A New Experimental/Computational Approach for Predicting Phase Evolution and Defect Thermodynamics: Application to Concentrated Multicomponent Ni-Based Superalloys

Northwestern University, Evanston IL

Investigators

Abstract

TECHNICAL: PIs will study temporal evolution of microstructures in concentrated Ni-based superalloys, which are used in the aviation, electric generation, chemical, and gas and oil industries. Their microstructures consist of coherent ordered gamma?(g?)-precipitates in a disordered FCC gamma(g)-matrix. There are significant unanswered questions concerning the early stage nucleation of the coherent g?-phase and phase separation from the g-matrix. Additionally, the temporal evolution of the crystallography and morphology of the g?-precipitates will be studied at all relevant length scales (micron to subnanoscale) using scanning electron, electron transmission, and high-resolution electron microscopies. In parallel with the experimental research, PIs will perform lattice kinetic Monte Carlo (LKMC) simulation, based on a vacancy diffusion mechanism, to simulate the temporal evolution of the microstructures at the same length scale as the LEAP tomograph reconstructions, where the kinetic parameters are experimental and/or calculated from first-principles density functional theory (DFT). PIs will study the kinetic pathways for nucleation in Ni-Al-Cr alloys whose mean compositions are close to the solvus line and therefore they may follow classical nucleation theory (CNT). Additionally, PIs will study two Ni-Al-Cr alloys with similar volume fractions of the g?-phase whose mean compositions are farther from the solvus line and hence less likely to obey CNT. All the experimental results are to be compared to LKMC simulation and diffusion theory. To analyze the data, a new theory of nucleation in concentrated multi-component alloys is developed that takes full advantage of a normal mode analysis to reformulate the theory of coherent phase separation that incorporates the complete details of solid-state diffusion. Additionally, PIs are exploring ways to rephrase CNT based on a diffusion equation. Finally, to make a link between the microstructure and the high-temperature mechanical properties of Ni-based superalloys PIs will calculate the Helmholtz free energy of anti-phase boundaries (APBs), sigma-APB. PIs present a new approach, the residence weight algorithm, which combines density functional calculations and Monte Carlo simulations, to calculate the APB free energy. PI?s atomic scale approach to studying the temporal evolution of microstructures is significantly different from existing methods in that PIs combine atomic-scale experimentation, simulation, and diffusion theory. The combined experimental/computational approach will allow an atomistic description that spans the subnanometer to mesoscopic length scales with unprecedented fidelity. NON-TECHNICAL: Through the Northwestern University Center for Atom-Probe Tomography (NUCAPT) PIs are educating undergraduate work-study, senior-thesis, research-experiences for undergraduate students (REU), and research experiences for teachers (RET) to perform atom-probe tomography using a LEAP tomograph. These programs have and will involve men, women and legal minority groups, Hispanic, and African-American. NUCAPT also provides APT services to other universities, national laboratories, and industrial firms. Additionally, PIs will develop a summer workshop on ?Linking Atomic-Scale Experiments and Computations,? to be held during the summer of the 2nd year of the grant at Northwestern. This course will be geared toward graduate students interested in atomic-scale and multiscale computation, and atomic-scale experimentation, as well as young faculty members and industrial participants. PIs are planning for ca. 50 participants and intend to charge a modest registration fee to cover costs.