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[en] Nb and Ti were added to 316 stainless steel, and then heat-treatments and surface treatments were performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples indicated enhanced electrical conductivity after surface treatments, whereas they showed low electrical conductivity before surface treatments due to the existence of non-conductive passive film on the allot surface. In particular, the Nb- and Ti-added alloys showed remarkable enhancement of electrical conductivity compared to the original alloy, 316 stainless steel. Surface characterization revealed that small carbide particles formed on the alloy surface after surface treatments, while the alloys indicated flat surface structure before surface treatments. Cr23C6 mainly formed on the 316 stainless steel, and NbC and TiC mainly formed on the Nb- and Ti-added alloys, respectively. We attribute the enhanced electrical conductivity after surface treatments to the formation of these carbide particles, possibly acting as a means of electro-conductive channel through the passive film. Furthermore, NbC and TiC are supposed to be more effective carbides than Cr23C6 as electro-conductive channels of stainless steel
[en] The FCP behavior of FSWed 5083-H32 and 6061-T651 specimens were examined with the fatigue crack growing either parallel to the DXZ at variable ΔK values and an R ratio of 0.1 and 0.8, respectively, or perpendicular or 45 .deg. to the DXZ at various constant ΔK values and an R ratio of 0.1. Residual stress was measured on the top surface of FSWed plate either perpendicular or parallel to the welding direction and the residual stress-corrected ΔK, ΔKcorr, is calculated based on the Kres, the stress intensity factor at residual stress, for the DXZ specimens to quantify the compressive residual stress contribution to the FCP rates. The present study suggests that the FCP behavior of FSWed 5083-H32 and 6061-T651 specimens in the DXZ is mainly determined by the beneficial compressive residual stress reducing effective ΔK and the detrimental grain refinement causing intergranular fatigue failure. The FCP behavior of FSWed 5083-H32 and 6061-T651 specimens is discussed based on residual stress measurement and fractographic observation
[en] The dry sliding wear tests were performed on as-cast samples of A356/20vol.% SiCp composites. The samples were tested by pin-on-disc wear test equipment with different applied load. The wear results show that as-cast samples exhibited better resistance to wear than the T6 treated samples. In addition, as-cast samples exhibited stable friction coefficient values in the range of 0.45-0.5. Wear behavior of as-cast A356/20vol.%SiCp is examined through the observations and analysis of the worn surface, worn subsurface and wear debris. In the low applied load region, wear is occurred mainly by oxidation. With increasing the applied load, Mechanical Mixed Layer(MML) is formed on the worn subsurface and crack is formed on MML. In the higher applied load region, wear is occurred by adhesive and delamination
[en] The corrosion behavior and oxide properties of HANA-3, HANA-6 and Zircaloy-4 alloys have been investigated. HANA-3 and HANA-6 alloys showed an excellent corrosion resistance when compared to the Zircaloy-4 alloy in a PWR-simulation loop condition without a neutron flux. In HANA alloys, the corrosion rate was increased with an increase of the final annealing temperature. Oxide microstructure observation showed that the oxides formed on all the alloys had a layer structure regardless of the alloy composition. HANA-6 which was annealed at 470 .deg. C showed the best corrosion resistance among the tested alloys and it had the highest thickness of the fully developed oxide layer even though the total oxide thickness was the smallest. The oxides were consisted mainly of columnar grains in the interface region and equiaxed grains in the outer surface region. The size of oxide grains was increased with an increase of the corrosion resistance and the grain morphology was changed periodically from the interface to the outer surface, which was well correlated the layer structure and the transition behavior in the corrosion kinetics. The lateral cracks were observed only in the regions where the equiaxed grains are dominant. The precipitates incorporated into the oxide of HANA alloys maintained the crystallite in the interface region but transformed to the amorphous state by oxidation when apart from the interface
[en] The effects of CH4 content at nitriding step in the low temperature two-step plasma treatment (carburizing+nitriding) on the surface characteristics of AISI304L stainless steel were investigated. The low temperature plasma carburizing was carried out at 550 .deg. C for 5h in a gas mixture of H2 Ar and CH4. The thickness of a carburized layer increased up to about 30 μm and corrosion resistance of the layer decreased due to the precipitation formed at the grain boundary. After carburizing, a low temperature plasma nitriding was subsequently performed in the same chamber at 400 .deg. C for 15h to improve corrosion resistance and to further increase the surface hardness. The surface hardness of a N-enriched layer after nitriding reached up to 1,200HV0.1, which is much higher than that of as-carburized layer(750 HV0.1). The post nitriding process had a beneficial effect on reducing the precipitates in the C-enriched layer. The thickness of the N-enriched layer was about 10 μm without containing CH4 in the nitriding atmosphere. With increasing the CH4 level in the atmosphere, the uniformity and thickness of the N-enriched layer decreased and finally reached 5 μm with 8% CH4. The amount of the precipitates in the N-enriched layer increased with increasing CH4 composition
[en] In this study, Binderless-WC hard materials were fabricated using the pulsed current activated sintering (PCAS) process for a Friction Stir Welding tool and difficult-to-cut tool application. Tungsten carbide (WC) hard materials are used in various industries and possess a superior hardness compared to other hard materials. They have particularly high melting points, high strength, and abrasion resistance. 100 mm diameter and 5 mm thick binderless-WC hard materials were fabricated using a 30,000 A pulsed current activated sintering machine and 0.3 to 0.5 μm size WC powders. Variation of properties in binderless WC hard materials by sintering temperature were increased slightly for 0.11 to 0.37 um with a grain size and densified completely for 70.0 to 99.5% for a relative density. Consequently, these materials were almost completely dense with a relative density of up to 99.5% after simultaneous application of 60 MPa pressure and an electric current for 2 min at 1600 .deg. C, almost without any significant change in the crystallite size. The average WC crystallite size that was produced through PCAS was 0.37 μm at 1,600 °C. In terms of mechanical properties, considering only the densified sintered body that the hardness and fracture toughness of binderless-WC hard materials were about 2,661.5 kg/mm2 and 3.90 MPa·m1/2, respectively.
[en] Nanopowders of TiCo were synthesized from Ti and Co by high energy ball milling. Highly dense nanostructured TiCo compounds were consolidated at low temperature by pulsed current activated sintering within 3 minutes from the mechanical synthesis of the powders (TiCo) and horizontal milled Ti+Co powders under 100 Mpa pressure. This process allows very quick densification to near theoretical density and prohibits grain growth in nanostructured materials. The grain sizes of the TiCo compounds were calculated. Finally, the average hardness values of the nanostructured TiCo compounds were investigated.
[en] Aluminum nitride(AlN) is a compound (III-V group) of hexagonal system with a crystal structure. Its Wurzite phase is a very wide band gap semiconductor material. It has not only a high thermal conductivity, a high electrical resistance, a high electrical insulating constant, a high breakdown voltage and an excellent mechanical strength but also stable thermal and chemical characteristics. This study is on the preferred orientation characteristics of AlN thin films by reactive evaporation using NH_3. We have manufactured an AlN thin film and then have checked the crystal structure and the preferred orientation by using an X-ray diffractometer and have also observed the microstructure with TEM and AlN chemical structure with FT-IR. We can manufacture an excellent AlN thin film by reactive evaporation using NH_3 under 873 K of substrate temperature. The AlN thin film growth is dependent on Al supplying and NH_3 has been found to be effective as a source of N_2. However, the nuclear structure of AlN did not occur randomly around the substrate a particle of the a-axis orientation in fast growth speed becomes an earlier crystal structure and is shown to have an a-axis preferred orientation. Therefore, reactive evaporation using NH_3 is not affected by provided H_2 amount and this can be an easy a-axis orientation method.
[en] Nanotubular structure formation on the Ti-6Al-4V and Ti-Ta alloy surfaces by electrochemical methods has been studied using the anodizing method. A nanotube layer was formed on Ti alloys in 1.0 M H_3PO_4 electrolyte with small additions of F-ions. The nanotube nucleation and growth of the α-phase and β-phase appeared differently, and showed different morphology for Cp-Ti, Ti-6Al-4V and Ti-Ta alloys. In the α-phase of Cp-Ti and martensite α , and in the α and β-phase of the Ti-Ta alloy, the nanotube showed a clearly highly ordered TiO_2 layer. In the case of the Ti-Ta alloy, the pore size of the nanotube was smaller than that of the Cp-Ti due to the β-stabilizing Ta element. In the case of the Ti-6Al-4V alloy, the α-phase showed a stable porous structure; the β-phase was dissolved entirely. The nanotube with two-size scale and high order showed itself on Ti-Ta alloys with increasing Ta content.
[en] Hydrogen was generated by the reaction of metal hydride with water. The variation of hydrogen generation with the kind of powders (milled MgH_2, and MgH_2 milled with various contents of MgO, MgCl_2 or Ni+Nb_2O_5) was investigated. MgH_2 powder with a hydrogen content of 6.05 wt% from Aldrich Company was used. Hydrogen is generated by the reaction of Mg as well as MgH_2 with water, resulting in the formation of byproduct Mg(OH)_2. For about 5 min of reaction time, milled 95%MgH_2+5%MgO has the highest hydrogen generation rate among milled MgH_2+x%MgO (x=0, 5, 10, 15 and 20) samples. Milled 90%MgH_2+10%MgCl_2 has the highest hydrogen generation rate among all the samples.