Andrea Školáková

Investigation of the Effect of Magnesium on the Microstructure and Mechanical Properties of NiTi Shape Memory Alloy Prepared by Self-Propagating High-Temperature Synthesis

This work aims to describe the effect of magnesium on the microstructure, phase composition, amount of undesirable Ti2Ni phase, martensitic transformation, mechanical properties, and corrosion resistance of NiTi alloy. To minimize the quantity of Ti2Ni phase, we use the magnesium as an element with high affinity to oxygen, because this phase is stabilized by oxygen. Various quantities of magnesium (1, 3, and 5 wt pct) were tested. Self-propagating high-temperature synthesis (SHS) was used as a production method of the alloys. The samples prepared by SHS were pulverized by a vibrating mill, and the obtained powders were used for consolidation by means of spark plasma sintering. Results showed a significant reduction of the content of undesirable Ti2Ni phase by the addition of magnesium. Further, magnesium increased corrosion resistance and yield strength.

Structure and Mechanical Properties of Al-Cu-Fe-X Alloys with Excellent Thermal Stability

In this work, the structure and mechanical properties of innovative Al-Cu-Fe based alloys were studied. We focused on preparation and characterization of rapidly solidified and hot extruded Al-Cu-Fe, Al-Cu-Fe-Ni and Al-Cu-Fe-Cr alloys. The content of transition metals affects mechanical properties and structure. For this reason, microstructure, phase composition, hardness and thermal stability have been investigated in this study. The results showed exceptional thermal stability of these alloys and very good values of mechanical properties. Alloying by chromium ensured the highest thermal stability, while nickel addition refined the structure of the consolidated alloy. High thermal stability of all tested alloys was described in context with the transformation of the quasicrystalline phases to other types of intermetallics.

Adaptive Method for Novelty Detection in Crystallographics Data

Při navrhu parametrů PID regulatoru (ktere jsou v praxi dosud nejobvyklejsi) pro regulacni obvody je zapotřebi stanovit parametry regulatoru tak, aby zajisťovaly dostatecnou kvalitu a robustnost regulace. Je definovano a v praxi zavedeno mnoho seřizovacich metod pro nastaveni PID regulatorů vychazejicich z chovani uzavřeneho regulacniho obvodu ci naopak z matematickeho modelu regulovane soustavy (a jejiho L-přenosu), ktere umožňuji stanovit parametry regulatoru přimým výpoctem. S rostoucim výkonem HW a SW nastrojů může ale být výhodne použit přistup, který nalezne optimalni hodnoty nastaveni regulacniho procesu na zakladě simulaci a analýzy chovani regulacniho obvodu pro různe kombinace možných parametrů regulatoru. Navržena metoda seřizeni PID regulatoru linearniho dynamickeho systemu „hrubou silou a od oka“ je založena na opakovane simulaci regulacniho pochodu při skoku řidici veliciny a poruchove veliciny pro měnici se konstanty regulatoru P, I, D, (a v připadě cislicoveho regulatoru i vzorkovaci periodu T) pro smysluplne rozpěti hodnot – odtud „hrubou silou“ v nazvu metody. Z každeho běhu simulace je uložen diagram průběhů zajimavých velicin (žadana velicina, regulovana velicina, porucha) jako obrazový soubor a jsou pro tuto simulaci vypocteny hodnoty ukazatelů kvality regulace. Po provedeni simulaci pro vsechny hodnoty parametrů z uvažovaneho stavoveho prostoru jsou výstupy simulaci setřiděny podle ukazatelů kvality regulace; simulace s parametry regulace, ktera skorovala podle ukazatelů kvality regulace nejlepe, je pak podkladem pro nastaveni parametrů regulatoru a ověřeni v realnem regulacnim obvodu. Protože mame k dispozici vizualizovane podoby průběhů regulacniho procesu pro mnoho kombinaci vstupů, můžeme odhadnout i polohu a podobu „ostrovů stability“ a pro dosaženi robustnějsi regulace volit parametry regulatoru i expertni volbou, tedy „od oka“.

Reactive Sintering Mechanism and Phase Formation in Ni-Ti-Al Powder Mixture During Heating

This work aims to describe the formation of intermetallics in the Ni-Ti-Al system in dependence on the heating rate, which has been determined previously as the crucial factor of thermal explosion self-propagating synthesis (TE-SHS). The tested alloys contained 1–7 wt % aluminum. Thermal analysis has been realized by the optical pyrometer under the conditions of high heating rates up to 110 °C·min−1. TE-SHS process in Ni-Ti-Al system is initiated by exothermic reaction of nickel aluminides Ni2Al3 and NiAl3 at the temperature of 535–610 °C. The next reactions occur in dependence on the heating rate. Samples containing 1–3 wt % of aluminum exhibit the similar initiation temperature as Ni-Ti binary mixture. The samples containing 5 wt % and more of aluminum were fully reacted after sintering at 800 °C with the heating rate of 300 °C·min−1 and the initiation temperature of the TE-SHS was observed close to Al-Al3Ni eutectic temperature (between 630–640 °C).

Effect of Zirconium, Niobium and Chromium on Structure and Properties of Ni-Ti Alloy

The Ni-Ti alloy belongs to the group of materials with shape memory. This alloy has found its application mainly due to the excellent properties associated with the shape memory effect. It is necessary to keep the ratio between nickel and titanium approximately equimolar. In the alloy there could exist many intermetallic phases – NiTi, Ti2Ni, Ni3Ti, Ni4Ti3 and Ni3Ti2 and Ni8Ti5 [1, 2]. The intermetallic phase NiTi is responsible for the ability of shape memory. In addition to this phase, there is always a Ti2Ni intermetallic phase, which is undesirable because of its hardness and brittleness. In the NiTi phase, the phase transformation could be indicated between the high-temperature modification (austenite) and the lowtemperature modification (martensite). It could be indicated by thermal energy or mechanical energy [3-5]. The superelasticity is the most important property of this alloy. The material is above the austenite finish temperature (austenite phase). After mechanical deformation austenite is changed to martensite. Martensite is not stable in these conditions and it is changed back to austenite [6, 7]. Manufacturing is difficult because of the need for the exact chemical composition of a product. It is necessary to keep the exact ratio between nickel and titanium because of the temperature of the phase transformation. For example 0.1 at. % of nickel could decrease the martensite start temperature for 10 °C. The problem is also with the reactivity of titanium. Titanium has a high affinity to oxygen, nitrogen and carbon, so it is necessary to do the production under vacuum or inert atmosphere. The most common techniques of production are vacuum induction melting (VIM) and vacuum arc remelting (VAR). In VIM, the ingot has a good homogeneity, but there is a danger of contamination due to the usage of a crucible. The crucible is made of graphite and it reacts with titanium and products carbide of titanium. In VAR, the crucible is made of copper, so there is less contamination, but the problem is with homogeneity [8, 9]. An alternative to melting metallurgy is powder metallurgy. The biggest advantage of this technique is the opportunity of making a product with an exact chemical composition. Reactive sintering method is used for the production of Ni-Ti alloys. The principle of this method is very easy. The powders of nickel and titanium are mixed in the exact chemical composition, then pressed by define pressure and heated up to the temperature of initiation the reaction between nickel and titanium. This temperature is lower than the melting temperature of the phase NiTi. This reaction is very exothermic. The propagation of heat is very fast and it causes the reaction in a whole bulk. This process runs in two stages. A slow diffusion part in which the temperature is lower than in the second part. The second part is faster and is called selfpropagating high-temperature synthesis (SHS). The advantages of this method are low price and low energy for production, but the negatives of this method are uncontrollable porosity and an incomplete reaction of all elemental particles is possible [1, 10]. There are also the alternatives to reactive sintering such as hot isostatic pressing (HIP), spark plasma sintering (SPS) or metal injection moulding (MIM), but the reactive sintering powder metallurgy was chosen in this work [1, 10].

MICROSTRUCTURE AND THERMAL STABILITY OF Al-Fe-X ALLOYS

In this work, Al-11Fe, Al-7Fe-4Ni and Al-7Fe-4Cr (in wt. %) alloys were prepared by combination of casting and hot extrusion. Microstructures of as-cast alloys were composed of aluminium matrix with large and coarse intermetallics such as Al 13 Fe 4 , Al 13 Cr 2 and Al 5 Cr. Subsequently, as-cast alloys were rapidly solidified by melt-spinning technique which led to the supersaturation of solid solution alloying elements. These rapidly solidified ribbons were milled and compacted by hot-extrusion method. Hot-extrusion caused that microstructures of all alloys were fine with uniform dispersed particles. Moreover, long-term thermal stability was tested at temperature 300 °C for as-cast and hot-extruded alloys and chromium was found to be the most suitable element for alloying to improve thermal stability.

Effect of Alloying Elements on the Reactive Sintering Behaviour of NiTi Alloy

NiTi alloy is usually prepared by casting and forming. As an innovative process, reactive sintering powder metallurgy is tested worldwide, aiming to prepare pure NiTi alloy easily from nickel and titanium powders. This process enables to prepare both porous and low-porosity alloy, depnding on the process conditions. However, the formation of NiTi phase in this process is always accompanied by the Ti2Ni phase, which is hard, brittle, less corrosion resistant and does not have the shape memory. In this work, various alloying elements (Al, Si, Mg, Fe, Nb, V) were added to Ni-Ti alloy in order to lower the amount of Ti2Ni phase or at least to minimize its undesirable effect on the alloy properties. The reactive sintering behaviour, phase composition and mechanical properties of Ni-Ti-X alloys were described.