Materials Science Forum Vols. 544-545

Abstract: The phases formed in the wide-gap region brazed with BNi-3 filler metal powder in IN738 superalloy were investigated by electron backscatter diffraction (EBSD). The morphology and chemical composition of the phase were investigated by field emission scanning electron microscopy (FESEM) and Auger electron spectroscopy (AES). The wide-gap region brazed with BNi-3 filler metal had a microstructure consisting of primary Ni3B, binary eutectic of Ni3B-Ni solid solution and ternary eutectic of Ni3B-Ni-Ni3Si structure. EBSD pattern analysis revealed that the Ni3B had orthorhombic structure with lattice parameter of a=0.439, b=0.522 and c=0.662 nm, and the Ni3Si phase had cubic structure with lattice parameter of a=0.350 nm.

Abstract: Multilayered coatings of WC-Ti and WC-Ti1-xAlxN were deposited to form a wear- and corrosion-resistant surface. The cathodic arc deposition (CAD) method was used to deposit WC-Ti1- xAlxN coatings with variable Al content onto high speed steel (HSS). Three kinds of WC-Ti1-xAlxN coatings, each with and without interlayer (Ti/TiN), were prepared. The corrosion behaviors of WCTi1- xAlxN coatings in a deaerated 3.5% NaCl solution were investigated by galvanic corrosion test and potentiodynamic polarization test. The porosity of the coatings, both with and without interlayer, generally decreased with increasing Al content. The multilayer coating with a Ti/TiN interlayer was responsible for the increased corrosion resistance due to its lower porosity and higher adhesion strength than the multilayer coatings without interlayer.

Abstract: Al-Mg alloy foams containing different Mg contents were synthesized via a conventional melt foaming method. The surface structures of pores formed in resultant foamed alloys was characterized by scanning electron microscopy and x-ray diffractometer. It was found that the pore structures were deteriorated with increasing Mg contents, while the percent porosities did not vary with increasing Mg contents, about 90% and 3~5 mm in pore size. The detailed microstructural examination conducted on increasing Mg containing alloy foams revealed presence of various oxide particles on the surface of individual cells, including MgAl2O4 particles in a form of fine spinel; its small amount for Al-1wt%Mg alloy foam but relative high amount of Al-4wt%Mg one. This suggested the negative effect of MgAl2O4 on the stable pore and thus cell structures in corresponding alloy foams. The possible mechanism associated with MgAl2O4 formation was discussed in the present study.

Abstract: In order to enhance the corrosion and wear resistance of the STVAX alloy, the CrN film has been deposited by UBM(unbalanced magnetron) sputtering. In this study, the influence of process parameters, such as N2 gas concentration and Cr interlayer insertion, on the adhesive strength was investigated. By increasing the concentration of N2 gas from 30% to 33%, the microstructure of the coating layer was changed from hexagonal Cr2N to cubic CrN. By increasing N2 gas concentration to 38%, a CrN coating layer with a preferred orientation of <111> plane was formed. Nanoindentation testing showed that the mechanical properties of coatings varied according to the coating type and crystallographic orientation. Superior adhesive strength was obtained at the of N2 gas concentration of 38%, and higher hardness at 33% concentration. The adhesive strength of the coating film was higher with a Cr interlayer than that without. These findings showed that the Cr interlayer between the CrN coating and the STAVAX substrate significantly improved the adhesive strength of coating to the substrate.

Abstract: Thixoextrusion, one of the thixoforming processes, has advantages of high productivity, reduction of the extrusion pressure and cost saving because of the low energy consumption compared with the conventional extrusion processes. For the thixoextrusion, the low liquid fraction (fL<0.3) should be achieved and also the liquid fraction and average grain size should be uniform according to the reheating profile at the desired low liquid fraction. The main emphasis of this study is to investigate the feasibility of thixoextrusion for 2024 Al wrought alloy without additional pretreatment. The results show that the liquid fraction and average grain size were almost uniform with respect to isothermal holding temperature and time. It was very useful for thixoextrusion in terms of process control such as billet temperature control and actual extrusion time.

Abstract: The brittleness of ceramics has been their weak spot when considering them for total hip replacements. There are several situations from which concentrated stresses can develop and induce failure of the components. It is not a problem of the material’s properties in particular but also its design. This means that even proof tested-third generation-ISO obeying ceramics may fail if the incidence of the design parameters on them is not carefully evaluated. In this paper, the finite element analysis technique is used to show how the change of ball diameter can affect the Vonmises stress on the ball head component when it is subjected to a high (trauma-like) load. The analysis is nonlinear and was carefully tested for convergence. 3D models of four different ball head diameters where constructed, two designs of the bore/cone interface with two sets of material properties (Alumina and Zirconia) were considered. The results from these analyses are given in the form of stress maps on the ball heads; such data may be useful for surgeons, patients, and designers because it shows a direct relationship between two important parameters on ball head design (diameter and bore) and material properties, on the stresses that can cause failure of the component when they are concentrated in small elements of volume.

Abstract: The effect of CaO, MgO and SiO2 as a flux on the sinterability of zirconia toughened alumina(ZTA) used for plasma reactors was investigated and the effect of zircon addition on thermal shock resistance of ZTA with 15wt.% of ZrO2 was also investigated. The resultant data revealed that ZTA shows the best sinterability at the composition of 2wt.% of CaO, 4wt.% of MgO and 2wt.% of SiO2 and at the sintering temperature of 1350°C. Thermal shock resistance of ZTA containing zircon was improved significantly. It is shown that ZTA containing 10wt.% of zircon shows better thermal shock resistance than others. This fact can be explained due to the low thermal expansion coefficient of zircon. It was concluded that zircon is an effective material to improve thermal shock resistance of alumina ceramics.

Abstract: We focused on the surface reinforcement of ligth weight casting alloys with Ni3Al intermetallic compounds by in-situ combustion reaction to improve the surface properties of nonferrous casting components. In the present work, by setting the mixture of elemental Ni and Al powders in a casting mold, the powder mixture reacted to form Ni3Al intermetallic compound by SHS reaction ignited by the heat of molten AZ91D Mg alloy and simultaneously bonded with the Mg casting alloy. The AZ91D Mg alloy bonded with the Ni3Al intermetallic compound was sectioned and observed by optical microscopy and scanning electron microscopy(SEM). The chemical composition of intermetallic compounds and diffusion layer formed around bonding interface were identified by energy dispersive spectroscopy(EDS), X-ray diffraction analysis(XRD) and electron probe micro analyzer(EPMA). The main intermetallic compound was Ni3Al phase and a little Ni2Al3 intermetallic compound was formed from the Ni and Al powder mixtures. Residual pores were observed in the synthesized intermetallic compound. The AZ91D Mg alloy and Ni3Al intermetallic compound were bonded very soundly by the interdiffusion of Mg, Ni and Al elements, but some cracks were observed around the bonded interface on the interdiffusion layer. The diffusion length formed between AZ91D Mg alloy and Ni3Al was different depending on the diffusivity of Ni and Al elements into the molten Mg alloy. Ni was more deeply diffused into the Mg alloy than Al. The diffusion layer was about 200
m thickness and various phases were formed by the interdiffusion of Mg, Ni and Al. From this challenge we have successfully produced a coating layer based on nickel aluminide on ligth weight Mg alloy using molten metal heat without any additional process. On the basis of the results obtained, it is concluded that near-net shaped nickel aluminide coating layer can be formed using this unique process.

Abstract: Homogeneous Cop/Cu coating alloys were fabricated by laser cladding rapid solidification technique and their microstructures were investigated, with the emphasis on the influence of processing on microstructure. The experimental results showed that a Cop/Cu composite coating can be successfully in situ synthesized when the processing conditions were controlled. The as-solidified microstructures were characterized by a homogeneous distribution of small spherical Co-rich particles dispersed in the Cu-rich matrix. The Cop/Cu coating was consisted of γ-Co, Cu-rich solution and et al. With the increasing of line energy, the spherical particles diameter in coating was increasing. The structural evolution and refining mechanism of the Cop/Cu coating were also discussed.

Abstract: The SiC/h-BN ceramic composites with different h-BN size were fabricated by Plasma Active Sintering (PAS) method. For the SiC/ nano-sized h-BN ceramic composites, when the h-BN content was increased, because the nano-sized h-BN crystals were homogeneously dispersed around the SiC grains of the matrix, the bending strength and fracture toughness of the composites decreased slowly, but the hardness decreased sharply, therefore the machinability and thermal shock resistance were improved noticeable.