The structural and thermoelectric properties of rare-earth (Sm)-substituted Ruddlesden-Popper-type Sr3Fe2O7, Sr3-xSmxFe2O7 (SSFO, x = 0, 0.1, 0.15), were investigated by measuring the temperature dependence of its crystallographic parameters, electrical conductivity, and Seebeck coefficient (5). A sharp increase in electrical conductivity (sigma) up to 60 S/cm (log sigma = 1.78 S/cm) at 400-500 degrees C occurred as a result of oxygen reduction in the x = 0 sample. S values changed from negative to positive with increasing temperature, presumably due to the increase in the number of oxygen vacancies, resulting in Fe ion hopping conduction. The introduction of Sm increased both S and sigma, and the power factor of the oxide prepared with x = 0.1 was approximately fivefold higher than that of the oxide with x = 0.
The causal relation between cross-slip structure and brittle-ro-ductile transition has been examined using NaCl single crystals with the [100] axis. Ar around room temperature, the crystals exhibit cleavage fracture after a few percent elongation, although their yield stress is lower than 2 MPa. When the temperature is raised, brittle-to-ductile transition takes place at around 400 and 500 K with the respective strain rates of 5.5 x 10(-6) and 5.5 x 10(-5) s(-1). The transition temperatures correspond well to those for the abundant operation of cross slip. The cross-slip lines are not parallel to a specific crystallographic plane, but are widely distributed in angles of 10 degrees similar to 40 degrees from the slip line of the primary plane (10(1) over bar) on the (001) surface. Even in high magnification images observed using an UHV-AFM, cross-slip lines appear to be wavy, indicating the cross slip on indefinite planes. Dislocations emitted from a crack tip can extend sideways along a moving crack front by the cross slip on indefinite planes, and reduce local stress intensities to impede the crack advance by crack tip shielding due to dislocations.
Since the micro/macro-structures affect the critical current density (Jc) and the mechanical properties of superconductive YBCO oxides, the following numerical and analytical studies of solidification process of faceted 123 (YBa2Cu3O7−X) crystals from liquid+211 (Y2BaCuO5) phases are essential to clarify the solidification mechanism and improve the properties of YBCO . To clarify the effects of growth mode and conditions on the microstructures of 123 crystals, two-dimensional numerical simulation of faceted peritectic growth of 123 crystal was performed by considering (a) growth of 123 crystal, (b) melting of 211 particles in the liquid, and (c) solute diffusion in the liquid. The growth rate (R) of 123 crystal was approximated by: R=ag·ΔTk2, where ag was kinetic growth constant, and ΔTk was kinetic undercooling of faceted interface. The kinetic melting constant (am) and superheating (ΔTm) was also used for evaluation of melting rate of 211 phase. Solute distributions in the liquid during the 123 growth were calculated by FDM, and the distributions of residual 211 particles and liquid pools in the faceted 123 crystals were evaluated from the experimentally obtained log-normal distributions of 211 particles in the liquid of YBCO . The calculated results agreed well with the experimental ones. Transition of macrostructures from columnar to equiaxed 123 crystals in unidirectionally solidified YBCO was also studied experimentally and analytically. Critical transition conditions (:relations of growth rate (R) and temperature gradient (G)) were calculated by equations obtained from nucleation and growth theories, and compared with experimental results.
The corrosion resistance of dental alloys was investigated by taking anodic polarization measurements in 1 mass%-lactic acid, artificial saliva, and cell-culture medium solutions. The role of alloying elements in the passive film that formed on the dental alloys by the anodic polarization was examined using X-ray photoelectron spectroscopy. The quantity of metals released from the dental alloy into the 1% lactic acid solution at each anodic potential was compared. In the anodic polarization curves fur the Au and Au-Ag-Pd alloys, the current density tended to decrease with higher Au content. In the Ag alloy, the current density sharply increased in the 1 mass%-lactic acid solution. On the other hand, the passivity zone was slightly seen in the artificial saliva and Eagle's medium solutions. Amalgam had a Low open-circuit potential, and a passivation peak was seen. The peaks of the Au4f and Ag3d orbital were high in the surface of the passive film formed oil the Au and Au-Ag-Pd alloys by anodic polarization in the artificial saliva solution. SnO2 and In2O3 peaks were observed in the passive film formed on the Ag alloy. SnO2 and CuO peaks were seen in the passive film on amalgam surface. In the Au and Au-Ag-Pd alloys, Cu was released most in the low potential region less than 1.0 or 0.5 V us. SCE, respectively. For the Ag alloys, Zn release was most, and Ag, Sn and In were also released. In amalgam, Sn release increased with higher anodic potentials. Cu release also increased at potentials over 0 V. Ag and Hg release increased from potentials over 0.5 V. Considering that the electrode potential measured in the pseudo-oral environment is a maximum of 0.2 V, it is important to examine the effect of Cu and Ag releases far the Au and die Au-Ag-Pd alloys, Zn release for the Ag alloy, and Sn and Cu releases for amalgam.
The influence of a plastic zone developed at a crack tip is analyzed under a micromechanical point of view. Mechanical interaction is described in terms of crack shielding caused by the stress intensity factor induced by a dislocation loop in the three opening modes. Numerical calculations and experimentally studied configurations quantify the total relaxation rate induced by this dislocation loop.
This paper reviews fracture features of a variety of conventional and advanced materials which consist of more than two different phases in terms of composite material. To be concrete, the materials include ductile cast irons, carburized steels, aluminum alloys, and several kinds of discontinuously-reinforced composites. all of which have been studied by the present authors to date. Firstly, features on fracture of the above-mentioned three conventional materials are introduced. Those have constructions common to artificial composites: particle reinforced composite and layered composites. In the latter half of this paper, the detailed mechanisms of deformation and fracture in artificial composites and those analyses mainly on the basis of continuum mechanics and fracture mechanics are shown. Deformation and fracture behaviors of "natural" (or in-situ) and artificial composites are discussed in terms of their similarities and discrepancies both from metallurgical and mechanical points of view in order to bring systematic understanding of the materials having complex microstructures. In addition, it is shown that certain phenomena that already constitute common knowledge in the field of composite materials, should be strongly considered in the field of conventional materials.
Surface morphology changes of silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composite materials occurring after simultaneous triple-ion-beam irradiation were studied. Irradiation tests were performed with helium (He) ions, hydrogen (H) ions, and self-ions (carbon (C) ions or silicon (Si) ions). The peak displacement damage was 10 dpa (displacements per atom), and the irradiation temperatures were 600, 800 and 950 degreesC. The concentrations of He and H at the damage peak region were 1000 atomic ppm and 385 atomic ppm, respectively. Observations of the irradiated surface and the measurement of morphology changes were performed. The shrinkage of SIC fibers and the apparent shrinkage of the interfacial material, carbon, between the matrix and the fibers at the irradiated surface were observed. These phenomena were mainly attributed to displacement damage caused by irradiation.
New Fe-based glassy alloys of Fe45Cr16Mo16C18B5 Fe45Cr16Mo14Nb2C18B5 and Fe5Cr16Mo14Ta2C18B5 were synthesized. They exhibit a large supercooled liquid region (DeltaT(x)) reaching 58 K before crystallization and high reduced glass transition temperatures (T-g/T-m) up to 0.62. These values indicate that these Fe-based alloys have a glass-forming ability which is high enough to enable die formation of bulk glassy alloys. The Fe-based glassy alloys have high corrosion resistance in 1, 6 and 12 mol L-1 HCl solutions at room temperature. HCl corrosion rates are in the range of 10(-4)-10(-2) mm.y(-1). The glassy alloys are spontaneously passivated in 1 and 6 mol.L-1 HCl solutions with wide passive region and low passive current density in the range of 10(-4)-10(-2) A.m(-2). No pitting corrosion is seen even in 12 mol.L-1 HCl solution. The addition of Nb or Ta to the glassy alloys is effective on enhancing the corrosion resistance.
Direct observation methods were successfully applied to investigate the pore structure in green and sintered bodies of alumina ceramics made through a powder granule compaction process. Fracture strength was measured for alumina sintered bodies prepared under three different processing conditions, using granules made through the same procedure. The strength variation among the sintered bodies was quantitatively correlated to the difference in the size and concentration of large pores determined by the observation methods. These pores were developed during sintering.
Dislocation structures near the tip of a crack in ionic crystals with the NaCl-type structure were investigated by using an etch-pit technique, photoelasticity and high voltage electron microscopy (HVEM). The characteristics of those dislocation structures are reviewed and their effect on the local stress intensity factor is discussed. we focused on two kinds of plastic zones developed near a crack lip. Firstly, in hulk NaCl crystals, the so-called hinge-type structure of slip bands were formed along {110} planes ahead of a {100} crack. The effect of crack tip shielding due to dislocations was demonstrated using a photoelastic method visualizing the internal stress held around a crack tip. Secondly. dislocation configurations in MgO thin crystals were analyzed using HVEM, where dislocation arrays of the {110}[1 (1) over bar0] slip system corresponding to the plastic zone of the 45 degrees -shear-type were formed around a {100} crack. 3-D stress analyses for the crack-dislocation interaction indicates that the dislocations observed contribute to mainly the mode I shielding to suppress the crack extension.
A new generating method of compression waves in a liquid metal has been proposed in which a static magnetic field and an alternating current are simultaneously imposed. The theoretical expressions of intensities and distributions of pressure and velocity accompanied with the compression waves have been derived. The pressure change in liquid gallium excited by the method proposed here was measured under different intensities of the magnetic field and the alternating current. The measured pressures approximately agreed with the theoretical evaluation. The structure of a Sn–Pb alloy that was solidified under the imposition of the compression waves, was completely refined.
The effect of hydrogen on the shear localization and associated crack nucleation has been investigated by means of a three point bending test of hydrogen-charged steels. The ductile crack growth resistance in terms of the slope of R-curve was lowered under the presence of hydrogen, the decrease being more pronounced in the steel with more abundant slip constraint phases along grain boundaries. Enlargement of size and reduction in depth/width ratio of primary dimples, occasionally associated with quasi-cleavage, were observed on the fracture surface of the hydrogen-charged steels. By means of a FEM calculation, the increase of the nucleation void volume fraction localized at the crack tip with strain localization as well was shown to take place in the hydrogen-charged steel in consistent with enhanced shear instablity. It was discussed that the evolution of vacancy-type defects, rather than void nucleation at second phase particles, in the course of plastic straining was enhanced under the presence of hydrogen, reducing the ductile crack growth resistance.
The intrinsic threshold behavior of fatigue cracks and the disappearance of any cyclic plastic deformation below a threshold value can be understood by taking into account the discreteness of plasticity with recourse to discrete dislocation models. The aim of this paper is to document the progress in the discrete dislocation modelling within the past twenty years and the resulting increase in the understanding of fatigue cracks. The problems addressed are (1) the nature of the intrinsic fatigue threshold, (2) the influence of microstructure and/or of the mean stress level on the crack tip deformation and (3) the physical reason for the minimum striation spacing at small stress intensity ranges. A particular purpose of this paper is to compare the different dislocation models proposed in the literature in order to differentiate aspects of fatigue crack growth that do and do not depend on modelling and on microstructural details.
A modified centrifugal combustion synthesis process has been developed that enables precisely casting synthesized materials and simultaneously joining them to a dissimilar metal. The material to be synthesized was a Ni-25 mol%Al alloy and that to be bonded was a stainless steel, an ultra-low carbon steel, pure nickel or a Ni-25 mol%Al alloy. The base material to be bonded; a graphite mold; and a green compact of reactants consisting of Al, Ni and NiO were set in a centrifugal easter. When the combustion synthesis reaction was induced in the centrifugal force field, synthesized molten NI-A1 alloy flew into the mold and collided with the base material. This process was successfully applied in joining the synthesized Ni-Al alloy and various base materials. Centrifugal force was also confirmed to assist the molten Ni-Al alloy fill the mold cavity and adhere to the surface of the base materials.
Cyclic deformation of ultra-fine grain (UFG) materials processed by severe plastic deformation is reviewed in light of recent experimental results and common concepts of fatigue. High strength bulk metals with a characteristic structural element size of 200-300 nm were obtained through the so-called equal-channel angular pressing (ECAP) technology. Fatigue properties are discussed in terms of stress-controlled and strain-controlled fatigue. Enhancement of fatigue life under constant stress amplitude is emphasized in comparison with some shortening in fatigue life under constant plastic strain amplitude. Fine structure and surface morphology of post-fatigued materials are characterized on different scale levels to account for the fatigue behaviour observed. Mechanisms of fatigue in EGA-processed materials are discussed within frameworks of a simple one-parameter model of dislocation kinetics.
A reactive plasma are melting process using a mixture of Ar and N-2 as the carrier gas for titanium and aluminum powders has been applied to synthesize nitride particle reinforced Ti-34mass%Al intermetallic matrix composites (IMCs). This technique successfully allowed to produce in-situ nitride particle dispersed IMCs and the volume fractions of nitride increased from 6 vol% to 54 vol% with increasing mixing ratios of N-2 gas from 10 vol% to 100 vol%. In the IMCs both rod-like Ti(2)AIN and coarse two-phase nitride particles consisting of the core of TiN and the outer shell of Ti2AlN were formed in the matrix of a full lamellar structure or a lamellar containing small amount of equiaxed gamma phase structure. The IMCs had significantly fine grains of which size was about 1/4 that of the unreinforced Ti-34 mass%Al. The Rockwell hardness of the IMCs increased abruptly from 36.5 to 48.4H(RC) with increasing volume fraction of nitride. Unlike the hardness, the tensile strength of the IMCs had a maximum value of 507 MPa, which was approximately 170 MPa higher than that of the unreinforced Ti-34 mass%Al, at 13 vol% nitride, beyond which the strength decreased. The strengthening is derived from complex reinforcing effects of both direct strengthening effects of nitride particles due to the interaction of dislocations with the particles and grain refinement. The degradation in tensile strength at higher volume fractions of nitride is considered to be attributed to higher population of clustered coarse two-phase nitride particles, which can act as crack initiation and propagation sites. As for the strength at elevated temperatures, the IMC with 13 vol% nitride had higher tensile strengths than the unreinforced Ti-34 mass%Al by 100 MPa at 1173 K and 61 MPa at 1273 K.
In order to develop a new kind of medical implant material, the microstructure and mechanical properties of cast Ti-Si alloys were investigated using smalt-size ingots prepared by a dental casting machine. The results show that the addition of silicon significantly changes the microstructure of titanium alloys. The Ti5Si3 intermetallic compound precipitation occurs in die matrix of alpha and beta phases, when the silicon content is over 1.33 mass%. The compound is observed as a netted structure around grain boundaries of me titanium matrix when die silicon content exceeds 2.35 mass%. In addition, the Ti-Si alloys show a good combination of strength and ductility in a wide range of silicon content hi contrast to the purl titanium and Ti-6Al-4V alloys. The cast Ti-Si alloys re promising candidates for dental applications because of a good balance between strength and ductility.
The effect of eutectic modification by strontium on nucleation and growth of the eutectic in hypoeutectic Al-Si foundry alloys has been investigated by electron back-scattering diffraction (EBSD) mapping. Specimens were prepared from three hypoeutectic AlSi base alloys with 5, 7 and 10 mass%Si and with different strontium contents up to 740 ppm for modification of eutectic silicon. By comparing the orientation of the aluminium in the eutectic to that of the surrounding primary aluminium dendrites? the growth mode of the eutectic could be determined. The mapping results indicate that the eutectic grew from the primary phase in unmodified alloys. When the eutectic was modified by strontium, eutectic grains nucleated separately from the primary dendrites. However, in alloys with high strontium levels, the eutectic again grew from the primary phase. These observed effects of strontium additions on the eutectic solidification mode are independent of silicon content in the range between 5 and 10 mass%Si.
