Experimental Investigation of Phase Equilibria in the Co-Ta-Si Ternary System

In this work, two isothermal sections of the Co-Ta-Si ternary system at 900 °C and 1100 °C are constructed in the whole composition range via phase equilibrium determination with the help of electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques. Firstly, several reported ternary phases G (Co(16)Ta(6)Si(7)), G″ (Co(4)TaSi(3)), E (CoTaSi), L (Co(3)Ta(2)Si) and V (Co(4)Ta(4)Si(7)) are all re-confirmed again. The G″ phase is found to be a kind of high-temperature compound, which is unstable at less than 1100 °C. Additionally, the L phase with a large composition range (Co(32–62)Ta(26–36)Si(10–30)) crystallizes with a hexagonal crystal structure (space group: P6(3)/mmc, C14), which is the same as that of the binary high-temperature λ(1)-Co(2)Ta phase. It can be reasonably speculated that the ternary L phase results from the stabilization toward low-temperature of the binary λ(1)-Co(2)Ta through adding Si. Secondly, the binary CoTa(2) and SiTa(2) phases are found to form a continuous solid solution phase (Co, Si)Ta(2) with a body-centered tetragonal structure. Thirdly, the elemental Si shows a large solid solubility for Co-Ta binary compounds while the Ta and Co are hardly dissolved in Co-Si and Ta-Si binary phases, respectively.