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Yu Qiao - One of the best experts on this subject based on the ideXlab platform.
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Resistance to cleavage cracking and subsequent shearing of High-Angle Grain Boundary
Engineering Fracture Mechanics, 2010Co-Authors: Jin Chen, S.s. Chakravarthula, Yu QiaoAbstract:In a previous experimental study, it was observed that the break-through process of a cleavage front across a High-Angle Grain Boundary can be highly nonuniform. While the central part of the Boundary can be cleaved quite smoothly, the rest parts must be sheared apart. In this paper, the trapping effect of Grain Boundary shearing is analyzed in considerable detail. Before the shearing is completed, the crack flanks are locally pinned together and a bridging stress must be provided. The bridging stress has a negative contribution to the local stress intensity at the cleavage front segment that penetrates across the Grain Boundary, and thus the crack growth driving force must be increased. A closed-form equation is derived to relate the overall fracture resistance to the fracture mode through an energy analysis.
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characteristic length scale in cleavage cracking across high angle Grain Boundary
Computational Materials Science, 2008Co-Authors: Jin Chen, Yu QiaoAbstract:The factors that govern cleavage cracking across High-Angle Grain boundaries are investigated theoretically. According to previous experimental observations, a cleavage front overcomes the resistance of a High-Angle Grain Boundary by first penetrating across it at a number of break-through points (BTP) and then separating apart persistent Grain-Boundary islands (PGBI). In the current study, this process is modeled as a competition between Grain Boundary shearing and crack front transmission. The numerical calculation shows that at a large Grain Boundary there exists an optimum BTP distance at which the Grain Boundary toughness is minimized, and when the BTP distance is relatively large its influence is secondary, fitting well with the experimental results.
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energy equilibrium of cleavage front transmission across a high angle Grain Boundary in a free standing silicon thin film
Engineering Fracture Mechanics, 2008Co-Authors: Yu Qiao, Jin Chen, Xinguo Kong, S.s. ChakravarthulaAbstract:In this article, the crack growth driving force and the resistance to cleavage cracking associated with crack front transmission across a High-Angle Grain Boundary in a silicon thin film are analyzed, and a closed-form solution of Grain Boundary toughness is obtained. It is noticed that the fracture resistance of the Grain Boundary is a function of the film thickness. This size effect is attributed to the nonuniform nature of cleavage front advance.
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mixed mode cleavage front branching at a high angle Grain Boundary
Scripta Materialia, 2007Co-Authors: Jin Chen, Yu QiaoAbstract:In a previous study it was observed that a cleavage front can penetrate through a High-Angle Grain Boundary in either regular or irregular mode. In this article, we report the third mode, which has a self-similar characteristic. In this mode, the cleavage front branches into a number of segments and the breakthrough points are in clusters, which can be regarded as the combination of regular and irregular processes. A first-order analysis is performed to estimate the Boundary fracture resistance.
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modeling of resistance curve of high angle Grain Boundary in fe 3 wt si alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2003Co-Authors: Yu QiaoAbstract:Abstract When a propagating cleavage front encounters a High-Angle Grain Boundary it first penetrates through the Grain Boundary stably. With increasing penetration depth the resistance of the Grain Boundary rises, resulting in the well-known R -curve. When the balance between the rates of the energy release rate and the Grain Boundary resistance is reached, the crack advance becomes unstable and the Grain Boundary is broken through. In this paper, this process is analyzed quantitatively based on the experimental observations of the cleavage cracking in Fe–3 wt.% Si bicrystals. The effects of the crystal misorientation, the crack length, and the crack front profile are discussed.
Jin Chen - One of the best experts on this subject based on the ideXlab platform.
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Resistance to cleavage cracking and subsequent shearing of High-Angle Grain Boundary
Engineering Fracture Mechanics, 2010Co-Authors: Jin Chen, S.s. Chakravarthula, Yu QiaoAbstract:In a previous experimental study, it was observed that the break-through process of a cleavage front across a High-Angle Grain Boundary can be highly nonuniform. While the central part of the Boundary can be cleaved quite smoothly, the rest parts must be sheared apart. In this paper, the trapping effect of Grain Boundary shearing is analyzed in considerable detail. Before the shearing is completed, the crack flanks are locally pinned together and a bridging stress must be provided. The bridging stress has a negative contribution to the local stress intensity at the cleavage front segment that penetrates across the Grain Boundary, and thus the crack growth driving force must be increased. A closed-form equation is derived to relate the overall fracture resistance to the fracture mode through an energy analysis.
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characteristic length scale in cleavage cracking across high angle Grain Boundary
Computational Materials Science, 2008Co-Authors: Jin Chen, Yu QiaoAbstract:The factors that govern cleavage cracking across High-Angle Grain boundaries are investigated theoretically. According to previous experimental observations, a cleavage front overcomes the resistance of a High-Angle Grain Boundary by first penetrating across it at a number of break-through points (BTP) and then separating apart persistent Grain-Boundary islands (PGBI). In the current study, this process is modeled as a competition between Grain Boundary shearing and crack front transmission. The numerical calculation shows that at a large Grain Boundary there exists an optimum BTP distance at which the Grain Boundary toughness is minimized, and when the BTP distance is relatively large its influence is secondary, fitting well with the experimental results.
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energy equilibrium of cleavage front transmission across a high angle Grain Boundary in a free standing silicon thin film
Engineering Fracture Mechanics, 2008Co-Authors: Yu Qiao, Jin Chen, Xinguo Kong, S.s. ChakravarthulaAbstract:In this article, the crack growth driving force and the resistance to cleavage cracking associated with crack front transmission across a High-Angle Grain Boundary in a silicon thin film are analyzed, and a closed-form solution of Grain Boundary toughness is obtained. It is noticed that the fracture resistance of the Grain Boundary is a function of the film thickness. This size effect is attributed to the nonuniform nature of cleavage front advance.
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mixed mode cleavage front branching at a high angle Grain Boundary
Scripta Materialia, 2007Co-Authors: Jin Chen, Yu QiaoAbstract:In a previous study it was observed that a cleavage front can penetrate through a High-Angle Grain Boundary in either regular or irregular mode. In this article, we report the third mode, which has a self-similar characteristic. In this mode, the cleavage front branches into a number of segments and the breakthrough points are in clusters, which can be regarded as the combination of regular and irregular processes. A first-order analysis is performed to estimate the Boundary fracture resistance.
P B Prangnell - One of the best experts on this subject based on the ideXlab platform.
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microstructure and texture evolution during annealing a cryogenic spd processed al alloy with a nanoscale lamellar hagb Grain structure
Acta Materialia, 2009Co-Authors: G H Zahid, Yan Huang, P B PrangnellAbstract:Abstract The Grain structure and texture evolution during annealing a Al–0.13% Mg submicron-Grained alloy, deformed by plane-strain compression (PSC) at cryogenic temperatures, has been investigated by transmission electron microscopy and electron backscatter diffraction. After deformation the alloy contained a lamellar Grain structure with a High-Angle Grain Boundary (HAGB) spacing of 190 nm and an area fraction of ∼80%. On annealing the Grain structure coarsened and transformed from lamellar to equiaxed. Remarkably, the fraction of low-angle Grain boundaries (LAGBs) progressively increased during annealing, to ∼50% above 300 °C, leading to instability and discontinuous recrystallization at higher temperatures. This resulted in a “bimodal Grain structure” comprised of bands of coarser Grains and fine subGrains, arising as a result of the increase in proportion of lower-mobility LAGBs. The surprisingly large increase in LAGB fraction on annealing is shown to be related to orientation impingement, originating from the strong texture present after PSC in liquid nitrogen.
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Grain structure formation during friction stir welding observed by the stop action technique
Acta Materialia, 2005Co-Authors: P B Prangnell, Chris P HeasonAbstract:Abstract Experiments have been carried out to ‘freeze’ the friction stir welding process by stopping the tool and immediately quenching the work piece in an Al-2195 plate welded under typical conditions. Sectioning through the ‘frozen’ weld keyhole with the tool in place has allowed the microstructure development, leading to the formation of the ultrafine Grained nugget material, to be directly observed as fresh material encounters the deformation field surrounding the rotating pin. The Grain refinement process is shown to be driven by Grain subdivision at the colder periphery of the tools deformation zone, and the geometric effects of strain, which together reduce the overall high angle Boundary spacing with increasing deformation. However, it also involves thermally activated high angle Grain Boundary migration, which increases as the temperature rises towards the tool. The higher temperature latter stages of the refinement process are closely reminiscent of geometric dynamic recrystallisation seen in high strain hot torsion experiments. The nugget Grain structure has also been found to become more equiaxed and coarsens slightly, due to static annealing in the thermal wake of the tool.
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Grain structure formation during friction stir welding observed by the stop action technique
Acta Materialia, 2005Co-Authors: P B Prangnell, Chris P HeasonAbstract:Abstract Experiments have been carried out to ‘freeze’ the friction stir welding process by stopping the tool and immediately quenching the work piece in an Al-2195 plate welded under typical conditions. Sectioning through the ‘frozen’ weld keyhole with the tool in place has allowed the microstructure development, leading to the formation of the ultrafine Grained nugget material, to be directly observed as fresh material encounters the deformation field surrounding the rotating pin. The Grain refinement process is shown to be driven by Grain subdivision at the colder periphery of the tools deformation zone, and the geometric effects of strain, which together reduce the overall high angle Boundary spacing with increasing deformation. However, it also involves thermally activated high angle Grain Boundary migration, which increases as the temperature rises towards the tool. The higher temperature latter stages of the refinement process are closely reminiscent of geometric dynamic recrystallisation seen in high strain hot torsion experiments. The nugget Grain structure has also been found to become more equiaxed and coarsens slightly, due to static annealing in the thermal wake of the tool.
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developing stable fine Grain microstructures by large strain deformation
Philosophical transactions - Royal Society. Mathematical physical and engineering sciences, 1999Co-Authors: F J Humphreys, P B Prangnell, Jacob R Bowen, A Gholinia, C HarrisAbstract:Methods of deforming metals to large strains are reviewed and the process of equal channel angular extrusion is analysed in detail. The development of microstructure during large strain deformation is discussed, and it is concluded that the main criterion for the formation of a sub–micron Grain structure is the generation of a sufficiently large fraction (> 0.7) of high–angle Grain Boundary during the deformation process. For aluminium alloys, it is found that a low–temperature anneal is required to convert the deformed microstructure into an equi–axed Grain structure. The material, microstructural and processing factors that influence the formation of such fine–Grain microstructures are discussed, and the stability of these microstructures at elevated temperatures is considered.
Christoph Kirchlechner - One of the best experts on this subject based on the ideXlab platform.
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Strain rate dependence of the slip transfer through a penetrable high angle Grain Boundary in copper
Scripta Materialia, 2017Co-Authors: Nataliya Malyar, Gerhard Dehm, Christoph KirchlechnerAbstract:Abstract Micro pillar compression is used to analyze the strain rate dependence of copper pillars containing a penetrable High-Angle Grain Boundary via in situ compression tests at strain rates ranging from 10 − 1 to 10 − 4 s − 1 . While the Grain-Boundary containing pillars exhibit a clear strain-rate dependence of m = 0.04 ± 0.02, their single crystal counterparts seem to have a weak strain rate dependence of m = 0.01 ± 0.01. The results strongly suggest that the movement of the dislocation line in the Grain Boundary, required to change its orientation from the incoming to the outgoing slip plane, is the critical process in deforming this kind of Grain-Boundary containing pillars.
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size effect in bi crystalline micropillars with a penetrable high angle Grain Boundary
Acta Materialia, 2017Co-Authors: Nataliya Malyar, Gerhard Dehm, Jeansebastien Micha, Christoph KirchlechnerAbstract:The implications of various size effects on the deformation behavior of and near Grain boundaries is not yet fully understood. In this manuscript, slip transfer mechanisms through a general high angle Grain Boundary (HAGB) allowing for easy transfer are investigated in order to understand the size dependence of the dislocation-Grain-Boundary interaction. Complementary in situ micro compression tests on copper single and bi-crystals in the scanning electron microscope and with x-ray Laue microdiffraction were used to correlate the mechanical response with the evolving microstructure. It is shown that no dislocation pile-up is formed at the Boundary. The lack of pile-up stresses results in a deformation process which is dominated by the initial dislocation source statistics. This is evidenced by similar size scaling of the single and bi-crystalline samples with the Grain size being the characteristic length scale.
Hiroshi Fujiwara - One of the best experts on this subject based on the ideXlab platform.
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the effect of ecap deformation route on microstructure mechanical and electrochemical properties of low cn fe 20 cr alloy
Materials Sciences and Applications, 2014Co-Authors: Muhammad Rifai, Hiroyuki Miyamoto, Hiroshi FujiwaraAbstract:The effect of the deformation route on the microstructure, and the mechanical and electrochemical properties of low CN Fe-20%Cr alloy by equal channel angular pressing, have been investigated focusing on the anisotropy of the microstructure. This alloy was pressed at 423 K from one, two and four passes via routes A, Bc and C, and the microstructure was observed in three orthogonal planes. As has been acknowledged, overall Grain fragmentation proceeded most effectively in route Bc. However, the degree of anisotropy of microstructural development was different among the three deformation routes. The fractions of the high angle Grain Boundary and mean Grain Boundary misorientation were high and nearly isotropic in route Bc, whereas they were consi- derably low in one direction and highly anisotropic in routes A and C. Most importantly, those two parameters were the highest in route C if limited to the transverse direction (Y-plane), i.e. normal to both the insert and extruding directions. This result contrasted with FCC materials, which was reported by most papers having the highest fraction of high angle Grain Boundary (HAGB) in route Bc. Ultrafine Grained structure with the highest HAGB on Y-plane in route C exhibited the most stabilized corrosion behavior.
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effect of ecap deformation route on the degree of anisotropy of microstructure of extremely low cn fe 20mass cr alloy
Metals, 2014Co-Authors: Muhammad Rifai, Hiroshi FujiwaraAbstract:The effect of the deformation route on the microstructural evolution of low CN Fe-20%Cr alloy by ECAP has been investigated, with a focus on the anisotropy of the microstructure. This alloy was pressed at 423 K from one, two and four passes via routes A, Bc and C, and the microstructure was observed three dimensionally. As has been acknowledged, overall Grain fragmentation proceeded most effectively in route Bc, and the highest hardness was obtained following routes C and A. However, the degree of anisotropy of microstructural development is different among the three deformation routes. The fractions of the high angle Grain Boundary (HAGB) and mean Grain Boundary misorientation were high and nearly isotropic in route Bc, whereas they are considerably low in one direction and highly anisotropic in routes A and C. Most importantly, those two parameters and hardness are the highest in route C if limited to the transverse direction, i.e., normal to both the insert and extruding directions. This result contrasts with FCC materials which most papers report as having the highest fraction of HAGB in route Bc. This result can be interpreted by the slip irreversibility of screw dislocations which is predominant in BCC metals.
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effect of ecap deformation route on the degree of anisotropy of microstructure of extremely low cn fe 20mass cr alloy
Metals, 2014Co-Authors: Muhammad Rifai, Hiroyuki Miyamoto, Hiroshi FujiwaraAbstract:The effect of the deformation route on the microstructural evolution of low CN Fe-20%Cr alloy by ECAP has been investigated, with a focus on the anisotropy of the microstructure. This alloy was pressed at 423 K from one, two and four passes via routes A, Bc and C, and the microstructure was observed three dimensionally. As has been acknowledged, overall Grain fragmentation proceeded most effectively in route Bc, and the highest hardness was obtained following routes C and A. However, the degree of anisotropy of microstructural development is different among the three deformation routes. The fractions of the high angle Grain Boundary (HAGB) and mean Grain Boundary misorientation were high and nearly isotropic in route Bc, whereas they are considerably low in one direction and highly anisotropic in routes A and C. Most importantly, those two parameters and hardness are the highest in route C if limited to the transverse direction, i.e., normal to both the insert and extruding directions. This result contrasts with FCC materials which most papers report as having the highest fraction of HAGB in route Bc. This result can be interpreted by the slip irreversibility of screw dislocations which is predominant in BCC metals.
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nano Grain formation in a fully ferritic sus 316l austenitic stainless steel produced by hs pm process
Materials Science Forum, 1999Co-Authors: Hiroshi Fujiwara, Kei AmeyamaAbstract:In this paper, the high strain powder metallurgy (HS-PM) process was applied to an SUS 316L stainless steel powder, and the microstructural changes during the process are discussed. The HS-PM process is applied to an SUS 316L austenitic stainless steel. In the case of the SUS 316L stainless steel, room temperature recrystallization and recovery of an austenite phase take place because of the increased high angle Grain Boundary area and the existence of excess vacancies, which are stored during the milling process. Very fine ferrite Grains are formed in the early stage of the milling. Although the amount of the α' martensite is not so large even after 98% cold rolling of the parent material, the extremely high density of defects introduced by the milling process increased free energy of the austenite phase, and thereby stabilized the α phase. In the late stage of milling, or in the case of higher energy milling, fully ferritic nano Grain structure with an average Grain size of approximately 20nm is formed. During heating, the fully ferritic powder showed complex microstructural changes. The powder compact sintered at 1173K for 3.6ks showed (α'+ M 2 C) microduplex structure with an average Grain size of 250nm.