An approximate analytical description of the unidirectional growth of arrays of needles is proposed for the solidification under high temperature gradients. Temperature and solute distribution for a needle are described by use of functions with exponential increase/decrease and integral exponential functions. Those for arrayed needles are described by the addition of distributions. Then, a modeling of the growth of arrays of needles is developed in order to predict needle dimensions. Local equilibrium conditions are applied to determine the tip radius and unknown coefficients included in descriptions of temperature and solute distribution. Minimum undercooling of needle tip is also applied to select the primary spacing of growing needles. Predicted dimensions are studied and compared with those given by other theoretical predictions. The model predicts the growth rate of transition from planar interface to cellular, which is very close to the growth rate given by the Mullins-Sekerka theory. The predicted dependency of tip radius of curvature on growth rate is similar to the dependency given by the Kurz-Fisher model for needle growth.

View full abstract

Predicted needle dimensions given by the model proposed in part I for the growth of arrayed needles under solidification with a high temperature gradient are compared with those given by solidification of alloys; Al–Cu, Al–Fe and SCN–Argon. Correspondences between predicted primary arm spacings and experimental ones are shown to be very high for arrayed solidification of needles under high temperature gradients.

View full abstract

Oscillation modes of the solid-liquid (S/L) interface using a non-linear equation for thermal transfer are analysed. Thus, the solidification is achieved by dark cnoidal oscillation modes and the melting by bright cnoidal oscillation modes of this interface. We show the S/L interface self-structurates as a non-linear Toda lattice and then specify some of its properties.

View full abstract

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 Al–Si 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.

View full abstract

The fabrication techniques using a droplet, which is formed at the nozzle tip of an injector-like crucible by pushing a melt from the crucible and a piston rod, is called “the suspended droplet method”. The changes of grain size, dendritic morphology and macrosegregation which dependent on the droplet size are investigated in high- and low-purity aluminum and Al–0.5 mass%Cu alloy. A single crystal might be formed in the high-purity Al when the droplet size becomes as small as 1 mm in diameter. As the purity of Al decreases, polycrystals are formed even though the droplet size decreases. The sample of Al–0.5 mass%Cu alloy shows coarse dendrites, which gradually degenerate as the droplet size decreases. Additionally, the profile of the Cu concentration becomes uniform as the droplet size decreases.

View full abstract

Since the micro/macro-structures affect the critical current density (J_c) and the mechanical properties of superconductive YBCO oxides, the following numerical and analytical studies of solidification process of faceted 123 (YBa₂Cu₃O_7−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·ΔT_k², where ag was kinetic growth constant, and ΔT_k was kinetic undercooling of faceted interface. The kinetic melting constant (am) and superheating (ΔT_m) 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.

View full abstract

The effect of the volume fraction of the solid phase (V_f) on coarsening phenomena of Y₂BaCuO₅ (Y211) particles in Ba–Cu–O melt has been investigated. The volume fraction was changed from 10% to 40% by means of controlling of the nominal composition. The size distribution depends on the nominal composition as well. In the samples with hyperperitectic composition (V_f>37%), two different size groups were recognized. On the other hand, it was not observed in the case of hypoperitectic compositions (V_f<37%). The coarsening rate constant (K) also strongly depends on V_f and classified into two groups even in the hypoperitectic compositions. The samples with lower V_f than 15% reveal a higher coarsening rate constants than those with higher V_f than 20%. The phenomena were qualitatively explained by the difference of coarsening mechanisms. Although the zero V_f assumption in a mean field model can be applied for the samples with lower V_f, it became difficult with increasing of V_f and the diffusion mode for the coarsening is changed from the diffusion through a mean field liquid to those among particles.

View full abstract

Yttrium-iron garnet, Y₃Fe₅O₁₂ (YIG), is formed via the peritectic reaction of YFeO₃ (YIP) and liquid phases. The direct growth of YIG from a highly undercooled melt with stoichiometric YIG composition was studied using an aero-acoustic levitator with a CO₂ laser heating system. Although a YIG droplet was successfully levitated and undercooled to 1327°C, about 250°C below its peritectic temperature (T_P), YIP was primarily solidified in the microstructure of dendritic YIP with interdendritic eutectics of YIP and FeO_X. However, when a droplet was quenched after it was undercooled below T_P, single phase of YIG without YIP and FeO_X was found. The X-ray diffraction pattern as well as microstructure indicated that YIG was solidified directly and congruently from the undercooled melt, bypassing the peritectic reaction.

View full abstract

There are two eutectic reactions in the Al₂O₃-rich portion of the Al₂O₃–Y₂O₃ pseudo-binary system; one is the Al₂O₃-YAG (yttrium-aluminum-garnet) equilibrium eutectic reaction, the other is the Al₂O₃-YAP (yttrium-aluminum-perovskite) metastable reaction. The selection of eutectic systems was examined, in terms of maximum melt temperatures before solidification and of cooling rates, for specimens in a composition range of 13.5 to 28.5 mol%Y₂O₃. YAG nucleation did not occur at a cooling rate of more than 5 K/s when eutectic composition specimens were cooled from 2173 K . Heating of the melt to temperatures above 2273 K also inhibited YAG nucleation. The inhibition of YAG nucleation led to the production of undercooling melt below the metastable eutectic temperature, and consequently resulted in a metastable solidification path. Selection of the eutectic system was controlled by YAG nucleation. At around 2273 K no heat release was detected by optical DTA measurement, although the effect of the maximum melt temperature on YAG nucleation suggested some change in the melt at about 2273 K.

View full abstract

A reactive plasma arc melting process using a mixture of Ar and N₂ as the carrier gas for titanium and aluminum powders has been applied to synthesize nitride particle reinforced Ti–34 mass%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 N₂ gas from 10 vol% to 100 vol%. In the IMCs both rod-like Ti₂AlN 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 γ 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 48.4 H_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.

View full abstract

Directional solidification studies were carried out in continuous alumina fiber reinforced Al–4.5 and 15 mass%Cu alloy composites in order to clarify the influences of fibers on crystal growth of matrix alloy in the composites. The specimens were designed to have an inner composite region and an outer unreinforced bulk region in order to characterize the dendrite morphology in both regions under the same conditions. The composite specimen was produced by a pressure infiltration process, subsequently remelted, and then directionally solidified. In the composite region of the specimens, the shape of the dendrite was distorted by the presence of fibers, with the primary dendrite tip position being located about 450–750 \\micron below that in the bulk region. The difference of undercooling was estimated to be 1.2–1.7 K . The concentration of copper at the dendrite tips was 0.05–0.15 mass% higher than that in the bulk region, and the tip composition increased as the fiber interstice became smaller. The solutal undercooling was estimated to be 0.9–2.7 K, so that solutal diffusion field around dendrite tip could have governed the dendrite tip undercooling. Furthermore, in the composite region, the primary dendrite arm spacing decreased to about 70% of that in the bulk region on an average. A model based on the continuity of liquid phase among fibers reveals how fibers influence both the concentration of copper on the dendrite tips and the size of the lateral solute diffusion field.

View full abstract

Numerical simulation was applied to evaluate the thermal and solutal influences of fibers on the dendrite growth of matrix alloy reinforced with continuous fibers. The results of temperature and solute distribution, and dendrite tip undercooling and tip radius were compared with experimental data obtained from directional solidification studies for polyvinylidene fluoride, Pyrex glass, and copper fiber reinforced succinonitrile-acetone alloy composites. In the polyvinylidene flouride fiber/pure succinonitrile composites, dendrite in the composite region grew behind that in the bulk region, and in the direction of the heat flow. However, dendrite between copper fibers grew faster than that in the bulk region. A computer simulation revealed that a difference in thermal diffusivity influences the thermal distribution of specimens and the apparent dendrite tip location. The gap in dendrite tips between the composite and bulk regions increased as the composition of acetone increased or the fiber interstices became smaller. Numerical analysis revealed that tip composition and undercooling increased as the fiber interstices became smaller than the primary dendrite arm spacing. The constraint of growth on the secondary arm and the change of dendrite morphology were also shown in the analysis.

View full abstract

Extremely superheated liquid of high-melting-point NiAl-base intermetallic alloys have been produced and cast into a cylindrical bar using the reactive casting method, which is based on the exothermic self-propagating high-temperature synthesis (SHS) reaction between elemental liquids. When liquid aluminum of 1023 K and a molten nickel-cobalt alloy of 1773 K are mixed, they exothermically react and produce a cobalt-containing NiAl liquid with a temperature over 2300 K . The liquid solidifies into a B2-ordered β-phase intermetallic alloy. When the cobalt concentration of the alloy increases, the density, hardness, wear resistance, coefficient of thermal expansion and corrosion resistance to hydrochloric acid increase, while the thermal conductivity decreases. The effect of cobalt concentration on the oxidation resistance of the alloy to hot air is negligible.

View full abstract

Mechanically alloyed Ti–25 mol%Al powders were low pressure plasma sprayed in order to produce nanostructured α₂ intermetallic compound on mild steel. Rapidly quenched sprayed layers with various cooling rates were formed by changing the substrate temperature and spray distance. Mechanically alloyed powders with convoluted structure of pure Al and Ti melted in the plasma flame and completely reacted to form α₂ intermetallic compound. The relative density of the sprayed layer increased with the substrate temperature and an almost 100% dense layer was obtained in the case of a substrate temperature over 650 K . The microstructure of the sprayed layer consisted of equiaxed nano α₂ grains, of which size increased with substrate temperature as well. The grain size was reduced down to a minimum of about 200 nm at the substrate temperature of about 500 K . Effect of cooling rate on the grain size was estimated using Boswell’s model, which agreed well with the experimental results.

View full abstract

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.

View full abstract

In order to improve the quality of cast metal and to control solidification of metals, two new casting processes controlling metal solidification by imposing electromagnetic field are developed. One is simultaneous imposition of multiple-electromagnetic fields from the outside of a cold-crucible copper mold during continuous casting of Sn–4.5 mass% Pb alloy, and the other is the imposition of a rotating magnetic field during the unidirectional solidification to make in situ surface composite with special mechanical and physical properties. The experimental results show that multiple-electromagnetic fields can not only eliminate surface defects, but also improve solidification structure of cast metal. Moreover, a new kind of composite pipe and gear of Al–12.6 mass% Si eutectic alloy was made by imposing electromagnetic stirring during unidirectional solidification. This suggests a new method of making surface composite with special mechanical and physical properties. It is also found that a separated eutectic occurs in the anomalous eutectic and the separated phase is the leading faceted phase with solution entropy over 23 J/mol·K.

View full abstract

Sn–Ag binary alloys, with Ag content in the range between 0 mass% and 4.0 mass%, were examined in order to understand the effect of Ag addition on microstructural and mechanical properties of the solders. Fine Ag₃Sn fibrous precipitates form the Ag₃Sn/Sn eutectic network surrounding the β-Sn primary grains. Increasing Ag content produces finer precipitates and finer networks. Sn–4.0 mass%Ag has additional large Ag₃Sn primary particles. Thermal expansion coefficient of the alloy decreases with increasing Ag content. The 0.2% proof stress of Sn–Ag alloy increases with increasing Ag content up to 4.0 mass%Ag, and is higher than that of Sn–37 mass%Pb solder above 2.0 mass%Ag. In contrast, tensile strength increases up to 3.5 mass%Ag but decreases at 4.0 mass%Ag slightly. The formation of primary Ag₃Sn is attributed to the degradation at 4.0 mass%Ag. The wettability of the Sn–Ag alloys on Cu is slightly improved by the Ag addition but is worse than Sn–37 mass%Pb solder. Two intermetallic layers are formed at the interface, Cu₃Sn adjacent to Cu and Cu₆Sn₅ adjacent to the solder. The Cu₆Sn₅ layer is thicker than the Cu₃Sn layer and grows into the solder forming scallop shape. The thickness of the reaction layers slightly increases with increasing Ag content. The composition of Sn–(2–3.5 mass%)Ag is the best selection for obtaining high joint strength. Sn–Ag alloy is superior to Sn–37 mass%Pb solder for establishing a rigid interface.

View full abstract

Transient liquid phase (TLP) bonding process of Ni using Ni–11.0 mass%P binary filler metal was simulated by using a mathematical model based on diffusion analysis. In the model, diffusion-controlled transformation was assumed, and the base metal dissolution in the early stage and the subsequent isothermal solidification stage of the TLP bonding process were simulated. The calculated width of the eutectic structure at the bonding region agreed well with the experimental result and showed the validity of the model. In order to obtain the condition to reduce isothermal solidification time, the effect of factors, such as the width of filler metal, diffusivity of solute element in Ni and partition coefficient of element, were investigated by numerical simulation. The simulation showed that isothermal solidification time remarkably decreases with increasing partition coefficient of solute element. In order to confirm the numerical prediction, the TLP bonding experiment of Ni using Cu filler metal was carried out (the partition coefficient of Cu in Ni is close to unity). The experimental result showed that Cu filler metal remarkably shortened the isothermal solidification time and showed the validity of the prediction.

View full abstract

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; that to be bonded with the synthesized material 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 caster. When the combustion synthesis reaction was induced in the centrifugal force field, synthesized molten Ni–Al 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.

View full abstract

Semi-solid processing of alloys is becoming one of the key technologies for producing advanced materials. The present study aims to clarify mould filling characteristics and solidification structure in semi-solid processing of gray iron. The results indicate that the apparent viscosity and primary particle morphology have pronounced effects on the filling ability of semi-solid slurry. The particle segregation in the solidified sample is observed between the upper and lower part of the sample and is shown to decrease with the increase of stirring in semi-solid state. Finally semi-solid processed cast iron with the radial change of both the microstructure and mechanical property is obtained under the centrifugal effect acting on liquid/solid slurry.

View full abstract

Semisolid processing using laser irradiation was proposed to fabricate porous media. This processing is based on semisolid processing and rapid prototyping technology. In the processing, laser irradiation is used to produce the semisolid state in objective particles. The particles can remain their spherical shape due to high viscosity of the semisolid state. The liquid phase in the semisolid particles locally flows into gap between the particles and consequently the particles are joined to each other. Porous media were experimentally fabricated for constantan, chromel, pure copper and Cu–5 mol%Ag alloy by using the proposed processing. The range of laser power and irradiation time to fabricate porous media became wider as the temperature range in which the semisolid state was obtained was larger. Preheating the green particles also increased the fabrication range. The large latent heat of metallic alloys contributed to the fabricability of porous media. The proposed semisolid processing using laser irradiation can fabricate porous media of metallic alloys with three-dimensional shape.

View full abstract

Nickel electrodeposits on copper plates were prepared from a sulfamate bath at a current density of 200 A m⁻². The bath temperature was varied from 30 to 60°C at increments of 10°C. The recrystallization behavior was studied via the microstructural characterization of Ni deposits after 1 h of annealing at temperatures ranging from 200 to 600°C at increments of 100°C. In addition, optical metallography and cross-sectional transmission electron microscopy (TEM) were used to characterize the microstructure of Ni deposits. The texture of Ni deposit was determined by conventional X-ray diffraction method. Ni deposited at 50°C exhibited a weak [110] texture, whereas Ni plated at temperatures lower than 40°C displayed a strong [100] texture. The microstructural and hardness changes of Ni deposits upon annealing differed for the deposits with different textures. After annealing at temperatures higher than 300°C, the recrystallized nuclei were observed on the [110]-oriented deposits, which contained high-density dislocations and numerous twins. An equiaxed grain structure was observed for the Ni deposits annealed at 600°C. Conversely, for [100]-oriented deposits, which contained less lattice defects, the recovery and grain growth prevailed in the absence of pronounced recrystallization after 1 h of annealing at temperatures up to 600°C, when the [100]-oriented deposits still retained their well-defined columnar grain structure. The different annealing behaviors associated with the distinct textured Ni deposits could be explained by their different lattice defects. That is, the population density of twins associated with the as-deposited Ni markedly affected the feasibility of the formation of twin-free recrystallized nuclei.

View full abstract

A single crystal of a Ti–51Ni (at%) alloy was grown by floating zone method, and Ti₃Ni₄ precipitates of a single variant were formed coherently to the matrix by aging treatment under a compressive stress applied along the [111] direction. By using this single crystal, two-way (all-round) shape memory property was investigated. The precipitates were lenticular in shape with 30 nm in thickness and 250 nm in diameter, and their volume fraction was about 9%. The matrix was expanded elastically about 0.2% on average along the [111] direction. The elastic energy accumulated in the matrix was estimated to be about 0.3 MJ/m³; it was about 1.5% of the latent heat of the B2→R transformation. The thermally induced R-phase was composed of a single variant, and the spontaneous strain along the [111] direction was about 0.8% of expansion. The maximum work we could gain from the spontaneous shape change was estimated to be about 0.17 MJ/m³.

View full abstract

Sintered PbTe materials with average grain sizes of 28–309 \\micron were prepared by the spark plasma sintering technique. The apparent densities of the sintered PbTe were 8.17–8.23 Mg/m³, which were higher than 99% of the theoretical one. Thermoelectric properties of the sintered materials and those of the as-grown boule by Bridgman method were measured in the temperature range from 77 to 350 K . Resistivity, ρ of the sintered materials increased with decreasing grain size. Temperature dependence of ρ of the sintered PbTe was remarkably different from that of the as-grown boule below 250 K because of potential barriers at grain boundaries. Hall coefficient, R_H of the sintered materials at room temperature increased from 1.4×10⁻⁶ to 3.4×10⁻⁶ m³/C, as the average grain size increased from 28 to 309 \\micron, which suggested that oxidation in the crystal grains was caused in the sintering process. Temperature dependence of the thermoelectric power, α of the sintered PbTe below 250 K reflected the effect of carrier scattering at grain boundaries. Lattice thermal conductivity of the sintered PbTe decreased with decreasing grain size below 250 K, while it was independent of the grain size above that temperature. Above 250 K, thermoelectric properties of the sintered PbTe, except those in the 28 \\micron grain case, were consistent with those of the as-grown boule. The values of α and R_H at 295 K were calculated by using a two-valence-band model involving a non-parabolic and a parabolic valence band. The calculation results were in good agreement with the experimental data.

View full abstract

In order to develop a new kind of medical implant material, the microstructure and mechanical properties of cast Ti–Si alloys were investigated using small-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 the 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 the titanium matrix when the 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 in contrast to the pure titanium and Ti–6Al–4V alloys. The cast Ti–Si alloys are promising candidates for dental applications because of a good balance between strength and ductility.

View full abstract

Tensile tests and detailed microstructure observation were performed for Al–7%Si–0.4%Mg alloy castings with systematically controlled microstructure. The semi-liquid die-cast alloy showed improved ductility compared to the cast alloys exhibiting ordinary dendrite structure. The microstructure of the alloy was characterized by colonies consisting of single or several globular dendrite cells. The EBSP analysis and slip line observation have revealed that the misorientation among these colonies are large and so they can be recognized as “effective grains”. Presence of refined effective grains reduces stress concentration at the grain boundaries and prevents the localized crack formation during tensile deformation. This is considered to be the origin of the improved ductility in the semi-liquid die-cast alloy.

View full abstract

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 for 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 on 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 vs. 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 for the Au and the Au–Ag–Pd alloys, Zn release for the Ag alloy, and Sn and Cu releases for amalgam.

View full abstract

The fracture morphology and quenched-in embrittlement in Zr_52.5Ni_14.6Al₁₀Cu_17.9Ti₅ bulk glass were investigated by tensile and compressive tests at room temperature at the same strain rates of 4×10⁻⁴ s⁻¹ and scanning electron microscopy (SEM) observation. SEM analysis based on the deformation and fracture features indicates that the normal stress and shear stress on the fracture surface play a different role in the shearing-off of the specimens in tension and compression. The shear stress is the main controlling factor for the fracture in compression, and the fracture surface is along the maximum shear stress plane. In tension, however, both the shear and normal stresses govern the fracture process together, and the fracture surface is along the plane with an angle of 56 deg away from the axial direction. The fracture firstly starts from a random region on the surface where there is a stress concentration due to the voids or shear bands. The shearing-off leads to a dilatation and softening of the local glass. The softening then promotes the shearing-off and leads to final catastrophic fracture. The crystalline precipitates significantly influence the tensile and compressive properties. With increases in the volumetric fractions and the sizes of the precipitates, both the tensile and compressive strength and fracture strains decrease. The fracture mode changes from ductile to brittle. Vein patterns, shear-bands and local melting can still be observed when the volume fractions of quenched-in precipitates are less than 3∼5%. When the precipitates exceed 5% in volume fraction, fracture surface becomes rock strata-like feature, and samples lose almost their strength. The precipitates with larger sizes and no-spherical shapes play a role in rising stress concentration, resulting in decreasing the fracture strength.

View full abstract

This paper investigates a new joining process for Ni based ODS alloy. This process is done without brazing and is designed to enhance the reliability of joints at high temperatures. The main feature of the process is the removal of surface oxide films by keeping ODS alloys at high temperature (1523 K) in vacuum (≈10⁻⁴ Pa) followed by successive solid state diffusion joining between the cleaned surfaces at 1473 K in the same vacuum chamber. Specimens joined by this new process exhibited both high tensile strength and high ductility comparable to the base metal. However, the ductility of the joints decreased slightly when heated at 1523 K for 2, 8, and 20 h in a vacuum, but this additional step substantially increased the tensile strength.

View full abstract

The paper reports Icosahedral phase (i-phase) formation in the nanocrystalline state in Ti₅₅Zr₂₀Ni₂₅ and Zr₅₅Ti₂₅Ni₂₀ alloys. While nanocrystalline i-phase forms directly from the melt during melt spinning at low wheel velocity of 20 m/s, nanoquasicrystallization of amorphous phase has been observed on subsequent annealing after melt spinning at higher wheel velocities of 30 and 40 m/s. The i-phase is stable up to 873 K in Ti-rich alloy, while it transforms to crystalline phases in Zr-rich alloy below this temperature. The i-phase formed both directly from the melt and from the amorphous phase is much finer in Zr-rich alloy compared to Ti-rich alloy.

View full abstract

New Fe-based glassy alloys of Fe₄₅Cr₁₆Mo₁₆C₁₈B₅, Fe₄₅Cr₁₆Mo₁₄Nb₂C₁₈B₅ and Fe₄₅Cr₁₆Mo₁₄Ta₂C₁₈B₅ were synthesized. They exhibit a large supercooled liquid region (ΔT_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 the 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. The 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 10⁻²–10⁰ 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.

View full abstract