Mechanical Behavior of Three-Dimensional Braided Nickel-Based Superalloys Synthesized via Pack Cementation
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Due to their low density and high surface area, porous metals are of interest for many thermostructural applications.[1–6] Use of three-dimensional (3-D) weaving and 3-D braiding technologies makes it possible to produce periodic cellular materials from metal wires with specifically tailored properties.[7–14] For applications at elevated temperatures, e.g., in the aerospace industry, properties, such as creep, oxidation, and corrosion resistance, are required. Nickel-based superalloys are well suited for such applications, but, as weaving and braiding require ductile materials to sustain high bending angles, commercial nickel-based superalloy wires cannot be used for this purpose. Hence, in this study, we decouple the braiding and alloying steps by using ductile Ni-20Cr (all compositions are given in weight percent) wires for 3-D braiding and subsequently alloying them
NICOLAS LIPPITZ is with the Institute for Materials, TU Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany. Contact email: [email protected] DINC ERDENIZ and DAVID C. DUNAND are with the Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208. KEITH W. SHARP is with TexTech Industries, Inc., 1 City Center, Portland, ME 04101. Manuscript submitted October 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
using pack cementation, a chemical vapor deposition process. A multistep heat treatment, consisting of transient liquid phase (TLP) bonding, homogenizing, solutionizing, and aging, is used to form bonds between adjacent wires and to achieve a braided structure made from c¢-strengthened wires with a near uniform composition and microstructure. This process was previously demonstrated for 3-D woven structures[8,15] and microlattices.[16] In this study, it is applied to braided tubes, examining the microstructure development as well as mechanical properties. In addition to monotonous tensile tests, an interrupted tensile test in combination with X-ray tomography is used to analyze the failure behavior of the superalloy braided tubes. Three-dimensional braiding produces a wire (fiber) architecture, where the thickness is equivalent to several layers of a traditional two-dimensional braid and each individual wire can traverse the full thickness of the part. The braided tubes used in this study were fabricated from Ni-20Cr wires with a diameter of 202 lm using a rotary braiding technique described in Reference 8. Each braid contained 160 wires and the final tubular structure had an outer diameter of ~ 6 mm and an inner diameter of ~ 3 mm. Each tube was cut to a length of 70 mm and subsequently coated with Al and Ti using pack cementation. The pack mixture consisted of 57 wt pct Al2O3 powder (filler), 30 wt pct Ti powder (source), 10 wt pct Raney Ni precursor powder (Ni-50 wt pct Al, source), and 3 wt pct NH4Cl powder (activator), as used in prior studies[15] of pack cementation of woven Ni-20Cr wires. The braid and the pack mixture were placed in an alumina crucible, which was then i
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