Papers by Author: Masahiko Morinaga

Abstract: A concept for alloy design of Nb-based hydrogen permeable alloys has been proposed based on the mechanical properties of niobium in hydrogen atmosphere and also on the hydrogen chemical potential in metal membrane. Following this concept, Nb-based alloys are designed and developed that possess excellent hydrogen permeability without showing any hydrogen embrittlement.

Abstract: The state of the microstructure of ferritic heat resistant steels during creep was evaluate by the system free energy, which composes mainly chemical free energy, surface energy and elastic strain energy, and its stress dependence was expressed quantitatively by using a relaxation time. The steels used in this study were P91 (9Cr-1Mo-C-N-V-Nb) steel and P92 (9Cr-Mo-W-C-N-V-Nb-B) steel. The obtained results are as follows: (1) the relaxation time of elastic strain energy was expressed as a function of stress and temperature, (2) surface energy of P92 scarcely decreased during creep due to the formation of the Laves phase, and (3) the relaxation time of the chemical free energy in P92 was larger than that in P91.

Abstract: Phase-field simulation of phase transformation during creep in Type 304 austenitic steel is performed and simultaneous nucleation and growth of both M23C6 carbide and ferromagnetic α phases are reproduced. Nucleation events of these product phases are explicitly introduced through a probabilistic Poisson seeding process based on the classical nucleation theory. Creep dislocation energy near the carbide is integrated into the nucleation driving force for the α phase. We examine the effect of the dislocation density on precipitation of the α phase, and it is found that a small difference in the dislocation density leads to a significant change in precipitation behavior of the α phase.

Abstract: The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, , and the difference of hydrogen concentration, C, between the inlet and the outlet sides of the membrane. It is found that the hydrogen diffusion coefficient during the hydrogen permeation is increased by alloying ruthenium or tungsten into niobium. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion decreases by the addition of ruthenium or tungsten into niobium.

Abstract: The modulated structure characterized by periodic concentration fluctuations is caused by the up-hill diffusion of solute atoms due to the spinodal decomposition, when the Gibbs free energy surface of the solid solution has a convex curve. This convex shape in the free energy surface will appear in multi-component systems, even though the energy in each binary system does not show the convex curve, when a binary system exhibits a strong attractive interaction, i.e., the Gibbs free energy shows a deep concave curve. This may be the case that steel containing an element having a strong tendency of carbide forming. The Fe-Ni-V-C system was selected in this study. V is a strong carbide forming element and forms VC carbide based on f.c.c. lattice. A series of experiments was carried out using Fe-25Ni-3V-3C (mol%) alloy after aging at elevated temperatures. A modulated structure was observed in specimens aged at temperature range of 550 °C to 650 °C, and the maximum temperature occurring the spinodal decomposition was estimated to be 715 °C in Fe-25Ni-3V-3C alloy. Furthermore, the coefficient of the gradient energy caused by composition fluctuations was estimated to be 2.8 x 10-15 J.m2/mol.

Abstract: Interdiffusion coefficients of Al replacing elements in Ni-Al-X (X=Ti, V and Nb) were estimated by a series of experiments using diffusion couples of Al rich pseudo-binary systems at three different temperatures of 1423, 1473 and 1523K. In order to obtain interdiffusion coefficients of the pseudo-binary systems, the experimental data was analyzed by the Sauer and Freise method, and also impurity diffusion coefficients of Ti, V and Nb in Ni3Al were estimated by applying the Darken-Manning equation. The magnitude of interdiffusion coefficient decreased in order of V, Ti and Nb at all three temperatures. Impurity diffusion coefficients were described by the expressions: , , . The activation enthalpies obtained from the experimental data confirmed the retardation of Ti, V and Nb diffusion in Ni3Al by the anti-site diffusion mechanism. These results are consistent with our previous work on diffusion of Re and Ru in Ni3Al .

Abstract: A phase-field simulation is performed to examine the effect of elastic inhomogeneity between the  and ’ phases on coarsening of the ’ phase in Ni-based superalloys. In the calculation of elastic strain energy, the mechanical equilibrium equation in elastically inhomogeneous system is solved by an iterative-perturbation scheme. On the basis of the elastic constants of a practical Ni-based superalloy, a series of simulations is performed in which both elastic anisotropy and shear modulus are varied independently. The variation of elastic anisotropy gives significant effect on both morphology and size distribution function of the ’ particles, whereas the variation of shear modulus gives little effect on them. Furthermore, it is found that the coarsening rate constant of the cubic growth raw changes and increases with increasing the standard deviation of the ’ size distribution.

Abstract: The hydrogen diffusion coefficients are investigated during the hydrogen permeation through Nb-based hydrogen permeable membranes at high temperature. It is found that the hydrogen diffusion coefficient for pure niobium under practical conditions is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.

Abstract: Interdiffusion coefficients of the refractory elements in Fe-W-Re and Fe-Cr-X (X=Mo, W) ternary alloys have been measured on the basis of the modified Boltzmann-Matano method for ternary system. Both the cross interdiffusion coefficients, Fe ReW ~D and Fe WRe ~D were negative in Fe-W-Re ternary alloys. This result indicates that attractive interaction exists between W and Re atoms in iron alloys [1]. This is consistent with our previous experimental results that Re suppresses W diffusion in Fe-15Cr alloy [1]. In addition, the value of cross interdiffusion coefficient Fe CrW ~D was positive in Fe-Cr-W diffusion system, whereas Fe MoCr ~D was negative in Fe-Cr-Mo diffusion system.

Abstract: Ni-based superalloys are strengthed by refractory elements such as Re, Ru and W [1]. Thus, the information on the interdiffusion coefficient as well as the thermodynamic interaction between the refractory elements is important for the future alloy design. In this study, interdiffusion coefficients of the refractory elements in Ni-X-Y (X, Y=Co, Re, Ru, W) ternary systems were estimated by a series of experiments. In the all systems studied in the present works, the main interdiffusion coefficients were much larger than the cross interdiffusion coefficients. In some systems, two cross interdiffuion coefficients had opposite signs each other. For example, in Ni-Co-Ru system, the main interdiffusion coefficients are 2.7 10 14 ~ Ru = × − CoCo D and 15 6.8 10 ~ Ru = × − RuRu D , while the cross interdiffusion coefficients are 16 6.6 10 ~ Ru = − × − CoRu D and 16 8.9 10 ~ Ru = × − RuCo D at 1523K. In Ni-Co-Ru and Ni-Re-Co systems, the activation energies and frequency factors for two main interdiffusion coefficients. For example, Q (kJ ) Co = 268 , 5 ( 2 1 ) 0( ) 4.4 10 D = × − m s − Co , 3 ( 2 1 ) 0( ) 2.9 10 D = × − m s − Ru in Ni-Co-Ru system.