Deformation structure in a Ti-24Al-11Nb alloy
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I.
INTRODUCTION
T H E alloy Ti-24Al-11Nb is a typical composition in a class of Ti3A1-Nb alloys developed recently for hightemperature applications, t~l A feature of this alloy is that on quenching from a temperature range between 1293 and 1333 K, the microstructure of the alloy consists of two ordered phases, a2 (based on Ti3A1 with a D019 structure) and /30 (based on Ti2NbA1 with a B2 structare).t2,31 Variation of the relative volume fractions of these phases leads to a ductility maximum at an optimum volume fraction condition, t21 Examination of the deformation behavior of such structures indicates that the slip mode in the/30 phase and the effectiveness of a2 particles in acting as obstacles to slip in/3o play a critical role in controlling the overall deformation mode of the alloy and, consequently, its tensile fracture, t21 We therefore describe in this paper a detailed characterization of the dislocation structure in the alloy after small strains ( - 0 . 0 2 ) by transmission electron microscopy (TEM). As considerable work has already been carded out on dislocation arrangements in the a2 phase, t4-71 emphasis has been placed in this work on the deformation of the/30 phase (which has not been examined earlier), and the conditions under which slip transfer occurs from/30 to a2. II.
EXPERIMENTAL
Tensile samples were prepared from consumable arcmelted alloy ingot, forged and hot rolled to 12-mmdiameter rod as described in Reference 2, and heat-treated in the two-phase a2 + /30 region at different solutiontreatment temperatures (1373 K for 120 minutes, 1333 K for 240 minutes, or 1293 K for 480 minutes) and water quenched. These samples were strained in an Instron machine to about 2 pct true strain at a strain rate of 5 x 10 -4 S-1 . Three mm discs were then cut from the gage length perpendicular to the tensile axis and electropolished to electron transparency in a twin jet electropolisher in a solution of 5 pct sulfuric acid in methanol tS1 D. BANERJEE, A.K. GOGIA, and T.K. NANDY, Scientists, are with the Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad 500 258, India. Manuscript submitted April 4, 1989. METALLURGICAL TRANSACTIONS A
at 223 K. Thin foils were examined in a PHILIPS* *PHILIPS is a trademark of Philips Instruments Corporation, Mahwah, NJ.
EM430T microscope equipped with a side entry goniometer stage capable of a ---45-deg tilt on the X-axis and a - 3 0 - d e g tilt on the Y-axis. The beam direction with respect to the foil for any tilt of the goniometer was determined by a stereographic technique developed for use with a side entry goniometer, t9j It was necessary to use such a technique rather than the more conventional Kikuchi line method since sharp Kikuchi lines were not always available, especially in the flo phase. Slip planes were then identified either by orienting them parallel to the electron beam direction or alternatively by determining the true direction of the trace of the slip plane intersection with the foil surface, t~~ In the latter method, the slip plane
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