1. The Effects of Cr Additions on the Morphologies of  ’(L1 2 ) Precipitates David N. Seidman, Northwestern University, DMR 0804610 The effects of Cr additions on the morphologies of  ’(L1 2 ordered) precipitates were studied in three Ni-Al based alloys utilizing atom-probe tomography and first-principles calculations. Figure. A comparison of the morphology of the precipitate phase with experimental 3-D atom-probe tomography reconstructions and calculated Wulff construction based on first-principles calculated values of the (100), (110) and (111) interfacial free energies at 600 o C: (a) experimental  ’(L1 2 ), aged in bulk for 4096 h at 823 K; (b) experimental  ’(L1 2 ) phase in a Ni-7.5 Al -8.5Cr at.% alloy. The alloy samples were aged at 600 o C for 1024 h. (c) (d) (f) calculated  ’-Ni 3 Cr(L1 2 ) precipitate phase in Wulff construction shape, which transform from cubic-to-spheroidal.

2. The Effects of Refractory Elements on Ni Excesses at γ'/γ Interfaces in Ni-based Superalloys David N. Seidman, Northwestern University, DMR 0804610 The atom-probe tomographic concentration profiles (right) across the γ/γ' interface for Ni for model Ni-based alloys aged at 1073 K for 256 h, with respect to Al iso-concentration surfaces. We observed that interfacial excesses exist due to strong R- Cr and R-Al binding, which significantly decelerate the diffusion fluxes across γ'/γ interfaces even after 256 hour of aging. Employing atom-probe tomography and first-principles calculations, we study the effects of refractory (R) elements on the interfacial excesses of the solvent element Ni in Ni-based superalloys..

3. Temporal Evolution of the γ(fcc)/γ’(L1 2 ) Interfacial Width in Binary Ni-Al Alloys David N. Seidman, Northwestern University, DMR 0804610 The interfacial width, δ(t), between the γ- and γ’-phases normalized by the mean radius, 〈 R(t) 〉, of the γ’-precipitates as a function of time, i.e., 〈 R(t) 〉. The solid- circles display experimental data points and the solid-triangles indicate vacancy- mediated LKMC simulations results. It is demonstrated that the δ(t)s decrease with increasing mean precipitate radius, 〈 R(t) 〉. The ratio δ(t)/ 〈 R(t) 〉 decreases, to first order, as 1/ 〈 R(t) 〉. The temporal evolution of γ’(L1 2 structure) precipitates is studied in Ni-12.5 Al and Ni-13.4 Al at.% alloys, aged at 823 or 873 K, utilizing three-dimensional atom-probe tomography (APT) and lattice kinetic Monte Carlo (LKMC) simulations.