, dmin, obtainable in FCC and BCC metals by high-pressure torsion. Such a consistency indicates that the dislocation model for ball milling is quantitatively applicable to the description of other severe plastic deformation (SPD) processes.

  • Refining efficiency and capability of top-down synthesis of nanocrystalline materials
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2011
    Co-Authors: Farghalli A Mohamed
    Abstract:

    Severe plastic deformation (SPD) has been employed in recent years as a top-down method for the synthesis of nanocrystalline materials. In this paper, data reported for various severe plastic deformation processes, including high pressure torsion (HPT), equal channel angular pressing (ECAP), ball milling (BM), and accumulative roll bonding (ARB), are analyzed in terms of: (a) the Minimum Grain Size obtainable by SPD processing, (b) the capability and feasibility of individual processing techniques, and (c) refining efficiency in terms of Grain Size decrease as a function of induced strain.

  • correlation between the behavior of nanocrystalline hcp metals and the dislocation model for the Minimum Grain Size obtainable by milling
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2010
    Co-Authors: Farghalli A Mohamed
    Abstract:

    Abstract Recently, a dislocation model that quantitatively relates the Minimum Grain Size obtainable by milling, d min , to several physical parameters, such as the activation energy for self-diffusion and the stacking fault energy, in a nanocrystalline (nc) material was developed. The development of the model was based on the suggestion that the Minimum average Grain Size, d min , is the result of a balance between the formation of dislocation structure and its recovery by thermal processes. The predictions of the above model were found to be in good agreement with experimental data and trends reported for nc-FCC and nc-BCC metals. In this paper, the validity of the model to the description of the behavior of nc-HCP metals for which data on the Minimum Grain Size, d min , are available is examined. Also, the general applicability of the model to other severe plastic deformation (SPD) processes is discussed.

Terence G Langdon - One of the best experts on this subject based on the ideXlab platform.

  • an evaluation of the saturation hardness in an ultrafine Grained aluminum 7075 alloy processed using different techniques
    Journal of Materials Science, 2015
    Co-Authors: Shima Sabbaghianrad, Terence G Langdon
    Abstract:

    A commercial Al-7075 alloy was processed by the severe plastic deformation (SPD) procedures of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) and by a combination of these two techniques. The results show samples processed by a combination of ECAP and HPT have smaller Grain Sizes and higher saturation hardnesses than samples processed separately by ECAP or HPT. Microstructural observations reveal Grain refinement after each SPD technique, and the Minimum Grain Size was ~200 nm after processing by a combination of ECAP for eight passes and HPT for 20 turns. It is demonstrated that the saturation hardness is dependent upon the microstructure of the sample introduced in any processing step prior to the HPT processing.

  • The Influence of Impurity Content on Thermal Stability of Low Stacking Fault Energy Silver Processed by Severe Plastic Deformation
    Materials Science Forum, 2012
    Co-Authors: Zoltán Hegedűs, Jenő Gubicza, Megumi Kawasaki, Nguyen Q. Chinh, Zsolt Fogarassy, Terence G Langdon
    Abstract:

    The effect of the impurity content on the evolution of the ultrafine-Grained (UFG) microstructure in low stacking fault energy Ag and its stability at room and elevated temperatures were investigated. Samples of silver having high (99.995%) and somewhat lower (99.99%) purity levels were processed by equal-channel angular pressing (ECAP) at room temperature (RT) up to 16 passes. Although, the Minimum Grain Size achieved by ECAP was ~200 nm for both series, the lattice defect structure was strongly influenced by the impurity content. In the samples processed by 4-16 passes of ECAP a self-annealing occurred during storage RT that was promoted by the higher twin boundary frequency. Both room-and high-temperature thermal stability of 99.99% purity Ag were much better due to the pinning effect of impurities. It was found that a large number of dislocation loops remained in the microstructure even after recrystallization at high temperatures.

  • Microstructure of low stacking fault energy silver processed by different routes of severe plastic deformation
    Journal of Alloys and Compounds, 2012
    Co-Authors: Zoltán Hegedűs, Jenő Gubicza, Megumi Kawasaki, Nguyen Q. Chinh, Zsolt Fogarassy, Terence G Langdon
    Abstract:

    Abstract Samples of 4 N purity Ag were processed at room temperature (RT) by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) up to 8 passes and 20 revolutions, respectively. It was found that the Minimum Grain Size was around 200 nm for both ECAP and HPT. However, the dislocation density and the twin boundary frequency were about three times larger in HPT due to the very high applied hydrostatic pressure. The maximum dislocation density (about 1.5 × 10 16  m −2 ) and twin boundary frequency (about 2%) achieved by HPT at RT are extremely high among pure fcc metals and this can be explained by the difficult annihilation of the highly dissociated dislocations due to the very low stacking fault energy in Ag.

  • The effect of impurity level on ultrafine-Grained microstructures and their stability in low stacking fault energy silver
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2011
    Co-Authors: Zoltán Hegedűs, Jenő Gubicza, Megumi Kawasaki, Nguyen Q. Chinh, Zsolt Fogarassy, Terence G Langdon
    Abstract:

    Abstract The effect of impurity content on the evolution of microstructure in low stacking fault energy silver processed by severe plastic deformation (SPD) was studied. The SPD-processing was carried out on 4N5 and 4N purity Ag samples by equal-channel angular pressing (ECAP) up to 16 passes. It was found that, although the Minimum Grain Size and the maximum dislocation density were not affected by the different impurity atom content, there is a lower degree of twinning in the less pure material for high number of passes. The small increase of impurity level from 4N5 to 4N in Ag resulted in a significantly better thermal stability at room temperature for the ultrafine-Grained microstructures obtained by ECAP.

  • Significance of stacking fault energy on microstructural evolution in Cu and Cu-Al alloys processed by high-pressure torsion
    Philosophical Magazine, 2011
    Co-Authors: Xianghai An, Z.f. Zhang, Suxiang Wu, Roberto B. Figueiredo, Terence G Langdon
    Abstract:

    Disks of pure Cu and several Cu–Al alloys were processed by high-pressure torsion (HPT) at room temperature through different numbers of turns to systematically investigate the influence of the stacking fault energy (SFE) on the evolution of microstructural homogeneity. The results show there is initially an inhomogeneous microhardness distribution but this inhomogneity decreases with increasing numbers of turns and the saturation microhardness increases with increasing Al concentration. Uniform microstructures are more readily achieved in materials with high or low SFE than in materials with medium SFE, because there are different mechanisms governing the microstructural evolution. Specifically, recovery processes are dominant in high or medium SFE materials, whereas twin fragmentation is dominant in materials having low SFE. The limiting Minimum Grain Size (dmin) of metals processed by HPT decreases with decreasing SFE and there is additional evidence suggesting that the dependence of dmin on the SFE decreases when the severity of the external loading conditions is increased.

Levente Vitos - One of the best experts on this subject based on the ideXlab platform.

  • determining the Minimum Grain Size in severe plastic deformation process via first principles calculations
    Acta Materialia, 2012
    Co-Authors: Song Lu, Qingmiao Hu, Erna Krisztina Delczegczirjak, Borje Johansson, Levente Vitos
    Abstract:

    Although the stacking fault energy (SFE) is a fundamental variable determining the Minimum Grain Size (dmin) obtainable in severe plastic deformation (SPD) processes, its accurate measurement is difficult. Here we establish the SFEs of binary Pd–Ag, Pd–Cu, Pt–Cu and Ni–Cu solid solutions using the axial interaction model and the supercell model in combination with first-principles theory. The two models yield consistent formation energies. For Pd–Ag, Pd–Cu and Ni–Cu, the theoretical SFEs agree well with those from the experimental measurements. For Pt–Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We discuss the correlation of the SFE and dmin in SPD experiments and show that the dmin values can be evaluated from first-principles calculations.

Alex V. Hamza - One of the best experts on this subject based on the ideXlab platform.

  • strength and thermal stability of nanocrystalline gold alloys
    Scripta Materialia, 2007
    Co-Authors: Y M Wang, A F Jankowski, Alex V. Hamza
    Abstract:

    By additions of Cu and Sn elements, we have syntheSized nanocrystalline Au–Cu and Au–Cu–Sn materials with a Minimum Grain Size down to ∼3 nm. Tensile measurements of these nanocrystalline materials indicate that the Hall–Petch scaling law is maintained to the smallest length scale. Though Cu and Sn solid solutions are observed to have little effect on the strength of nanocrystalline Au, they substantially enhance the thermal stability of these alloys. Abnormal Grain growth no longer becomes the characteristic feature of alloyed nanocrystalline Au.

Kenji Higashi - One of the best experts on this subject based on the ideXlab platform.

  • Effect of Solute Elements on Grain Refinement during Friction Stir Processing in High-Purity Aluminum
    Materials Science Forum, 2016
    Co-Authors: Tokuteru Uesugi, Yorinobu Takigawa, Hideaki Iwami, Kenji Higashi
    Abstract:

    Friction stir processing (FSP) is one of the severe plastic deformation (SPD) processes. It has been reported that SPD-processed Al with various purities attained a Minimum Grain Size when Zener-Hollomon parameter is larger than 1016 s-1. The Minimum Grain Size is different by purity level and alloying elements. We investigated the influence of Fe solute atoms on Grain refinement of high-purity Al on the condition that Zener-Hollomon parameter was larger than 1016 s-1. FSP was conducted on Al-0.01%Fe, which was fabricated by using 5N Al (99.999% purity). FSP-ed Al-0.01%Fe exhibits the Minimum Grain Size of 1.4 μm, although high-purity aluminums with more than 99.998% exhibits much larger Minimum Grain Sizes of 30-40 μm. Only 101 at.ppm Fe played a critical role in the Grain refinement of pure aluminums.

  • effect of mg content on the Minimum Grain Size of al mg alloys obtained by friction stir processing
    Scripta Materialia, 2011
    Co-Authors: Taiki Morishige, Tomotake Hirata, Tokuteru Uesugi, Yorinobu Takigawa, Masato Tsujikawa, Kenji Higashi
    Abstract:

    Friction stir processing (FSP) is one of the severe plastic deformation processes that have been developed to improve the mechanical properties of both metals and alloys by producing an ultrafine-Grained structure. In this study, it was used to realize the Minimum Grain Size in Al–Mg alloys. The results indicate that the Grain Size in Al–Mg alloy decreases with increasing Mg content because of the influence of stacking fault energy in the Al–Mg alloy.

  • comprehensive analysis of Minimum Grain Size in pure aluminum using friction stir processing
    Materials Letters, 2010
    Co-Authors: Taiki Morishige, Tomotake Hirata, Masato Tsujikawa, Kenji Higashi
    Abstract:

    Abstract A friction stir processing (FSP) technique has been developed for use with aluminum and magnesium alloys, with the goal of high-strain-rate processing. In this study, we treat the microstructures of aluminum samples, of three levels of purity, manipulating their Grain Sizes, making them finer, using FSP. Grain Sizes decreased with an increase in the Zener–Hollomon parameter, at all purity levels. However, the recrystallized Grain of FSP-ed ultra-high-purity (99.999%) aluminum was particularly large, compared to the Grain Sizes of lower-purity (99% and 99.99%) aluminum, when subjected to the same processing conditions. The Grain Size reached a certain Minimum value at high-strain-rates of processing, which we report for each purity level herein.