Phase Diagram Optimization And Material Design Of Ti-Al-Nb-Zr Quaternary System

TiAl-based alloys have been widely used in aerospace and automotive industry due to their low density,high temperature specific strength and high oxidation resistance.However,the defects of room temperature brittleness and high temperature creep resistance limit its application in engineering.The research shows that alloying is an important method to improve the properties of TiAl-based alloys.The addition of high melting point element Nb can not only maintain the low density and high strength of TiAl-based alloys,but also effectively improve its room temperature plasticity and machinability.It is found that the strength of TiAl-based alloys are greatly improved by solution strengthening of Zr element.It is well known that phase diagram is a guide book for material design and processing.In the present paper the thermodynamic description of the quaternary Ti-Al-Nb-Zr system were obtained by CALPHAD method,which is of great importance for the development of Ti-based biomedical materials.The main research contents are described as follows:(1)The thermodynamic description of the ternary Ti-Al-Nb system was obtained by Pandat software^?,according to the thermodynamic and experimental data from the literatures.The calculated phase equilibrium and thermodynamic properties are consistent with most of the experimental data reported in the literatures.(2)8Nb-TiAl alloys with different Al contents were prepared by the combination of investment casting and centrifugal casting.The morphology of the 8Nb-TiAl alloys were analyzed by means of X-ray diffractometer(XRD)and scanning electron microscopy(SEM).Moreover,the actual solidification paths were obtained by the calculated phase diagram.(3)The thermodynamic descriptions of the binary Ti-Zr system were obtained by Pan Optimizer,according to the experimental data.Subsequently,the thermodynamic descriptions of the ternary Ti-Nb-Zr system were evaluated by combining the thermodynamic parameters of Ti-Nb and Nb-Zr binary systems.The calculated phase equilibria and thermodynamic properties are in good agreement with the most of the experimental data reported in the literatures.(4)The thermodynamic descriptions of the ternary Al-Ti-Zr system in literature were adopted in present work,while had some inconsistency in thermodynamic parameters which had been optimized as well.It is found that the calculated isothermal sections are in good agreement with the experimental data.The thermodynamic descriptions of the ternary Al-Nb-Zr system were described using a simple ternary extrapolation of the constituent binary systems with no changing.And then,the thermodynamic database of quaternary Ti-Al-Nb-Zr system was established.The isothermal section of 2073K was described by the 3D-tetrahedron,as well as the relative phase surfaces.The vertical sections of quaternary Ti-Al-Nb-Zr system were also calculated.(5)According to the equilibrium phase diagram,several Ti-Al-Nb-Zr quaternary alloys were designed,as well as the equilibrium solidification paths and the prediction of the microstructures and properties.