Microstructure, Tensile, and Creep Behavior of Boron-Modified Ti-15Al-33Nb (at.%)
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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 hexagonal intermetallic a2 phase, based on Ti3Al, and the body-centered-cubic (bcc) phase, whose ordering is dependent upon composition (B2 designates the ordered structure while b designates the disordered structure).[1–5] Such alloys have shown potential to exceed the elevatedtemperature capabilities of conventional a + b Ti alloys. In particular, a Ti-15Al-33Nb (at. pct) alloy has been shown to exhibit an attractive balance of elevatedtemperature tensile and creep strength while maintaining room temperature (RT) strength and elongation-tofailure (ef) values comparable to Ti-6Al-4V (wt pct).[2,6] The creep stress exponent (n) and apparent activation energy (Qapp) values for many O-based Ti-Al-Nb alloys have suggested grain boundary sliding to be the dominant secondary creep deformation mechanism for lowto-intermediate applied stresses.[2,7–10] One study has shown that grain boundary sliding accommodated by grain boundary cracking contributes significantly to the overall creep strain of a Ti-23Al-27Nb (at. pct) alloy.[7] Such observations were made through interrupted experiments performed using a creep testing apparatus C.J. Cowen, formerly a Ph.D. graduate student with the Department of Chemical Engineering and Materials Science at Michigan State University, is currently an NRC Postdoctoral Research Associate in the Process Development Division of the US DOE National Energy Technology Laboratory, Albany, OR 97321, USA. C.J. BOEHLERT is an Associate Professor in the Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824-1226, USA. Contact e-mail: [email protected] Manuscript submitted May 3, 2007. Article published online January 5, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
contained within a vacuum chamber. The sample was unloaded then removed from the chamber after achieving 4 and 10 pct creep strain and imaged using a scanning electron microscope (SEM). Thus, true in-situ imaging of the evolving surface deformation was not performed. One objective of this work was to observe and characterize the deformation events in-situ during creep deformation of a Ti-15Al-33Nb (at. pct) alloy. 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.[11,12] This dramatic reduction in the as-cast grain size leads to significant benefits including increased yield strength. The addition of B also leads to the formation of the titanium boride (TiB) phase within the conventional a + b microstructure.[11–13] The presence of the TiB phase within these microstructures has led to substantial increases in RT strength and YoungÕs modulus (E), while maintaining adequate ef levels.[12,14] Relatively few studies have focused on investig
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