Tomonori Kitashima

Coupling of the phase-field and CALPHAD methods for predicting multicomponent, solid-state phase transformations

The development of an effective microstructure design method for multicomponent alloys is of considerable importance for improving both the design of alloys and the design of processes for producing alloys with unique properties. The coupling of the phase-field method and the calculation of phase diagrams (CALPHAD) method can be used for predicting the evolution of microstructures in multicomponent alloys. Such predictions make use of CALPHAD thermodynamic information with the chemical free energy function in the phase-field method. This article reviews several of these coupling methods, focusing on solid-state phase transformations in multicomponent systems, such as phase separation and disordered or ordered phase precipitation from a matrix. When calculating disordered phase transformations, the Gibbs energy function derived from the CALPHAD database can be used directly in the phase-field method. On the other hand, when dealing with an order/disorder transition, the degrees of freedom of the element site fraction for an ordered phase in the CALPHAD method can be reduced using the Gibbs energy single formalism for constituent phases, by using a database that stores the Gibbs energy and chemical equilibrium conditions, or by obtaining the driving force calculated using the Thermo-Calc software. The current status and future directions for further development of these coupled methods are discussed.

Effects of Ga and Sn Additions on the Creep Strength and Oxidation Resistance of Near-α Ti Alloys

The effects of Ga and Sn additions, with an almost constant value regarding Al equivalence, on the creep properties and oxidation resistance of the near-α Ti alloy Ti-5Al-4Zr-1Mo were investigated. The creep strain rate increased due to the replacement of Sn with Ga, which was accompanied by an increase in the volume fraction of primary equiaxed α phase. In addition, the replacement of Sn with Ga decreased the activation energy of the steady-state creep rate for the similar volume fraction of the equiaxed α phase. Ga addition improved the oxidation resistance without Ga segregation at the TiO2/substrate interface, whereas Sn promoted oxide spallation with metallic Sn segregation at the interface. Ga uniformly dissolved into the internal TiO2 and into the external Al2O3 as a result of solid solution formation between Ga2O3 and the aforementioned oxides, which suppressed oxide growth. The formation of (GaAl)2TiO5 in TiO2 and Al2O3 was also discussed on the basis of the phase relationships in the TiO2-Al2O3-Ga2O3 system.

First principles study of oxidation of Si-segregated α-Ti(0001) surfaces

The oxidation of the α-Ti(0001) surface was studied using the density functional theory. To enhance the oxidation resistance, we substituted Ti atoms with Si atoms on the Ti(0001) surface. We observed that Si prefers to segregate at the surface layer of Ti(0001) compared with other subsurface layers. The Si solubility on the Ti(0001) surface, based on the segregation energy, was found to be much larger than the corresponding bulk solubility. Si segregation was found to reduce the binding between the oxygen atom and the Ti(0001) surface and hinder the diffusion of oxygen atoms into the slab. The dissociation of the oxygen molecule on the clean and Si-segregated surfaces of Ti was found to be barrierless. Overall, the Si segregation on the Ti(0001) surface was found to hinder the dissolution of oxygen in Ti.

Virtual Jet Engine System

In 1999, we proposed the concept of a virtual gas turbine system which is a combination of turbine design and material design programs. Using this system, it has become possible to design a gas turbine engine and a combined cycle automatically, by inputting some basic information such as power output, turbine inlet temperature and material specifications. The derived outputs are turbine gas path dimensions, gas and cooling air flow rates, thermal efficiency, CO2 emissions, etc. We use the system to evaluate the potential improvement if a newly developed material is to be used in building the engine. Based on the virtual gas turbine system we have begun developing the virtual jet engine system, which can simulate the operation of a jet engine or a gas turbine engine to predict the degradation of materials used in the high temperature parts of the engine. The system consists of a thermal and aerodynamic analysis of the engine, a thermal and stress analysis of hot parts, and a material degradation analysis. Actual engine dimensions, operation data and material specifications are used to perform the analyses. In this paper, we will show some of the results of the use of the virtual gas turbine system, and then describe the development plan and the preliminary output of the virtual jet engine system.

Multiscale Multiphysics Simulations for Development of High Temperature Alloys in Jet Engines

High temperature materials such as nickel-base superalloys and titanium alloys are used in the compressor and/or the turbine of a jet engine. Multiscale multiphysics simulations to be used for development of such high temperature alloys, which combine theoretical and empirical methods, are described in this paper. In addition, a new phase-field method coupling with the CALPHAD method to simulate gamma-prime precipitation of multicomponent Ni-base superalloys in two dimensions is proposed as part of the multiscale multiphysics simulations. This phase-field method demonstrates both diffusion-controlled precipitate growth under local equilibrium at phase interfaces and precipitation interaction such as coalescence.