Materials Science Forum Vol. 1016

Abstract: Multilayered laminate structures obtained by coating of ultrafine-grained metallic materials with bioactive and multifunctional composite coatings are considered for biomedical applications. Laser-assisted densification of multiple materials using laser cladding and selective laser melting is an alternative route to reduce the risk of early implant failure allowing for faster and cheaper fabrication. To understand the cooperative relationships between different factors that cam influence the manufacture of such bioactive laminates reflecting in their bioactivity and mechanical properties, the multi scale numerical modelling is applied. This work presents resent advances on development of integrated numerical models including generation, melting and solidification of the powder bed, considering surface flow, wettability, surface tension and other physical phenomena, specific mechanical and thermo-mechanical aspects and microstructure evolution.

Abstract: The methanation reaction has recently received considerable attention as a perspective CO₂ utilization technology leading to the formation of renewable natural gas methane. This reaction is favorable at low temperature, but it is hindered of slow kinetic rates, whereas below a temperature of 270°C, the CO₂ conversion is practically 0, and at higher temperatures, 350-400°C, the co-existence of secondary reactions favors the formation of CO. This is the reason why new catalysts and process conditions are continuously being investigated to maximize the methane selectivity, preferably at low reaction temperatures and at atmospheric pressure. Thus, this work is focused on the use of a heterogeneous catalyst Ni/ Zirconia supports modified by rare earth metals such as Lanthanum, tungsten and Yttrium combined to a Dielectric Barrier Discharge plasma. Three catalysts were prepared by a conventional wet impregnation method, using 15 wt% of Ni loading over zirconia supports modified with different promoters. To better define the physical, textural and chemical properties, the catalysts were characterized by the means of BET, XRD, H₂-TPR, CO₂-TPD. The influence of basicity, Ni crystallite size and the Ni-support interaction on the catalytic activity was clearly evidenced.

Abstract: One of the possibilities of the dieless drawing is the production of ultrafine wire. In this case, it is possible for additionally stretch the wire, obtained in the conventional way. This may allow to obtain a wire of smaller diameter than the industry produces. However, the significant problem is the increase of the strain induced roughness of wire during dieless drawing. This problem has become important due to the fact that the resulting roughness can be comparable to the diameter of the wire and significantly reduce the workability. Thus, the solution of these problems requires plastometric studies of the material, physical and numerical modeling for prediction the roughness of the wire under conditions of dieless drawing. The experimental study shown, that the surface roughness of the copper wire after dieless drawing increases significantly at a deformation temperature above 300°C. The total roughness is associated both with the formation of oxides and the strain induced roughness.

Abstract: In this paper,effect of the different forging processes on the microstructure and mechanical properties of the flat flat billets of TA15 titanium alloy was investigated.The flat billiets of 80 mm×150 mm×L sizes of TA15 titanium alloy are produced by four different forging processes.Then the different microstrure and properties of the flat billiets were obtained by heat treatment of 800 °C~850 °C×1 h~4h.The results show that, adopting the first forging temperature at T1 °C、slow cooling and the second forging temperature at T2°C 、quick cooling, the primary αphases content is just 10%, and there are lots of thin aciculate phases on the base. This microstructure has both high strength at room temperature and high temperature, while the properties between the cross and lengthwise directions are just the same. So the hot processing of the first forging temperature at T1 °C、slow cooling and the second forging temperature at T2°C 、quick cooling is choosed as the ideal processing for production of aircraft frame parts.

Abstract: In general, a flux is used to braze a copper alloy. In many cases, when the molten brazing filler metal spreads in the set joint gap, vaporised flux and its residue are produced, and defects (mainly voids) are formed. Voids, which are formed on the brazed layer, cause deterioration in the strength and other properties. However, with conventional evaluation methods (e.g. ultrasonic or X-ray radiography tests), the behaviour of the molten brazing filler metal during the brazing process cannot be visually observed from the outside of the joint. Therefore, the void formation process cannot be clarified. To improve the quality of the brazed layer, it is necessary to elucidate the mechanism of void formation. The purpose of this study is to observe the behaviour of the molten brazing filler metal by performing an X-ray radiography test at the same time as brazing and to study how to reduce voids. In this study, a brass specimen was brazed with a Cu–P-based brazing filler metal. The specimen was brazed by heating in an electric furnace, and the specimen was irradiated with X-rays. The state where the molten brazing filler metal spread into the gap was photographed as the transmission image. Thereafter, the behaviour of the molten brazing filler metal was analysed.

Abstract: In this study, tensile tests of rolled Mg-Li alloy and Mg-Ce alloy sheets were carried out at room temperature to investigate effects of alloying additions on the relationship between mechanical properties and activities of slip systems in magnesium polycrystals. In Mg-Li alloy, ductility increased while strength decreased by lithium addition. Frequency of non-basal slips increased with increasing lithium content. In Mg-Ce alloy, strength and ductility were similar pure magnesium, and non-basal slips were hardly activated. Since critical resolved shear stress of non-basal slips were decreased by lithium addition, ductility of magnesium was increased while its strength was decreased.

Abstract: This paper is aimed to study the creep behavior of two-phase ultrafine-grained VT8M-1 (Ti-5.7Al-3.8Mo-1.2Zr-1.3Sn) titanium alloy obtained by rotary swaging (RS). It is shown that the 100-hour creep strength of the ultra-fine grained (UFG) VT8M-1 alloy retains high values at temperatures up to 400 °C. An increase in the testing temperature to >450 °C leads to a decrease in the creep rupture strength. The relationship between the microstructure and creep resistance of UFG alloy is discussed.

Abstract: An Al-3Mg (wt. %) alloy was studied after equal channel angular pressing and subsequent cold rolling. The mechanical behavior of the alloy in the temperature range from 223 K to 373 K (from –50°C to 125°C) at strain rates 2.1×10^–1 – 5.2×10^–5 s^–1 was investigated. The analysis of stress-strain curves was performed to determine the conditions of manifestation of the Portevin – Le Chatelier (PLC) effect in investigated alloy. The deformation curve at a temperature of 298 K (25°C) and a strain rate of 1×10^–3 s^–1 is characterized by instability of plastic flow in contrast to the deformation curves obtained under other studied strain rate/temperature conditions. Stress oscillations at the necking stage were observed at high temperatures (>323 K (50°C)) and lower strain rates (1×10^–4 s^–1 and 5.2×10^–5 s^–1) forming the left border of the PLC effect domain. In general, deformation curves are characterized by the absence of stress serrations during the uniform elongation.

Abstract: The natural oxide layer, which spontaneously covers the surface of aluminium and its alloys, is well adherent to the surface, but it does not show adequate corrosion resistance in many conditions. In order to improve the material anti-corrosion performance, it is necessary to replace the surface oxide by a conversion coating. The first step to do this is to prepare the alloy surface to subsequent treatments. The pre-treatment implemented before the conversion treatment is one of the main factors responsible for the performance of the conversion coating. It involves several phases, including desmutting that encompasses the part immersion in an acid or alkaline solution bath. In this work, the influence of hydrofluoric acid concentration in the desmutting bath's formulation was investigated. The samples surface were characterised by using electrochemical techniques, glow discharge optical emission spectrometry and contact angle measurements.

Abstract: In this work, the low-density Al_0.5NbTa_0.8Ti_1.5V_0.2Zr RHEA prepared by mechanical alloying combined with hot uniaxial pressing (HP) was investigated. The alloy was subjected to different sintering temperatures on the range of 1200°C up to 1600°C, while keeping a constant densification time. The influence of the increase in sintering temperatures on the RHEAs’ microstructural features, composition and basic mechanical properties was explored by means of x-ray diffraction and scanning electron microscopy, hardness testing, density determination by Archimedes’ principle and elastic moduli by ultrasonic measurements. Full density samples were obtained in all sintering temperatures. The alloy has shown a homogeneous distribution of phases, with presence of dispersed oxides inside the matrix, leading to a fine grain size distribution due to grain-boundary pinning effect, even during the exposure of the alloy at the highest sintering temperature. The mentioned effect is responsible for the achievement of high-hardness in all conditions. Powder metallurgy has been shown to be an advantageous technique for production of alloys for high-temperature applications, as the inherent in-situ formed oxides homogeneously distributed within the matrix may be beneficially used as reinforcement, thus potentially enhancing the mechanical properties of the parts.