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Sudbrack, Chantal K. |
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Decomposition Behavior in Model Ni-Al-Cr-X Superalloys: Temporal Evolution and Compositional Pathways on a Nanoscale |
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2004 |
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209 |
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In model Ni-Al-Cr-X superalloys, the compositional pathways and temporal evolution of coherent γ' (L12)-precipitation from an isothermally aged solid-solution, γ (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 γ'-precipitates’ morphology is a mixture of isolated spheroids and spheroids in various stages of coalescence. Sub-nanometer scale compositional profiles across the γ/γ' 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 γ'-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 γ/γ' 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 γ'-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 γ'-precipitates. The coarsening kinetics of the mean radius and interfacial area per unit volume obey t1/3 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 γ'-phase and Cr to the γ-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. |
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Northwestern University |
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Ph.D. thesis |
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I want to bring to your attention a mistake in my thesis with respect to the units on the compositional rate constant, kappa, which was a caught in the review process of recent submission of mine to Acta Mater. Kappa is obtained by fitting experimental data of the supersaturation del C of a particular phase, to: delC = C(t)- C(eqm)= kappa * t^(-1/3) so the units of kappa are at.% s^(1/3) NOT at.% s^(-1/3). Please make note of this. In my thesis, it applies to Figures: 4.1 (page 89), 4.2 (page 90), 4.3 (page 94) and Table 4.1 (page 95). |
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