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Influence of Microstructure Instability on Creep Behaviour of UFG Pure Materials
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Kvapilová, Marie ; Sklenička, Václav
Experiments were conducted to determine an effect of creep temperature on creep behaviour of pure Cu. The ECAP pressing was performed at room temperature by route Bc. Constant load creep tests in tension were conducted at 373-573 K under different stresses. The values of the stress exponent n of the minimum creep rate for ultrafine-grained (UFG) and coarse-grained material were determined. Microstructure of samples was characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM) equipped with the electron backscatter unit (EBSD). The microstructure analyses showed that microstructure of pure Cu processed by 8 ECAP passes and subsequent creep exposure contained large fraction of boundaries with coincidence sites lattice (CSL). The results showed that creep in UFG materials is influenced by additional creep mechanisms up to 0.5 Tm when the UFG microstructure in more or less stable.
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Influence of Heterogeneity of Microstructure on Creep Behaviour of SPD Metals
Král, Petr ; Dvořák, Jiří ; Kvapilová, Marie ; Svoboda, Milan ; Sklenička, Václav
The methods of severe plastic deformation enable a production of bulk metals with the submicrometer or even nanometer grain size. Experiments were conducted to determine an effect of equal-channel angular pressing (ECAP) on the creep behavior of pure metal. The ECAP deformation was performed at room temperature by route Bc. Constant load creep tests in tension were conducted at 373 K under different stresses. The values of the stress exponent n of the minimum creep rate for ultrafine-grained material were determined. Microstructure of samples was characterized by scanning electron microscope equipped with the electron backscatter diffraction unit. Heterogeneity of microstructure was evaluated by coefficient of variation CVa. It was found that the fracture behavior in UFG materials is influenced by GBS, formation of mesoscopic shear bands and cavitation.
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Creep Behaviour and Microstructure of Ultrafine Grained Iron Processed by ECAP
Král, Petr ; Dvořák, Jiří ; Kvapilová, Marie ; Svoboda, Milan ; Sklenička, Václav
Pure iron (99.99%) was subjected to equal-channel angular pressing (ECAP) with a die having an internal channel of 90°. The subsequent extrusion passes were performed by route A up to 4 passes. The microstructures of UFG iron were investigated using EBSD (electron back scatter diffraction) and transmission el. microscopy (TEM). Tension creep tests were performed on the as-pressed samples at temperature of 773 K and under applied load 60 MPa. For comparison purposes the same creep test was performed also on the unpressed state. The effect of number ECAP passes on creep behavior of pure Fe was investigated. Results showed that the creep resistance of pure Fe after ECAP is improved in comparison with the unpressed state.
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Different creep behaviour and microstructure of ECAP aluminium with the identical ECAP history
Král, Petr ; Dvořák, Jiří ; Sklenička, Václav ; Svoboda, Milan
Experiments were conducted on extremely coarse - grained Al (99.99%. The subsequent extrusion passes were performed by route A (no rotation between subsequent passes) up to 2 ECAP passes. Creep tests in tension were performed on the as-pressed samples at 473 K and applied stress 15 MPa. It was found that the ultrafine - grained Al specimens with the same ECAP history showed the scatter in the creep properties. In this work the microstructure changes within grains with different crystallographic orientations and near their high - angle boundaries of coarse-grained state in the sample after different ECAP passes were studied. Mutual comparison of the results of microstructural characterization of microstructure of as-received coarse-grained state and the state after different ECAP passes were performed. It is concluded that the scatter in creep properties could be influenced by inhomogeneity of ECAP microstructure.
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Effect of different number of ECAP passes on creep in Cu-Zr alloy
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Kvapilová, Marie ; Sklenička, Václav
A coarse-grained binary Cu-0.2wt%Zr alloy was processed by equal-channel angular pressing (ECAP) in order to evaluate the effect of severe plastic deformation on the thermal stability of resulting ultrafine-grained microstructure after ECAP and on creep behaviour of the pressed alloy. Constant stress creep tests in tension were conducted at temperature 673 K and at the different applied stresses. It was found that the Cu-0.2Zr alloy after the first four ECAP passes exhibited higher creep resistance by comparison with unpressed state. However, the alloy processed by 8 ECAP passes showed faster creep rate than unpressed material under the same loading conditions. The measured value of stress exponent of the creep rate n~6 indicates possibility of an intragranular deformation mechanism as the rate-controlling process in creep of an alloy after 8 ECAP passes. Furthermore, creep behaviour of ECAPed material may be influenced by mesoscopic boundary sliding along shear bands.
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