Effects Of Boron On The Phase Equilibria Of The Al-Cr-Nb-Ti System

Titanium aluminide alloys based on the gamma TiAl phase attract a lot of attentions due to its high melting point,low density and good structural stability.Additions of alloying elements of B,Cr and Nb will greatly improve the properties of gamma TiAl alloys.B and Cr will improve the plasticity and fracture toughness,while Nb will improve resistance against oxidation.This dissertation aims to investigate the effects of boron on the phase equilibria of the Al-Cr-Nb-Ti system.Under the framework of CALPHAD(CALculations of PHAse Diagrams)method,a thermodynamic database for the Al-B-Cr-Nb-Ti system were built by the hybrid approach of key experiments and CALPHAD assessment.The main contents of this dissertation include as follow:(1)Thermodynamic descriptions of the Al-B,Al-Nb,Al-Ti,Al-B-Ti and B-Nb-Ti systems were built based on the systematic assessment of the related experimental information of phase diagram and thermochemistry and thermodynamic assessment of the sub-binary and sub-ternary of the Al-B-Cr-Nb-Ti system.The B-Cr binary system was reoptimized and the thermodynamic parameters were obtained.(2)The phase equilibria of the B-Cr-Nb system at the Cr-Nb rich side,and the B-Cr-Ti system at the Cr-Ti rich side were determined by X-ray diffraction and electron probe micro analysis.The corresponding isothermal sections at 1000? were built.(3)Substitutional solution model was used to describe the solution phases and the sub lattice model to describe the intermediate phases.On this basis,the thermodynamic parameters for the phases presented in the B-Cr-Nb system at the Cr-Nb rich side,and the B-Cr-Ti system at the Cr-Ti rich side were obtained.The calculated phase diagrams were in reasonable agreement with the experimental data.