Comparison of the Microstructure, Tensile, and Creep Behavior for Ti-22Al-26Nb (At. Pct) and Ti-22Al-26Nb-5B (At. Pct)
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TRODUCTION
THE addition of trace amounts of boron (B) to conventional titanium (Ti) alloys, such as Ti-6Al-4V (wt pct), has been shown to decrease the as-cast grain size by approximately an order of magnitude.[1] This drastic reduction in the as-cast grain size leads to significant benefits including increasing yield strength while reducing or avoiding time spent on expensive and energyintensive thermomechanical processing. The addition of B produces titanium boride (TiB) whiskers within the conventional a + b microstructure.[1,2] The presence of these TiB whiskers within these microstructures has led to substantial increases in room-temperature (RT) strength and Young’s modulus (E), while maintaining adequate elongation-to-failure (ef) values.[3] Such benefits of the TiB phase are exhibited through either ingot metallurgy or powder metallurgy processing routes. Titanium-aluminum-niobium (Ti-Al-Nb) alloys differ from conventional Ti alloys in that their constituent phases may include the ordered intermetallic orthorhombic (O) phase, based on Ti2AlNb; the ordered hexagonally-close-packed (hcp) intermetallic a2 phase, based on Ti3Al; and the body-centered-cubic (bcc) phase, whose ordering is dependent upon composition.[4–8] TiAl-Nb alloys have shown potential to exceed the elevated-temperature capabilities of conventional a + b Ti alloys.[5,9–15] In particular, Ti-Al-Nb alloys of nominal compositions close to Ti-22Al-26Nb (at. pct) have exhibited an attractive balance of RT and elevatedtemperature properties.[16–22] However, relatively few studies have focused on investigating the effects of B C.J. COWEN, Graduate Assistant, and C.J. BOEHLERT, Assistant Professor, are with the Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824-1226, USA. Contact e-mail: [email protected] Manuscript submitted August 8, 2006. 26—VOLUME 38A, JANUARY 2007
additions on the microstructure and mechanical properties of Ti-Al-Nb alloys.[23–28] The purpose of this work was to determine the effect of a nominally 5 at. pct B addition on the microstructural features of a nominally Ti-22Al-26Nb (at. pct) alloy (henceforth, all compositions will be given in atomic percent) and the impact this causes on the tensile and creep deformation behavior. The alloy composition was selected based upon studies by Rowe,[19] Rowe and Larson,[20] and Smith et al.,[16,17,18] which suggested that this alloy exhibited an exceptional balance of mechanical properties compared to other ternary O-based Ti-Al-Nb alloys. In order to examine a fabrication route alternative to ingot metallurgy, powder metallurgy processing was performed on the Ti-22Al-26Nb and Ti-22Al-26Nb-5B materials. This latter approach has the potential to produce significant cost reduction compared to some alloy and metal matrix composite (MMC) processing methods.[16] A limited amount of work has been done to understand the effects of powder microstructures on the mechanical performance of O-based alloys and MMCs.[16,17] II.
EXPERIMENTAL PROCE
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