Notranja oksidacija Cu-C in Ag-C kompozitov

The internal oxidation in copper-carbon and silver-carbon composites occurs when they are exposed to air or oxygen at high temperature. Solubility of carbon in copper or in silver is very low. The kinetics of oxidation at high temperature and activation energy were determined and the mechanism of internal oxidation was analysed. The kinetics of internal oxidation was determined for both cases and it is depended from the diffusion of oxygen following parabolic time dependence according to Wagner\u27s theory. The activation energy for Cu-C composite is 70.5 kJ/mol, and for Ag-C composite is 50.1 kJ/mol, what is in both cases close to the activation energy for the volume diffusion of oxygen in copper or in silver. In both cases gas products are formed during the internal oxidation of composites. In the internal oxidation zone pores, bubbles occur. The carbon oxidates directly with the oxygen from solid solution as long there is a contact, which breaks down with the presence of gas products. Then the oxidation occurs over the gas mixture of CO and CO2.Pri visokih temperaturah kompoziti bakra in srebra z ogljikom na zraku ali v kisiku reagirajo po mehanizmu notranje oksidacije. Topnost ogljika v trdnem bakru in trdnem srebru je zelo majhna. Analizirali smo kinetiko oksidacije kompozitov, določili aktivacijsko energijo in mehanizem notranje oksidacije. Kinetika oksidacije je pri obeh skupinah materialov odvisna od difuzije kisika in sledi parabolični odvisnosti od časa v skladu z Wagnerjevo teorijo. Aktivacijska energija procesa je za kompozit Cu-C enaka 70,5 kJ/mol, za kompozit Ag-C pa 50,1 kJ/mol, kar je blizu aktivacijski energiji za volumsko difuzijo kisika v trdnem bakru oziroma srebru. Pri oksidaciji kompozita nastajajo plinski produkti. Oksidacija ogljika poteka neposredno s kisikom iz trdne raztopine, ko pa se zaradi nastanka plinske faze stik prekine, pa preko plinske zmesi CO in CO2

The Methodology of Monitoring the Implementation of a Strategy in an Electricity Distribution Enterprise

Lately, a greater emphasis has been placed on the quality of environment and sound management of resources, and above all on energy sources as the basis of the human life quality and of sustainable development. The sustainable development of an organization is monitored through the organization’s performance, which is the result of its strategy that has incorporated all stakeholders’ requirements beforehand. Although a great number of companies apply a strategic management concept and business success measurement methods, researches show that only every tenth company manages to implement the defined strategy. The main causes of unsuccessful strategy implementation can be found in the lack of adequate strategic control mechanisms. The paper presents the methodology developed with an aim to remove the main obstacles to successful implementation of the defined strategy and to the monitoring of an enterprise\u27s effectiveness

Mechanical properties and microstructure characterisation of Au-Pt dental alloy

Development of a dental alloy with high Au content is based on the ternary system of Au-Pt-Zn with a nominal composition of 86,9Au-9,9Pt-1,5Zn, and about 1,5 wt.% micro-alloying elements (in, Ir, Rh). The results analyses of different heat-treated states showed that the optimal mechanical properties and hardness of an Au-Pt-Zn alloy can be reached with combinations of heat treatment for 20 minutes at 723 K and then slowly cooling, if the alloy was annealed at 1223 K for 30 minutes and the water quenched. Research results confirmed that the microstructure of the Au-Pt-Zn alloy consists of two phases: alpha(1)-phase rich in Au (main phase) and alpha(2)-phase rich in Pt (minor phase). During XRD analysis and use of the Rietveld method, it was found that the alpha(1)-phase content is about 98,5 wt.% while the content of alpha(2)-phase is 1,5 wt.%. STA analyses show that the Au-Pt-Zn alloy has a solidus temperature of about 1292 K and a liquidus temperature of about 1412 K

Special finite elements: Theoretical background and application

Specijalni konačni elementi (KE) koriste se za modeliranje posebnih fenomena u ponašanju konstrukcija. Izraz "specijalni" označava KE sa specifičnim svojstvima (parametri krutosti, ponašanje pod opterećenjem, itd.) i posebnom ulogom u modeliranju, po čemu se i bitno razlikuju od "standardnih" KE. Postoji više tipova specijalnih KE, ali se najčešće koriste: "nelinearna opruga" KE, "zazor" KE i "veza" KE. Na različitim će primjerima biti pokazane prednosti primjene specijalnih KE, posebno u modeliranju konturnih i prijelaznih uvjeta. Teorijska podloga data je samo u neophodnom obimu, zbog dobrog razumijevanja numeričkih aspekata primjene specijalnih KE.Special finite elements (FEs) are used for modeling of certain phenomena in structural behavior. The term "special" denotes FEs with specific characteristics (stiffness parameters, behavior under load, etc.) and a specific role in modeling, which is their main difference from "common" FEs. There are many types of special FEs, but the following are often used: so-called "nonlinear spring" FE, "gap" FE and "link" FE. Various examples will show many advances in the application of these FEs, especially in modeling of boundary and interface conditions. Theoretical background is given only in the amount necessary for proper understanding of numerical aspects of the application of special FEs

Comparison of mechanical and microstructure properties of tungsten alloys for special purposes

Tungsten belongs to group of refractory metal that possess extraordinary resistance to heat and wear and it is the heaviest engineering material. Because of its properties tungsten is used for special purposes. This paper presents the results of mechanical and microstructure research on the example of the characteristic heavy tungsten alloys 91W-6Ni-1.8Fe-1Co and 93W-5Ni-1.6Fe-0.3Co with different Ni/Co ratios. The proper Ni/Co ratio is important to obtain a favorable microstructure and mechanical properties of these materials. The distribution of the W, Ni, Co and Fe elements in tungsten phase and binder phase, which can influence on mechanical properties of tungsten alloys. The SEM analysis and mechanical results show that the alloy, which has Ni/Co within the given limits,  posses a finer microstructure and better mechanical properties that is very important for the maintenance of the quality of tungsten alloys for special purposes

ANALIZA MIKROSTRUKTURE ZLITINE CuAlNiMn Z OBLIKOVNIM SPOMINOM PRED NATEZNIM PREIZKUSOM IN PO NJEM

In this paper the results of a microstructural analysis before and after fracture along with the mechanical properties and hardness of the CuAlNiMn shape-memory alloy are presented. The melting of the alloy was carried out in a vacuum-induction furnace in a protective atmosphere of argon. The alloy was cast into an ingot of 15 kg. After casting the alloy was forged and rolled into rods with a diameter of approximately 10 mm. A microstructural characterization was performed with light microscopy (LM) and scanning electron microscopy (SEM) equipped with energy-dispersive spectrometry (EDS). Martensitic microstructure was observed in the rods after the deformation. The fractographic analysis of the samples after the tensile testing revealed some areas with intergranular fracture. However, the greater part of the fracture surface indicated the pattern of transgranular brittle fracture. The results of the tensile tests showed the tensile strength of 401.39 MPa and elongation of 1.64 %. The hardness of the CuAlNiMn alloy is 290.7 HV0.5.V prispevku so predstavljeni rezultati analize mikrostrukture pred prelomom in po njem skupaj z mehanskimi lastnostmi in trdoto zlitine CuAlNiMn z oblikovnim spominom. Taljenje zlitine je bilo izvedeno v vakuumski pe~i v za{~itni atmosferi argona. Zlitina je bila ulita v ingot mase 15 kg. Po litju je bila zlitina kovana in zvaljana na premer pribli`no 10 mm. Karakterizacija mikrostrukture je bila izvedena s svetlobno mikroskopijo (SM) in vrsti~no elektronsko mikroskopijo (SEM), opremljeno z energijskim disperzijskim spektrometrom (EDS). Analizirana je bila martenzitna mikrostruktura zlitine CuAlNiMn pred izvedenim nateznim preizkusom. Izvedena sta bila natezni preizkus in meritve trdot. Fraktografska analiza je pokazala ve~ podro~ij z interkristalnim in pogosto transkristalnim krhkim prelomom. Rezultati nateznega preizkusa so pokazali, da je natezna trdnost 401,39 MPa in raztezek 1,64 %. Trdota zlitine CuAlNiMn je 290,7 HV0,5

Influence of tool wear on the mechanism of chips segmentation and tool vibration

The tool wear has a significant impact on the cutting process and therefore tool wear monitoring is especially important for building intelligent machine tools which are capable of assessing their own states and reacting to important changes. This approach is based on the assumption that there exists a relationship between the spectrum of high-frequency vibrations measured on tool holder, in immediate vicinity of the cutting zone, and the tool wear degree. The wear causes changes in tool tip geometry, which has significant influence on the process of chip forming. At the same time, the erratic nature of chip forming process excites the cutting zone, generating a very broad spectrum of vibrations. Due to high input energy, these vibrations are very intensive, and spread through the entire machining system. In the paper, experimental results are shown which pertain to the relationship between the power spectral density (PSD) within 5 kHz to 50 kHz interval.Web of Science20111210

The mechanics behind formation of secondary ledeburite during tool steel welding

U ovom prilogu opisujemo mikrostrukturne promjene u zoni toplinskog utjecaja (ZTU) alatnog čelika W. Nr. 1.2379, koji je bio navaren ili zavaren po postupku zavarivanja pod praškom s različitim parametrima zavarivanja. Mikrostrukturu zavara i navara analizirali smo pomoću optičnog i rasterskog (skenirajućeg) elektronskog mikroskopa. U analiziranju smo posebno utvrđivali mikrostrukturne promjene u okolini primarnih kromovih karbida u zoni toplinskog utjecaja i njihov utjecaj na kristalizaciju zavara. Utvrdili smo, da su temperature u zoni toplinskog utjecaja za vrijeme zavarivanja dovoljno visoke da se za vrijeme rastapanja primarnih karbida u okolnoj matici do te mjere povećala koncentracija karbidotvornih elemenata i ugljika, da je nastala tz. talina eutektičnog sastava, koja se stvrdnula u sekundarni eutektik (ledeburit), na kojemu se na granici ZTU/zavara pokrene kristalizacija zavara.In this article we describe the microstructural changes in the heat affected zone (HAZ) of the tool steel W. Nr. 1.2379, which was surfaced or welded by the submerged arc welding technique (SAW) with different welding parameters. Microstructure of the welds and of the surfacing welds was analysed by optical and scanning electron microscope. In this research, we particularly studied microstructural changes in the area of primary chromium carbides in the HAZ and their effect on the weld crystalization. We came to the conclusion that the temperature in the HAZ is high enough during the welding process that it caused primary carbides to dissolve and concentration of the carbide-forming elements and carbon increased in the surrounding austenite matrix area to eutectic composition, which remelts and solidified as secondary eutectic (ledeburite)

Effect of heat treatment on Charpy impact energy of microalloyed steel

U ovom radu istraživan je niskolegirani visokočvrsti čelik Niomol 490K koji sadrži dodatak niobija (0,032 mas. %). Toplinska obrada čelika se sastojala od austenitizacije pri 1250 °C u trajanju od 5 minuta i ponovnog zagrijavanja do 750 °C u trajanju od 5 s. Nakon austenitizacije provedeno je kaljenje u vodi kod temperature od 70 °C i u olovnoj kupci do temperature od 400 °C uz naknadno hlađenje na zraku. Utvrđeno je da je Charpy energija udara viša, a prijelazna temperatura niža za transformaciju austenita u bainit nego kod transformacije austenita u martenzitnu mikrostrukturu. Eksperimentalni rezultati su pokazali različite prijelomne površine. Pokazano je da je preferentno cijepajući prijelom zapažen po ravnini rešetke (001), neovisno o mikrostrukturi.In this paper the microalloyed high-strength low-alloy (HSLA) Niomol 490K steel with an addition of niobium (0,032 wt. %) has been investigated. Heat treatment of the steel consisted of austenitisation at 1250 °C for 3 minutes and reheating up to 750 °C for 5 s. After austenitization the quenching was carried out in water with temperature of 70 °C and then in a lead bath up to 400 °C following cooling on the air. It was found that Charpy impact energy is higher and the transition temperature is lower for the transformation of austenite to bainite than to martensite microstructure. Experimental results showed that different fracture surface was observed. It is shown that preferred cleavage fracture occurred in the lattice plane (001) independently of steel microstructure

The Influence of Heat Treatment on Microstructure and Phase Transformation Temperatures of Cu-Al-Ni Shape Memory Alloy

This paper presents the results of thermal and microstructural analysis of Cu-Al-Ni shape memory alloy before and after heat treatment. After casting, a bar of Cu-12.8 Al-4.1 Ni (wt.%) alloy, obtained by the vertical continuous casting technique, was subjected to a certain heat treatment procedure. Solution annealing was performed at 850 °C for 60 min, followed by water quenching. Tempering was then performed at four different temperatures (150 °C, 200 °C, 250 °C and 300 °C). The microstructural results were obtained by optical and scanning electron microscopy. Thermodynamic calculation of ternary Cu-Al-Ni system under equilibrium was performed using Thermo-Calc 5 software. Phase transformation temperatures were determined by differential scanning calorimetry (DSC). The DSC results show the highest values of transformation temperatures in as-cast state. After solution annealing and tempering, the transformation temperatures show lower values with exceptional stability of Ms temperature (martensite start temperature)