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Experimental study of the deformation mechanisms in cobalt by advanced in-situ techniques
Greš, Adam ; Knapek, Michal (advisor) ; Mathis, Kristián (referee)
The behaviour of pure polycrystalline cobalt in compressive deformation is influenced by the presence of high temperature fcc phase. To investigate this influence samples are prepared using annealing at different temperatures (600 řC- 1100 řC) with ≈ 6-10% of residual fcc phase. Additional sets of samples are also subjected to thermal cycling around the phase transformation temperature for ten or twenty cycles with the goal of stabilising the microstructure and creating material with no fcc fraction. Samples are tested in compression and a decrease in ductility and strength is observed after the the fcc phase is removed using thermal cycling. The interrupted deformation experiment shows that the transformation is rather sluggish and the main contribution of fcc grains to deformation is their ability to accommodate shape change. The presence of 71ř boundaries also sug- gests that the transformation is not fully stabilised and further thermal cycling could lead to further evolution of microstructure. Additionally the slow transfor- mation does not seem to produce detectable acoustic emission (AE) signal. The energies of AE events (recorded mostly around the yield point) follow a power law distribution. This is in line with recent investigations of the collective nature of dislocation motion and the...
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Mechanical properties of WN43 magnesium alloy prepared by spark plasma sintering
Knapek, Michal ; Minárik, P. ; Greš, A. ; Zemková, M. ; Cinert, Jakub ; Král, R.
The spark plasma sintering (SPS) method was used to prepare bulk materials form WN43 magnesium alloy atomized powder. Compression tests were carried out in order to investigate the effect of different sintering regimes (10 min at 400, 450, or 500 degrees C) on the mechanical properties of the material. Furthermore, complementary in-situ acoustic emission (AE) recording was employed to reveal the dynamics of deformation processes during compression. It was shown that by increasing the sintering temperature, the ultimate compressive strength and ductility were significantly improved. The AE data and microstructure observations suggest that pronounced twin nucleation takes place around the yield point whereas twin growth and dislocation activity are the dominant deformation mechanisms in the later stages of deformation.
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Mechanical properties of a novel magnesium WN43 alloy prepared by spark plasma sintering
Greš, Adam ; Knapek, Michal (advisor) ; Dobroň, Patrik (referee)
Title: Mechanical properties of a novel magnesium WN43 alloy prepared by spark plasma sintering Author: Adam Greš Department: Department of Physics of Materials Supervisor: RNDr. Michal Knapek, Ph.D., Department of Physics of Materials Abstract: Properties such as low modulus of elasticity, low toxicity, and biode- gradability make magnesium alloys interesting candidates for medical applicati- ons. To avoid potentially harmful effects of rare earth mischmetal present in the commercial WE43 alloys, it has been replaced in this study with pure neody- mium, which exhibits lower toxicity. To achieve high control over the material microstructure, it was prepared using the spark plasma sintering technique. This study focuses on exploring the mechanical properties of the novel WN43 alloy using compression and tensile testing accompanied by the acoustic emission tech- nique. Ultimate compressive strength and ductility were observed to improve significantly with higher sintering temperatures. In order to obtain better insi- ghts into microscopic aspects of the deformation mechanisms, EBSD images of the samples were taken at multiple points during deformation. It was shown that around the yield point twin nucleation produces strong acoustic emission signal, while later stages of deformation are mostly accommodated by...
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