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The Effects of Forging and Rolling on Microstructure in O+BCC Tialnb

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The Effects of Forging and Rolling on Microstructure in O+BCC Tialnb Materials Science and Engineering A279 (2000) 118–129 www.elsevier.com/locate/msea The effects of forging and rolling on microstructure in O+BCC TiAlNb alloys C.J. Boehlert * Department of Mechanical Engineering, Johns Hopkins Uni6ersity, 3400 North Charles Street, Baltimore, MD 21218, USA Received 1 July 1999; received in revised form 8 September 1999 Abstract The effects of hot upset forging and hot pack rolling on microstructure of orthorhombic (O)+body-centered cubic (BCC) TiAlNb alloys was investigated. The starting materials were melted ingots of nominal compositions: Ti25Al25Nb(at.%), Ti23Al27Nb(at.%), and Ti12Al38Nb(at.%). Smaller cigar-shaped Ti25Al25Nb ingots were examined to understand the effect of rolling preheat treatment on microstructure. It was found that super-transus preheat treatment results in large prior BCC grains and surface edge cracking. For larger castings, forging and rolling procedures were carried out after heating the materials between 932–1000°C. These temperatures were below the BCC-transus temperature for Ti23Al27Nb and Ti25Al25Nb and above the transus for Ti12Al38Nb. This resulted in a significantly larger grain size for the as-processed Ti12Al38Nb compared with the other two alloys. The Ti25Al25Nb alloy required the greatest forging and rolling loads, while the fully-BCC Ti12Al38Nb alloy exhibited the best workability and required the lowest forging and rolling loads. This was related to the alloys’ aluminum contents and O-phase volume fractions. Sub-transus processing of the near Ti2AlNb alloys proved to be a viable technique for obtaining homogeneous microstructures containing fine O and BCC phases and lacking large prior BCC grains, which can be detrimental to the mechanical performance. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Titanium alloys; BCC phases; Orthorhombic phase 1. Introduction the thermomechanical processing has been based on previous methodologies developed for conventional a– b a Since the discovery of the orthorhombic (O) phase in titanium alloys and the intermetallic 2 titanium aTi25Al12.5Al(at.%)1 alloy by Banerjee et al. [1], aluminides. Early alloy development efforts focussed on titanium aluminides containing the O phase (based on extrusion, forging, or rolling operations on arc-melted ingots, with the primary intent being to characterize the Ti2AlNb) have been of interest for high-temperature structural applications, primarily because of their high equilibrium phases, phase transformations, and me- specific strength and stiffness as well as their creep and chanical behavior [5–8,10,11]. Later, Smith et al. [2–4] oxidation resistance. Recent results have shown that O used foil processing to examine the development of alloys offer major performance improvements over microstructure in Ti 22Al 23Nb and Rhodes et al. [12] commercial titanium alloys [2–10]. Like commercial studied microstructural evolution and crystallographic titanium alloys, the properties of O alloys depend texture in Ti 22Al 23Nb and Ti 22Al 27Nb sheet and strongly on the microstructure and therefore the pro- foil products during hot rolling, cold rolling, and subse- cessing. To date, the relationship between the process- quent heat treatment. More recently detailed studies of ing and microstructure of O alloys has been the development and control of microstructure during forging of a Ti22Al27Nb alloy [9] and hot pack investigated to a limited extent [1,5,6,8–16]. Much of rolling of a Ti22Al23Nb alloy have been performed [13]. These studies have focussed on a relatively narrow * Tel.: +1-410-5162876; fax: +1-410-5167316. a E-mail address: [email protected] (C.J. Boehlert) range of O alloy compositions, containing the O, 2 (ordered hexagonal close packed), and body-centered 1 All alloy compositions are given in atomic percent. cubic (BCC) phases, which are being considered for 0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0921-5093(99)00624-3 C.J. Boehlert / Materials Science and Engineering A279 (2000) 118–129 119 metal matrix composite applications using foil-fiber-foil tions of the near Ti2AlNb alloys were Ti 25Al 25Nb processing. and Ti23Al27Nb. The initial portion of this study In this study, the processing-microstructure relation- involved examining the effect of processing parameters, ship for O alloys containing a wide range of composi- and in particular the rolling preheat temperature, on tions; namely Ti25Al25Nb, Ti23Al27Nb, and microstructure using smaller ingots prior to attempting Ti12Al38Nb alloys, was examined. Due primarily to large-scale deformation on larger castings. The term the high Nb content in such alloys, the two-phase used to describe the smaller ingots is ‘cigar melts’ O+BCC regime has a wider temperature range than because the dimensions, 150 mm in length and 30 mm a a the 2 +B2, 2 +B2+O, and O-phase fields. Hence diameter, are similar to that of a cigar. Three 300-g such alloys are termed ‘O+BCC’ alloys. The evolution cigar-melts were triple-melted using a vacuum induction of microstructure, from melted ingot to forged pancake melter at the Air Force Research Laboratory Materials to rolled sheet on the order of millimeters thick, was Directorate of Wright–Patterson Air Force Base, OH. examined. In attempt to avoid large prior-BCC grains, The heats were formulated using elemental Ti, Al, and which have shown to be detrimental to mechanical Nb according to the stoichiometric mixture Ti2AlNb behavior [17–19], conservative thermomechanical pro- and their measured compositions are listed in Table 1. cessing techniques comprising non-isothermal forging The larger castings consisted of 175–500 mm long and hot pack rolling at relatively low processing tem- cylinders of 75 mm diameter. The large, near Ti2AlNb peratures were chosen. In addition to further develop- ingots, whose compositions are listed in Table 2, were ing the understanding of microstructural evolution ‘induction-skull’ melted at Flowserve (formerly Duriron during processing of O+BCC alloys, this work de- Corp.), Dayton, OH. Note that for Ti23Al27Nb, the scribes how processing temperature affects the ability to Ti, Al, and Nb contents adhered well to the target control microstructural features, especially grain size, composition, while for Ti25Al25Nb, the measured which strongly influence the mechanical behavior. composition was close to Ti25Al23Nb. The large Ti12Al38Nb ingot, whose measured composition was close to Ti13Al39Nb (see Table 2), was vacuum arc 2. Experimental procedures melted at Pittsburgh Materials Technology Inc., Large, PA. Several samples, diamond cut from each material, 2.1. Materials and microstructural characterization were analyzed for their constituent elements. The Ti, Al, Nb, and Fe contents were analyzed by means of The studied alloys were grouped into two categories: solution X-ray fluorescence spectrometry and the data near Ti AlNb and Ti12Al38Nb. The target composi- 2 were obtained using a Kerex Corporation Model 770 Table 1 Delta Analyst. The amounts of nitrogen and oxygen were quantified using a Leco Corporation Model TC- Chemical analysis of the Ti2AlNb cigar-melted ingots and the corre- sponding as-processed sheetsa 136 oxygen/nitrogen analyzer. Chemical composition distribution between the different phases was measured Material Atomic percent Weight (ppm) using a Japan Electron Optics Ltd electron microprobe TiAl Nb N Fe H O analyzer (JEOL 733). Grain size (d) and phase volume fractions were determined quantitatively using NIH Ingot ABal 24.8 24.5 140 nana 250 image analysis software of digitized, high-contrast, Sheet ABal 25.4 24.2 110 350 na 280 back-scattered-detector (BSD) images taken using a Ingot BBal 26.6 23.5110 na 650 135 Leica 360 field-emission scanning electron microscope Sheet BBal 26.2 25.1 110 460 na 230 Ingot CBal 24.6 25.1 100 na na 290 (SEM). Transmission electron microscopy (TEM), per- Sheet C Bal 24.6 25.0 110 530 na 890 formed using a JEOL JEM-2000FX electron micro- scope, and X-ray diffraction (XRD) were used to a na, not available. confirm the presence of the different phases. Table 2 2.2. Procedures Chemical analysis of the large ingots Material Atomic percent Weight (ppm) 2.2.1. Forging and rolling procedures for the cigar-melted ingots TiAl Nb N Fe O Following melting, the cigar melts were cut by a wire electron discharge machine (EDM) to a rectangular Ti25Al25NbBal 24.7 23.3 150 290 930 Ti23Al27NbBal23.2 27.2 200 1100 1160 geometry, measuring 125×25×20 mm, and coated Ti12Al38Nb Bal 13.2 39.2 70 255 575 with high-temperature glass for lubrication and protec- tion from the environment. They were then sealed in 6 120 C.J. Boehlert / Materials Science and Engineering A279 (2000) 118–129 Table 3 Rolling procedures and parameters for the Ti2AlNb cigar-melted ingots Ingot A B C Preform Forged (50%)Forged (50%) Forged (50%) Can dima (mm3) 140×60×12 122×66×12 150×66×12 Pre-rolling – — 1200°C/24 h Heat treatment Rolling-preheat 815C/1h/1000°C/0.25 h1040C/1h/1000°C/0.25 h 1040°C/1h/1000°C/0.25 h Interpass reheating 1000°C/2–3 min 1000°C/2–3 min 1000°C/2–3 min Intermediate anneal – 1060°C/0.5 h/1000°C/0.25 h 1060°C/.5 h/1000°C/0.25 h Reduction per pass 10%10% 10% Roll speed (m2 min−1) 2.3 2.3 2.3 Can dim (mm3) 412×64×3391×66×3 419×74×3 Sheet dimb(mm3) 381×46×2.7353×48×2.8 389×50×2.8 a After forging, the cans were weld repaired, reevacuated, and sealed for use in rolling. b All rolled sheets were reheated at 1000°C for 3 min and then cooled slowly in vermiculite (3°C min−1). mm thick stainless steel cans and unidirectionally was performed on a cold die and therefore after each forged according to a 2:1 ratio from 25 to 12.5 mm in pass the pancakes were reheated at 1000°C for 2–3 air at a rate of 150 mm min−1.
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