The effect of pre-heat treatments, followed by a hot forging process, on the microstructure and strength of Co-Cr-Mo alloys was investigated. Four pre-heat treatments were conducted at 1443, 1473, 1503 and 1533 K for three different time condition: 7.2 ks, 21.6 ks and 54.0 ks and then water cooled to room temperature. Tensile tests and XRD analyses were carried out on both as-cast alloy and heat-treated alloys. The volume fraction of retained γ phase increases with an increase in temperature of heat treatment, suggesting that the γ→ε martensitic transformation is suppressed after experiencing the heat treatment at higher temperatures. Tensile strength slightly decreases with increasing the temperature and the time of heat treatment, whereas the ductility slightly increases. The σ phase completely dissolves into matrix when the alloy is heat-treated at temperature of 1533 K longer than 21.6 ks.
A dry wear behavior of a forged Co-Cr-Mo alloy without Ni and C additions have been investigated using a ball-on-disc type wear testing machine with an alumina ball in ambient air. The wear factor of the Co-Cr-Mo forged alloy without Ni and C additions (hereafter, designated the forged alloy) shows negative contact load dependence. The coefficient of friction decreases with increasing contact load. Worn surfaces are hardened during the wear tests, forming oxide films. This results from significantly high work hardening rate of the forged alloy, caused by the strain-induced martensitic transformation from an fcc-γ phase to an hcp-ε phase, which contributes to the improvement in the dry wear resistance. Wear mechanisms of the forged alloy are discussed on the basis of Hertzian contact theory and observations of the wear scars formed on the alloy disc and the alumina ball surfaces. It is considered that the dominant wear mechanism of the forged alloy is the mild adhesive wear, though the extrinsic abrasive wear mediated by the wear debris, i.e., third-body abrasive wear, is exerted as an extrinsic wear mechanism. In addition, it is suggested that a delamination wear resulting from the fatigue fracture likely occurs under the present dry wear condition.
Co-29Cr-6Mo alloy powder was fabricated by the gas atomizing process on the compacts were hot pressed at 1273 K to 1465 K with an applied pressure of 40 MPa for 7.2 ks. The atomized alloy powder possesses relatively small concentration scattering of chromium and molybdenum. The powder dominantly consists of fcc phase. The compacts hot pressed at 1273 K possess the relative density of 92%, meanwhile the compacts with high relative density (>98%) are obtained by sintering above 1323 K. The grain size of the compacts tends to increase with increasing sintering temperature. The σ phase in the fcc matrix are observed in the compacts hot-pressed at 1273 K and 1323 K. Twins and stacking fault are observed and the σ phase disappear in the compacts hot-pressed over 1323 K. The compact hot-pressed at 1323 K shows the highest hardness value (374 Hv), resulting from the existence of the σ phase in the compact.
Porous Co-29Cr-6Mo (ASTM F75) compacts were fabricated by spark plasma sintering method (SPS) with plasma rotating electrode processed (PREPed) Co-29Mo-6Mo (mass%) powder with a diameter range (610-700 μm). The porosity of the compacts could be changed between 10.8% and 39.4% when the SPS temperature varies from 1023 K to 1273 K. Relatively large neck was formed by post annealing at 1473 K for 7.2 ks for the compacts containing the porosity of 32.4%, although the porosity change was only 0.6%. This annealing improves the bending strength of the compact. Young's modulus of the annealed compact containing the porosity of 31.4% shows 54.0 GPa, which is relatively higher than that of human cortical bone but approximately one-forth as much as that of as-cast Co-Cr-Mo alloy. The compact possesses the bending strength of 223.4 MPa, which is superior to that of human cortical bone.
Ingots of ferritic stainless steels, Fe-24Cr-2Mo in mass%, were worked to various dimensions for test specimens. Nitrogen was absorbed by the specimens in a furnace filled with nitrogen gas with a pressure of 101.3 kPa at 1473 K to develop a simple and convenient manufacturing process of nickel-free austenitic stainless steels. Changes in the mechanical properties of the alloy with nitrogen absorption treatment are discussed on the basis of the resultant microstructure. Ferritic Fe-24Cr-2Mo was austenitized with 129.6-ks nitrogen absorption to a 2-mm depth from the surface. On the other hand, Fe-24Cr-2Mo was completely austenitized with 7.2-ks nitrogen absorption to a 0.5-mm depth from the surface. The hardness, tensile strength, 0.2% proof stress, and elongation to fracture increased, and the reduction of area decreased in Fe-24Cr-2Mo by austenitization due to nitrogen absorption. Tensile properties of the alloy after nitrogen absorption were improved by grain refinement process before nitrogen absorption. The tensile strength and 0.2% proof stress of the alloy with nitrogen absorption for 129.6 ks were much larger than those of 316L steel, while the elongation to fracture was much smaller than that of 316L steel. Therefore, small devices and parts with a maximum thickness or diameter of 4 mm were manufactured with this process in this study.
The relationships between mechanical properties and microstructure of Ti-29Nb-13Ta-4.6Zr in under aged, peak aged and over aged conditions at various aging temperatures were investigated. The maximum Vickers hardness values of Ti-29Nb-13Ta-4.6Zr aged at 573 K, 673 K and 723 K are obtained at aging times of around 2419.2 ks, 3024 ks and 1209.6 ks, respectively. ω phase precipitates in β phase of Ti-29Nb-13Ta-4.6Zr at relatively low aging temperature, 573 K, while α phase precipitates in β phase of Ti-29Nb-13Ta-4.6Zr at relatively higher aging temperature, 723 K. At an aging temperature of 673 K that lies between aging temperatures of 573 K and 723 K, ω phase precipitates at the early stage of aging, but α phase precipitates at relatively longer aging time. The precipitation site of α phase changes from intragrain to grain boundary at around peak aging time when Ti-29Nb-13Ta-4.6Zr is aged at 673 K and 723 K. The tensile strength of Ti-29Nb-13Ta-4.6Zr aged at 573 K, 673 K and 723 K increases up to a peak aging time with increasing aging time, but under over aging conditions, the tensile strength decreases. While the elongation decreases with increasing aging time at every aging temperature. The fatigue strength of Ti-29Nb-13Ta-4.6Zr increases greater when α phase precipitates than when ω phase precipitates. The fatigue strength of Ti-29Nb-13Ta-4.6Zr decreases a little due to the coarsening of α phase precipitated in β grain.
In order to produce a novel bioactive beta-type titanium alloy-Ti-29Nb-13Ta-4.6Zr - and pure titanium for biomedical applications, an electrochemical treatment involving anodic oxidizing and cathodic polarization was applied to the alloy in calcium nitrate solution under various conditions. For creating a hydroxyapatite (HAP) coating on the alloy, the treated alloy and pure titanium were dipped into simulated body fluid (SBF) and then into 1.5SBF, wherein the concentration of constituents was 1.5 times that of SBF. The characterization and morphology of HAP formed on the alloy were examined in comparison with those of pure titanium. In Ti-29Nb-13Ta-4.6Zr, the formability of HAP in SBF after the electrochemical treatment is less than that in pure titanium. The thickness of a calcium hydroxide layer formed by the cathodic polarization can be controlled by the cathodic potential. In both pure titanium and Ti-29Nb-13Ta-4.6Zr, the formability of HAP increases up to a certain increase in the thickness of the oxide layer formed by the anodic oxidizing. Therefore, the formability of HAP decreases due to a reduction in the formability of the calcium hydroxide layer. The number of hydroxyl groups on the surface of Ti-29Nb-13Ta-4.6Zr is less than that in pure titanium after the anodic oxidizing. The tensile bonding strength of HAP in Ti-29Nb-13Ta-4.6Zr is similar to that in pure titanium.
In recent years, the opportunity of the hard tissues such as bones, hip joints and tooth to be replaced by metallic biomaterials is increasing. In general, metallic biomaterials lack bioactivity, which is the ability to directly form a chemical bond with bones. Therefore, the bioactive surface modifications on surface of metallic biomaterials have been investigated and applied. Among them, the calcium phosphate invert glass coating method, which is dip-coating treatment (DCT), can be granted the bioactivity on the surface of a β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ) alloy, for biomaterial applications. In this study, aging characteristics and mechanical properties of TNTZ alloy surface-modified with DCT were investigated. There is an oxide layer and α case near boundary between calcium phosphate invert glass layer and TNTZ surface in TNTZ surface-modified with DCT (TNTZDCT). A very fine ω phase precipitates in an equiaxed β phase with an average diameter of 20 μm at a distance of 200 μm from specimen surface. On the other hand, the microstructure of aged TNTZDCT has an α phase. The tensile strength of TNTZDCT is around 30% greater than that of as-solutionized TNTZ (TNTZST). On the other hand, the elongation is around 48% smaller than that of TNTZST. Tensile strength of aged TNTZDCT is around 26% greater than that of TNTZDCT. While the elongation of aged TNTZDCT is around 52% smaller than that of TNTZDCT. Fatigue limit of TNTZDCT is nearly equal to that of TNTZST. Fatigue limit of aged TNTZDCT is around 80 MPa greater than that of TNTZST. Fatigue limits of TNTZDCT and aged TNTZDCT, where their fatigue specimen surfaces are mirror-polished, rise as compared with those of TNTZDCT and aged TNTZDCT, where their fagigue specimen surfaces are shot-blasted.
A novel method of preparing a calcium phosphate glass coating on a metallic titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ), was developed by CO2 laser irradiation. It was difficult to melt 60CaO-30P2O5-3TiO2-7Na2O (mol%) glass powders dip-coated on TNTZ for joining by the irradiation of 5 W power. After a trace amount of sodium phosphate was dispersed between the glass particles on TNTZ, the CO2 laser was irradiated to the surface; sodium phosphate was melted by the irradiation to react with the surfaces of the glass particles and with the thin oxide layer on TNTZ, resulting in the formation of a strong calcium phosphate glass coating on the substrate.
Amorphous and crystallized calcium phosphate (Ca-P) coatings on Ti substrate have been investigated by X-ray photoelectron spectroscopy (XPS). The Ca-P coating on Ti substrate was performed by radio frequency magnetron sputtering of a sintered hydroxyapatite (HAp) target. As-sputtered Ca-P coating having 1.81 of Ca/P ratio was amorphous. Annealing the as-sputtered amorphous coating at 600°C resulted in crystallized coating into HAp and tetracalcium phosphate (TTCP). The XPS profile of amorphous coating is almost the same as that of annealed coating. The XPS profile of oxygen in the amorphous coating shows that oxygen atom combines with both phosphorus and calcium atoms, indicating the amorphous coating contains fundamental chemical bonds of both HAp and TTCP.
Tensile tests and static fracture toughness tests were carried out on variously heat-treated type IV gold alloys. The effects of microstructures on tensile characteristics and static fracture toughness were discussed. The tensile strength of type IV gold alloy increases with increasing solutionizing temperature. Moreover, the tensile strength of type IV gold alloy increases with increasing ageing time when the solutionizing temperature is 1023 K. While the elongation of type IV gold alloy decreases with increasing solutionizing temperature. Static fracture toughness of type IV gold alloy increases with increasing solutionizing temperature. Static fracture toughness of type IV gold alloy of aged for 0.3 ks is the greatest, and that of aged for 1.8 ks is the smallest when the solutionizing temperature is 1023 K.
This study theoretically examined the relationship between mold permeability and the casting pressure acting on the molten titanium in two types of pressure casting equipment [two-chamber and one-chamber] for preparing titanium dental castings in order to select the most effective investment material and optimal casting conditions. The casting pressure exerted on the melt can be defined as the pressure difference between the melting chamber and the mold cavity after the molten titanium drops and seals the entrance to the cavity sprue. Differential equations describing the pressure in the mold cavity were derived from the equation of the state of gas as a function of time. Analysis revealed that mold permeability and the operation of each casting unit affect how the casting pressure acts on the melt: a low-permeability mold is appropriate for the one-chamber type but intermediate permeability molds are desirable for the two-chamber type. Using the results of this study and published permeability data on investments, an optimum investment material can be selected for each type of equipment.
An understanding of how fatigue cracks propagate in the bone is important because fatigue is thought to be the main cause of clinical stress fractures. In this study, fatigue crack initiation and propagation behaviors of bovine humerus and femur were investigated with respect to their microstructures. The initiation and propagation of fatigue cracks were observed in a specimen surface by using the replica method. Fatigue tests were also conducted on the bovine humeral and femoral compact bones in order to obtain S-N curves. Based on their microstructures, the bovine humeral and femoral compact bones are classified into the haversian and plexiform bones. In haversian bone, when the microstructure is inclined toward the bone axis, the fatigue crack initiates at the osteon-ossein interface and osteon lamellae interface. In case the microstructure is parallel to the bone axis, the fatigue cracks initiate at the haversian canals, Volkmann's canals, osteon-lamellae interface, and osteon-ossein interface. Among these, the fatigue crack that initiates at the Volkmann's canal may receive a relatively greater maximum shear stress; therefore, it has a tendency to be the main propagating fatigue crack that connects the fatigue cracks initiated at the other above mentioned sites. In the plexiform bone, when the microstructure is inclined toward the bone axis, fatigue crack initiates at the interface of the lamellae. When the microstructure is parallel to the bone axis, fatigue cracks initiate at the blood vessel and interface of the lamellae. The fatigue crack that initiates at the blood vessel may receive a relatively greater maximum shear stress; therefore, it has a tendency to be the main propagating fatigue crack that connects the fatigue crack initiated at the other site. In the haversian and plexiform bones, the crack propagation rate when the microstructure is inclined toward the bone axis is greater than that when the microstructure is parallel to the bone axis.
In this study, the effect of Al content on isochronal heat treatment behavior in Ti-13Cr-1.2Fe-Al alloys was investigated by measurements of electrical resistivity and Vickers hardness, X-ray diffraction and transmission electron microscope observation. The results obtained are as follows. In the solution treated and quenched state (STQ), resistivity (ρ) at liquid nitrogen (LN) and room temperatures (RT) increased monotonously with Al content. 0%Al and 3.0%Al alloys consisted of retained β phase and athermal ω. In the isochronal heat treatment, two minima of resistivity were observed at 623 K and 823 K. The former is due to the precipitation of isothermal ω phase and the latter is α precipitation. The temperature of resistivity minimum attributing to ω phase precipitation gradually increases with increasing of Al content. In 4.5%Al and more, the minimum of resistivity by ω precipitation disappears and the minimum caused by α precipitation remained. It is considered that Al content above 4.5% was suppressed ω precipitation and enhanced α precipitation.
The aggregation phenomena of several metal oxide nanoparticles were analyzed and discussed based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. The attractive van der Waals (VDW) and the repulsive electrostatic double-layer (EDL) forces acting upon a pair of approaching particles are two kinds of the fundamental interaction forces that control their aggregation. The surface potential and the Debye length of the particles are also important factors. The particles that disperse in an aqueous solution were forced to aggregate under unstable dispersion conditions wherein the EDL interaction becomes much weaker than the VDW interaction between two particle surfaces. Under this condition, the agglomerated nanoparticles having a disorderly arranged structure, i.e. a lot of small pores, are created. In order to control the surface potential and the Debye length on the nanoparticles of the Ti-, Zn-, Al-, Sn- and Sb-oxides, the electrolyte (KBr) and acid/alkali were added into the colloidal solutions. Agglomerated their nanoparticles with pore structure were prepared after evaporating the solutions and removing KBr. In the case of the Ti- and Zn-oxides, a strong aggregate force was observed among the nanoparticles. The pore size and the pore volume of the agglomerated nanoparticles increased with decreasing energy barrier of the interaction energy among these particles. The other oxide nanoparticles also formed an agglomerate body in each pH-controlled (metal oxide/KBr) solution. However, the aggregate force was so weak that the particles could not keep the aggregate state in the removal stage of KBr, which resulted in the re-dispersion of the agglomerated particles into their primary particles. From the DLVO calculations of the experimental systems, it was confirmed that the dispersion/aggregation phenomena of the nanoparticles observed in each stage of the experiments are determined by the balance of the attractive and the repulsive interactions among them and their thermal kinetic energy.
The electromigration of Cu interconnection was investigated to solve the Huntigton's equation by numerical analysis. The Cu atoms moved toward anode from cathode and accumulated at the anode end. This result was in good agreement with the result previously derived by our theoretical analysis. The accumulation rate of Cu atoms increased with increasing in current density, but it was not so influenced by temperature. The characteristic of the rate of electromigration showed a liner relationship with current density and exponential relationship with temperature. This result agreed with the equation experimentally derived by Black. This result proved that numerical analysis of Huntignton's equation enables to predict of electromigration failure time.
The effects of yttrium addition on high temperature properties of the 4th generation nickel-base superalloy were studied. Nickel-base superalloys, which possess both creep strength and oxidation resistance, are being required to satisfy high efficiency and reliability of jet engines. 4th generation superalloys contain platinum groups metals such as ruthenium. In order to satisfy creep strength and phase stability, however ruthenium worsens oxidation resistance. Yttrium doping was found to increase the oxidation resistance due to the improved adhesive strength of Al2O3 oxide, and the effects of yttrium doping on creep strength are established in this paper. Yttrium-added TMS-138 alloy (TMS-138Y) showed high creep strength of 4th generation superalloys, however the creep strength was slightly worse than the alloy without yttrium in the high-temperature low-stress creep condition. Microstructural observations confirmed the increase of Topologically Close Packed (TCP) phases in TMS-138Y. This was due to the formation of a yttrium-containing compound, Al0.06Ni0.73Co0.04Y0.18 (at.), which made incoherent interfaces that allowed TCP phases to precipitate by heterogeneous nucleation.
Microstructure of a recently developed silver-based conductive adhesive, which is expected as a substitute for conventional soldering, has been studied by advanced transmission electron microscopy (TEM). Energy-filtered TEM has revealed the feature of dispersion of silver particles. Conductivity between neighbored particles is evaluated inside the electron microscope by using two microprobes that can be operated independently. The results shed light on the development of the silver-based conductive adhesive.
SCC susceptibility of Type 304 stainless steel depends on NaCl concentration in aqueous solution composed of H2SO4 and NaCl. Immersion potential and electrode impedance were measured during SSRT(Slow Strain Rate Tensile) test in two kinds of solutions; i.e., one is 2.5 kmol•m-3 H2SO4+0.2 kmol•m-3 NaCl solution in which SCC occurs and the other is 2.5 kmol•m-3 H2SO4+3.0 kmol•m-3 NaCl solution in which SCC does not occur. In case of non-SCC, the enhancement of cathodic reaction was observed during the measurement of corrosion potential in both solutions, while in case of SCC, the enhancement of anodic reaction was observed. In addition, Rct, Cdl and τ0 slightly depend on the stress loaded on specimens in case of non-SCC, but they greatly depend on the stress loaded on specimens in case of SCC.