Materials Science and Engineering A267 (1999) 82 – 98 Microstructure, creep, and tensile behavior of a Ti – 12Al – 38Nb (at.%) beta + orthorhombic alloy C.J. Boehlert * Department of Mechanical Engineering, Johns Hopkins Uni6ersity, 3400 North Charles Street, Baltimore, MD 21218, USA Received 13 September 1998; received in revised form 31 December 1998 Abstract The microstructural evolution, creep, and tensile deformation behavior of a Ti – 12Al – 38Nb (at.%) alloy were studied. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisother- mal forging and pack rolling. TEM studies showed that depending on the heat-treatment schedule, this alloy contains two constituent phases including: b (disordered body-centered cubic) and O (ordered orthorhombic based on Ti 2 AlNb). Heat treatments at all temperatures above 800°C, followed by water quenching, resulted in fully-b microstructures. Below 800°C, fine Widmanstatten O-phase needles precipitated within the b grains. Fully-b microstructures exhibited room-temperature (RT) elongations of more than 27%. The second-phase O precipitates provided strengthening at the expense of elongation. However, RT elongations of more than 12% were recorded for aged microstructures containing 30 vol.% O-phase. Metallographic observations revealed that slip was compatible between the two phases. Tensile creep tests were conducted in the temperature range 650 – 705°C and stress range 50 – 172 MPa. The deformation observations and measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress. For applied stresses less than 123 MPa, the creep exponents were between 1.6 and 2.0 and low dislocation densities were observed. For higher stresses, the stress exponents were between 3.5 and 7.2 and higher dislocation densities were observed. The calculated activation energies for the low-stress regime were approximately half those calculated for the high stress regime. These data suggest that Coble creep operates at low stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behavior are discussed in the light of existing models. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Titanium; Orthorhombic; Body-centered-cubic; Creep; Tension; Microstructure 1. Introduction Within the aerospace industry, there is a need for materials with high strength and stiffness and low density, especially for high-temperature applications. Titanium alloys and their composites are examples of such materials which have found use in a variety of elevated-temperature applications because of their high specific strength and stiffness, as well as their creep and oxidation resistance. Due to the excellent creep resis- tance of the orthorhombic (O) phase [1,2], titanium alloys containing a significant amount of the O phase (henceforth termed O alloys) are being considered for use in elevated-temperature structural applications both as monolithic materials as well as matrix alloys in continuously reinforced metal matrix composites (MMCs). The intermetallic alloy based on the O- phase’s stoichiometric composition, Ti–25Al–25Nb (at.%) 1 , is one of the O alloys being considered. The excellent creep resistance of this alloy is balanced against its low fracture toughness and elongation-to- failure at ambient temperatures [3,4]. Low matrix duc- tilities prevent effective fiber strength utilization of MMCs [5]. Therefore, lower Al-containing alloys, such as Ti–23Al–16Nb, Ti–22Al–26Nb and Ti–12Al– 38Nb, are also being considered because of their en- hanced RT ductility and fracture toughness compared with Ti–25Al–25Nb [3–8]. One of the primary reasons why such alloys offer enhanced RT ductility is because * Tel.: +1-410-516-8284; fax: +1-410-516-4316. E-mail address: [email protected] (C.J. Boehlert) 1 alloy compositions are given in atomic percent. 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0921-5093(99)00024-6
Microstructure, creep, and tensile behavior of a Ti–12Al–38Nb (at.%) beta+orthorhombic alloy
Jun 18, 2023
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