Effect of compositional modification on Young's modulus of Ti 2 AlNb-based alloy

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The Young's moduli of three Ti 2 AlNb-based alloys were investigated. When they were heat treated to have a B2 single-phase structure, their Young's moduli were identical. In an O þ B2 structure, the Young's modulus of O phase changed
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  Effect of compositional modification on Young’s modulus of Ti 2 AlNb-based alloy Feng Tang  a,* , Tohru Awane  b , Masuo Hagiwara  a a Mechanical Properties Research Group, Materials Engineering Laboratory, National Institute for Materials Science,1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Kobe Material Testing Laboratory Co. Ltd., Harimacho, Kakogun, Hyogo 675-0155, Japan Received 22 August 2001; accepted 19 October 2001 Abstract The Young’s moduli of three Ti 2 AlNb-based alloys were investigated. When they were heat treated to have a B2single-phase structure, their Young’s moduli were identical. In an O  þ  B2 structure, the Young’s modulus of O phasechanged with alloy, and showed a correlation with the lattice parameter rate  a = b .    2002 Acta Materialia Inc. Publishedby Elsevier Science Ltd. All rights reserved. Keywords:  O phase; B2 phase; Young’s modulus; Composition; Lattice parameter 1. Introduction Alloys based on the orthorhombic Ti 2 AlNbphase (O phase) have been developed recently aspotential high temperature materials for applica-tions in the aircraft engines. These alloys usuallyhave an O  þ  B2 two-phase microstructure, andexhibit better tensile strength and creep resistancein comparison with Ti 3 Al alloys [1]. Recently, themicrostructure, tensile properties, and creep be-haviors of Ti 2 AlNb-based alloy have been inves-tigated extensively [2–4]. In contrast, the elasticproperties of Ti 2 AlNb alloy are studied only inrare cases.Elastic properties are fundamental physicalproperties that relate to the interatomic forces insolid materials. Knowledge of elastic constantsis essential for the understanding of mechanicalproperties concerning deformation under load,thermoelastic stress, and fracture toughness. It hasbeen known that the Young’s modulus and shearmodulus of Ti 2 AlNb-based alloy are smaller thanthat of TiAl and Ti 3 Al alloys [5]. In this paper,we will give the experimental results about theYoung’s modulus of three Ti 2 AlNb-based alloys.The factors affecting the Young’s modulus of thesealloys will be discussed. 2. Experimental Three alloys are used in the present study.Alloy 1, with a composition of 54.58Ti-22.92Al-20.43Nb-2.07W (at.%), was prepared by plasma Scripta Materialia 46 (2002) 143–147www.elsevier.com/locate/scriptamat * Corresponding author. Fax: +81-298-592501. E-mail address:  tang.feng@nims.go.jp (F. Tang).1359-6462/02/$ - see front matter    2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(01)01211-8  melting after skull melting. Alloys 2 and 3, withcompositions of Ti-22.5Al-20.0Nb-2.0W-2.0Vand Ti-22.0Al-17.1Nb-1.7W-2.0V respectively,were prepared by adding additional alloying ele-ments to Alloy 1, using a non-consumable electricarc furnace in a high-purity argon gas environ-ment.The ingots were roll-milled into a 12-mm squarebar, after heating at 1150   C (over B2 transustemperature) for 30 min. After the hot processing,the alloys were heat treated under conditionsshown in Table 1.The Young’s modulus was determined from theslope of the elastic area of stress–strain curve ob-tained in tensile test. The tensile sample, which was3.5 mm in diameter and 16 mm in gage length, wascut with an NC lathe. The sample’s axial directionwas along the roll-milling direction. The tensilestrain was measured by strain gage with gagelength of 2 mm. To ensure the accuracy, two straingages were struck on the opposite sides of sample’ssurface, and the average value from the two gageswas taken as the tensile strain. 3. Results and discussion The grain size and phase constitution aftervarious heat treatments are shown in Table 2. Thephase constitution was identified by X-ray dif-fraction (XRD) pattern. After STQ-treatment, aB2 single phase was retained in all of the threealloys. After STA-, BCA-, and DA-treatments, allof the three alloys showed O  þ  B2 two-phasestructures. Scanning electron microscope (SEM)image showed that the three alloys had a similarmorphology when they were heat treated under thesame condition. The typical microstructures aftervarious heat treatments are shown in Fig. 1. Thevolume fraction of B2 phase in an O  þ  B2 two-phase structure was determined by the area frac-tion of B2 phase in the SEM image. In an alloy,the volume fraction of B2 phase remained to be thesame, whether the heat treatment was STA, BCA,or DA. After the STA-, BCA-, or DA-treatment,the volume fraction of B2 phase was 35% in Alloy1, and 41% in Alloys 2 and 3.The constituent in the O and B2 phases in anO  þ  B2 two-phase microstructure has been ana-lyzed by an energy dispersive X-ray spectrometer(EDS). As an example, the EDS spectra for the Oand B2 phases in Alloy 2 are shown in Fig. 2. TheEDS analysis shows that, in the present three al-loys, the O phase is rich in Ti and Al, and the B2phase is rich in Nb and W (and V in Alloys 2 and3). This result is consistent with a previous reportwhich shows that the O phase is rich in Al and theB2 phase is rich in Nb in some ternary O  þ  B2alloys [6]. Table 1Heat treatments and their abbreviationsHeat treatment Abbreviation850   C/33h/WQ DA1130   C/1h/WQ STQ1130   C/1h/WQ þ 850   C/33h/WQ STA1130   C/1h/CC þ 850   C/33h/WQ BCAWQ: water quenching, CC: controlled cooling (cooling rate: 3  C/min).Table 2The grain diameter, phase constitution, and Young’s modulus after various heat treatmentsAlloy Composition (at.%) Heat treatment Graindiameter( l m)PhaseconstitutionVolume frac-tion of B2phase,  V   B2  (%)Young’s modu-lus,  E   (GPa)1 Ti-22.9Al-20.4Nb-2.1W STQ 274 B2 100 98BCA 274 B2  þ  O 35 1242 Ti-22.5Al-20.0Nb-2.0W-2.0V STQ 251 B2 100 101STA 251 B2  þ  O 41 117DA 163 B2  þ  O 41 1183 Ti-22.0Al-17.1Nb-1.7W-2.0V STQ 318 B2 100 99STA 318 B2  þ  O 41 109BCA 318 B2  þ  O 41 112DA 183 B2  þ  O 41 112144  F. Tang et al. / Scripta Materialia 46 (2002) 143–147   Texture can be formed in the Ti–Al–Nb alloysduring the roll-milling process [7,8]. Due to thecrystallographic anisotropy in the B2 and O pha-ses, the Young’s modulus in a textured alloy mayshow dependence on load orientation. However, asthe present three alloys were roll-milled under thesame condition, the texture in these alloys is be-lieved to be the same. Therefore, in a comparativestudy of the Young’s modulus, we can neglect theeffect of texture.Typical stress–strain curves obtained in thetensile test are shown in Fig. 3. The Young’smodulus has been determined by measuring theslope of the linear part of stress–strain curve, andthe results are shown in Table 2.When the three alloys had B2 single phase afterSTQ-treatment, their Young’s moduli were ap-proximately the same. This finding indicates thatthe Young’s modulus of B2 phase is insensitive tothe compositional modification within the presentrange.However, when the alloys were heat treated tohave an O  þ  B2 two-phase structure, the Young’smodulus changed with the alloy as shown in Table2. Considering that the Young’s modulus of B2phase is insensitive to compositional modification Fig. 2. The EDS spectra of (a) O phase and (b) B2 phase in Alloy 2 after BCA-treatment.Fig. 1. Microstructures of Alloy 3 after (a) DA-, (b) STA-, and (c) BCA-treatment. The light areas are B2 phase, and the dark areas areO phase. F. Tang et al. / Scripta Materialia 46 (2002) 143–147   145  as mentioned above, we believe that the differencein the Young’s modulus between the O  þ  B2 two-phase alloys is due to the compositional depen-dence of the Young’s modulus in the O phase.According to the rule of mixtures, the Young’smodulus of an O  þ  B2 two-phase structure can beexpressed as,  E   ¼  E  B2 V   B2  þ  E  O ð 1    V   B2 Þ ð 1 Þ or,  E   ¼ ½ V   B2 =  E  B2  þ ð 1    V   B2 Þ =  E  O   1 ð 2 Þ where  E  B2  or  E  O  is the Young’s modulus of B2 orO phase, and  V   B2  is the volume fraction of B2phase.  E   in Eq. (1) is the upper bound (Voigtbound), and  E   in Eq. (2) is the lower bound (Reussbound) [9].As the Young’s modulus of B2 phase is insen-sitive to composition, we consider that  E  B2  in analloy with O  þ  B2 two-phase structure is identicalto the Young’s modulus of this alloy with B2 singlephase after STQ-treatment (Table 2). Thus, usingEqs. (1) and (2), the lower and upper bounds of   E  O in an O  þ  B2 two-phase structure can be calcu-lated. The average value of the lower and upperbounds is shown in Table 3.  E  O  is highest in Alloy1 and lowest in Alloy 3.The Young’s modulus is related to the inter-atomic bond, which may change with the latticeparameters. It has been experimentally found thatthe lattice parameters of O phase are dependent onthe chemical composition [10]. For the O phasewith composition of Ti-25Al- x Nb  ð 12 6  x 6 30 Þ ,while the lattice parameter  c  is essentially identical,the lattice parameter  a  increases and the latticeparameter  b  decreases with increasing Nb content(Fig. 4). Table 3The lattice parameters of O and B2 phases, and the average value of the lower and upper bounds of Young’s modulus in O phasecalculated by Eqs. (1) and (2)Alloy Heat treatment O phase (nm) B2 phase (nm) Average  E  O  (GPa) a b c 1 BCA 0.605 0.964 0.467 0.325 1412 DA 0.603 0.965 0.467 0.325 132STA 0.602 0.965 0.466 0.324 1303 DA 0.597 0.973 0.466 0.324 122STA 0.598 0.974 0.467 0.325 117BCA 0.598 0.971 0.467 0.325 122   Fig. 3. Typical stress–strain curves obtained from the tensile testof Alloy 3.Fig. 4. (a) Projection of O structure along [001] direction. Smallsymbols indicate atoms lying in the layer half the distanceabove/below the projection direction. (b) Lattice parameters  a and  b  as functions of Nb content in alloy Ti-25Al- x Nb [10].146  F. Tang et al. / Scripta Materialia 46 (2002) 143–147   The lattice parameters of O and B2 phase in thepresent alloys were determined by XRD test. Thelattice parameter of B2 phase was calculated usingthe Bragg angle of (110) reflection, and the latticeparameters of O phase were calculated using theBragg angles of (002), (221), and (041) reflec-tions. The calculation result is listed in Table 3. Itwas found that, for the O phase, the lattice pa-rameters  a  and  b  changed with alloy, and the latticeparameter  c  was essentially identical. The latticeparameter of B2 phase did not change with alloy.In Fig. 5,  E  O  is plotted as a function of   a = b . It isclear that there is a correlation between  E  O  and a = b . When  a = b  increases from 0.614 to 0.628, theYoung’s modulus increases from 119 to 138 GPa.The present study gives only a rough imageabout the relation between the Young’s modulusand the lattice parameters. For the purpose of further understanding of the interrelation betweenthe elastic properties and lattice parameters in theO phase, meticulous study is necessary. 4. Conclusions The Young’s modulus of three O-basedalloys, Ti-22.9Al-20.4Nb-2.1W (Alloy 1), Ti-22.5Al-20.0Nb-2.0W-2.0V (Alloy 2), and Ti-22.0Al-17.1Nb-1.7W-2.0V (Alloy 3), has been investigated usingtensile samples. The results are summarized asfollows:(1) When the three alloys are heat treated tohave a B2 single phase, their Young’s moduli areapproximately the same (about 99 GPa). Thisfinding indicates that the Young’s modulus of B2phase is insensitive to the compositional modifi-cation in the range of the present study.(2) The Young’s modulus of O phase changeswith composition, and is respectively 138, 129, and119 GPa in Alloys 1, 2, and 3. The Young’smodulus of O phase shows a correlation with thelattice-parameter rate  a = b . References [1] Banerjee D, Gogia AK, Nandy TK, Muraleedharan K,Mishra RS. In: Structural intermetallics. Champion PA:The Minerals Metals and Materials Society; 1993. p. 19.[2] Gogia AK, Nandy TK, Banerjee D, Carisey T, Strudel JL,Franchet JM. Intermetallics 1998;6:741.[3] Tang F, Emura S, Hagiwara M. Scripta Mater2001;44:671.[4] Tang F, Nakazawa S, Hagiwara M. Mater Sci Eng A2001;315:147.[5] Chu F, Mitchell TE, Majumdar B, Miracle D, Nandy TK,Banerjee D. Intermetallics 1997;5:147.[6] Boehlert CJ, Majumdar BS, Seetharaman V, Miracle DB.Metal Mater Trans A 1999;30:2305.[7] Suwas S, Ray PK. Metal Mater Trans A 2000;31:2339.[8] Nicolaou PD, Semiatin SL. Metal Mater Trans A1997;28:885.[9] Clyne TW, Withers PJ. In: An introduction to metal matrixcomposites. Cambridge University Press; 1993. p. 14.[10] Kestner-Weykamp HT, Ward CH, Broderick TF, Kauf-man MJ. Scripta Mater 1989;23:1697.Fig. 5. The Young’s modulus of O phase as a function of thelattice parameter rate  a = b . The error bars show the lower andupper bounds calculated with Eqs. (1) and (2). F. Tang et al. / Scripta Materialia 46 (2002) 143–147   147
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