Study On The Phase Equilibria Of The Al-Cr-Nb-Ti Quaternary System

Titanium aluminide alloys based on the gamma TiAl phase attact a lot ofattention from the research community due to its high melting point, low density,high specific strength and modulus, good structural stability, good resistance againstoxidation and corrosion, and high ignition resistance. It is crucial to investigate theproperties-composition-structure interrelationship and the alloying mechanism.Phase diagram is the basis for alloying. The CALPHAD method, proposed in the1970s, provides an effective way to study the multi-components phase equilibria andmaterials design. As a part of the project entitled Investigation on the phase equilibriaand alloying mechanism of new TiAl alloy （No.51071142）, this thesis built thethermodynamic database for the Al-Cr-Nb-Ti system over the whole compositionrange under the framework of CALPHAD method via the key experiments, firstprinciple calculations and thermodynamic assessment. The main content of the thesisare as follows.（1） Based on the critical review of the related experimental information of phasediagram and thermochemistry and thermodynamic assessment of the sub-binary andsub-ternary of the Al-Cr-Nb-Ti quaternary system, a preliminary thermodynanmicdatabase for the Al-Cr-Nb-Ti quaternary system was built which included thethermodynamic descriptions of the six binary systems Al-Cr, Al-Nb, Al-Ti, Cr-Nb,Cr-Ti, Nb-Ti and one ternary system Al-Nb-Ti.（2） Phase equilibria experiments was designed to clarify the reportedcontradictionary information in the Cr-Nb-Ti ternary system. The phase constitution,phase composition and microstructure of the samples heat treated at1200℃weremeasured by X-ray diffraction analysis and electron probe micro analysis.（3） The Gibbs energies of the liquid, bcc and hcp phases were described by thesubstitutional solution model. The Laves_C14, C15and C36were described by thetwo sublattice model of （Cr,Nb,Ti）2（Cr,Nb,Ti）. The Gibbs energy of the end membersNb₂Ti and Ti₂Nb were obtained by the first principles calculations. Thethermodynamic parameters for the liquid, bcc, hcp and laves phases of the Cr-Nb-Ti system were obtained via the classical assessment. The calculated phase diagramswere in reasonable agreement with most of the experimental data.（4） The Al-Cr-Nb and Al-Cr-Ti systems were reassessed to solve theinconsistency in the thermodynamic descriptions for the sub-binary sytems used. Athermodynamic database for the Al-Cr-Nb-Ti system over the whole compositionrange was built.