The competitive precipitation behavior observed in microstructures with high dislocation density and ultrafine grains has been studied experimentally and computationally for cold-rolled and severe plastic deformed Al-Mg-Si alloys. The age-hardenability at 443 K was reduced by the two deformation processes due to the accelerated formation of larger precipitates on dislocation lines and grain boundaries, in place of the transgranular precipitation in the matrix. The developed numerical model based on a classical heterogeneous nucleation theory clarified the dislocation density and grain size dependences of volume fraction of precipitates nucleated at different sites, in good agreement with experimental results. From such dependencies, it becomes possible that three strengthening mechanisms of precipitation hardening, strain hardening and strengthening by grain size reduction are optimally exploited by controlling microstructural parameters such as dislocation density and grain size.
Oxidation behavior and strength of ZrB2-SiC ceramics in high temperature reduced pressure plasma around 2000°C were investigated. Thick oxidized layer forms on the material even at 2000°C. The material endured up to 4 times repetitive plasma oxidation around 1900°C. The tensile strength of ZrB2-SiC ceramics was around 200 MPa and did not change with thermal history up to 2000°C. The tensile strength of ZrB2-SiC ceramics at 2000°C was 65 MPa. ZrB2-SiC ceramics will be a candidate for heat protective material of future space re-entry vehicle with adequate thermal stress analyses for proper thermal protection structure.
Closed-cell Zn-22Al alloy foams are produced through powder metallurgy process. Zn-22Al alloy powders and titanium hydride powders were mixed to prepare a precursor. The precursor was manufactured using uniaxial hot pressing. Macro- scopic porosity is as large as 70% in the condition of heating temperature at 873 K for 20 min. Average pore aspect ratio is 0.48 and the pores extended in the direction of perpendicular to hot pressing were observed. Compressive tests were carried out in the different loading axes and crosshead speeds. Plateau stress in the direction of hot pressing was lower than the direction of perpendicular to hot pressing. Anisotropic deformation behavior is due to the anisotropic cell morphology which was induced during the uniaxial hot pressing. In contrast, difference of strain rate sensitivity exponent (m value) in the compressive directions was not observed. This is because the m value does not depend on cell morphology but on cell wall material.
For extending the life of a lead-acid battery, the control of the creep of a Pb-base alloy as the grid material is of great importance. In this study, Pb-1.50 mass%Sn alloys were produced by two different processes, namely the cast-rolling process and the powder-rolling one, in order to study the influence of the crystallographic structure on the creep behavior of the Pb-base alloy. The structure produced by the cast-rolling, which is the commonly used process in the lead-acid battery industry, consists of relatively large grains. On the other hand, the powder-rolling is a new process for the preparation of the grid material, which uses air-atomized powders as the raw materials. The mean size of crystal grains of the powder-rolled Pb-base alloy is extremely small in comparison with that of the cast-rolled one. The tensile strength and the Vickers hardness of a Pb-1.50 mass%Sn powder-rolled alloy are 23 MPa and ca. Hv 9.0, respectively. Although these values are twice larger than those of a Pb-1.50 mass%Sn cast-rolled alloy, they are lower than the values of the Pb-base alloy in common use of the VRLA battery, or a Pb-0.08 mass%Ca-1.20 mass%Sn cast-rolled alloy. However, the corrosion creep test showed that the steady creep rate of a Pb-1.50 mass%Sn powder-rolled alloy was extremely smaller than those of all the Pb-base cast-rolled alloys. This finding suggests that the powder rolling process is very effective to improve the creep resistance of a Pb-base alloy as a grid material in a lead-acid battery. It can be also presumed that fine grain size in the powder-rolled alloy of Pb-1.50 mass%Sn function as an obstacle to the movement of dislocation, resulting in remarkably the suppression of the creep rate.
When metal is exposed at the atmospheric environment, thin water film is formed on the metal surface. This film is strongly affected by environmental parameters such as temperature, relative humidity and rainfall. Because a film that consists of aqueous solution on a metallic surface is extremely thin, it is difficult to investigate the surface reaction by the conventional electrochemical technique. In this paper, a surface potential measurement device with non-contact probe was applied to measure the potential of the passive metals. The corrosion behavior of the materials surface was also investigated by using the surface potential. The surface potential distribution of high corrosion resistance materials showed different behavior between titanium and type 304 stainless steel (SUS304). The potential on the SUS304 became less noble during drying up. In the case of Ti, the potential didn't descend intensely. From these results, the determination of the surface potential using a surface potential measurement device is quite suitable to investigate the atmospheric corrosion behavior of high corrosion resistance materials.
This paper presents the characteristics of 1.2 μm-laser pumped wide frequency tunable terahertz (THz) light source based on difference frequency generation (DFG) with an excitation of phonon-polariton in gallium selenide (GaSe) crystal. The pump and signal lasers are 1.2 μm Cr: Forsterite lasers as a novel frequency. The frequency can be tuned by changing wavelength of signal laser and incidence angle to GaSe crystal. The collinear phase matching conditions of generating terahertz wave in this system are shown. The tuning range of the THz-wave frequency covers from 0.3 THz to 4.8 THz (type oee phase matching) and 8.3 THz to 10.2 THz (type eoo phase matching). The maximum conversion ratio is ~10−6 (J−1), which is about 3 orders in magnitude larger than that of gallium phosphide (GaP) crystal.
We are proposing a centrifugal mixed-powder method as a novel processing technique to fabricate a nano-particle distributed FGM. If the mixed-powder has been changed into machining chips, an eco-centrifugal casting method can be achieved, since the reduction of necessary power for casting will be expected if recycled materials are used in the melting and casting process. Pure Al or Al-Si alloy melt of 750°C or 800°C is poured into the spinning mold with or without the machining chips. It is found that Al pipe can be fabricated by the centrifugal casting with machining chips. It is also found that the machining chips act as the grain-refiner. Although some pores were found for the cast with the machining chips, the defect can be reduced when the mold is preheated. In this way, Al pipes without casting defect can be fabricated by the proposed method.