AN INTEGRATED APPROACH FOR MICROSTRUCTURE SIMULATION: APPLICATION TO NI-AL-MO ALLOYS

Open Access
Author:
Wang, Tao
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 03, 2006
Committee Members:
  • Zi Kui Liu, Committee Chair
  • Long Qing Chen, Committee Chair
  • Padma Raghavan, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
Keywords:
  • Microstructure evolution
  • Ni-base superalloy
  • First-principle
  • CALPHAD
  • Phase-field
Abstract:
The properties and performance of a material are strongly dependent on its microstructure. For example, the gamma' precipitate coherently embedded in the gamma matrix is the primary strengthening phase in Ni-base superalloys, and its volume fraction, morphology and size distribution largely determine the strength, fatigue and creep properties of an alloy. In the present study, a multiscale computational approach was proposed to predict the microstructure evolution in Ni-base superalloys. It integrated a quantitative phase-field model with first-principles calculations as well as the CALPHAD (CALculation of PHAse Diagram) technique. Fundamental materials property databases such as lattice parameters and atomic mobility were developed. A phenomenological model was developed to describe the lattice parameter in solid states as a function of temperature and composition, and successfully applied to Ni-Al binary system by evaluating the model parameters using experimental data. An integrated computational approach was also proposed for evaluating the lattice misfit between the matrix and precipitates by combining first-principles calculations, existing experimental data and phenomenological modeling when the experimental data is limited. The lattice parameters and the local lattice distortions around solute atoms in gamma-Ni solutions were studied using first-principles calculations. The solute atoms considered include Al, Co, Cr, Hf, Mo, Nb, Re, Ru, Ta, Ti and W. The effects of the atomic size and the electronic and magnetic interactions on lattice distortion have been discussed. Atomic mobility in disordered gamma and ordered gamma' phases was modeled for the Ni-Al-Mo ternary system, and a kinetic database was developed. The diffusion of Al in gamma' was simulated, and the formation energies of vacancy in different sublattices were calculated by first-principles approach, both of which indicate the anti-site diffusion mechanism being dominant for diffusion of Al. The phase-field model for binary Ni-base superalloys was extended to ternary systems and integrated with the corresponding thermodynamic, kinetic and lattice parameter databases. The microstructure evolutions and coarsening kinetics of gamma' precipitates in Ni-Al-Mo alloys were studied by two-dimensional phase-field simulations. The effects of volume fraction of precipitates and Mo concentration have been analyzed.