Fatigue strength of notched specimen and smooth specimen of Zr-based bulk metallic glass, Zr55Cu30Ni5Al10, were conducted under either plane-bending or axial-loading. Fatigue notch factor, Kf, for axial-loading was almost equal to the elastic stress concentration factor, Kt, and the value of Kf for plane-bending was 5% lower than the value of Kt. It indicates that the effect of stress gradient around notch root is negligible, and the fatigue limit of notched specimen is determined by the stress at the notch root. On the contrary, the fatigue strength and the fatigue limit were lower for axial-loading than plane-bending either for notched or smooth specimen. It is considered that the fatigue strength of BMG depends on the cooling rate in the casting process of the material, which is different along the thickness direction, and the fatigue strength of plane-bending reflects the strength at the specimen surface while that of axial-loading is determined by the weakest strength in the thickness direction.
As-cast bulk metallic glasses fracture without plastic strain under tensile loading at room temperature. However, it is inferred from molecular dynamics calculation that the yield surface of the glass is similar to that of one of the ductile metals. In this study, uni-axial tensile, uni-axial compressive, and simple shear tests were carried out for zirconium-based bulk metallic glass at room temperature and the yield surface (the fracture surface) was inspected experimentally. As a result, the deformation of the zirconium-based bulk metallic glass at room temperature is dominated mainly by shear stress just like in the case of a ductile metal.
Both tensile and compressive tests of Zr70Ni16Cu6Al8 BMG were carried out under a strain rate of 10-4, 10-2 and 10-1 s-1 to investigate the effect of strain rate on plastic deformation. As the results, it was confirmed that there are the different effects of strain rate on tensile and compressive plastic deformation. In tensile test, although plastic deformation did not occur under strain rate 10-4 s-1, clear plastic deformation occurred under 10-2 and 10-1 s-1. In high strain rate condition, some shear bands were observed on the specimen surface. On the other hand, in compressive test, the BMG showed some plasticity in all strain rate conditions. Particularly, large plastic deformation occurred under 10-4 s-1. Furthermore, many multiple shear bands were observed the specimen surface under 10-4 s-1 in comparison with those under 10-2 and 10-1 s-1.
Zr50Cu50-XAlX (X = 2 to 16) and Zr90-YCuYAl10 (Y = 30 to 45) bulk metallic glasses (BMGs) have been prepared by tilt casting techniques. The viscosity of the supercooled liquids has been measured by using a penetration viscometer with a column-shaped tungsten indenter with 0.5mm in diameter and an applied load of 0.020N at a high-speed heating at a rate of 400K/min in a highly-purified He-gas atmosphere. In the Zr50Cu50-XAlX (X = 2 to 16) system, the viscosity of the supercooled liquids exhibited almost the same values when the Al-content was increased from 2 to 10 at. %. When the Al-content increased to 12 at. %, the viscosity of the supercooled liquids increased largely. A large amount of the ZrCu (B2) phase after crystallization of this BMG system was detected when the Al-content was 10 at. % and below. While, when the Al-content was increased to 12 at. %, the main precipitation phase was changed to the ZrCu (B19') phase. Similar increase in the viscosity of the Zr90-YCuYAl10 (Y = 30 to 45) supercooled liquids was also observed when the Cu-content was increased from 30 to 45 at. %. This may be due to the strong relationships between the compositional dependence of structure and the viscosity of the supercooled liquids in these BMG systems.
The freeze drying process for hardened bodies of cement consists of a freezing step by liquid nitrogen and a drying step by vacuum. The use of liquid nitrogen during the process involves the risk of cracking due to heat impact. The authors made improvement to existing freeze-drying equipment to minimize heat impact on specimens and tested various freezing conditions and drying temperatures in the current study. The specimens used in this experiment were hardened bodies of hauyne-containing cement with ettringite, which was susceptible to structural destruction by drying. As the result of comparative study, the authors determined optimum conditions for drying where peak area measured by the powder X-ray diffraction remained almost unreduced successfully. This revealed that the proposed conditions were almost non-altering to ettringite. The pore size distribution of the specimens prepared by this method was found to differ significantly in major pore diameter and total pore volume from those of specimens dried by other common methods : D-drying or drying at 105°C. The pore size distribution data for ettringite obtained by the freeze drying under the conditions proposed in the current study further allowed prediction that the data would represent the extremely fresh condition in which most of ettringite remained intact.
An exposure test was conducted on test specimens simulating marine structures to which cathodic protection was applied. Test specimens of seawater resistant stainless steel plates and those of carbon steel plates exposed to the tidal zone environment were analyzed. We measured the density of cathodic current flowing into the test specimens, corrosion rate of the specimens and weight loss of sacrificial anode. The cathodic current density was influenced by the reduction of rust and calcareous deposits. Weight loss of the anode calculated using cathodic current was found to be approximately the same as that obtained from the experiment. Weight loss of the anode for seawater resistant stainless steel as a test specimen was approximately 60% for those of carbon steel plates. Furthermore, we investigated the design method for cathodic protection taking into account the current flowing into the plate placed at the tidal zone.
A cold spray process has been developed for dense and clean and tightly bonded coatings in contract with other established thermal spray processes. The cold sprayed copper is particularly well suited for metallic coatings, such as electric conductor and heat-sink. However, the thermal and electrical properties of copper coatings formed by the cold spray process have not always been clarified. Four kinds of free-standing copper specimens, which were formed by the cold spray process and an atmospheric plasma spray process, were machined from the thick coatings and were used for measuring the thermal conductivity, specific heat and electrical resistivity etc. The experimental results suggested that the porosity and oxide content had important effects on the thermal and electrical properties. Namely, high thermal conductivity and the low electrical resistivity of coatings could be obtained from the cold spray process, because of the low porosity and the low oxide content. Also, it was confirmed that the thermal conductivity of cold sprayed copper could be improved at high temperature by the sintering effect.
In this study, we develop an extended version of homogenization theories for composite materials consisting of inclusions and a matrix. The extension allows discrete dislocations to move in a matrix based on dislocation dynamics. Macroscopic stress-strain response and microscopic stress distribution in representative volume elements (RVEs) are analyzed on the assumption that the periodic arrangement of RVEs is subjected to macroscopic uniform deformation. The periodicity, which enforces the periodic distributions of microscopic stress and strain as well as dislocations, is used as boundary conditions on the surface of RVEs. It is shown that elastic fields of periodically arranged dislocations in an infinite medium have no contribution to macroscopic relations, and that discontinuous displacements resulting from dislocation motions in each RVE are introduced as a macroscopic plastic strain. From these relations, a set of homogenization equations is derived, in which two boundary value problems in integral form are solved for analyzing perturbed displacements due to the presence of inclusions and due to the interaction of inclusions with dislocations, respectively. Finally, the influence of periodic boundary conditions on macroscopic and microscopic responses is investigated by performing the RVE analysis of a model composite.
Homoepitaxial growth on gallium nitride substrate is simulated by molecular dynamics (MD) method. Crystal growth configuration is evaluated qualitatively according to the atomic configuration and the radial distribution function. Moreover, crystal structure is evaluated quantitatively by considering variance of local atomic density as a monitoring index. As a result, we found that the growth layer is formed two-dimensionally and like film formation when the substrate temperature is high. It is found that the dynamics of crystal growth can be understood from not only variance value but also gradient of temporal evolution of variance.
This paper describes a molecular-dynamics study on the relationship between lattice mismatch and adhesion strength of interfaces between organic materials and ceramics. Aromatic resins with benzene-ring (phenyl-ring) connected structures are used as examples of organic materials. A lattice constant of the aromatic resins is defined as the distance between the second-nearest-neighbor carbon atom pairs of a benzene ring (phenyl ring). The value of the lattice constant of a wholly aromatic polyester resin is about 0.24 nm. On the other hand, a lattice constant of ceramics is defined as the distance between the nearest neighbor atoms in the crystal plane connected with the resin. The lattice constants of the SiO2(111) and TiO2(111) planes are about 0.253 nm and 0.295 nm, respectively. The adhesion of aromatic polyester resin with the SiO2(111) plane is stronger than that with the TiO2(111) plane because the lattice mismatch of the resin with the SiO2(111) plane is smaller than that with the TiO2(111) plane. Reducing the lattice mismatch is found to be effective in strengthening the adhesion.