čas: 23.4.2021 01:48:27
Obnovit | RAW
Università Politecnica delle Marche
List of available PhD theses
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.
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.
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.