Materials Science Forum Vols. 706-709

Abstract: Cadmium sulfide (CdS) semiconductor thin films were grown on sapphire single-crystal substrates. Indium oxide (In₂O₃) and zinc oxide (ZnO), which are typical transparent conducting materials, were heteroepitaxially grown on sapphire substrates as underlayers before the cadmium-based semiconductor thin films were grown on them. Subsequently, cadmium-based thin films were heteroepitaxially grown on the transparent conducting under-layers. Crystallinity and photoluminescence properties of the cadmium-based thin films grown on the underlayers were evaluated and compared with those of the cadmium-based thin films grown directly on sapphire substrates.

Abstract: The interdiffusion behaviors and interfacial reaction in solid Cu/solid Ni, liquid Bi/solid Bi_0.4Sb_0.6, liquid Al/solid Cu, and gas Al/solid Cu diffusion couples have been experimentally studied under a high magnetic field of up to 12T. The effects of magnetic flux density and the direction of magnetic field (B) on the evolution of interfacial microstructure (including interfacial migration, phase composition and thickness of diffusion layers) have been examined systematically. We found that (1) The shift distance of Kirkendall marker and the inter-diffusion coefficient in solid Cu/solid Ni diffusion couples increased with increasing magnetic flux density in case of the direction of diffusion parallel to B; (2) The migration of the Bi/Bi_0.4Sb_0.6 interface due to the self-diffusion of the liquid metal into the solid alloy decreased markedly with the increase of magnetic flux density; (3) High magnetic fields exerted a non-monotonic influence on the thickness of diffusion layers during the reactive diffusion process between liquid Al and solid Cu; (4) The application of a high magnetic field during chemical reactive-diffusion process of gas Al/solid Cu system induced a significant change in the final products.

Abstract: We have studied the properties of photoluminescence (PL) from individual isoelectronic traps formed by nitrogen-nitrogen (NN) pairs in nitrogen atomic-layer doped (ALD) GaAs. Micro-PL measurements were performed to investigate the properties of single photons generated from individual isoelectronic traps. Twin PL peaks were observed from individual isoelectronic traps in nitrogen ALD GaAs (001). The PL transitions at longer and shorter wavelength sides were linearly polarized in the [110] and [1-10] directions, respectively. The peak splitting and polarization properties can be explained by some in-plane anisotropy most likely due to strain in host crystal. From individual isoelectronic traps in nitrogen ALD GaAs (111), a single PL peak with random polarization was observed, showing that the growth on (111) surface is an effective way to obtain unpolarized single photons. As for nitrogen ALD GaAs (110), different polarization properties were obtained depending on the atomic configuration of NN pairs. In addition, we have used AlGaAs layers to diminish the in-plane anisotropy and could successfully obtained single emission lines with unpolarized character. Introducing AlGaAs layers was also useful for improving the luminescence efficiency.

Abstract: On-line quality assessment becomes one of the most critical requirements for improving the efficiency of automatic resistance spot welding (RSW) processes. Accurate and efficient model to perform non-destructive quality estimation is an essential part of the assessment. Besides the usual welding parameters, various measured variables have been considered for quality estimation in RSW. Among these variables, dynamic resistance (DR) gives a relative clear picture of the welding nugget formation and presents a significant correlation with the RSW quality indicators (QI). This paper presents a structured approach developed to design an effective DR-based model for on-line quality assessment in RSW. The proposed approach examines welding parameters and conditions known to have an influence on weld quality, and builds a quality assessment model step by step. The modeling procedure begins by examining, through a structured experimental design, the relationships between welding parameters, typical characteristics of the RD curves and multiple welding QI. Using these results and various statistical tools, different integrated quality assessment models combining an assortment of DR attributes are developed and evaluated. The results demonstrate that the proposed approach can lead to a general model able to accurately and reliably provide an appropriate assessment of the weld quality under variable welding conditions.

Abstract: In this study, specific energy was calculated and correlated with the geometries, defects, microstructures, hardness values and tensile properties of the laser welded Ti-5553 alloy. It was found that specific energy is a good parameter to characterize the laser welding process.

Abstract: As an integral part the railway network infrastructure, insulated rail joints (IRJs) electrically isolate track segments providing critical feedback to both track signaling and train position detection systems. Because of the discontinuous nature of IRJs, accumulated damage at the railhead is high. Failure modes include plastic flow of metal across joints, bolt and fishplate failures, delamination of insulated material and, as a result of rolling contact fatigue, end post and endpost surface damage. In the current investigation, microstructural changes in the vicinity of endposts of IRJs made from both surface coated and uncoated rail are investigated using techniques of optical and scanning electron microscopy. Damaged IRJs made from pearlitic head hardened rail steel are compared with head hardened rail steel laser coated with martensitic stainless steel, the latter having an increased service life. Problems associated with the surface coating are identified and approaches to further improving IRJ resistance to rolling contact fatigue suggested. Keywords: Insulated rail joints, rail, head hardened, surface coated rail

Abstract: A simple three dimensional thermomechanical model for FSW is used in the present paper. It is developed from the model proposed by Heurtier [1] improved by Jacquin [10] to account for the eulerian cooling flux due to the tool motion during welding. The velocity fields used to describe the bulk flow around the tool are introduced in the particular derivative of the thermal equilibrium equation. The complete thermomechanical history of the material during the process can then be accessed by temperature and strain rate contours.

Abstract: Welding residual stress and subsequent distortions are a common problem during fabrication. Several techniques for controlling residual stress and distortion during welding, amongst which transient thermal tensioning is one of the more promising methods. Thermal tensioning can be achieved by two side heaters moving along with the weld heat source. The concept of transient thermal tensioning induces a redistribution of the stress field in the welded plate by altering the thermal field. In this paper, the results are presented on the mitigation of out-of-plane deformation during gas tungsten arc butt welding of 2 mm thick AISI-316L stainless steel. Acetylene-air burners are applied as side heaters. The thermal tensioning process induces a number of additional process parameters including the intensity, the size and the location of the heaters. A thermo-mechanical (TM) finite element model is used to cope with the complexity of the process. The model provides a better understanding of the development of the stress field, both in conventional and transient thermal tensioned welds and of the mechanisms which mitigate distortion. The model is validated by temperature, stress and distortion measurements. It appears that the redistribution of stresses and the introduction of tensile stresses by side heating is the main mechanism responsible for the reduction of distortion.

Abstract: Laser joining for dissimilar materials between metals plate and polypropylene plate using a newly developed insert sheet was investigated. It was not possible to join the plate of various metals to the nonpolar plastic plate such as the polypropylene directly under the various laser-irradiation conditions. However, the use of insert sheet held between the metal plate and the polypropylene plate made it possible to join each metal plate to polyethylene plate by laser-irradiation. The joining strength increased with the increase in the input energy by laser-irradiation, and the joining strength cloud be improved since the reactivity between each metal and the insert material allows improvement by giving the insert material polarity with such as a-NH₂ group, -COOH group.

Abstract: A novel bonding process using Ag₂O paste composed of Ag₂O particles and a reducing agent has been proposed as a Pb-free alternative of high melting point solders in electronics packaging. Ag₂O paste formed Ag nanoparticles through the redox reaction in the bonding process and in-situ formed Ag nanoparticles sintered immediately. While the bonding process using Ag metallo-organic nanoparticles, which have been proposed, was unfavorable to the bonding at 250 degree Celsius or lower in terms of requiring removal of stable organic shells, the bonding process using Ag²O paste demonstrated the possibility of further low-temperature bonding.