Microstructural evolution in wire-drawn Ti-22Al-26Nb powder
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I. INTRODUCTION
II. EXPERIMENTAL
ORTHORHOMBIC titanium aluminide alloys, based on Ti2AlNb, have exhibited mechanical properties that render them attractive for use as the matrix in silicon carbide fiber-reinforced composites.[1–5] However, the two most common approaches to fabricating such composites—a foilbased matrix and a powder-based matrix—have their drawbacks. For instance, the processing of these alloys to foil is expensive, because the yield is not particularly high. This is at least partially due to the iterative hot-working and coldrolling steps required to get 0.005-in.-thick foil.[6,7] On the other hand, using powder directly as the matrix in a composite results in poorer than required mechanical properties in the matrix, especially for the cases of room-temperature tensile ductility and elevated-temperature creep resistance.[8] This is apparently due to microsegregation in the powder during gas atomization, to incomplete consolidation, and to a lack of redundant work in the powder material during consolidation. Atlantic Research Corporation has developed a technique for getting redundant hot and cold work into orthorhombic powder by drawing the powder into wire.[9] The wire, with its improved properties over as–hot isostatically pressed (aship’ed) powder, is then used as the matrix material in the fabrication of silicon carbide fiber-reinforced composites. Because the drawing of wire from powder is a new approach for orthorhombic titanium aluminides, it is imperative to follow the evolution of the microstructure and its effect on properties. The objective of this work, then, was to characterize a Ti-26Al-22Nb (at. pct) alloy as it progressed from powder to a 0.127-mm (0.005-in.) gage wire.
Ti-22Al-26Nb (at. pct) prealloyed gas-atomized powder was obtained from Crucible Research (Pittsburgh, PA) in the form of -35 mesh. The powder was hip’ed at 1032 8C into a 3.8-cm-diameter by 40.6-cm-long billet prior to extrusion at 1032 8C. The extruded bar was then drawn down to wire using successively smaller dies, with intermediate anneals at 1032 8C. The final wire diameter was 0.127 mm (0.005 in.). Microstructures were analyzed using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Samples were prepared for SEM by means of the standard metallographic techniques of grinding and electropolishing in a solution of 5 pct H2SO4 and 1.25 pct HF in methanol[10] or grinding and mechanical polishing followed by etching in Kroll’s reagent. The SEM analyses were performed using a CamScan, a PHILIPS* XL30FE
C.G. RHODES, Consultant, is with the Structural Metals Department, Rockwell Science Center, Thousand Oaks, CA 91360. P.R. SMITH, Senior Materials Research Engineer, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson, AFB, OH 45433-7817. W.H. HANUSIAK, Chief Engineer, is with Advanced Materials, Atlantic Research Corporation, Wilmington, MA 01887. M.J. SHEPARD, Materials Research Engineer, is with the Southwestern Ohio Council for Hig
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