RADA NOVAKOVIC - Academia.edu (original) (raw)

Papers by RADA NOVAKOVIC

Surface tension and viscosity of complex Ti-based industrial alloys are important for simulation ... more Surface tension and viscosity of complex Ti-based industrial alloys are important for simulation of liquid assisted industrial processes such as casting, joining, crystal growth and infiltration. Modelling of the interface and mass transport during liquid-solid phase transition requires reliable surface tension and viscosity data. Therefore, to obtain accurate predictions of microstructural evolution during solidification related processes, only reliable input data are necessary. In the case of liquid Ti-Al alloys, the experimental difficulties related to high temperature measurements and reactivity of these alloys with supporting materials or containers as well as inevitable presence of oxygen may lead to data gaps including a complete lack of property data. An alternative for container-based methods are containerless processing techniques that offer a significant accuracy improvement and / or make possible to measure temperature and composition dependent thermophysical properties ...

Metallurgical and Materials Transactions A, 2019

The contact heating sessile drop and capillary purification methods were applied for a fundamenta... more The contact heating sessile drop and capillary purification methods were applied for a fundamental study concerning the wettability and reactivity of liquid Si-16.2at%Ti (eutectic composition) in contact with Glassy Carbon and SiC at T = 1450°C under an Ar atmosphere. Different spreading stages with different slopes, depending on the starting conditions of the materials used, where observed. On the contrary, the final contact angle value seemed not affected and the equilibrium contact values of θ ≈ 44°± 2 and θ ≈ 42°± 2 where displayed on Glassy Carbon and SiC, respectively. The solidified Si-Ti eutectics/GC and Si-Ti eutectics/SiC samples were examined both at the top of the drop and at the cross-section by SEM/EDS. The presence of a SiC layer as unique reaction product at the Si-Ti eutectics/GC interface, confirmed that wettability is mainly driven reactivity. Contrarily, as non-reactive system, at the Si-Ti eutectics/SiC interface a weak dissolution of SiC substrate was detected.

Metallurgy in Space, 2022

Journal of the European Ceramic Society, 2015

Among manufacture techniques for SiC ceramics, reactive infiltration has proven to be the most ef... more Among manufacture techniques for SiC ceramics, reactive infiltration has proven to be the most efficient. However, materials manufactured with this technique present minimum 10% of unreacted silicon. Si hinders the performance of the material. The replacement of Si with a Me x Si y can improve the performance of the composite. Since Me-Si alloys wet carbon materials, they are suitable candidates to be used as infiltrants in SiC/Me x Si y composite production via reactive infiltration. To optimize the process, it is necessary to identify the mechanisms involved in infiltration and the parameters governing them. A basic wetting study can provide a clear understanding of the operative mechanisms and allow obtaining accurate parameters. In this work, the wetting behavior of silicon-cobalt alloys on carbon materials was studied using the sessile drop technique. The influence of temperature, composition and carbon crystallinity was evaluated, observing a strong dependence of process kinetics and reactivity with temperature and carbon crystallinity.

Microgravity Science and Technology, 2020

The present study is an overview of the surface properties of liquid Al-Ni alloys, which are of g... more The present study is an overview of the surface properties of liquid Al-Ni alloys, which are of great importance for the design and development of new Al-Ni and Ni-based industrial alloys, widely used as functional and structural materials. The solidification and thus, the microstructural evolution are directly dependent on the interface/surface properties of metallic melts. Therefore, numerical simulation of microstructure evolution requires reliable property data as input to such models. Taking into account the experimental difficulties related to a high reactivity of liquid Al-Ni alloys and the effects of impurities on their surface properties, the surface tension over the whole concentration range has been determined in the frameworks of three international research projects. Namely, the surface tension measurements have been carried out by both traditional container-based and as an alternative, containerless methods within the ESA-MAP ThermoProp and ESA-MAP Thermolab Projects and also under the EU FP6- IMPRESS Project. The obtained datasets were analysed and subsequently compared with the model predicted values as well as with the literature data. A strong exothermic mixing characterises the Al-Ni system and the presence of a few intermetallic compounds in the solid state leads to the formation of short range ordered elements or complexes in the liquid phase, at least near the melting temperature, which significantly affects the surface properties of alloy melts. Aiming to estimate the effects of short range ordering on these properties, the Compound Formation Model (CFM) and the Quasi Chemical Approximation (QCA) for regular solution were applied.

Journal of Alloys and Compounds, 2015

Lead-free solders based on Bi-Sn bimetallic nanoclusters with eutectic composition (Bi 43 Sn 57) ... more Lead-free solders based on Bi-Sn bimetallic nanoclusters with eutectic composition (Bi 43 Sn 57) were synthesized at low temperature by simultaneous reduction reaction from aqueous solution containing bismuth and tin chlorides, using potassium borohydride as a reducing agent. By the same processing route, pure bismuth and tin nanoparticles have also been prepared. Microstructure, morphology and composition of the samples were characterized by X-ray powder diffraction (XRD), transmission (TEM) and scanning electron microscopy (SEM). TEM images of Bi-Sn nanoparticles show average size ranging from 30 to 100 nm. Thermal behaviour of Bi-Sn nanopowders was studied by DSC (differential scanning calorimetry) and a melting temperature (135°C) lower than that of the corresponding microcrystalline sample (139°C) was observed. SEM micrographs of the thermally treated sample up to 400°C show fine spherical grains in the micrometer range with finer powder particles on the surface. XRD powder diffraction analysis indicates the formation of bismuth and tin nanophases with an average particle size of 85 and 126 nm, respectively. The oxidation behaviour of the samples was also investigated. The results obtained have been analyzed in view of theoretical models describing the melting temperature depression of nanoparticles.

High Temperatures-High Pressures, 2021

Conventional high-chromium ferritic / martensitic steels, such as T91 and AISI 316 L as well as t... more Conventional high-chromium ferritic / martensitic steels, such as T91 and AISI 316 L as well as the new oxide dispersed strengthened (ODS) steels, i.e. Fe-Cr (ferritic or martensitic) with a fine Y2O3 dispersion, are promising candidates as structural materials for applications in aggressive environments. In order to improve the efficiency and lifetime of structural materials for liquid metal fast reactor (LMFR), where liquid lead (Pb) is used as coolant, the investigations are currently focused on the interfacial phenomena occurring at the liquid Pb/steel interfaces. The better understanding of the interfacial phenomena is the key issue for the selection of structural materials meeting the requirements of the demanding application. In the present work, a fundamental study on the wetting characteristics and the interfacial phenomena of liquid Pb in contact with T91, AISI 316L, ODS- 12Cr and ODS-14Cr steels over the temperature range 550�820�C and under different oxygen containing at...

Viscosity of liquid Ag-Cu alloys is determined over the entire compositional range as function of... more Viscosity of liquid Ag-Cu alloys is determined over the entire compositional range as function of temperature and composition by means of the oscillating cup technique. It is found that the viscosities of the pure elements can be related to their self-diffusion coeffcients via the StokesEinstein relation. The isothermal viscosities of the alloys investigated agree only reasonably with values predicted by models which are based mainly on thermodynamic principles. In particular, the viscosity isotherm exhibits a concave shape in the Cu-rich concentration range and a convex one in the Ag-rich range. Using a model based on the Stokes-Einstein relation, it can be shown that the behaviour mentioned above corresponds to a transition between two different mechanisms, i.e. a dynamic and a kinetic one. It is evident from the present study, in contrast to what is commonly practised, that the cross correlation term in the Darken equation has to be taken into account.

Among the thermophysical properties, the surface / interfacial tension, viscosity and density / m... more Among the thermophysical properties, the surface / interfacial tension, viscosity and density / molar volume of liquid alloys are the key properties for the modelling of microstructural evolution during solidification. Therefore, only reliable input data can yield accurate predictions preventing the error propagation in numerical simulations of solidification related processes. Due to experimental difficulties related to reactivity of metallic melts at high temperatures, the measured data are often unreliable or even lacking. The application of containerless processing techniques either leads to a significant improvement of the accuracy or makes the measurement possible at all. On the other side, accurate model predicted property values could be used to compensate the missing data; otherwise, the experimental data are useful for the validation of theoretical models. The choice of models is particularly important for the surface, transport and structural properties of liquid alloys r...

Materials and Corrosion

The corrosion behaviour of oxide dispersion strengthened (ODS) steels with different chromium con... more The corrosion behaviour of oxide dispersion strengthened (ODS) steels with different chromium contents (9, 12 and 14 wt% Cr) is investigated in stagnant liquid lead at a temperature of 973 K for 2000 h. Under an oxygen concentration close to the passivation domain (i.e. Co ∼10−5–10−4 wt% O), neither an internal oxidation zone in the steel matrix, nor dissolution attack caused by liquid lead or its penetration is observed. Indeed, thin layers of chromium rich oxides with different compositions are detected.

Objective: Due to the allergic nature of the pollen of Cryptomeria japonica, the most important J... more Objective: Due to the allergic nature of the pollen of Cryptomeria japonica, the most important Japanese forestry conifer, a pollen-free cultivar is preferred. Mutant trees detected in nature have been used to produce a pollen-free cultivar. In order to reduce the time and cost needed for production and breeding, we aimed to develop simple diagnostic molecular markers for mutant alleles of the causative gene MALE STERILITY 1 (MS1) in C. japonica to rapidly identify pollen-free mutants. Results: We developed PCR and LAMP markers to detect mutant alleles and to present experimental options depending on available laboratory equipment. LAMP markers were developed for field stations, where PCR machines are unavailable. The LAMP method only needs heat-blocks or a water bath to perform the isothermal amplification and assay results can be read by the naked eye. Because the causative mutations were deletions, we developed two kinds of PCR markers, amplified length polymorphism (ALP) and allele specific PCR (ASP) markers. These assays can be visualized using capillary or agarose gel electrophoresis.

Journal of Phase Equilibria and Diffusion, 2021

A new experimental investigation of the Sb-Sn phase diagram has been performed. Special attention... more A new experimental investigation of the Sb-Sn phase diagram has been performed. Special attention was focused on the phase equilibria within SbSn-Sb 3 Sn 4 composition range. Phase equilibria have been investigated by DSC measurements combined with a thorough optical and electronic microscopy examination with a special attention to the SbSn-Sb 3 Sn 4 phase transformation. Both DSC and metallographic results have been critically evaluated and debated. The presence of two peritectic reactions L ? Sb 3 Sn 4 (bSn) at 243 ± 1°C and L ? (Sb) (SbSn) at 420 ± 1°C have been confirmed. The invariant reaction at 321 ± 1°C has been observed and an additional small effect at about 330-340°C was systematically recorded. These results have been extensively discussed.

Journal of Molecular Liquids, 2020

Abstract The surface tension of liquid Ag Cu eutectic alloy has been measured in a wide range of ... more Abstract The surface tension of liquid Ag Cu eutectic alloy has been measured in a wide range of oxygen partial pressures using two different methods: the constrained drop method and the electromagnetic levitation method. In order to investigate the influence of oxygen potential on the surface tension of the Ag 40Cu (at.%) eutectic alloy, the oxygen partial pressure was varied between 8.62 × 10−15 and 2.42 Pa. At low oxygen partial pressure, the surface tension of the Ag Cu eutectic alloy measured in the temperature range of 1223–1476 K, shows temperature dependence in a good agreement with previously reported results. At higher oxygen partial pressures, the decrease in surface tension is more pronounced and the oxygen adsorption on the surface accordingly increases. Moreover, at high oxygen partial pressure, the oxygen adsorption on the Ag Cu eutectic alloy was found to be far greater than the expected saturation limit taking into account the theoretically calculated surface concentration of Cu in the Ag Cu eutectic alloy and the surface excess of oxygen on the molten Cu at saturation. This phenomenon can be explained with a model where the surface of the Ag Cu eutectic alloy undergoes Cu-substitution at the Ag sites during the oxygen adsorption process.

Advanced Engineering Materials, 2020

Titanium alloys generally exhibit high strength and toughness within a large composition and temp... more Titanium alloys generally exhibit high strength and toughness within a large composition and temperature range. Combined with their high corrosion resistance and low density, titanium alloys are important for many applications, ranging from lightweight constructions over aerospace applications to biomedical implants. [1-5] One of the most commonly used titanium alloys is the alpha-beta titanium alloy Ti64 (Ti-6Al-4V, in wt% or Ti-10.2Al-3.6V, in at%). Developed in 1954, it is an established alloy and a prototype for a large group of newer alpha-beta Ti-Al alloys with good strength to weight ratio. [2,5,6] Different fabrication routes to produce parts and components from Ti64 exist (casting, additive manufacturing, metal injection molding, etc.), mainly including the presence of the liquid phase. [6] Ti64 is also very commonly used in 3D printing and additive manufacturing technologies that continue to evolve rapidly. The final microstructure, defect density, residual stresses, surface roughness, porosity, and consequently the mechanical properties of Ti64 parts are depending on the used process conditions. [7-12] Manufacturing processes can be optimized by simulations of heat and mass flow during the manufacturing process. [13-17] Also, the microstructure evolution, including porosity and other defects, can be predicted by numerical methods. [18] These simulations need precise thermophysical property data of the simulated alloy as input parameters. [5,13-18] Although Ti64 is a widely applied alloy, not too much literature data are available for its thermophysical properties in the liquid phase. Also, there are some disagreements in the published literature concerning the thermophysical property data, such as thermal conductivity or specific heat capacity. One reason is that the measurement of thermophysical properties of liquid Ti alloys using classical thermoanalytical equipment (containerbased methods) is challenging or even impossible. The inevitable contact of the material with the container walls leads to contaminations and parasitic container-wall reactions that will prevent precise measurements of thermophysical properties. [19] We present measurements of Ti64 using the electromagnetic levitator (ISS-EML) inside the European science module "Columbus" on board the International Space Station (ISS). This containerless method is the methodology of choice for precise measurement of thermophysical properties of hightemperature reactive liquid metallic melts. The microgravity environment is used to diminish the strong forces necessary

Journal of Materials Science: Materials in Electronics, 2020

The new Bi–Sn surface tension experimental data have been obtained by the pinned drop method for ... more The new Bi–Sn surface tension experimental data have been obtained by the pinned drop method for temperatures ranging from 623 to 1123 K and were subsequently compared with the model predicted values as well as with the literature data. Taking into account endothermic mixing in liquid Bi–Sn alloys, the surface properties (surface tension and surface segregation) were described by the Quasi-Chemical Approximation (QCA) for regular solution and the Self Association Model (SAM) aiming to estimate the effects of demixing on these properties. In addition, the transport and structural properties of Bi–Sn melts were analysed by theoretical models developed in the framework of statistical mechanics in conjunction with the Quasi-Lattice Theory (QLT) and Faber-Ziman Theory. The abovementioned properties are involved in the development of complex solder materials based on the Bi–Sn system.

Thermochimica Acta, 2019

Abstract The aim of this study is to correlate the thermodynamics of liquid Ag-Ge alloys with the... more Abstract The aim of this study is to correlate the thermodynamics of liquid Ag-Ge alloys with their thermophysical properties such as the surface tension, viscosity, electrical resistivity and microscopic functions. For this purpose the Quasi-Chemical Approximation (QCA) for the regular solution and Faber-Ziman theory have been applied to describe the mixing behaviour of Ag-Ge melts in terms of the energetics and structure. Concerning the Ag-Ge system, the most important is Ag-24.5Ge (in at %) eutectic alloy, widely used as a brazing filler material. The melting temperature and the heat of melting of Ag-Ge eutectic alloy were measured by Differential Scanning Calorimetry (DSC). From a technological point of view, particular attention should be paid to the surface tension, a key property of the joining processes. Accordingly, the QCA and Butler’s model calculations have been done and subsequently, the model predicted values were compared to available literature data.

Philosophical Magazine, 2018

The thermodynamic model in conjunction with Butler equation and the geometric models were used fo... more The thermodynamic model in conjunction with Butler equation and the geometric models were used for the surface tension calculation of Cd-Sn-Zn liquid alloys. Good agreement was found between the experimental data for limiting binaries and model calculations performed with Butler model. In the case of ternary alloys, the surface tension variation with Cd content is better reproduced in the case of alloys lying on vertical sections defined by high Sn to Zn molar fraction ratio. The calculated surface tension is in relatively good agreement with the available experimental data. In addition, the surface segregation of liquid ternary Cd-Sn-Zn and constituent binaries has also been calculated.