Takateru Umeda

Phase selection during solidification of peritectic alloys

Abstract Formation of stable and metastable phases under directional solidification conditions in materials showing peritectic phase equilibria is discussed. Based on interface response functions for single phase growth morphologies (dendrites and plane front) and on competitive growth arguments, a kinetic stabilisation of metastable peritectic phases with respect to stable ones is predicted for various growth conditions. This method is applied to two industrially important classes of materials such as steels and NdFeB permanent magnets, the properties of which depend closely on the phases formed.

Magnetostriction of Tb–Dy–Fe crystals

〈111〉-oriented twin free Tb–Dy–Fe single crystals, 〈112〉- and 〈110〉-oriented twinned “single” Tb–Dy–Fe crystals were prepared using floating zone melting technique. Magnetostrictive performances of the crystals were investigated. Better low-field properties were observed in the 〈110〉 twinned crystals than in the 〈112〉 crystals. The highest properties were achieved in the 〈111〉 twin free single crystals. Even though there are still oxidized particles in the present 〈111〉 single crystals, a large magnetostrictive jump of 1700 ppm and a very low saturation magnetic field of 500 Oe were obtained. To understand magnetization and magnetostriction of different Tb–Dy–Fe crystals, theoretical modeling was carried out based on a simplified domain rotation model. Magnetization moment rotation paths of different domains were simulated and hence the resultant magnetostriction was obtained, which can well account for the experimental results of different crystals. The limitation of the domain rotation model was also di...

Directional solidification of TbDyFe magnetostrictive alloy

Abstract Grain-aligned rods of the huge magnetostrictive TbDyFe alloy have been prepared by directional solidification techniques. Axial grain orientation, phase spacing and elemental distribution of the alloy rods have been investigated. It was found that unseeded rods usually show a strong 〈112〉 orientation at higher growth rate regime. At lower growth rate regime, 〈110〉 axial grain orientation was often observed. In all the rods, a range of phase spacing existed. The average spacing was more sensitive to the temperature gradient than the growth rate. Large spacing was achieved with lower temperature gradient and lower growth rates. It was also found that different partition of Tb and Dy in the Laves phase and RE-rich phase existed, as well as the microsegregation of Tb and Dy. These led to a higher Tb/Dy atomic ratio in the RE-rich phase and a variation of the Tb/Dy ratio in the cellular Laves phase sheets. To explain the different axial grain orientations, a model was proposed. An orientation selection principle was adopted such that an orientation with a higher local interface temperature among competing orientations was preferred. The effects of the kinetic attachment anisotropy and the surface tension anisotropy were considered.

Dendrite growth from the supercooled melt

The growth rates for silver and copper are measured as a function of the bath supercooling. The measured growth rates are scattered, but the growth rates of free dendrite are determined from the maximum value at each supercooling. Thus the relations v = 0.95 (ΔT)1.7 and v = 0.53 (ΔT)1.8 are obtained respectively. Using the solution by Trivedi, the linear kinetic coefficient μ0 (cm/sec°C) and the solid-liquid interfacial free energy γ (erg/cm2) are determined by fitting the solution to the experimental data for silver, copper, nickel and cobalt. The values of (μ0, γ) are obtained as (165,111), (158,136), (183,110) and (188,40) respectively.

Effect of oxygen partial pressure on the neodymium solubility in BaCuO solvent

Abstract The neodymium (Nd) solubility in BaCuO solvent with a Ba to Cu ratio of 3 to 5 was investigated under different oxygen partial pressures (P(O2) = 1, 21, 100%). A small amount of the solution was taken out by dipping thermally equilibrated MgO single crystals, and compositions of these specimens stuck with liquid on the MgO, were analyzed by inductively coupled plasma spectrometry (ICP). Sampling the solution for ICP analysis was performed in the temperature range from approximately 1000°C up to 1200°C. The peritectic temperature of NdBa2Cu3O7−δ (Nd123) increased with increasing oxygen partial pressure. The Nd solubility at the peritectic temmperature of Nd123 increased with increasing oxygen partial pressure. The slope of the Nd 123 liquidus lines near the peritectic temperature decreased with increasing oxygen partial pressure. Assuming a regular solution, expressions for the solubility and the enthalpy of dissolution of Nd123 and Nd4Ba2Cu2O10 (Nd422) were derived from classical thermodynamic calculations. The enthalpies of dissolution ΔHdiss for Nd123 and Nd422 decreased with increasing oxygen partial pressure.

Numerical models for casting solidification: Part II. Application of the boundary element method to solidification problems

A new numerical model, which is based on the boundary element method, was proposed for the simulation of solidification problems, and its application was demonstrated for solidification of metals in metal and sand molds. Comparisons were made between results from this model and those from the explicit finite difference method. Temperature recovery method was successfully adopted to estimate the liberation of latent heat of freezing in the boundary element method. A coupling method was proposed for problems in which the boundary condition of the interface consisting of inhomogeneous bodies is governed by Newton’s law of cooling in the boundary element method. It was concluded that the boundary element method which has several advantages, such as the wide variety of element shapes, simplicity of data preparation, and small CPU times, will find wide application as an alternative for finite difference or finite element methods, in the fields of solidification problems, especially for complex, three-dimensional geometries.

95 K NdBCO single crystal grown in air by controlling liquid composition

Using the crystal pulling method, we achieved high superconducting transition temperatures (T c) of Nd1+x Ba2-x Cu3O6+δ (NdBCO) single crystals in air. By controlling the liquid composition of the ratio of Ba to Cu (Ba/Cu) at approximately 0.80, the NdBCO crystal displays high T c of about 95 K with a sharp transition width ΔT of less than 0.5 K. The high Ba/Cu in the liquid leads to the low amount of substitution of Nd for Ba, and consequently results in high superconducting properties. This finding suggests that the Ba/Cu in the liquid is a very important processing parameter for the NdBCO system.

Microstructure of Cu-added Pr-Fe-B magnets: Crystallization of antiferromagnetic Pr6Fe13Cu in the boundary region

Abstract The relations between microstructures and magnetic properties have been investigated in Pr17Fe76B5.5Cu1.5 magnets produced by a process of casting, hot-pressing and heat treatment. The crystallization of Pr6Fe13Cu during heat treatment at 753 K for 2 h increases the coercivity. This explains the enhancement of coercivity with the addition of small amounts of Cu. Pr6Fe13Cu has a tetragonal structure, space group 14/mcm, and lattice parameters a = b = 0.81 nm, c = 2.31 nm. Mossbauer spectroscopy and VSM measurements indicate that Pr6Fe13Cu is an antiferromagnetic compound with a Neel temperature of 391 K. Pr6Fe13Cu crystallizes in a peritectic reaction at 918 K, and is stable below that temperature. In Pr-Fe-B-Cu magnets, Pr6Fe13Cu is formed only within a low boron composition range (below 5.5 at%B). In the system Nd-Fe-Cu a compound Nd6Fe13Cu with the same crystal structure is observed.

Numerical models for casting solidification: Part I. The coupling of the boundary element and finite difference methods for solidification problems

An investigation on the coupling technique for boundary element and finite difference methods was carried out to simulate solidification processes. In the coupling model, solidification problems in casting and unsteady-state heat conduction problems in mold regions were analyzed by explicit finite difference and boundary element methods, respectively. A comparison was made between the coupling model and the finite difference method on the solidification of castings in metal and sand molds. The proper range of time steps for boundary elements in transient problems was presented for a simple geometry. And two types of time marching schemes were proposed for application of the boundary element method to solidification processes.

Construction of a quasi-ternary phase diagram in the NdO1.5BaOCuO system in the air atmosphere Part II. Phase equilibria of the neodymium-rich Nd1+xBa2−xCu3Oz solid solution

Abstract In the present work phase relations in a copper-rich corner of the NdO 1.5 BaOCuO system were studied at 970–1060°C in air by DTA, quenching experiments, EPMA, XRD, ICP AES and finally confirmed by growth of several test single crystals. It was found that in the analyzed region the Nd 1+ x Ba 2− x Cu 3 O z solid solution (Nd123ss) with maximum x values co-exists at 970–1060°C with Cu-rich melt and different solid phases: CuO (970–990°C), Nd 2 CuO 4 (990…995–1045°C) or Nd 4 Ba 2 Cu 2 O 10 (1060°C). The decomposition temperature of Nd123ss decreases with increasing the neodymium content, and the limit composition of Nd123ss extends up to Nd 2 BaCu 3 O z at 990–995°C. The Nd123ss unit cell volume deviations from the ideal solution of the Nd123 and Nd213 phases were found positive by XRD, as well as their excess Gibbs energy of mixing, estimated thermodynamically using the experimental phase diagram. This provides a probability of a spinodal decomposition phenomena of such a solution.