Crystallography Determination, Phase Diagram Measurement And Thermodynamic Modeling Of The Al-Cr-Si, Al-Cr-Ti, Al-Cu-Fe, Al-Cu-Ni And Nb-Ni Systems

CALPHAD technique is an effective and widely utilized method for assessment and calculation of phase diagrams.It provides a thermodynamic basis for materials kinetics and the computational simulation of the microstructure evolution.Furthermore,it can be widely applied to the design and the synthesis of new materials.Cu,Fe,Nb,Ni, Si,Cr and Ti are important alloying elements or additives in the Al-based alloys.To design alloy compositions,optimize the synthesis process,and effectively improve alloys' performance,an accurate knowledge of the phase diagram and thermodynamics of the multi-component Al-based system is needed.As part of the project "experimental investigation, thermodynamic modeling and simulation of microstructure evolvement of multi- component Al-based alloy system",this work was involved in the investigation of the ternary Al-Cr-Ti,Al-Cr-Si,Al-Cu-Fe and Al-Cu-Ni systems.This thesis is comprised of five parts:(1)The Al-Cr binary phase diagram,especially for the crytal structure and phase equilibria in theγphase region,was experimentally determined.The phase equilibria at 900℃of the Al-Cr-Ti system were determined using XRD and SEM/EDX,and an entire isothermal section was correspondingly constructed.Based on the combination of DTA of the equilibrated alloys and the observation of microstructures of as-melt alloys,information on solidification paths and phase transitions were obtained,and therefore the liquidus projection and Scheil reaction scheme were constructed for the Al-Cr-Ti system.(2)Crystallography of the solid phases in the ternary Al-Cr-Si system was experimentally examined.Three ternary compounds,Ï„₂,Ï„₃ andÏ„₄ were newly found and the crystal structures ofÏ„₂ andÏ„₄ were preliminarily determined.The site occupancies ofÏ„₃ were determined by full profile Rietveld refinement.The phase equilibria at 800℃in the ternary Al-Cr-Si system were investigated and the entire isothermal section was determined.Based on the transition temperatures resulting from DTA of the equilibrated ternary alloys,and the solidification paths from the microstructure analysis of the as-melt alloys,the projection of liquidus surface and the Scheil reaction scheme were constructed.The microstructure evidences from the Cr-Si binary alloys yield that the Cr₅Si₃ phase forms from Cr₃Si via a peritectic reaction.The Cr-Si binary phase diagram was thus revised and its thermodynamic description was updated.(3)The isothermal section at 600℃and the phase relations at the Al-rich corner in the ternary Al-Cu-Fe system were experimentally determined.A thermodynamic modeling of the Al-Cu-Fe system was carried out by fitting the phase equilibria in the entire compositional range and the temperature range from 400 to 1300℃.The A2/B2 order-disorder transition was described by using one equation to model both ordered and disordered phases.The immiscibility gap of the bcc phase and the second order transition between A2 and B2 were well fitted. A series of isothermal sections and the projection of the liquidus surface were calculated for comparison with experimental data.(4)The phase equilibria at the Al-rich corner of the Al-Cu-Ni system were experimentally studied.Based on the experimental results,the phase diagram data,crystallographic data and thermodynamic data were evaluated once again.A set of self-consistent thermodynamic description was obtained for the Al-Cu-Ni system by reproducing all the accepted experimental data.Comprehensive comparisons between the calculations and experimental data showed that the enthalpies of mixing of the liquid were well reproduced,and the isothermal sections at 500,600,700,800, 900,1100 and 1300℃,the invariant equilibria,as well as the projection of the liquidus surface were generally fitted.A Scheil reaction scheme was proposed based on the experimental data and thermodynamic calculations.(5)The Nb-Ni binary system had been experimentally reinvestigated and the stability of Ni₈Nb was confirmed.Based on the present experimental results,the Nb-Ni system was reviewed and a thermodynamic reassessment was conducted.A set of self-consistent thermodynamic parameters for the Nb-Ni system were obatained by utilizing a four-step optimizaing procedure.