Materials Science Forum Vol. 879

Abstract: Aluminum and titanium alloys are among the most important and the most frequently used construction materials due to their physical and mechanical properties. Especially in the automotive and aerospace industry these materials allow to reduce the weight of structure which leads to reducing fuel consumption and environment pollution. These materials are often used together which leads to problems with junction between these materials. In addition to the mechanical joints, there is an effort to produce quality welded joints. Series of works focused to welding of Al/Ti joints by conventional and nonconventional welding methods were published [1, 4, 5, 6, 7]. By reduction of dimensions of molten material is possible to reduce the amount of emerging intermetallic phases and welding defects. Electron beam welding appears as suitable method for welding Al/Ti joints because it allows production of very narrow welds. The benefit is also necessity to perform electron beam welding in vacuum which is required for decrease energy losses of incident beam and simultaneously prevents reaction of molten metal with ambient atmosphere. This paper is focused to determine of appropriate parameters for electron beam welding of heterogeneous welds of titanium alloy Ti-6Al-4V and aluminum alloy 6061. Metallographic evaluation, analysis of chemical and phase composition were performed on the test welds for purpose to describing present phases. On the selected welds was evaluated the influence of intermetallic phases on the mechanical properties. The obtained results will be used for further experiments focused to optimize the process of electron beam welding of Al/Ti alloys.

Abstract: Thermal barrier coatings (TBCs) on turbine blades in gas turbine engines are in some cases damaged at high temperatures exceeding 1200°C by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals. In this study, material interaction between molten CMAS and yttria-stabilized zirconia (YSZ) was investigated using a single crystal YSZ material and a synthetic CMAS product. Reaction between the molten CMAS and YSZ was significant at high temperature resulting in the infiltration of CMAS into the dense bulk-YSZ. The extent of interaction between CMAS and YSZ was found to be dependent on the crystallographic plane of the YSZ. The change in elastic stiffness due to the CMAS infiltration was also found by using a vibrating reed technique. The CMAS infiltrated layer had elastic stiffness higher by approximately five times of the non-infiltrated one. An attempt to detect the CMAS damage progression was also made through an AC impedance method. The proposed AC impedance technique is expected to be a useful technique to evaluate the CMAS infiltration as well as the associated delamination of TBC top coat.

Abstract: The influences of electric current on the microstructures and mechanical properties of 6061 aluminum alloy were investigated by electric conductivity measurement, tensile test, scanning electron microscope and transmission electron microscope. When applying electric current both at solid solution and ageing treatment, the alloy has the highest peak strength, and the time to peak strength shortens by 12h. The electric current applying during either solid solution or ageing, increases the density and size of β'' phase.

Abstract: High-Pressure Torsion (HPT) is one of the most effective severe plastic deformation techniques in grain refinement. The goal of this study was to investigate the influence of HPT on the microstructure and hardness of a Ti-rich High-Entropy Alloy (HEA). The evolution of the grain size due to 1 turn of HPT was studied by transmission electron microscopy. Besides the refinement of the microstructure, a phase transition also occurred during HPT, as revealed by X-ray diffraction. The initial bcc structure transformed into a martensitic phase throughout the material. The features of this phase transformation were studied on a sample compressed to low strain values. The hardness as a function of the distance from the center in the HPT-processed disk was measured and correlated to the microstructure.

Abstract: Magnetic shape memory effect is general name for several effects in which the most visible feature is huge strain induced by magnetic field. Magnetic field-induced structure reorientation (MIR) occurs due to motion of twin boundaries in single phase. As the magnetic field is a relatively weak force compared with mechanical stress, very high mobility of twin boundaries is crucial. Here we study the properties of martensite relevant for this effect using X-ray diffraction, optical and electron microscopy, magnetic observation and mechanical testing. In 10M modulated martensite, two types of mobile twin boundary (type I and type II) are observed with complex layered microstructures consisting of a hierarchy of twinning systems. We search for analogue with non-magnetic Cu-Ni-Al shape memory alloy.

Abstract: The microstructure and microtexture evolution of a Fe-36%Ni alloy processed by cross accumulative roll-bonding was investigated using Electron BackScatter Diffraction. Deformation led to the development of elongated ultrafine grains parallel to the rolling direction that subsequently became more equiaxed. The grains were more effectivelly refined after CARB than after ARB processing. The grain aspect ratio (l/L) decreased (which means a trend towards elongated sub-grain structure) after 2 and 3 CARB processing cycles and then increased (which means a trend towards equiaxed subgrain structure) from 4 to 5 cycles. The fraction of HAGB, CSL boundaries and the estimated deformed volume fraction gradually increased with increasing number of CARB cycles. Copper-type texture was observed after CARB odd cycles (RD//RD), while after even cycles (RD//TD) a new texture component named H ({012}<221>) was observed.

Abstract: Micrometric surface topologies are required for a wide range of technical applications. While lowered surface features have been used for many years to improve the tribological behavior of contacting surfaces, there are also other fields of application, where the potential of elevated surface features is known, e. g. for metal forming tools. However, the demand for a high wear resistance of these structures often inhibits an industrial application. A solution is offered by structuring techniques that use additional material. A promising approach is the localized dispersing of hard ceramic particles by pulsed laser radiation, the so-called laser implantation. This paper describes the potential to adjust the geometry as well as the mechanical properties of laser implanted surfaces by means of microstructural and topological investigations. Afterwards, results of a wear test are given and different applications for this structuring technique are discussed. It can be shown that dome-shaped or ring-shaped structures on a micrometric scale can be produced with high hardness and wear resistance.

Abstract: For the purpose of application for structural members such as the automobile, effect of carbon modification to the AZ91D magnesium alloy chip surface on fatigue property was examined by the rotary bending test. The carbon modification to the chip surface was quite useful for the control of the internal defect for the thick part on the thixomolding process. The depression of the internal defect by the carbon nanoparticle modification is closely related to the improvement of flowability, and that is presumed to be based on promoting the filling of the molten metal into the metal die inside. The tensile strength and elongation were respectively improved by the carbon addition. In addition, the fatigue strength also improved over 15%. This improvement of the mechanical property is based on the refinement of the crystal grain as well as the depression of the internal defects.

Abstract: Mechanical properties of extruded WZ72 magnesium alloy with long-period stacking ordered (LPSO) phase were investigated during compression loading at room temperature and at a constant strain rate of 10^-3 s^-1. The samples of (8 x 4 x 4) mm³ were compressed along three directions with respect to the lamellar LPSO-phase: parallel (ED), perpendicular (TD) and under 45°. Concurrently with the deformation tests, the acoustic emission (AE) response of the specimens was recorded. The AE measurements revealed that both the twinning activity and the kinking of the LPSO phase significantly depend on the orientation of LPSO phase. The highest strength was observed for the sample which was compressed parallel to the LPSO phase (extrusion direction). The highest AE activity was also measured in this sample.

Abstract: To clarify microscopic elastic properties of a Pd₄₀Ni₄₀P₂₀ bulk metallic glass, inelastic X-ray scattering (IXS) experiments were carried out using high-resolution IXS spectrometer installed at BL35XU/SPring-8. Clear longitudinal acoustic (LA) excitation modes are observed in the whole momentum transfer Q range up to ~22 nm^-1. The microscopic sound velocity of the LA modes obtained from the Q → 0 limit of the dispersion relation is about 12% faster than the macroscopic value, suggesting the microscopic heterogeneity in the elastic properties of this BMG. In addition, transverse acoustic (TA) modes can be deduced from a detailed analysis of the IXS spectra. The microscopic Poisson’s ratio obtained from the ratio of the excitation energies of the TA and LA modes is 0.47 ± 0.04, larger than the macroscopic value of 0.40. Thus, the fragile nature of this BMG already found in the macroscopic sense is much enhanced in the microscopic view.