Allowable tensile stress at the elevated temperatures used for design of high temperature structural components is determined based on a long-term creep strength, such as 100,000 hours creep rupture strength. Aim of the Creep Data Sheet project conducted in National Research Institute for Metals and National Institute for Materials Science is to obtain long-term creep strength data of engineering creep resistant steels and alloys. A time-temperature parameter method is applied for creep rupture life prediction, and several parameters are proposed. A wide variety of equations are examined to describe and to predict a long-term creep deformation behavior. Creep strength of ferritic creep resistant steels after very long-term exposure at the elevated temperatures is controlled by an inherent creep strength that is almost the same independent of chemical composition, heat treatment, initial microstructure and short-term creep strength. A region splitting analysis method was proposed as accurate life prediction method of ferritic creep resistant steels by consideration of 50% of 0.2% offset yield stress. Allowable tensile stress of some ferritic steels was revised according to reevaluation of long-term creep strength by means of region splitting analysis method.
Magnetotactic bacteria have one or more chains of magnetosome, consisting of nano-sized magnetic crystal covered with a phospholipid bilayer and use it to sense the geomagnetic fields. In order to elucidate the molecular process to make magnetosome from the iron compounds found in the bacteria, laser Raman spectroscopic measurements were performed with the magnetotactic bacterium, Magnetospirillum magnetotacticum MS-1 and the fractions separated from it. The clear Raman signals were observed at 662 cm-1 and 740 cm-1. The former was observed in whole cell and magnetosome fraction, but not in membrane and cytoplasmic fraction and assigned to the Raman signal of magnetite. The Raman signal of the latter was observed not only in the magnetosome fraction, but also in the cytoplasmic fraction and membrane fraction. This signal was assumed to ferrihydrite. Based on the results, the pathway of the magnetosome synthesis and possible roles of ferrihydrite in the magnetotactic bacteria were discussed.
When stainless steels are sensitized, intergranular corrosion resistance decreases and sensitization is assumed to be one of the causes of stress corrosion cracking under severe environment in nuclear reactors. For various stainless steels, Laser surface melting (LSM) with CO2 laser have been reported. In this study YAG laser that is able to transmit energy with the optical fiber is used. Influence of LSM condition on intergranular corrosion resistance was examined. Experimental results indicated that by LSM sensitized Type 316 stainless steel restored intergranular corrosion resistance as the material after solution treatment. Moreover, The material that sensitized in low temperature (LTS) 500°C for 24 hours after LSM exhibited the same degree of intergranular corrosion resistance as the material after solution treatment. This would be caused by lower δ ferrite volume fraction in the solidified microstructure after LSM than that in arc welding metal.
We have studied the formation and carried out an in vivo evaluation of carbonate apatite (CO3-Ap) and CO3-Ap/CaCO3 composite coatings on titanium substrates using the thermal substrate method. The coatings were formed on commercial pure titanium rods (diameter=2 mm, length=5 mm) and plates (thickness=0.3 mm) by the thermal substrate method in an aqueous solution that contained Ca(H2PO4)2, CaCl2, and NaHCO3. The coating experiments were conducted at 40-140°C and pH=8 for periods of 15 or 30 min. The coating temperature and NaHCO3 of the solution had a significant influence on the surface morphology (net-like, plate-like, needle-like, or sphere-like), the phase (single phase of CO3-Ap or binary phase of CO3-Ap and CaCO3), and the carbonate content in the precipitated films. A subsequent autoclave treatment also had an effect on the films. A coated rod was implanted in a 10 weeks old male rat's tibia with a non-coated titanium rod being used as a control. The constructs were retrieved after a period of 14 d postimplantation and examined for new bone formation and for tissue response in the cancellous and cortical bone parts, respectively. Single-phase sphere-like CO3-Ap had high osteoconductivity in the cortical bone region, and this increased with increasing carbonate content in the films. However, the osteoconductivity of the CO3-Ap/CaCO3 composite coatings decreased with increasing total carbonate content.
Corrosion resistance of anodized surfaces on high-purity magnesium (99.95 mass%), rolled sheets of ASTM AZ31B (Mg-2.9Al-0.85Zn) magnesium alloy and die-cast plates of ASTM AZ91D (Mg-9.1Al-0.75Zn) magnesium alloy has been studied. Anodization was conducted by environment-friendly electrolysis whose electrolyte consists of phosphate and ammonium salt. The anodized surface was covered with amorphous film, and showed only discoloration during salt spray test where formation of corrosion product (magnesium hydroxide) was well suppressed within 605 ks. Even when the anodized surfaces were trenched with ceramic knife to form locally exposed substrate, corrosion was well suppressed by formation of new type of dense protective films for each substrate which consist of oxygen, magnesium, aluminum and phosphorus. Anodic polarization curves indicate that the anodized surfaces show sacrificial function due to the thermodynamically unstable state of phosphorus in the anodized layers and its resulting release of electrons. From the viewpoint of kinetics in corrosion on the anodzed surfaces, the curves show that the anodized layers dissolve quite slowly into the electrolyte compared with the case of the untreated substrates. The excellent corrosion protection obtained by the anodization is considered to be based on the formation of a dense protective film on the exposed area, as well as sacrificial function of the amorphous anodized layer.
To study the corrosion property of high strength type304 stainless steel for automotive applications in a chloride environment, cold rolled type304 stainless steel pipe with shot peening were used. The corrosion property of a sample, a pipe with a crevice between the outside and an O-ring and in which a press fitting part was inserted to create a tensile stress, was evaluated in an automotive field test in Okinawa. Cracking from a corrosion pit was observed in the crevice. It was thought that pitting corrosion was caused by chloride (from sea salt) concentrated in the crevice. The crack occurred in the residual compressive stress layer created by shot peening. A crack generated at a corrosion pit was reproduced in a wet/dry cyclic corrosion test after one flash of artificial seawater. To investigate the crack generating mechanism, a corrosion pit was generated on the sample by cyclic corrosion test, after which a cathodic charge test in artificial sea water was done. Similar cracking from a corrosion pit was observed on the sample after this test. Therefore, the cracking is presumed to be Hydrogen Embrittlement-Stress Corrosion Cracking (HE-SCC).
A cold rolled type304 stainless steel pipe with shot peening was prepared. A crevice was created between the outside of the pipe and an O-ring. Cracking from a corrosion pit was observed previously on a sample inserted with a press-fitting part in a chloride environment, and it was presumed to be Hydrogen Embrittlement-Stress Corrosion Cracking (HE-SCC). Using the samples with no press-fitting parts, cyclic corrosion tests were conducted in this work. To estimate the hydrogen density at the initiation point of crevice corrosion and cracking, the hydrogen amount diffused from the corrosion pit was measured using Thermal Desorption Spectroscopy (TDS), and the number of pits was measured. The average hydrogen quantity in a pit was derived, and diffused hydrogen density was calculated using a simulation model which is based on diffusion in a half sphere shape (simulating diffusion of hydrogen at the pit surface). The calculation results show that at 0.3 ppm of hydrogen density, Hydrogen Embrittlement can occur.
In the field of electronics, it is crucial to guarantee long-term joint reliability of the Sn-Ag-Cu solder alloy. Creep tests in low-stresses are necessary to guarantee the reliability. However, a very long period of time is required for the creep tests. To decide a steady-state creep strain rate, an approximate equation of a creep curve was newly applied to the helical spring creep test known as a method of a low-stress creep test. Obtained results are as follows. (1) The torsional strain component is decreased with the deformation of the helical spring shaped specimen. Therefore, the steady-state creep rate cannot be detected in the helical spring creep test. The approximate equation of a creep curve introduced by Li's group was found to be effective to obtain the steady-state creep rate for Sn-based solder alloy. (2) The helical spring creep testing method that uses the torsional deformation of the test specimen is more effective for the Sn-based solder alloy with a strong anisotropy of strength than the uniaxis creep testing method. (3) In the heat-treated Sn-3.0 mass%Ag-0.5 mass%Cu solder alloy, the creep stress exponent changed from 19 into 1.1 on the boundary of about 14 MPa. This result implies that the creep deformation mechanism was changed from the dislocation creep with dispersion-strengthening to the grain boundary sliding. This phenomenon was found for the first time by using the helical spring creep testing method.
The microstructure of bronze mirror fabricated in the Korai period (10~14th centuries) has been investigated. Transmission electron microscope, X-ray diffraction, scanning electron microscope, electron dispersive X-ray analyses are used to determine the structures of the specimen. The composition of the specimen is Cu-15.2 mass%Pb-9.3 mass%Sn-0.5 mass%S, and Fe and As are detected as trace elements. The optical microstructure of the mirror consists of αCu, Cu41Sn11 (δphase), Pb and a phase containing Cu and S. The phase containing S has never been observed in Japanese mirrors or Chinese mirrors. This phase agrees with Cu2S listed in JCPDS No. 33-0490. Most of the Cu2S grains exist around Pb grains. The PbS nano crystal containing Cu phase that does not correspond to any known compound is observed between the Pb and the Cu2S grains. It is thought that Cu2S is an intermediate product matter in the refinement of copper ore, chalcopyrite (CuFeS2).
The paper painted with green pigment containing Cu discolours and its mechanical properties also deteriorate. The microstructure and decomposition of the green pigment have been investigated. Specimens are obtained from a hand-coloured wood-block print from the late Edo era. A transmission electron microscope and EDX spectrometer are used to analyse the green pigment and changes in the quality of the pigment. The green pigment is polycrystalline copper acetoarsenite Cu(C2H3O2)2•3Cu(AsO2)2, and contains Cu-As-O compound, the composition of which is different from that of copper acetoarsenite. Small crystals grow radially and form a spherical and its crystal. Cu2O, As-S compounds and the Cu-As-O compound produced in the decomposition process of copper acetoarsenite are detected around the discoloured cellulose fibers. Of these, it is considered that the coloured Cu2O and As-S compounds stain the fiber.
The effect of continuous rotation evolutional control (CREO) on the pitting corrosion resistance of anodized Al-Mg alloy was investigated by electrochemical techniques in a solution containing 0.2 mol/L of AlCl3 and by surface analysis. The potentials for pitting corrosion of Al-Mg alloy was evidently shifted to the less noble direction by CREO and the time required before initiating pitting corrosion was shorter with CREO, indicating that the corrosion resistance was worse with CREO than without. The precipitates of Fe-Al intermetallic compounds remained in anodic oxide films of Al-Mg alloy. Cracks occurred in the anodic oxide films through the precipitates during initial pitting corrosion. The pitting corrosion was accelerated by cracks. The internal stress present in the anodic oxide films was higher in the alloys with CREO than without. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from the CREO.
Shot peening is a surface treatment effective to obtain compressive residual stress on shot peened surface and to improve fatigue strength, and it is applied to automobile parts such as springs and gears. Based on recent trend for further reduction of automobile parts weight, higher hardness of automobile parts is required through various treatments such as carburizing. In this study, the effects of the amount of B addition on properties of gas atomized Fe-8Cr-B(mass%) powders for shot peening were examined. Including Fe-8Cr-6.5B as base composition, alloys with the amount of B addition from 4 to 9.5 mass% were gas-atomized and examined. The amount of Fe2B phase and Vickers hardness increased with the increase of the amount of B addition, and high hardness of 1200HV was obtained in alloys with more than 6.5 mass% B addition. Density decreased with the increase of the amount of B addition, and 7.14 Mg/m3 was obtained in the alloy with 9.5 mass% B addition. No crack initiated near indentation by 1000 g load in the alloys with the amount of B addition from 5.5 to 6.5 mass%.