An Atom-Probe Tomography and Lattice Kinetic Monte Carlo Study of Phase Separation in Ni-Al-Based Alloys from the Atomic Scale Up to Link with Continuum Theories

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TECHNICAL SUMMARY: The kinetic pathways in the early stages of phase separation of a concentrated multicomponent supersaturated solid-solution are critical to the development of the final microstructure, which is of significant technological importance for controlling the physical and mechanical properties of a material at the macroscopic scale. On the theoretical side, kinetic pathways are described in terms of nucleation, growth and coarsening, whose theory implies diffusion in the matrix, transfer of atoms across a matrix/precipitate interface together with the assumption of local equilibrium. The most recent achievements of high-yield atom-probe tomography (APT) and fast lattice kinetic Monte Carlo (LKMC) simulations make it possible to describe phase separation at the atomic scale. The necessary parameters for the thermodynamics and kinetics are deduced from first-principles calculations and experimental fits for the LKMC parameters. In the case of coherent precipitation, phase separation is simply the footprint left by the myriad of vacancy jumps in the inhomogeneous distribution of atomic species. Because the same vacancy jumps are at work in the equilibrium solid-solution, a link is established between features of kinetic pathways and diffusion in a solid-solution. Our APT experimental results demonstrate directly that correlation effects (flux-couplings) in solid-state diffusion control the early stage kinetic pathway for coherent-phase separation. Such effects are neglected by all extant variants of nucleation theory, which all rely on a simplified form of the diffusion matrix. We have developed a transformative approach that employs state-of-the-art APT coupled with LKMC simulations and diffusion theory for phase separation, which studies the kinetic pathways experimentally and models it via simulations and diffusion theory. And we have demonstrated the uniqueness of this approach for concentrated Ni-Al-Cr alloys. The combined use of APT and LKMC in model Ni-base superalloys provides a unique data-base (experimental and LKMC results) for developing a new theory of nucleation in binary (Ni-Al) and concentrated multi-component alloys (Ni-Al-Cr and Ni-Al-Mo). Classical nucleation theory assumes that the atomic fluxes are such that the decomposition path follows the steepest slope of the configurational free energy surface, whereas kinetic coupling of the fluxes is one reason why this isn't generally correct: also the disparate diffusivities involved is another reason. Our approach will take into account the coupled-flux effects that lead to nucleation of a second phase. In this new approach the interfacial width should be affected by the diffusion mechanism (unlike in the Cahn-Hilliard formalism). Additionally, we propose to implement a parallel LKMC simulation code for multi-component systems based on perfect time synchronicity, which is based on a code developed by Martinez et al. at LANL that will accelerate the LKMC simulations. The proposed research will lead to a completely different way of viewing kinetic pathways for phase separation of a solid-solution, and hence, nucleation, growth and coarsening.NON-TECHNICAL SUMMARY: Our unique approach to phase separation makes extensive use of a local-electrode atom-probe (LEAP) tomograph in the Northwestern University Center for Atom-Probe Tomography (NUCAPT). NUCAPT is unique university facility in the US, which has had and will have a significant impact on the research efforts of undergraduate work-study, senior thesis, research experiences for undergraduates (REU), M.S., Ph.D. and postdoctoral students: during the last four years many underrepresented groups of students have used NUCAPT. In the Department of Materials Science and Engineering at Northwestern six professors and their students make extensive use of NUCAPT. Additionally, NUCAPT has users from other US universities, and both national and industrial laboratories. Research results based on atom-probe tomography are published in the archival literature and our web site, http://nucapt.northwestern.edu , has a library from which reprints can downloaded as PDF files. Through NUCAPT we have educated a large number of people in the application of atom-probe tomography to a wide range of materials science and engineering problems, which is important for characterizing materials at the subnanoscale scale. Additionally, we are interacting with a materials scientist in France on the subject matter of this research and are supplying service to industrial companies in the US and abroad. There is no one else in the US doing this to the extent that we are accomplishing this objective.