Showing papers in "Materials Transactions in 2004"

Mechanical Properties and Shape Memory Behavior of Ti-Nb Alloys

Abstract: Mechanical properties and shape memory behavior of Ti-(20–29)at%Nb alloys were investigated in order to develop Ni-free biomedical shape memory alloys. The Ti-Nb alloys were fabricated by arc melting method. The ingots were cold-rolled with a reduction up to 95% in thickness and then solution treated at 1173 K for 1.8 ks. The martensitic transformation temperature decreased by 43 K per 1 at% increase of Nb content. The shape memory effect and superelastic behavior were observed at room temperature in the Ti-(22–25)at%Nb alloys and Ti-(25.5– 27)at%Nb alloys, respectively. A small enthalpy of the martensitic transformation and a large difference between Ms and Mf were observed in the Ti-Nb alloys compared to Ti-Ni shape memory alloys. The maximum recovered strain of 3% was obtained at room temperature in solution treated Ti-(25–27)at%Nb alloys. The heat treatment at 573 K for 3.6 ks stabilized superelastic behavior of Ti-Nb alloys by increasing the critical stress for slip.

Beta Ti Alloys with Low Young's Modulus

Abstract: Composition dependence of Young's modulus inTi-V and Ti-Nb binary alloys and Sn-added ternary alloys quenched fromphase region was investigated at room temperature in relation to the stability ofphase. A minimum of Young's modulus in the binary alloys appears at such a composition that athermal ! phase transformation is almost completely suppressed. Formation of isothermal ! phase by aging after quenching increases Young's modulus. Sn addition to the binary alloys suppresses or retards ! transformation, thereby decreasing Young's modulus. Optimization of alloy composition in Ti-Nb-Sn alloys leads to low Young's modulus of about 40 GPa. The composition dependence of Young's modulus obtained experimentally in this study can be qualitatively explained by the theoretical discrete-variational Xcluster method.

Carbon Nanotube/Aluminium Composites with Uniform Dispersion

Abstract: Carbonnanotubes(CNTs)areattractingmuchinterestasfibrousmaterialsforreinforcingmetalmatrixcompositesduetotheirremarkablepropertiessuchasveryhighstrength,elasticmodulus,flexibilityandhighaspectratios.However,duetotheintricateentanglementsof longandfine CNTs and resulting aggregation, disentanglement and uniform dispersion of CNTs in aluminium (Al) matrices have been found quitedifficult. In addition, the poor wetting property of carbon for Al has been a great obstacle to forming composites. On a totally new principle, wesucceeded in producing nano-scale composites in which carbon nanotubes were uniformly dispersed in Al matrices. We named this methodNano-Scale Dispersion (NSD) method, which can also be employed to disperse various fillers such as whiskers, ceramic fibres, and powders inmetalmatrices as well as Al.Thecompositesobtainedwerefound tobe highlyreinforcedandnot tomelt at a temperaturefarabovethe meltingpoint of Al. Here we report the procedure of their fabrication and mechanical properties.(Received October 10, 2003; Accepted January 7, 2004)Keywords: nano-composites, metal matrix composites, light metals, carbon nanotubes, aluminium, uniform dispersion, reinforcement

Formation, Thermal Stability and Mechanical Properties of Cu-Zr and Cu-Hf Binary Glassy Alloy Rods

Abstract: Glassy alloy rods with diameters up to 1.5 mm exhibiting a large supercooled liquid region before crystallization and high mechanical strength were formed in Cu-Zr and Cu-Hf binary alloy systems by the copper mold castingmethod. The large supercooled liquid region exceeding 40 K was obtained in the composition range of 30 to 70 at%Zr and 35 to 60 at%Hf. The largest value of the supercooled liquid region defined by the difference between glass transition temperature (Tg) and crystallization temperature (T x ), ΔT x (= T x - Tg), was 58 K for Cu 6 0 Zr 4 0 and 59 K for Cu 5 5 Hf 4 5 . The reduced glass transition temperature (T g /T 1 ) of the two alloys was 0.61 and 0.59, respectively. The alloys with large ΔT x above 50K were formed into a bulk glassy alloy form with diameters up to 1.5 mm by copper mold casting. The Cu 6 0 Zr 4 0 , Cu 4 5 Zr 5 5 , Cu 6 0 Hf 4 0 and Cu 5 5 Hf 4 5 glassy alloy rods exhibited high fracture strength of 1920, 1880, 2245 and 2260 MPa, respectively, Young's modulus of 107, 102, 120 and 121 GPa, respectively, a nearly constant elastic elongation of about 1.9% and plastic elongation up to 2.2%. The formation of these binary glassy alloy rods can he interpreted in the framework of the concept of the formation of the unique glassy structure consisting mainly of icosahedral atomic configuration as similar to that for special multi-component alloys with the three component rules.

Effect of Grain Refinement on Thermal Stability of Metastable Austenitic Steel

Abstract: In martensitic steels, it is well known that a certain chemical driving force (about 180 MJ/m 3 ) is required to start martensitic transformation (Ms), and additional driving force has to be charged further to complete the transformation (Mf). In the case of metastable austenitic steels with Ms temperature at around room temperature, however, only the chemical driving force needed to start martensitic transformation has been stored at room temperature. Hence, the state of austenite is very unstable thermally. It has already been known that such a metastable austenite undergoes a partial martensitic transformation during isothermal holding at room temperature or cooling to a low temperature. It is very convenient to investigate the behavior of martensitic transformation of austenite. In this study, the effect of austenite grain size on martensitic transformation is introduced from the viewpoint of microstructural analysis and thermo-dynamics. The steel used in this investigation is an Fe-16 mass%Cr-10 mass%Ni ternary alloy, which has Ms temperature at around room temperature. The grain size of this steel can be controlled from 0.8 mm to 80 mm using the technique of reversion of deformation induced martensite. In the material with coarse grain size (80 mm), about 18% of martensite was detected at room temperature and the amount of martensite was increased to 50% by the following subzero treatment to 77 K. However, martensite was hardly detected in the material with ultra fine grains (0.8 mm) even after the subzero treatment. It was found that such a stabilization occurs in the materials with the grain size below 10 mm and the stabilization was reasonably explained by considering the relation between austenite grain size and elastic strain energy which is required on the single variant martensitic transformation.

Eutectic Modification of Al-Si Alloys with Rare Earth Metals

Abstract: The effects of different concentrations of individual additions of rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) on eutectic modification in Al-10mass%Si has been studied by thermal analysis and optical microscopy. According to the twin-plane re-entrant edge (TPRE) and impurity induced twinning mechanism, rare earth metals with atomic radii of about 1.65 times larger than that of silicon, are possible candidates for eutectic modification. All of the rare earth elements caused a depression of the eutectic growth temperature, but only Eu modified the eutectic silicon to a fibrous morphology. At best, the remaining elements resulted in only a small degree of refinement of the plate-like silicon. The samples were also quenched during the eutectic arrest to examine the eutectic solidification modes. Many of the rare-earth additions significantly altered the eutectic solidification mode from that of the unmodified alloy. It is concluded that the impurity induced twinning model of modification, based on atomic radius alone, is inadequate and other mechanisms are essential for the modification process. Furthermore, modification and the eutectic nucleation and growth modes are controlled independently of each other.

Mechanical Properties of a Ti-Nb-Al Shape Memory Alloy

Abstract: Ni-free Ti-base shape memory alloys (SMA) have been systematically developed by our group for biomedical applications in order to replace Ti-Ni SMAs which posses the possibility of Ni-hypersensitivity. In this study, superelastic behavior of solution-treated Ti-24 mol%Nb-3 mol%Al alloy was investigated by means of tensile tests at room temperature (RT) as well as microstructural observation. The alloy was fabricated by Ar arc-melting followed by a homogenization at 1273 K and then cold-rolled with the reduction of 99% in thickness without intermediate annealing. The cold-rolled sheets were solution treated at 1273 K for 1.8 ks in vacuum. Then, cyclic loading-unloading tensile tests were performed at RT. In the tensile tests, the tensile direction was systematically changed from rolling direction (RD) to transverse direction (TD) in the plane of the cold-rolled sheets. It was found by the tensile tests that the superelastic behavior strongly depends on the tensile direction and the number of deformation cycles. The solution-treated alloy after 99% cold rolling exhibits the best superelasticity when loaded along RD. The nature of the anisotropy in the superelastic behavior is discussed related with the texture developed during the fabrication process. It is concluded that the thermo-mechanical treatment performed in this study is quite useful as a superelastic treatment for the Ti-base SMAs, and that this alloy should be used industrially by taking into account such anisotropy of superelasticity.

Toughness of Ultrafine Grained Ferritic Steels Fabricated by ARB and Annealing Process

Abstract: A plain IF steel and a P-added IF steel having various ultrafine grain sizes from 0.24 to 11 mm were fabricated by the accumulative roll bonding (ARB) process followed by annealing. Dynamic fracture toughness of the ultrafine grained IF steels was investigated as a function of grain size by miniaturized Charpy impact test. The static strength of the IF steels significantly increased with decreasing the grain size, while the uniform elongation was limited in the ultrafine grained samples. A number of delamination appeared in the impact-tested specimens, especially in the ultrafine grained materials at low temperatures. It was concluded that the frequent delamination is not owing to insufficient roll-bonding in the ARB specimens but it is rather a characteristic feature of the ultrafine grained materials fabricated through heavy deformation. Because of the delamination, the absorbed energy in the impact test continuously decreased with decreasing the test temperature. On the other hand, an obvious change from the ductile fracture surface characterized by dimples into the brittle fracture surface mainly due to intergranular fracture was recognized at a certain low temperature. The ductile-brittle transition temperature determined from the microscopic fracture surfaces greatly decreased with decreasing the grain size, and finally no brittle fracture happened even at � 190 � C when the grain size was smaller than 5 mm or 2 mm in the plain IF steel or the P-added IF steel, respectively. It was concluded that the ultra-grain refinement is quite effective to improve the low-temperature toughness of ferritic steels and that it is possible to make phosphorus substantially harmless by grain refinement.

Shape Memory Properties of Biomedical Ti-Mo-Ag and Ti-Mo-Sn Alloys

Abstract: Ti-Mo (mol%) Ti-Mo-Ag (mol%) Ti-Mo-Sn (mol%) Ti-5.0Mo + Ti-5.0Mo-0.5Ag + Ti-5.0Mo-1.0Sn + Ti-6.0Mo + Ti-5.0Mo-1.0Ag + Ti-5.0Mo-2.0Sn + Ti-5.0Mo-2.0Ag + Ti-5.0Mo-3.0Sn + Ti-5.0Mo-3.0Ag + Ti-5.0Mo-4.0Sn + Ti-5.0Mo-4.0Ag + Ti-5.0Mo-5.0Sn + Ti-5.0Mo-5.0Ag + Ti-6.0Mo-1.0Sn + Ti-6.0Mo-1.0Ag + Ti-6.0Mo-2.0Sn + Ti-6.0Mo-2.0Ag + Ti-6.0Mo-3.0Sn + Ti-6.0Mo-3.0Ag + Ti-6.0Mo-4.0Sn Ti-6.0Mo-4.0Ag + Ti-6.0Mo-5.0Sn

Mechanical Properties and Shape Memory Behavior of Ti-Mo-Ga Alloys

Abstract: Mechanical properties and shape memory behavior of Ti-Mo-Ga alloys were investigated in order to develop Ni-free biomedical shape memory alloys. The Ti-Mo-Ga alloys were fabricated by arc melting method. The ingots were cold-rolled up to 95% reduction in thickness. The cold-rolled specimens were heat treated in the temperature range 673–1273 K for 60 s–3.6 ks. The martensitic transformation temperature decreased with increase in Mo and Ga content. The maximum shape recovery strain was obtained in a solution treated Ti-6 at%Mo-3 at%Ga alloy. Mechanical properties and shape memory behavior strongly depend on heat treatment condition in the Ti-6 at%Mo-3 at%Ga. Premature failure was observed in specimens heat treated in the temperature range 673–773 K. Ultimate tensile strength decreased and fracture strain increased with increasing heat treatment temperature. Shape memory effect was obtained in specimens heat treated in the temperature range 1073–1273 K. The shape memory effect was due to the stress induced martensitic transformation yielding tensile deformation and the reverse transformation upon heating after unloading. The martensitic transformation start temperature increased and the yield stress decreased with increasing heat treatment temperature and time. Stable superelastic behavior was obtained in a Ti-7 at%Mo-4 at%Ga alloy at room temperature by cyclic tensile tests. The recovery strain exceeding 4% was achieved in the pre-strained Ti-7 at%Mo-4 at%Ga alloy.

Stored Energy and Taylor Factor Relation in an Al-Mg-Mn Alloy Sheet Worked by Continuous Cyclic Bending

Abstract: The relation between the stored energy and the Taylor factor (TF) has been investigated using the SEM/EBSP analysis in an Al-Mg-Mn alloy sheet worked by the continuous cyclic bending (CCB). The analysis reveals that the stored energy is high for the high TF region, whereas a significant increase of the stored energy with the decrease of the Taylor factor appears in the vicinity of the minimum TF value of 2. This observation is discussed using the Schmid factor calculated. Further, the local strain accommodation during deformation is analyzed for grains of different orientations. The stored energy is derived from the calculation based on the kernel average misorientation (KAM).

Mechanical Properties Related to Microstructural Variation of 6061 Al Alloy Joints by Friction Stir Welding

Abstract: The microstructural change related with the mechanical properties of a friction stir welded 6061 Al alloy has been investigated under various welding conditions. Frictional heat and plastic flow during friction stir welding produced fine and equiaxed grains in the stir zone, macroscopically upset and elongated grains in the thermo-mechanically affected zone caused by dynamic recovery and recrystallization. The heat-affected zone, characterized by coarse precipitates, was formed beside the weld zone. Hardness distribution near the weld zone was strongly related to the behavior of precipitates and dislocation density. Especially, hardness of the SZ at a higher tool rotation speed was higher than that of a lower tool rotation speed due to higher density of spherical shaped re-precipitates. The joint strength was approximately 200 MPa which was lower than that of the base metal, 270 MPa, because softening region was formed around the weld zone.

Surface Tension and its Temperature Coefficient of Liquid Sn-X (X=Ag, Cu) Alloys

Abstract: The surface tension of liquid Sn-X (X=Ag, Cu) alloys was measured by the constrained drop method in the temperatures between 700 and 1500 K across whole composition range. Surface tension of the alloys increased with the content of Ag and Cu, and the temperature coefficient of the surface tension (d�= dT) had both positive and negative values. Experimental results were compared with the calculated results based on Butler’s model. The calculated results reasonably accorded with the measurements. The effect of thermo-physical parameters on the surface tension and the temperature coefficient were examined using the model. It was found that the temperature coefficient increases as the difference in the surface tension of component metals or the excess free energy increases in the high composition range of the component metal having higher surface tension, because of the surface enhancement of the other component metal.

Electrical and Optical Properties of Germanium-Doped Zinc Oxide Thin Films

Abstract: Impurity doped zinc oxide films show a maximum of conductivity at a proper dopant concentration, namely excess of impurity doping causes deterioration in the electrical property. With a view to improve the electrical property of impurity doped zinc oxide films, it is required to understand the relationship between various properties of the films and the true concentration of doped impurities. In this work, Ge-doped zinc oxide thin films with Ge content of 0� 8.1 at% were deposited by an RF magnetron sputtering method. Electrical, optical, and structural properties of the films were investigated. The concentration of Ge in the films was estimated by X-ray photoelectron spectroscopy (XPS) and the relationship between those properties and the Ge content was discussed. In order to deepen understanding on the conduction mechanism in the films, the temperature dependence of the electrical properties was studied as well.

Effects of Pore Morphology and Bone Ingrowth on Mechanical Properties of Microporous Titanium as an Orthopaedic Implant Material

Abstract: Successful bone formation which leads to functional osseointegration is determined by the local mechanical environment around bone-interfacing implants. In this work, a novel porous titanium material is developed and tested and then impact of porosity on mechanical properties as a function of bone ingrowth is studied numerically. A superplastic foaming technique is used to produce CP-Ti material with rounded, interconnected pores of 50% porosity; the pore size and morphology is particularly suitable for bone ingrowth. In order to understand the structure-property relations for this new material, a numerical simulation is performed to study the effect of the porous microstructure and bone ingrowth on the mechanical properties. Using ABAQUS, we create two-dimensional representative microstructures for fully porous samples, as well as samples with partial and full bone ingrowth. We then use the finite element method to predict the macroscopic mechanical properties of the foam, e.g., overall Young's modulus and yield stress, as well as the local stress and strain pattern of both the titanium foam and bone inclusions. The strain-stress curve, stress concentrations and stress shielding caused by the bone-implant modulus mismatch are examined for different microstructures in both elastic and plastic region. The results are compared with experimental data from the porous titanium samples. Based on the finite element predictions, bone ingrowth is predicted to dramatically reduce stress concentrations around the pores. It is shown that the morphology of the implants will influence both macroscopic properties (such as modulus) and localized behavior (such as stress concentrations). Therefore, these studies provide a methodology for the optimal design of porous titanium as an implant material.

Hydrogen-Assisted Degradation of Titanium Based Alloys

Abstract: Titanium base alloys are among the most important advanced materials for a wide variety of aerospace, marine, industrial and commercial applications, due to their high strength/weight ratio and good corrosion behavior. Although titanium is generally considered to be reasonably resistant to chemical attack, severe problems can arise when titanium base alloys come in contact with hydrogen containing environments. Titanium base alloys can pick up large amounts of hydrogen when exposed to these environments, especially at elevated temperatures. If the hydrogen remains in the titanium lattice, it may lead to severe degradation of the mechanical and fracture behavior of these alloys upon cooling. As a consequence of the different behavior of hydrogen in α and β phases of titanium (different solubility, different diffusion kinetics, etc), the susceptibility of each of these phases to the various forms of and conditions of hydrogen degradation can vary markedly. This paper presents an overview of hydrogen interactions with titanium alloys, with specific emphasis on the role of microstructure on hydrogen-assisted degradation in these alloys.

Recrystallization of Sn Grains due to Thermal Strain in Sn-1.2Ag-0.5Cu-0.05Ni Solder

Abstract: The formation of fine Sn grains in a Sn-1.2 mass%Ag-0.5 mass%Cu-0.05 mass%Ni solder due to thermal strain was investigated from the viewpoint of recrystallization. After thermal fatigue, small general grains recrystallized at the strain concentrated location in Sn-1.2Ag-0.5Cu-0.05Ni. Through isothermal annealing, however, grains, which had near ‹110› orientation at a chip-substrate direction before isothermal annealing, coarsened preferentially. Hence, not isothermal annealing but thermal strain was a driving force for recrystallization. Both grain growth after recrystallization and coarsening of recrystallized grains in Sn-1.2Ag-0.5Cu-0.05Ni were slower than those in Sn-1.2 mass%Ag-0.5 mass%Cu, which suppressed crack initiation and increased fatigue life of Sn-1.2Ag-0.5Cu-0.05Ni.

Relationship between Texture and Macroscopic Transformation Strain in Severely Cold-Rolled Ti-Nb-Al Superelastic Alloy

Abstract: Textures of severely cold-rolled Ti-24 mol%Nb-3 mol%Al superelastic alloy were examined by X-ray pole figure measurements for the bcc parent phase (� ) and the relationship between transformation strain and loading direction was evaluated on the basis of the lattice correspondence and change in the potential energy of an external stress. A well-developed recrystallization texture of h110if112gtype was confirmed by X-ray pole figure measurements for the material which was 99% cold-rolled followed by a solution-treatment at 1273 K. Crystallographic analysis showed that the maximum dilatation component of the martensitic transformation fromto � 00 is h110iparallel to the rolling direction (RD). An energy consideration revealed that only one � 00 -variant (lattice-correspondence variant) can be induced by the tensile stress along RD when the recrystallization texture appears. The transformation strain generated under the RD tension was about three times larger than that generated under the tension along transverse direction (TD). These calculations were in good agreement with experimental evidences.

Fe-B-Si-Nb Bulk Metallic Glasses with High Strength above 4000 MPa and Distinct Plastic Elongation

Abstract: The glass-forming ability and mechanical properties of Fe-B-Si-Nb glassy alloys have been investigated. The Fe72B20Si4Nb4 glassy alloy was prepared in a cylindrical form with a diameter of 2 mm. Young’s modulus, compressive fracture strength and plastic elongation of the bulk metallic glass were 200 GPa, 4200 MPa and 1.9%, respectively. Many shear bands were observed along the shear plane, which was declined by about 43 degrees to the direction of applied load, and the fracture occurred along the shear plane. The local-ordered regions were recognized in the high-resolution TEM image of the Fe72B20Si4Nb4 bulk metallic glass with a diameter of 2 mm. The good mechanical properties are attributed to the suppression effect of the local-ordered regions on the propagation of shear bands.

New Ti-Based Bulk Glassy Alloys with High Glass-Forming Ability and Superior Mechanical Properties

Abstract: Ti-based glassy alloy rods with diameters up to 5 mm were prepared for Ti41:5Zr2:5Hf5Cu42:5Ni7:5Si1 in a mutli-component system of (Ti, Zr, Hf)-(Cu, Ni)-Si by conventional copper mold casting. The new multi-component glassy alloy exhibits moderate thermal stability with a supercooled liquid region (� Tx) above 50 K. The glass transition temperature (Tg), crystallization temperature (Tx), melting temperature (Tm), liquidus temperature (Tl), and the reduced glass transition temperature (Tg=Tl) are 680 K, 730 K, 1143 K, 1199 K and 0.57, respectively. The cast glassy alloy exhibits compressive strength of 2080 MPa, tensile strength of 2040 MPa and Young’s modulus of 100 GPa. The reason for the high glass-forming ability (GFA) of the Ti-based multi-component alloy is discussed on the basis of knowledge available for bulk glass formation.

Comparison of Nanocrystalline Surface Layer in Steels Formed by Air Blast and Ultrasonic Shot Peening

Abstract: Surface nanocrystallization in various steels by shot peening (both air blast (ABSP) and ultrasonic (USSP)) was investigated. In all the shot-peened specimens, the equiaxed nanocrystals with grain size of several 10 nm were observed at the surface regions. The depth of nanocrystalline (NC) layers was several mm. The NC layers have extremely high hardness and were separated from the deformed structure regions just under the NC layers with sharp boundaries. By annealing, the NC layers show the substantially slow grain growth without recrystallization. These characteristics are similar to those observed in the specimens treated by ball milling, ball drop and particle impact deformation. Comparing ABSP and USSP at the similar peening condition, the produced volume of NC region in ABSP is larger than that in USSP.

Tensile Deformation Behavior of Ti-Nb-Ta-Zr Biomedical Alloys

Abstract: The composition of Ti-30Nb-10Ta-5Zr, which is simplified that of the Ti-29Nb-13Ta-4.6Zr alloy developed for biomedical applications, was selected, and then Nb content in the basic composition was varied from 20 to 35%. The deformation mechanisms of such Ti-Nb-Ta-Zr system alloys were investigated by loading-unloading tensile tests and characterizing deformed microstructures. The behavior of unloading and reloading of the stress-strain curves up to strain about 2% of Ti-20Nb-10Ta-5Zr and Ti-25Nb-10Ta-5Zr alloys is similar to that obtained in

IMC Growth and Shear Strength of Sn-Ag-Bi-In/Au/Ni/Cu BGA Joints During Aging

Abstract: The growth kinetics of intermetallic compound (IMC) layers formed between Sn-3Ag-6Bi-2In ball-grid-array (BGA) solder and Au/Ni/ Cu substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0 to 100 days. A quantitative analysis of the IMC layer thickness as a function of time and temperature was performed. The intermetallic layer exhibited a parabolic growth at the given temperature range. Because the values of the time exponent (n) are approximately 0.5, the layer growth of the IMC was primarily controlled by diffusion over the temperature range studied. The apparent activation energy value calculated for the Sn-Ag-Bi-In/Au/Ni/Cu BGA joint was 64.8 kJ/mol. Also, the reliability of the solder ball attachment was characterized by mechanical ball shear tests. The brittleness of the solder joints increased with increasing aging temperature and time, and the fracture occurred within the IMCs and Ni layer. The deterioration of the solder ball shear strength was found to be predominantly caused by the formation of the IMC layer.

Evaporation of Phosphorus in Molten Silicon by an Electron Beam Irradiation Method

Abstract: The evaporation behavior of phosphorus in molten silicon during electron beam irradiation was investigated with the aim of producing solar grade silicon (SOG-Si) from metallurgical grade silicon (MG-Si) by a sequential metallurgical process. Batch experiments showed that the evaporation rate of phosphorus increased in proportion to the power of the electron beam and phosphorus content. The phosphorus removal rate was controlled by free evaporation from the molten silicon surface. Electron beam irradiation makes it possible to secure a higher temperature at the free liquid surface, which results in more efficient dephosphorization. A continuous flow experiment indicated that the phosphorus concentration at the outlet increased when the silicon feed rate was raised, which was attributed to the fact that the hearth residence time of the molten silicon was proportionally shorter. Thus, the flow of molten silicon in the hearth did not behave as a complete mixed reactor flow type reaction, but was close to a plug flow type reaction. With a 150 kg scale pilot manufacturing plant, MG-Si containing about 25 mass ppm of phosphorus was successfully purified to P < 0.1 mass ppm.

General rule of phase decomposition in Zn-Al based alloys. (II): On effects of external stresses on phase transformation

Abstract: Microstructural changes and phase transformation of Zn-Al based alloys (ZA alloys) were systematically investigated during various thermal and thermo-mechanical processes using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro-analysis (EMPA), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and differential scanning calorimeter (DSC) etc. techniques. Phase decompositions of the alloys were studied under various thermal and thermo-mechanical circumstances. General rule of phase decomposition (II) (On effects of external stresses on phase transformation) was summarized with explanations from point view of Gibbs free energy.

Effect of Deformation Temperature and Strain Rate on Evolution of Ultrafine Grained Structure through Single-Pass Large-Strain Warm Deformation in a Low Carbon Steel

Abstract: Ultrafine grained structure formed dynamically through a severe warm deformation in the temperature range from 773 K to 923 K has been investigated in a 0.16%C-0.4%Si-1.4%Mn steel. The effects of the deformation conditions such as deformation temperature and strain rate on microstructural evolution were examined using a single-pass compression technique with a pair of anvils. A large plastic strain up to 4 was imposed on the specimen interior at a strain rate of 1 or 0.01 s � 1 . Ultrafine ferrite grains surrounded by high angle boundaries, whose nominal grain size ranged from 0.26 to 1.1 mm, evolved when the equivalent plastic strain exceeded the critical value about 0.5 to 1, and increased with an increase in strain without any large-scale migration of high angle boundaries. The effects of deformation conditions on microstructural evolution of ultrafine grained structures can be summarized into the Zener-Hollomon(Z-H) parameter dependences. The average size and the volume fraction of newly evolved ultrafine grains depend on the Z-H parameter. Decreasing Z-H parameter enhances the formation of equiaxed ultrafine grains. These indicate that the mechanism forming ultrafine grained structures through the warm severe deformation in the present study is similar to ‘‘continuous recrystallization’’ or ‘‘in-situ recrystallization’’ and that some activation process during or after the deformation plays an important role in the microstructural evolution.

Boron Removal in Molten Silicon by a Steam-Added Plasma Melting Method

Abstract: Oxidation and removal of boron from molten silicon by a steam-added plasma melting method was investigated as an important part of a sequential metallurgical process for producing high-purity solar grade silicon (SOG-Si) from commercially available metallurgical grade silicon (MG-Si). Experiments were carried out with the mass of silicon per charge varied in the range from 0.6 to 300 kg, corresponding to the laboratory scale to industrial scale. Boron was removed to ½B� < 0:1 mass ppm, which is the permissible boron content for SOG-Si. The deboronization rate was proportional to the steam content, 3.2th power of the hydrogen content of the plasma gas, boron content of the molten silicon, and area of the dimple formed by the plasma gas jet, and was inversely proportional to the mass of the molten silicon. A thermodynamic study showed that preferential oxidation of boron in molten silicon is positively related to higher temperatures, supporting the conclusion that this plasma method, which causes a local increase in the temperature of the reaction surface, is in principle advantageous.

Enhanced Piezoelectric Properties of Piezoelectric Single Crystals by Domain Engineering

Abstract: Various engineered domain configurations were induced into barium titanate (BaTiO 3 ) single crystals, and their piezoelectric properties were investigated as a function of (1) the crystal structure, (2) the crystallographic orientation and (3) the domain size. As a result, the orthorhombic mm2 BaTiO 3 crystals showed the highest piezoelectric properties among three kinds of BaTiO 3 crystals such as tettagonal 4mm, orthorhombic mm2 and rhombohedral 3m phases. On the other hand, the [001] c oriented BaTiO 3 crystals always exhibited the larger piezoelectric properties than the [111] c oriented BaTiO 3 crystals. Moreover, the domain size dependence on the piezoelectric properties was discussed, and this result revealed that the piezoelectric property was strongly dependent on the domain size, i.e., the piezoelectric properties significantly increased with decreasing domain size. On the basis of the above results, the most suitable engineered domain configuration was proposed for the high-strain high-coupling piezoelectric application.

Ti-Based Bulk Metallic Glasses with High Specific Strength

Abstract: (x ¼ 10, 15) BMG cylinders with thediameter up to 6mm are successfully fabricated by the injection casting method, exhibiting high Vickers hardness (681–700) and yield strength(2.0–2.1GPa). Ti-Zr-Be-Cu-Ni alloys with >50at% Ti content have higher specific strength than any other Ti-based BMGs reported so far.(Received October 6, 2003; Accepted December 11, 2003)Keywords: bulk metallic glass, glass forming ability, specific strength, titanium-zirconium-beryllium-copper-nickel

Effect of Pass Strain on Grain Refinement in 7475 Al Alloy during Hot Multidirectional Forging

Abstract: Effect of pass strain (�" ) on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490 � C under a strain rate of 3 � 10 � 4 s � 1 . Samples of rectangular shape were deformed up to accumulated strains of around 6 with subsequent changes in loading direction 90 � from pass to pass. The pass strains in each compression (�" ) were 0.4 and 0.7. The cumulative flow curves integrated by each compression exhibit significant work softening just after yielding, followed by apparent steady state plastic flow at high strains. Structural changes were characterized by grain fragmentation due to frequent development of deformation and/or microshear bands followed by full evolution of new fine grains in the original grains. Increasing �" accelerates significantly the kinetics of grain refinement, leading to more clear reduction of flow stresses at moderate to high strains. MDF of �" ¼ 0:7 results finally in formation of a finer grained structure with an average size of around 7.5 mm at strains of above 3.5, while, the processing with �" ¼ 0:4 develops a slightly coarser grain structure at higher strain of about 6. The effect of MDF on new grain evolution and the mechanisms of grain refinement are discussed in details.