Materials Science Forum Vol. 879

Abstract: The effects of effects of additional elements on hydrogen storage properties and crystal structures for vanadium alloys and their hydrides were investigated in order to obtain high hydrogen capacity. With increasing Cr content in V-xCr binary alloys, fully hydrogen content of the alloys slightly decreased until less than 9 at.%Cr. A clear distinction of the PC isotherm curves between the 15 at.%Cr alloy and the other alloys is observed. V alloys with an excessive Cr addition would come not to form gamma hydride (dihydride). This led the drastic decrement of the hydrogen content in the alloys. Meanwhile, the Cr addition in V alloys was effective to low hydrogen concentration in unstabilizing the beta hydride phases. In addition, it was found that the addition of X elements in V-Cr alloys (X=Al. Mo, Ti, W) was effective to expand the gamma-phase forming range of Cr amounts. As the results, high reversible hydrogen-capacity, 2.68mass% H was obtained in a V-18Cr-2Ti-0.5Al alloy.

Abstract: In this report, rolling contact fatigue (RCF) progression in two-cylinder type RCF testing is evaluated by using an X-ray diffraction ring analyzer, which can rapidly obtain tri-axial stress and the orientation of crystallite. The large compressive and three principal stresses on the RCF surface are observed under boundary lubrication. It is considered that the crack occurrence and its propagation by asperity contact of surface roughness are caused by residual principal shear stress and the repeated contact stress. In addition, the behavior of RCF progression from the point of view of the X-ray measurements is similar for the driving and driven specimen until the generation of peeling begins. This supports the conclusion of Kaneta et al. that the RCF progression for the driven cylinder is the same as that for driving cylinder until peeling occurs.

Abstract: In this study, the effect of electroless Ni-P plating on the mechanical properties of Al-4%Ge alloy was investigated. As the results, the following points were clarified. (i) Tensile strength of the specimen subjected to the Ni-P plating after aging treatment or furnace cooling treatment was improved by about 10% in comparison to one of the non-processed specimens. (ii) Breaking elongation of the specimen subjected to the Ni-P plating after aging treatment showed no significant changes in comparison to one of the non-processed specimens. On the other hand, breaking elongation of the specimen subjected to Ni-P plating after a furnace cooling treatment was reduced to 70% in comparison to one of the non-processed specimens. (iii) Fatigue strength of the specimen subjected to the Ni-P plating after a furnace cooling treatment was overall reduced rather than one of non-processed specimens. (iv) Fatigue strength of the specimen subjected to the Ni-P plating after aging treatment was overall reduced, except for the low-stress region, rather than one of the non-processed specimens. (v) In the specimen subjected to Ni-P plating after a furnace cooling treatment or aging treatment, clear hydrogen desorption was recognized. On the other hand, there was only hydrogen desorption from a few of the non-processed specimens. Especially, it is considered that the poor fatigue strength and ductility of the plating materials are mainly due to the interaction between the surface precipitates and hydrogen gas.

Abstract: The present work investigates the effects of laser beam power, focus position and advance speed on the geometry, microstructure and mechanical properties of fiber laser beam welded Ti-6Al-2Sn-4Zr-2Mo (denoted as Ti-6242) butt joints used for high temperature applications. Detailed microstructural and mechanical studies were performed on welds produced using optimized parameters (a laser beam power of 5 kW, a focus position of 0.0 mm and an advance speed of 6.2 m/min). The Ti-6242 base material is characterized by a globular (α+β) microstructure. The heat input during laser beam welding led to the formation of a martensitic α’-phase fusion zone. The heat affected zone consisted of globular grains and acicular crystallites. These local transformations were connected with a change in the micro-texture, average grain size and β-phase content. Furthermore, the microhardness increased from 330 HV 0.3 to 450 HV 0.3 due to the martensitic transformation. The mechanical behavior of the laser beam welded Ti-6242 butt joint loaded in tension was determined by the properties of the Ti-6242 base material. The local increase in hardness provided a shielding effect that protected the Ti-6242 butt joint against mechanical damage.

Abstract: This work focuses on the production of new high conductive carbon based MMC (Metal Matrix Composites) or co-cast components obtained by casting processes. These novel thermally conductive structures are designed to face modern heat management challenges in critical fields such as power micro-electronics, automotive and aerospace industries, renewable energy generation as well as highest performance combustion engines. The sought parts will be assembled by different heat conductive aluminum-carbon composites and for this reason different heat conductive MMCs have been studied. Their combination into once cast aluminum part may allow the part to meet applicative needs for heat management challenges. The cast production routes as well as thermal behavior of the obtained materials has been studied by means of numerical (Finite Element Methods) approaches in order to determine the effective thermal conductivity in the different directions of heat dissipation. Some kinds of casting methods have been FEM simulated and then performed practically. TPG/aluminum interface microstructure has been studied.

Abstract: In this paper, surface microstructural evolution induced by processes based on repeated sliding (friction) contact such as burnishing or machining is investigated. A set-up designed for simulating contact pressures and cutting speed occurring during machining is used to create a gradient of nanomicro-structure. It is composed of a top surface recrystallized layer and a sub-surface made of ultrafine grains over a depth larger than 100 μm. Induced-mechanical properties as well as resulting wear resistance are discussed. A conclusion is brought on the benefits of this new kind of sliding-based surface mechanical treatments (SMT).

Abstract: The grain coarsening temperature in Nb microalloyed steels is investigated by multiphase-field model. In this study, the pinning force is treated as time-dependent using mean-field kinetics of precipitates including volume fraction and their size. The grain size is calculated with time under various temperature range. The grain coarsening temperature is determined by the ratio of the largest radius of grain vs. the average grain radius criteria (R_max / R_avg > 2.94) in two-dimensional growth. Through this model, it is possible to simulate grain growth in microalloyed steels more precisely.

Abstract: Alloy 600 (UNS N06600) is an austenitic nickel-based alloy with superior corrosion resistance and high-temperature endurance, which determines its widespread applications in aeronautical, aerospace, marine and nuclear industries. Particularly, a number of nuclear components used Alloy 600 as their structure materials due to their high corrosion resistance, high-temperature endurance and excellent fabricant characteristics. Many failures have occurred in Alloy 600 with various forms of environmental degradations during long-term operation. In this study, an ultrasonic nanocrystal surface modification (UNSM) technique was applied to Alloy 600 at a room and a high temperature of 500 ^OC. The effects of UNSM treatment temperature on the microstructure and wear behavior including a compressive residual stress were investigated. The hardness, compressive residual stress with respect to depth from the top surface were measured. Also, the wear behavior of UNSM-treated at a room and a high temperature Alloy 600 specimen was compared to that of the untreated specimen. The increase in wear resistance by UNSM technique was discussed in terms of increased hardness, refined grain size and induced compressive residual stress.

Abstract: This present work was carried out to characterize the microstructure and mechanical properties of austenitic stainless steel (SS) 316L to dissimilar welding of Inconel 52 and Inconel 82 fillers that were treated by ultrasonic nanocrystalline surface modification (UNSM) technique. The microstructure and hardness map of the specimens were characterized by optical microscope (OM) and micro-hardness tester. The increase in hardness and refinement in grain size after UNSM treatment were found for the SS 316L/52 and SS 316L/82. In addition, the induced compressive residual stress in the surface layer of base SS 316L was measured. It is expected that the increased hardness into several microns in depth and modified microstructure with a severe plastically deformed surface layer by UNSM technique can increase the resistance to corrosion and extend the service life of pressurized water reactors (PWRs).

Abstract: In previous study, the formation behavior of texture and microstructure in AZ80 magnesium alloy under high temperature deformation was investigated. It was found that the basal texture was formed at stress of more than 15-20MPa and the non-basal texture was formed at stress of less than 15-20MPa. This means that stress of 15-20MPa is the change point of deformation mechanism. Therefore, in this study, uniaxial compression deformation of AZ80 magnesium alloy was carried out at high temperature deformation (stress of 15-20MPa). Behaviors of microstructure and texture development are experimentally studied. The material used in this study is a commercial magnesium alloy extruded AZ80. The uniaxial compression deformation is performed at temperature of 723K and strain rate 3.0×10^-3s^-1, with a strain range of between-0.4 and-1.3. Texture measurement was carried out on the compression planes by the Schulz reflection method using nickel filtered Cu Kα radiation. EBSD measurement was also conducted in order to observe spatial distribution of orientation. As a result of high temperature deformation, the maximum value of the flow stress is observed at the true stress-strain curves, and the main component of texture and the accumulation of pole density vary depending on deformation condition.