| In model Ni-Al-Cr-X superalloys, the compositional pathways and temporal evolution of coherent gamma' (L12)-precipitation from an isothermally aged solid-solution, gamma (FCC), is investigated at: (i) 600°C, to study nucleation, growth, and coarsening; and (ii) 800°C, to study the influence of W on coarsening of a Ni-Al-Cr alloy.;In the quenched Ni-5.2 Al-14.2 Cr at.% alloy, radial distribution functions establish Ni3(Al,Cr)-type short-range ordering that extends 0.6 nm and is Cr depleted. Phase separation at 600°C occurs by nucleation and growth, and the gamma'-precipitates' morphology is a mixture of isolated spheroids and spheroids in various stages of coalescence. Sub-nanometer scale compositional profiles across the gamma/gamma ' interfaces reveal: (i) transient chemical gradients of Al depletion and Cr enrichment adjacent to the precipitates; (ii) trapped Cr atoms in the growing precipitates; (iii) the interfacial width is component dependent; and (iv) increased Al solubility in the gamma '-precipitates resulting from capillarity. For a quasi-steady state, the governing power-law time dependencies during coarsening are compared to extant models and discussed in light of recent KMC simulations performed at Northwestern. Independent of the solute diffusivities, the gamma/gamma ' interfacial free-energy is determined from coarsening data to be 22 to 23 mJ m-2.;In Ni-9.8 Al-8.3 Cr at.% and Ni-9.7 Al-8.5 Cr at.%, spheroidal precipitates (5--15 nm diameter) form during quenching. Initially, chemical gradients exist in the gamma'-precipitates, however, they disappear after 1 h. After 16 h aging at 800°C, the precipitates have a cuboidal morphology and align along the elastically soft <100>-type directions. Particle size distributions and spatial pair correlation functions evolve temporally, and are discussed in context of the morphological development of the gamma'-precipitates. The coarsening kinetics of the mean radius and interfacial area per unit volume obey t 1/3 and t-1/3 law, where the addition of W decreases the coarsening rate by a third. The slower kinetics are attributed to W's influence on elemental partitioning, which leads to stronger partitioning of Al to the gamma'-phase and Cr to the gamma-phase, and to its smaller diffusivity. Finally, an inflection-point method for determining reproducible phase compositions from three-dimensional atom-probe data is described, which is important for determining partitioning ratios. |
