Processing effects on the grain-boundary character distribution of the orthorhombic phase in Ti-Al-Nb alloys
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I. INTRODUCTION
THE Ti-Al-Nb alloys containing a significant volume fraction of orthorhombic (O; Cmcm symmetry based on Ti2AlNb; a 0.63 nm, b 0.97 nm, and c 0.48 nm) phase have shown attractive mechanical properties for structural applications.[1–10] It has been demonstrated that when a sufficient volume of bcc phase is added to O-dominated microstructures, a balance of elevated-temperature (700 °C) tensile and creep strength and room-temperature (RT) strength and ductility can be achieved.[8,9,10] There is potential for improving the mechanical behavior through microstructural manipulation and, in particular, through grain-boundary engineering (GBE), where thermomechanical processing (TMP) treatments may alter the grain-boundary character distribution (GBCD). Few attempts have been made to characterize the GBCD, let alone GBE O-based material systems. In order to begin such a task, it is important to identify how the O-phase evolves in such systems. For the Ti2AlNb intermetallic system, the O phase is generally formed by phase transformation from either of two higher-temperature phases: the hcp 2 phase (Ti3Al, P63/mmc; DO19 structure; a 0.58 nm; c 0.46 nm) or the bcc phase (Pm3m; CsCl symmetry; a 0.32 nm), depending upon alloy composition and heat treatment.[11,12] Orientation relationships (ORs) exist between the 2/O ([0001]a2//[001]O; (10 10)a2//(110)O)[11,13] and bcc/O ([111]bcc//[1 10]O; (110)bcc//(001)O)[14,15] structures. The bcc/O OR is depicted in Figure 1. The geometrical relaDINGQIANG LI, Postdoctoral Research Associate, is with the School of Engineering, Alfred University, Alfred, NY 14802. C.J. BOEHLERT, Assistant Professor, is with the Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824-1226. Contact e-mail: [email protected] Manuscript submitted December 13, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
tionship between phases is likely to significantly influence the GBCD whenever O-phase transformation occurs. In addition, the sequential TMP treatments, which dictate not only the pretransformation microstructure but also the transformation behavior, are also expected to be influential. This work describes the effects of processing on the Ophase GBCD. In particular, the GBCD of subtransus- and supertransus-processed microstructures was evaluated, and the twin-related variant interfacial planes were quantified. This work was expected to identify whether the character of the O/O boundaries in fully-O microstructures differed from the O/O boundaries in O bcc microstructures, in order to shed light on the potential to process, through GBE, the O phase in multiphase alloys targeted for structural applications.
II. EXPERIMENTAL PROCEDURE Details of the alloy processing treatments for the nominally Ti-25Al-24Nb* and Ti-23Al-27Nb alloys were provided in *All alloy compositions are given in atomic percent.
previous work.[16,17] Briefly, the vacuum arc–melted ingots were pancake forged and hot-pack–rolled through a sequence of r
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