Papers by Author: Gabriela Popescu

Abstract: During the last years, mechanical alloying technique for high entropy alloys (HEAs) has been more often approached due to the good homogenous chemical distribution and near net shape technology provided by the respectively process. A new composite material having the matrix as HEA reinforced with graphite particles was designed. The graphite particles addition in the high entropy matrix (AlCrFeNiMn) improves the particles weldability during mechanical alloying and assures a good creep behavior for the final product. The aim of this paper is to investigate the pressure influence on the microhardness as dependence of sintering parameters which can be reflected also on the microstructure. The high entropy composite was completely alloyed after 40 hours of milling. The obtained composite was pressed using different pressures values in order to investigate the pressure influence on the microhardness and microstructure. The samples were investigated using optical microscopy, scanning electron microscopy, X-rays diffraction and microhardness tests. The microhardness values for all the samples were between 300 – 700 HV.

Abstract: Mechanical alloying (MA) is a high-energy ball milling process results in the obtaining of simple and stable microstructures having increased homogeneity compared to other non-equilibrium synthesis methods. The aim of this paper was to develop a high entropy alloy with an improved hardness value suitable for coating turbine blades working in geothermal steam. CoCrFeNiMo high entropy alloy was processed in solid state, using mechanical alloying. After 40h milling time in a planetary ball mill the alloyed sample was consolidated using spark plasma sintering process. The samples obtained were investigated with the aid of optical and electron microscope, X ray diffraction and the hardness value was determined. The results obtained revealed that the powder was completely alloyed after 40 hour milling and the mixture between BCC and FCC phases resulted in 34% improved hardness value in comparison with a stainless steel usually used for turbine blades working in geothermal environment.

Abstract: High Entropy Alloys (HEAs) represent a new concept of metallic materials, that contain 5 or more elements, in proportions from 5 at.% to 35 at.%, and form simple solid solutions (BCC and/or FCC) instead of complicated intermetallic phases. The high degree of randomness atomic HEA, gives them excellent properties: electrical, mechanical, electrochemical, ductility, anti-corrosion properties, stable structure etc, with applications in peak thus representing a growing research. These specific features provides HEA with excellent hardness, strength and wear strength, malleability, oxidation and corrosion resistance, with potential applications in diverse industrial areas [1÷4]. Considering these properties we decide to improve biomedical alloys with this new class of HEAs.

Abstract: Traditional alloys is based on a single element called matrix and to improve some mechanical properties (strength, ductility, strength) are added and other metallic elements in the system. High entropy alloys have become a field of increasingly explored in the world of materials. Excellent mechanical properties obtained of the high entropy alloys recommend them to be from year to year as investigated. In the last decade more than 500 high entropy alloys journal and conference papers have been published [1]. High entropy alloys are alloys who have in their composition 5 to 13 metal elements and the concentration of each component is between 5% and 35%. These elements in the composition of high entropy alloys are divided into elements of minority and majority elements. They are called minority elements because their molar fraction is less than 5%. High entropy alloys have mixing entropy higher than traditional alloys, ΔScons≥1.61R (R = 8.314 J / (mol • K)) [1]. High entropy alloy have been obtained in the laboratory of Science and Materials Engineering faculty from Iasi using a medium frequency induction furnace with 8000 Hz. Because they have excellent mechanical properties high entropy alloys can be used in various fields with high wear and corrosion degree or electronic, magnetic applications [1]. In this work we selected pure metallic elements like: Fe, Ni, Cr, Mn and Al. The quantity of alloy developed varied between 0.5 and 1.5 kg. Metal load necessary for the preparation of metal alloys were formed technical grade, industrial accessible prices and satisfying. Friction and wear rezistance were studies by using a reciprocating sliding test machine , in a pin on disk configuration, using aluminum as counter face.In this paper it investigated the wear resistance of high entropy alloys obtained, microstructure and their mechanical properties.

Abstract: A few numbers of countries in the world are involved in geothermal exploration and geothermal development projects. In order to provide a stable power supply without increasing carbon dioxide on global environment problem, a basic condition need to be fulfilled. This condition is high reliability and high maintainability for the geothermal energetic pumps and turbines. Effective efficiency improvement and geothermal turbines system upgrading are very important and also fundamental economical factor. This means that CO₂ emissions into the atmosphere are minimal and a higher reliance on geothermal power generation would work on preventing global warming.The solution to climate changes threat is based now, mainly, on renewable and ecological sources of energy. Geothermal energy has the potential to play a significant role in moving the Europe and other regions of the world toward a cleaner and more sustainable energy system. In order to increase the reliability of geothermal steam turbines, assessing the materials life under geothermal environment condition will be an important step. The corrosion process in the geothermal turbine and pumps depends on temperature, pressure, chemistry, mechanical and vaporous carryover of impurities and water treatment (distribution between the vapors, the surface film and rotor blades material, heat transfer properties etc).The aim of this paper is to present a new coating method for geothermal turbines and pumps components using multi composite technology in order to obtain a protective layer to reduce corrosion damages. The results were very promising and the technique used, plasma jet spraying is a very good method to be used on the geothermal turbines and pumps components.

Abstract: This paper aims to present a way of obtaining aluminum based composite material obtained by powder metallurgy technique. We start mixing and milling the powders, compressing and sintering the samples. We established a sintering cycle for this type of composite material and studied obtained samples by SEM microscopy. The reaction between the matrix and the precursor of the reinforcing materials was studied with the aid of HSC 4.0 program, available at Engineering and Management of Metallic Materials department.

Abstract: The paper’s aim is to present the processing of a new master alloy similar to STROBLOY. This alloy represents a combination of two master alloys, already known in aluminum industry (AlTiB and AlSr). The benefits of this new alloy are the cut of Ti, B and Sr consumption, as well as a grain refining/modification ecological technology for Al-Si and Al-Mg-Si alloys. So, this alloy was obtained from binary AlB8, AlSr10 and AlTi10 master alloys melted in an electric resistance furnace and argon atmosphere. Samples were cast in an iron mould. As STROBLOY, this new quaternary alloy contains fast dissolving SrAl4 particles important in modification stage, and nucleating particles such as TiB2 and (Al, Ti)B2 essential for grain refining of aluminum alloys.

Abstract: Thermal spraying technologies are an effective way to ensure surface protection against destructive effects of wear, corrosion and oxidizing phenomena which can be applied in majority industrial sectors for improving properties of new parts or for reconditioning worn out parts technology. The powders used in this study are mainly used in the repair of cast iron parts (e.g. engine blocks) and in build - up of press tools for car bodies, threads, turbine-guided vanes, rotating valves and shafts in eccentric presses. Three types of tubular composites based on WC powders were studied which demonstrated that this technology improved their properties.

Abstract: The aim of the paper is to present the influence of a new multifunctional material, a master alloy named Al-Sr-Ti-B, in aluminum foundry alloys. The Al-Sr-Ti-B master alloy represents a new combination of two master alloys, already known in aluminum industry, AlTiB and AlSr, used in treatment of aluminum alloys for grain refining and modification. As Strobloy, our master alloy contain fast dissolving SrAl4 particles and also nucleating particles as TiB2 and (Al,Ti)B2 which are important first in modification and second in grain refining of aluminum alloys. The paper presents optic and electron microscopy studies realized on AlSi7Mg alloy treated with this new multifunctional material