Microstructure Evolution During Al, Ti, and Mo Surface Deposition and Volume Diffusion in Ni-20Cr Wires and Woven Struct
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INTRODUCTION
NOVEL architectured materials can be developed efficiently using topological optimization methods to predict ideal material architectures, and advanced 3D textile-based manufacturing, such as weaving and braiding, can fabricate large volumes of these periodic architectures, which may possess unusual combination of high permeability (thus heat transfer properties) and superior mechanical properties.[1–6] Successful fabrication of architecturally designed Ni-base superalloy woven structures is of particular interest to actively cooled structural materials operating in high-temperature, high-stress, oxidative, and corrosive environments, e.g., shrouds, and load-bearing thermal management units including heat exchangers and heat pipes.[7–9] The most direct route would be to braid and weave structures with wires made from the final, desired Ni-base superalloy. However, this approach is impractical as these alloys are not sufficiently ductile to be drawn into fine wires and/or to achieve the small bending radii needed for braiding. Thus, a novel fourstep method is studied, where (i) ductile, commercial Ni or Ni-Cr wires are woven, taking advantage of their high ductility and low cost, (ii) they are then alloyed via surface deposition with further desirable elements, (iii) homogenized via annealing, and (iv) aged to create a desired microstructure. Often, steps (iii) and (iv) can be done in a single operation. Wire bonding is an optional step that can be carried out before, during, or after steps (ii to iii).[10,11]
Here, two methods are studied to alloy woven Ni-Cr structures into a superalloy composition. First, gas-phase deposition is used to add Al and Ti, which are the two main alloying elements necessary for the c + c¢ superalloy microstructure. This process has already been demonstrated by chromization and aluminization of Ni-foams[12,13] and Ni or Ni-20Cr wires.[10,11,14–16] In a parallel trial, a refractory element, Mo, is also added by the same method to achieve superalloy compositions optimized for enhanced coarsening resistance, as recently demonstrated by us.[15,16] We perform a systematic investigation to achieve the superalloy structure via sequential pack cementation of Mo, Al, and Ti on Ni-20Cr wires, either individually or as 3D woven structures. We show that samples in both cases develop significant amount of Kirkendall pores after Mo gas-phase deposition. By contrast, uniform triple-layered coatings are created after Al and Ti codeposition without additional Kirkendall porosity. Upon homogenization and aging, the layered coatings dissolve and diffuse into the wire until full homogenization is reached, and the desired c + c¢ microstructure is created. The successful implementation of the in situ alloying technique paves the way for creating architecturally designed woven or braided superalloy structures combining unusual pairs of properties, such as high permeability and high-specific stiffness or strength. II.
EXPERIMENTAL PROCEDURES
A. Pack Cementation CONG WANG, Professor, formerly with the D
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