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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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Orientation of Grains in the Al-Mg-Si-Mn Alloy by Scanning Low Energy Electron Microscopy
Müllerová, Ilona ; Matsuda, K. ; Horiba, K. ; Mikmeková, Šárka ; Frank, Luděk
Electron Backscatter Diffraction (EBSD) is a technique allowing the crystallographic infomiation to be obtained from samples in the scanning electron microscope (SEM). The main disadvantages of this method include the specimen tilt by 70°, requiring to operate at large working distances and hence with reduced lateral resolution, and a long acquisition time needed to obtain the full infomnation about grain orientations. However, the crystal orientation can be recognized upon energy dependence of the electron reflectance in the very low energy range. Information can be acquired at a high lateral resolution, high contrast and short acquisition time in a dedicated SEM equipped by the cathode lens.
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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 inhomogeneity. The results indicate that an inhomogeneity of the ECAP microstructure may influence the creep behaviour of the ECAP material.
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