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Low Strain Creep of ex-service 0.5CrMoV Steel
Kloc, Luboš
Helicoid spring creep testing technique with high strain sensitivity was applied on specimens of low-alloy steel after 197 thousands hours in service. The creep testing conditions were selected to be close to that of the service, that is 560 °C and 35MPa. The ex-service specimens creep much faster than the unused one and an anisotropy in creep properties was also observed. The method is capable to provide some additional information for the residual creep life assessment, but interpretation of the results is quite difficult. The large volume of the material needed for testing is also considerable disadvantage.
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Mechanical and creep properties of aluminium alloys processed by Equal-Channel Angular Pressing (ECAP)
Dvořák, Jiří ; Král, Petr ; Kvapilová, Marie ; Sklenička, Václav ; Svoboda, Milan
This work examines mechanical and creep behavior in UFG aluminium alloys. UFG microstructure was achieved after application 8 ECAP passes. The coarse-grained materials were subjected to ECAP at room temperature in a die, which had a 90° angle between the channels. The microstructural investigations were performed using TEM and EBSD. The experiments show that ECAP reduced the average grain size to approximately 300 nm and led to the higher occurrence of high-angle boundaries in comparison with the coarse-grained material. The mechanical properties of these alloys were investigated using hardness measurements and tensile testing at room temperature. In addition, constant stress creep tests in compression were conducted at the applied stresses from 10 to 80 MPa and at temperature of 473 K. The aim of this investigation was to determine the effect of ECAP on the mechanical behavior and creep resistance of aluminium alloys containing Mg and/or Sc.
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Creep failure of Al-0.2%Sc alloy after severe plastic deformation (ECAP)
Král, Petr ; Dvořák, Jiří ; Kvapilová, Marie ; Svoboda, Milan ; Sklenička, Václav
Experiments were conducted on a coarse grain Al-0.2%Sc alloy which was processed using ECAP method (equal-channel angular pressing). ECAP deformation was conducted at room temperature using a die with two channels inclined an angle 90°. Tensile creep tests were performed at 473 K and 20 MPa. The microstructure and the fracture surfaces were observed by light microscopy, SEM and using EBSD technique. Creep fracture elongation of Al-0.2%Sc alloy was increasing with the number of ECAP passes. The fracture behaviour of samples after ECAP deformation and subsequent creep exposure was controlled by the formation of the mesoscopic slip bands on the specimen surface. The analyzes of microstructures of samples after 2 and 4 ECAP passes showed that the failure probably occurred at the places with the maximum heterogeneity of the microstructure.
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Creepové chování hliníkových slitin zpracovaných metodou ECAP
Král, Petr ; Dvořák, Jiří ; Kvapilová, Marie ; Svoboda, Milan ; Sklenička, Václav
This work examines creep behaviour in ultrafine-grained aluminum alloys (with Mg and Sc). The coarse-grained materials were subjected to ECAP (equal-channel angular pressing) at room temperature. The microstructural investigations were performed using transmission electron microscopy (TEM). The experiments show that ECAP reduced the average grain size to ~300 - 400 nm and led to the higher occurrence of high-angle boundaries in comparison with the coarse-grained material. The movement of boundaries and the growth of grains, created by ECAP, was restricted by nano Al3Sc coherent precipitates (up to 10 nm).Constant stress creep tests in compression were conducted at the applied stresses from 10 to 80 MPa and temperature of 473 K . In this work the values of the stress exponent for UFG materials and the unpressed material were determined. creep rates recorded for the as-pressed samples were significantly faster, by about two orders of magnitude, than the unpressed alloys.
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Effect of Number of ECAP Passes on Creep Behaviour and Microstructure of Pure Aluminium and Al-Sc Alloy
Král, Petr ; Dvořák, Jiří ; Kvapilová, Marie ; Svoboda, Milan ; Sklenička, Václav
Experiments were conducted to determine an effect of equal-channel angular pressing (ECAP) on the creep behaviour and the microstructure of pure aluminium and an Al-Sc alloy. The coarse-grained materials in their as-received states were subjected to ECAP at room temperature using a die, which had a 90° angle between the channels. Experiments show that ECAP reduced the average grain sizes of ~1µm and ~0.3µm for pure Al and Al-Sc. The grain sizes after 8 ECAP passes and subsequent creep exposure are ~12µm and ~2µm for Al and Al-Sc alloy, respectively. The fracture analysis and morphology of creep fracture surfaces of an Al-Sc alloy revealed the formation of shear bands which were orientated along the direction of the last ECAP pass. The application of ECAP method causes a substantial continuous decrease in the creep resistance of Al and Al-Sc alloy with increasing numbers of ECAP passes which can be explained by increasing contribution of grain boundary sliding to the total creep strain.
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Vliv metody ECAP na mikrostrukturu a creepové chování slitiny AL-3Mg-0,2Sc
Dvořák, Jiří ; Král, Petr ; Kvapilová, Marie ; Sklenička, Václav
The coarse-grained material (Al-3Mg-0.2Sc alloy) was subjected to ECAP at room temperature. Compression creep tests were conducted on both states of material at the applied stresses from 16 to 80 MPa and temperature of 473 K. The results demonstrate that the minimum creep rate of the ultrafine-grained (UFG) material is up to two order at magnitude higher than that of the unpressed coarse-grained material. A microstructural investigation of this alloy was performed using electron microscopy (TEM and EBSD). The experiments show that ECAP reduced the average grain size to ~ 300 nm and led to the higher occurrence of high-angle boundaries. It should be stressed that the application of eight ECAP passes causes a substantial decrease in the creep resistance of the alloy. Based upon the results it may be possible to suggest, that the creep behaviour of UFG alloy is influenced by grain boundary sliding accommodated by diffusion-controlled movement of intragranular mobile dislocations.
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Creep behaviour of Al-3Mg-0,2Sc alloy processed by ECAP method
Dvořák, Jiří ; Král, Petr ; Sklenička, Václav
Ternary aluminium alloy Al-3Mg-0.2Sc was subjected to equal-channel angular pressing (ECAP) at room temperature to refine its original coarse grain size. For this purpose the ECAP die was used which had a 90° angle between the die channels. By application of this method an ultrafine-grained microstructure was achieved with the grain size ~ 0.3mm after 8 ECAP passes by route BC. Subsequent compression constant stress creep tests were conducted at 473 K and in the applied stress range of 16–40 MPa. It was found, that resulting microstructure of this alloy leads to a decrease of minimum creep rate by an order of magnitude in comparison with creep behaviour of an ultrafine – grained ECAP pure aluminium. Further, the creep behaviour of Al-3Mg-0.2Sc alloy at lower stresses indicates a possible occurrence of the threshold stress for creep.
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Influence of Microstructure Changes on the Mechanical Behaviour of Aluminium Processed by ECAP Method
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Sklenička, Václav
Extremely coarse – grained aluminium (99.99 %) was subjected to severe plastic deformation (Equal-Channel Angular Pressing) at room temperature. The properties of the ECAP material may considerably differ from properties of material with a standard coarse grain size. The largest changes of microstructure take place during the first four ECAP passes. Thus, the microstructure after twelve passes consists of roughly equiaxed grains (~??m) with high fraction of high angle boundaries (~70%). In this work the behaviour of ECAP material with different number of passes at the tensile and creep tests has been studied. It is shown that ECAP aluminum provides a significant increase of the ultimate strength, however, there is low strain hardening during tensile deformation. The creep resistance of aluminum increased considerably already after one ECAP pass. However, repetitive ECAP pressing lead to a noticeable degrease in the creep properties. ECAP microstructure was examined in terms of the
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Creep Behaviour of Aluminium after Severe Plastic Deformation (ECAP)
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Sklenička, Václav
In this work the behaviour of ECAP aluminium with different number of passes at the creep tests was studied. The data indicated that an increase fraction of high angle boundaries has a strong effect on the creep properties of ECAP aluminium which can be explained by extensive grain boundary sliding contributing to the total creep deformation. However, the samples of ECAP aluminium with the identical number of ECAP passes showed significant differences of creep properties. The observed scatter in the creep behaviour is probably a direct consequence of different microstructures in the as-pressed state after ECAP.
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