Thermodynamic Assessment Of Co-Ge Binary System And Phase Relations Of Ti-Co-X (X=Ge, Si) System

Heusler alloy is a popular and new functional material for both ferromagnetic and thermoelectricity areas in recent thirty years, especially in spin electronic devices (such as magnetic recording and magnetic sensor), the wide applications of which has become a hotspot. Ti-Co-X (X=Ge/Si) system contains Co2TiX compound, which is a typical semi metal with ferromagnetic properties. The spin electrons in band structure of the alloy are the main cause to produce this special property, and the doped X atoms are usually taken to improve the polarized electron transport abilities. Traditional production methods could not effectively predict and hardly prepare a series of half metallic ferromagnetic alloys with different elements combination. Therefore, it is significant that constructing database of Ti-Co-X ternary system by thermodynamic calculation will lead and instruct the frontier research of half metallic ferromagnetic alloy formation with multi-components doping at different temperatures. By experimental measurement and the calculation of phase diagram (CALPHAD) method, this paper did thermodynamic calculation of Co-Ge binary system and the phase equilibrium relations research of Ti-Co-X (X=Ge, Si) system as follows:1. Two intermetallic compound CoGe and CoGe2, their standard enthalpies of formation and crystal lattice parameters are obtained by using the First Principles calculation, respectively. According to the reported experimental data and assessment of Co-Ge binary system, CALPHAD method was used to optimize the system above. Not only a set of self-consistent parameters is obtained, but also a better thermodynamic description could help reproduce Co-Ge phase diagram with reasonable literature data.2. By using alloy method, the isothermal section of Ti-Co-Ge ternary at1273K has been constructed. A new phase with component approximation Ti7Co12Ge14was found there. The diffusion couple approach was adopted to determine the diffusion path of Ti-30at.%Ge/Co diffusion couple at1073K, which turned out to be Co/β Co5Ge3/TiCo2Ge/Ti-30at.%Ge.3. By using alloy method, the isothermal section of Ti-Co-Si ternary at1273K has been constructed. It verified that the high temperature phases Ti4CoSi4(τ8) and Ti3CO2Si (τ9) existed at1273K.