čas: 23.4.2021 01:37:34
Obnovit | RAW
Department of Metals and Corrosion Engineering
List of available PhD theses
Advanced high-entropy alloys with modifiable properties
High entropy alloys belong to a relatively new group of materials which are characterized by the preferential formation of solid solutions instead of intermetallic compounds. These materials exhibit several excellent properties, foremostly high strengths while maintaining sufficient ductility, good corrosion resistance and others. By suitable processing of these alloys, it is possible to achieve further substantial improvement of these already very good properties. The work will be focused on the preparation of new advanced high-entropy alloys combining significantly higher strengths while maintaining sufficient plasticity.
Analysis of damage causes of historical objects
The damage causes of historical objects are not always easy to identify. When the damage is caused by microscopic changes in the materials microstructure, it is necessary to use advance analytical techniques, e.g. transmission electron microscopy. In this work, the results obtained from historical materials will be compared with model samples.
Development and verification of hydrometallurgical routes for recovering critical metals from mineralized waters
The thesis deals with development and verification of hydrometallurgical routes for recovering critical metals from mineralized waters (brines) on the territory of the Czech republic. The metals of interest are mainly lithium, rubidium, boron and others, which are present in koncentration up to few hundreds ppm in the chloride and sulfate solutions. The methods such as membrane separation, ion-exchange and solvent extraction will be studied for separation and concentration of the metals of interest. From the concentrated solutions the metals will be obtained in their compounds form by suitable combination of traditional hydrometallurgical methods.
Development of atmospheric corrosion monitoring techniques
Majority of metallic objects, like metallic structures, vehicles, construction elements, cultural heritage, etc., is exposed in atmosphere. It deals with extreme costs and safety risks originated in corrosion. Corrosion protection is applied to avoid or minimize the impacts. There are many corrosion monitoring techniques available which might be applied for verification of corrosion countermeasures efficiency. However, the techniques must be accommodated to specific conditions of the particular fields. The project is focused on development of new device for atmospheric corrosion monitoring. The principle of the technique based on the electrical resistance method will be inovated. New corrosion sensors, electronics, software and data transfer and treatment will developed as well.
Development of new ultralight-weight magnesium-based alloys for aviation
Magnesium alloys are characterized by low density and a relatively high strength-to-weight ratio. From this point of view, they are interesting materials for applications in the automotive and aerospace industries. To improve the mechanical and corrosion properties, other metals like Al, Zn, Mn, Y, Nd, Gd, etc are very added. An interesting alloying element is Li, which as the lightest metal further reduces the density of the resulting alloy. Such behaviour is desirable for the potential reduction of CO2 emissions. Depending on the Li concentration, it is also possible to achieve a bcc structure and thus significantly improve the plasticity of the material. However, the strength and corrosion resistance of Mg-Li alloys are relatively poor, which prevents their wider use. This work aims to design and prepare by various processes including conventional casting and extrusion, as well as unconventional methods of powder metallurgy (mechanical alloying, spark plasma sintering - SPS, selective laser melting - SLM) magnesium alloys with Li and other alloying elements. The work will characterize in detail the influence of the microstructure on the mechanical and corrosion properties of the prepared materials. Besides, the optimization of selected preparation procedures leading to the required improvements of properties will be performed. Close cooperation with the organizations like Institute of Metals and Technology, Ljubljana and the Institute of Materials and Machine Mechanics, SAS, Bratislava is expected.
High-entropy alloys prepared by powder metallurgy techniques
Since the year 2004, the materials research gained a new way of focusing on special alloys made initially of five elements with equiatomic compositions. The newly discovered group of materials has been since then known as high-entropy alloys (HEA) due to a high entropy of mixing which allows creating predominantly solid solutions instead of intermetallic phases. These materials are known to be exhibiting a variety of excellent properties that often combine high strengths while maintaining good ductility, good corrosion resistance and others.
The work will aim at the description of the microstructure-related properties of high-entropy alloys prepared by powder metallurgy techniques combining mechanical alloying and compaction via spark plasma sintering.
Influence of Corrosion Products on Hydrogen Absorption to High Strength Steel
This project originates from industrial projects on hydrogen embrittlement (HE) of high strength steels, which are especially prone to hydrogen(H)-induced degradation. Since the development of these advanced steel grades with a huge potential of material and energy savings in many fields of human activity, the number of industry-funded studies on HE has been rising significantly. However, their practical focus does not allow for studying underlying mechanisms, although they are crucial for future development of products with improved application properties. This work aims at fundamental understanding of atmospheric corrosion as source for H insertion into iron, the principal element of all steel grades, as well as into selected steel grades. The role of corrosion products (CPs) in the H insertion during atmospheric corrosion will be clarified: How do composition and structure of CPs affect corrosion processes in view of H adsorption and absorption by e.g. reactions that involve a change of pH or a ratio between H reduction and other depolarization reactions? Which atmospheric conditions trigger these reactions? What is the mechanism of atomic H formation and entry?
Influence of radiolysis and bacterial extremophiles on lifetime of canister for radioactive waste repository
The work will be focused on the influence of bentonite pore solution radiolysis by gamma radiation on the oxidation ability, stability and semiconductive properties of passive layer on the 316L stainless steel. In addition will be studied the possibility of the formation of sulphate-reducing bacteria biofilm and the effect of metabolites on the susceptibility to pitting and stress corrosion cracking.
Mechanism of atmospheric low-temperature stress corrosion cracking of stainless steel
Stainless steels provide excellent service when selected and applied properly. Sufficient knowledge base is generally available for material selection for immersion conditions. This is not true for applications of stainless steels under atmospheric weathering conditions. Several cases of ceiling collapse and other component failures have shown that austenitic stainless steels are prone to stress corrosion cracking (SCC) under specific atmospheric conditions characterized by the spontaneous formation of concentrated chloride solutions under highly soluble chloride deposits even at room or only slightly elevated temperatures. This was observed in indoor swimming pools, for outdoor climbing anchors and under evaporative conditions in oil and gas production, storage, and processing. In collaboration with UCT Prague, the Safety Commission of Union Internationale des Associations d'Alpinisme (UIAA) introduced a new standard, which classifies climbing anchors into classes according to their resistance to SCC and/or corrosion. In the next step, particular environments will be classified. However, this requires deeper understanding into the degradation mechanism of stainless steels under relevant conditions is necessary. Factors influencing the SCC initiation and propagation such as the composition and concentration of deposits, tensile stress, role of crevices and aggressive ion accumulation, rock chemistry, periodic washing, stainless steel composition and microstructure and others will be systematically studied. In particular, in situ experiments will be carried out using X-ray micro tomography (μ-CT), which will allow for real time monitoring of crack initiation and propagation. In addition, outdoor exposures of numerous stainless steel grades and alternative materials organised around the world by UIAA will be followed and supported by failure analyses and specific measurements and tests.
Mechanism of the formation of intermetallics in mechanical alloying
Mechanical alloying is a popular technology for the preparation of powders of alloys or intermediary compounds (e.g. intermetallics, carbides or borides) by high energy mechanical milling. The high popularity of the method is given by the fact that it usually leads to nanostructured materials and that even non-miscible elements can create solid solutions during mechanical alloying. Even though the result is known and there are many descriptions available, the mechanism of the formation of intermetallics in this process is not fully understood yet. The reasons are probably in a large variety of possible process parameters and by impossibility to measure the temperature inside the powder in the milling jar. This work proposes following concept: indirect determination of the dependence of the peak powder temperature on milling conditions (rpm, ball-to-powder ratio, ball size) by the use of thermally decomposing salts, comparison of phase composition of the mechanically alloyed powder with the reference powder mixture exposed in the furnace to the detected peak temperature and observation of the time development of the microstructure and phase composition by XRD and electron microscopy (SEM, TEM). The mechanism will be observed on several different systems containing brittle and ductile powders (e.g. Ti-Al, Ti-Si, Ti-Al-Si) and the general conclusion regarding the mechanical alloying mechanism will be formulated.
Metallic materials and hydrogen
Metallic materials prepared by advanced 3D printing technologies
3D printing methods like selective laser melting (SLM) or other, are prospective for the fabrication of complex structural parts and medical implants because of their ability to produce very complex shapes. In the work, structures, mechanical, corrosion and biological properties of Ti based alloys, stainless or high-strength steels, biodegradable alloys or light-weight alloys prepared by various 3D printing processes will be investigated. In addition, the relationships between process parameters of 3D printing and characteristics of resulting products will be evaluated. Results of the study will propose process parameters the most suitable for obtaining desirable products. The work will also be focused on the development of novel 3D printing methods based on metal deposition.
New tool steels and their processing by non-conventional techniques
Tool steels reach very good mechanical properties, especially hardness and wear resistance, due to the content of carbide-forming elements and suitable heat treatment. The used scale of the carbide-formers comprises W, V, Mo and Cr, while some of them (W, V) are currently listed as critical raw materials by European commission. So it is desirable to substitute them. The suibstitution of the alloying elements is the subject of research for many years for various reasons and led to the formation of many grades of currently used tool steels. However, some of the potential alloying elements cause problems during processing by conventional metallurgical routes. This work is focused on the development of a new tool steel alloyed by non-conventional elements, aiming to form the material, which will be interesting from the viewpoints of both the mechanical proporties and the sustainability. For the processing, the techniques comprising 3D printing by "Direct Energy Deposition" method, which application for high-carbon tool steel will be unique, and the powder metallurgy methods comprising melt atomization or mechanical alloying and spark plasma sintering.
Properties and structure of rapidly solidified aluminium alloys with natural alloys
Rapidly solidified aluminium alloys with transition metals have excellent mechanical properties and elevated thermal stability when compared to conventional alloys. In this work, the alloys prepared by alloying aluminium by different amount of natural alloys obtained by reduction of polymetallic ore will be studied. Structures, mechanical properties and elevated temperature behaviour of these materials will be described. Study of these materials and their properties will enable the design of technology to obtain materials with the required properties from natural sources.
Příprava slitin na bázi Al se slitinami z hlubokomořských přírodních zdrojů a jejich korozní odolnost
Deep sea manganese nodules are formed by Mn and Fe oxide/hydroxides with minor amounts of critical elements like transition metals (TM) and rare earth metals (RE). The traditional way of processing this natural source is to separate the individual metals. This work is focused on innovative strategy of deep sea nodules utilization: a rough metallic alloy will be obtained by their reduction of natural mineral sources and it can be used for production of Al-based alloys. These alloys will be prepared by casting and extrusion with subsequent processing by rapid solidification and mechanical alloying, followed by spark plasma sintering compaction. Their microstructure and chemical-physics properties will be characterized and compared to commercial Al-alloys, which are widely used for different technological and industrial applications, due to their lightness, good mechanical properties and their low cost. Furthermore, considering all these applications, these metallic materials have a great interest in terms of corrosion performances in critical exposure conditions, therefore these studies will be performed as well.
Study of initiation and propagation of microstructural defects in metallic materials by X-ray microtomography
Rapid development of X-ray microtomography (μCT) in past two decades enabled its application in studies of metallic materials. Thanks to affordable μCT apparatuses with high resolution around or even below 1 μm, it is now possible to follow initiation and propagation of localized corrosion and environmentally assisted cracking in real time. A new, unique device available in Technopark Kralupy of UCT Prague with high resolution and equipped with integrated tensile testing stand and climatic chamber allows for in situ experiments in atmospheric conditions. The PhD thesis will be focused on two groups of modern iron-based materials, high strength steels and stainless steels. High strength steels are prone to hydrogen embrittlement, which will be studied in situ as a consequence of entry of atomic hydrogen generated by atmospheric corrosion. The application window of stainless steel in atmospheric exposure conditions is limited by the risk of pitting corrosion and stress corrosion cracking initiation under deposits rich with chloride ions. The study will focus mainly on the crack initiation in locations of microstructural defects and corrosion pits and the role of climatic parameters in crack propagation. The goal of the thesis will be to describe mechanisms of the initiation and propagation of these corrosion-induced defects.
Surface modifications of metallic biomaterials
Metallic biomaterials still play an irreplaceable role in medicine. The surface state significantly influences the properties and behaviour of biomaterials. This is especially the interaction on the metal-electrolyte phase boundary, i.e. biocompatibility and corrosion behaviour, although mechanical properties can also be affected. In the course of the work, the surfaces of metallic biomaterials will be modified in order to increase their utility properties. The modified surfaces will then be evaluated using standard material, electrochemical and spectroscopic methods.
Tribological properties of metallic biomaterials
Metallic biomaterials (titatium alloys, stainless steel, cobalt alloys) are frequently applied as joint replacement in human body. During the use of these materials, the wear occurs depending on the material and the wear debris can enter the tissue, causing the inflammation. Therefore there is an aim to apply the materials with highest possible wear resistance and even to improve it by suitable coatings. However, wear resistance of metallic biomaterials is not so easy to be determined, because the corrosion attack by the componets of the body environment plays important role in addition to the contact with the counterpart (acetabulum implant). This work is focused on the determination of suitable test conditions, simulating tribological and tribocorrosion loading joint and other specific (e.g. spinal) implants and determination of their tribological and tribocorrosion behaviour. The effect of various parameters - mechanical properties, microstructure and phase composition of the alloys and processing route (casting, forming, powder metallurgy, 3D print) - on resulting tribological characteristics will be described.
Zinc alloys for medical applications prepared by modern methods of powder metallurgy
Zinc and its alloys are considered as the biodegradable materials that can find applications such as orthopaedic implants or stents. The main advantage of zinc lies mainly in the acceptable corrosion rate and harmless degradation products. Corrosion of zinc does not lead to the release of hydrogen gas, which otherwise complicates the situation in the case of the degradation of magnesium-based materials. The main disadvantages of zinc and its alloys are poorer mechanical properties, especially yield strength, ultimate strength and ductility. Improvement of these properties can be achieved, for example, by suitable alloying. However, considering biocompatibility, the most acceptable alloying elements (Mg, Ca, Sr) are characterized by negligible solubility in the solid solution of zinc, which leads in conventional processing to the formation of various intermetallic phases. The aim of this work is to prepare and characterize in detail new biodegradable materials based on Zn-Mg-Ca / Sr prepared by powder metallurgy processes including mechanical alloying, atomization, melt spinning and powder compaction by extrusion processes, spark plasma sintering (SPS) and selective laser melting (SLM). ). The prepared materials should be characterized by a very fine microstructure positively affecting the mechanical properties and the uniformity of corrosion attack. The work will further focus on the potential addition of a small amount of other alloying elements such as Ag and Cu, which can further contribute to the improvement of the antibacterial properties of the material. Close cooperation with the organizations like Institute of Metals and Technology, Ljubljana and the Institute of Materials and Machine Mechanics, SAS, Bratislava is expected.