Temporal Evolution of Microstructures on a Nanoscale: Experiments and Simulations

Northwestern University, Evanston IL

Investigators

Abstract

This grant explores the fundamental problem of the temporal evolution of decomposing microstructures in multicomponent alloys on a nanoscale, from both the experimental and simulation points-of-view. A basic understanding of the decomposition processes, involving nucleation, growth, and coarsening of precipitates of metastable or equilibrium phase(s) is technologically and scientifically interesting. A major goal is to analyze nanostructural changes at the discrete atomic level by a combination of atom-probe (3DAP) microscopy and simulation techniques: specifically 3DAP microscopy yields the nanostructure in 3D, based on the millions of atoms collected; in concert with kinetic Monte Carlo (KMC) simulations a nearly complete atomistic picture can be obtained. To obtain a complete analysis of the evolving nanostructures on all relevant length scales, transmission electron microscopies are used. The major focus concerns the effects of the alloying elements Ta, Nb, W, Re or Ru on the evolution of g' (ordered L12 structure) precipitates in a g?matrix (disordered FCC) in twelve different model nickel-base superalloys. Ternary, quaternary, quinary and sexinary alloys are fabricated by adding one or several of these elements to an 81.5 Ni-8.5 Cr-10.0 Al (at.%) base composition. The kinetics of decomposition in model Ni-Al-Cr base alloys are also studied using KMC simulations, where diffusion of atoms on a rigid lattice is mediated by a monovacancy mechanism. KMC simulations allow one to study in direct lattice space the temporal evolution of a microstructure starting from a random solid solution and then following the decomposition at a selected aging temperature in real time and direct space. A subset of the alloys will be subjected to creep testing under constant compressive or tensile stress in air over the relevant temperature range (980 to 1150 degrees C) and strain rates (10-8 s-1 to 10-4 s-1). %%% The 3DAP microscope is utilized to determine: (1) temporal evolution of the distribution of all elements with subnanoscale spatial resolution; (2) partitioning of the alloying elements between the g? and g' phases; (3) segre-ga-tion at the g?/g' heterophase interfaces; (4) number density of the g' precipitates as a function of time; (5) super-saturation of each element as a function of time; (6) mean diameter of g' precipitates as a function of time. This research program involves undergraduate students through the REU program along with outreach at Evanston Township High School (ETHS), which has resulted in the employment of both juniors and seniors at ETHS for developing and maintaining full-featured data analysis application, ADAM 1.5 (Atom Probe Data Manipulation). An important goal of this research is to provide an experimental and quantitative verification of the properties and performance of alloys with fully characterized nanostructures. ***