Patterson Air Force Base, OH USA

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1 Low Temperture Superplsticity of Ti-6Al-4V Processed y Wrm Multidirectionl Forging G.A. Slishchev 1,, E.A. Kudrjvtsev 1,, S.V. Zheretsov 1,c, S.L. Semitin 2,d 1 Belgorod Stte University, Poed 85, Belgorod 3815, Russi 2 Air Force Reserch Lortory, Mterils nd Mnufcturing Directorte, AFRL/RXLM, Wright- Ptterson Air Force Bse, OH USA Slishchev@su.edu.ru, Kudryvtsev@su.edu.ru, c ser_z@mil.ru, d Lee.Semitin@wpf.f.mil Keywords: Low-temperture superplsticity, Multidirectionl forging, Ultrfine grin structure, Microstructure evolution Astrct. Multidirectionl forging hs een developed to produce n ultrfine-grin (UFG) microstructure in the two-phse titnium lloy Ti-6Al-4V. A microstructure with grin size of 135 nm ws ttined, enling low-temperture superplsticity (LTSP) t 55 C. A totl elongtion of 1% nd strin-rte-sensitivity coefficient m=.47 were otined t the optiml strin rte of s -1. Importnt fetures of the microstructure nd superplstic ehvior of the lloy re summrized in the present work. It is shown tht microstructure evolution during low-temperture deformtion plys key role in superplstic flow ehvior. Introduction Mnufcturing methods involving severe plstic deformtion nd the susequent mechnicl properties of two-phse titnium lloys with n UFG microstructure hve een studied extensively during the lst two decdes [1-5]. Different processes, including rolling, multidirectionl forging (MF), equl-chnnel-ngulr pressing (ECAP), nd high-pressure torsion t low tempertures hve een pplied to produce UFG microstructures. Recent work hs shown tht UFG microstructure with grin size of 1-15 nm cn e produced y MF in the / titnium lloy Ti-6Al-4V [3]. The gret interest in microstructure refinement is ssocited with significntly reduced superplstic (SP) forming tempertures y s much s few hundred degrees [6]. For exmple, in Ti-6Al-4V with grin size of 3 nm produced y MF, SP ehvior ws oserved t 6 C nd strin rte of s -1 ; the totl elongtion nd strin-rte-sensitivity coefficient m were 5% nd.34, respectively [7]. The sme lloy with the sme grin size produced y ECAP showed = 296% nd m =.34 t 6 C nd s -1 [4]. An increse in the deformtion temperture resulted in n enhncement of SP properties. Sheets of Ti-6Al-4V with grin size of 3-4 nm produced y MF nd susequent pck rolling showed 8% t 65 C nd >1% t 75 C for strin rte of s -1 [2]. A decrese in SP properties t lower tempertures my e expected due to oth the nturl retrdtion of diffusion-controlled processes nd the increse in the frction of the phse in which diffusion is much slower thn tht in the phse [8]. Another fctor which my contriute to the decresed is the propensity for grin growth when the frction of phse decreses noticely. Despite extensive studies of constitutive ehvior during LTSP, detiled microstructure evolution dt re known only for 775 nd 815 С [9]. The im of the present work, therefore, ws to quntify microstructure evolution nd mechnicl ehvior of Ti-6Al-4V with grin size of 135 nm produced y MF during SP deformtion t the extremely low temperture of 55 C. Mterils nd Procedures The progrm mteril consisted of the / titnium lloy Ti-6Al-4V with nominl composition (in weight pct.) of 6.3 Al, 4.1 V,.18 Fe,.3 Si,.2 Zr,.1 C,.18 O,.1 N. It ws supplied

2 in the form of hot-rolled 4-mm dimeter r with -trnsus temperture of 99 C. Bulk specimens mesuring 4 6 mm with UFG microstructure were produced initilly y MF under isotherml conditions [3]. For this purpose, preforms wter quenched following et nneling were worked in the temperture intervl of 7-55 C using hydrulic press equipped with isotherml-forging tooling. The nominl strin rte ws ~1-3 s -1. Following MF, 4-mm-thick rolling performs were sectioned long the longitudinl xis y electric-dischrge mchining. To increse microstructure homogeneity nd to form rolling texture efore SP tension testing, ech plte ws rolled to sheet t 475 C under isotherml conditions. The reduction per pss ws 5-1%. The totl reduction of ws ~5%, thus yielding finl sheet thickness of 2 mm. Tension specimens with gge section mesuring mm were mchined from the sheets. Tension tests were performed in n Instron mechnicl-testing mchine t tempertures etween 45 nd 6 С with crosshed speed of.5-1 mm/min. The strin rte sensitivity m ws evluted using either the slope of log -log curves or strin-rte-chnge tests [6]. The pprent ctivtion energy of plstic deformtion Q ws clculted from the semi-log dependence of the ultimte tensile strength ln( /G) on the inverse solute temperture (1/T) for =5 1-4 s -1 nd T = 45-7 С. X-ry investigtion ws crried out using DRON-3 diffrctometer with Cu-Kα rdition. The detils of structurl evolution were determined using JEOL JEM-21FX trnsmission electron microscope (TEM) nd Qunt 6 field-emission-gun scnning-electron microscope (SEM). Results nd Discussion Microstructure Evolution during Anneling. After MF nd rolling (referred to s the initil condition), the microstructure consisted of gloulr nd grins with men size of 135 nm (Fig. 1). Due to microscopic residul stresses, some frgments did not hve cler oundries. A histogrm of the grin-size distriution (Fig. 1) suggested quite good homogeneity of the microstructure. However, smll frction of grins with size of 3-4 nm ws present Frction, % Grin size, nm Figure 1 Microstructure of Ti-6Al-4V in the initil condition (fter MF nd rolling): (а) TEM right-field imge nd () grin-size distriution. Heting to Т=55 C resulted in noticele grin growth; the kinetics of this process depended considerly on sok time (Fig. 2). In the intervl of - 4 hours, the grin size incresed rpidly; during further soking t 55 C, the rte of grin growth decresed. After 2 hours, the men grin size hd incresed y pproximtely fctor of three compred to the initil condition (Fig. 2). A ckscttered electron (BSE) imge reveled the distriution of the phse in the microstructure (Fig. 2а). After short sok time, the phse ppered in the form of seprte prticles or thin lths situted etween the prticles (Fig. 2а). An increse in het tretment time to 2 hours led to corsening nd redistriution of the prticles minly to triple points. The volume frction of the phse (determined y the liner intercept method) ws ~1% in the initil condition nd did not chnge noticely during nneling.

3 4 Grin size, nm Figure 2 - () BSE imge of Ti-6Al-4V fter nneling t Т=55 С for.5 hour nd () grin size s function of sok time t 55 C. Superplstic Behvior. Figure 3 shows the mechnicl properties of the UFG lloy s function of temperture nd strin rte. An increse in deformtion temperture incresed the ductility of the lloy nd resulted in mrked decrese in flow stress (Fig. 3). At Т=55 С nd =5 1-4 s -1, the totl elongtion ws 64%. A further increse in temperture led to smll increse in, trend likely mitigted y the oxidtion effect nd the formtion of lph cse. An exmintion of specimens strined t different tempertures (Fig. 4) showed tht deformtion t 55 C occurred homogeneously with ovious signs of SP flow. The effect of strin rte on the mechnicl ehvior t 55 C ws lso typicl of tht for SP deformtion (Fig. 3). The optiml SP prmeters for the UFG lloy were =1% nd m =.47 t strin rte of s -1., h True stress, MP Temperture, о С Stress Elongtion Elongtion, % True stress, MP m vlue stress Strin rte, s -1 elongtion Figure 3 Mechnicl properties of the UFG lloy s function of () deformtion temperture for =5 1-4 s -1 nd () strin rte for Т=55 С. The stress level ws defined t ~.2. The shpe of the flow curves for the UFG mteril t Т=55 С (corrected to constnt true strin rte) depended noticely on strin rte. At s -1, the flow curve exhiited pek flow stress t the initil stges of deformtion nd then ner stedy-stte flow until frcture. A decrese in the strin rte to the optiml vlue of s -1 chnged the mechnicl ehvior to mrked flow-hrdening response suggestive of dynmic corsening [9]. A similr ehvior ws oserved t the yet lower strin rte of s -1. In generl, such ehviors re typicl of SP flow. However, there re lrge differences in the levels of flow stress. For the optiml condition t s -1, the flow stress ws ~13 MP. This is n order of mgnitude higher thn tht typiclly oserved for high-temperture superplsticity t the sme strin rte [6]. The higher flow stresses cn e scried lrgely to the mrked decrese in the diffusivity t the lower tempertures utilized Elongtion, %,5,4,3,2,1, m Vlue

4 True stress, MP х1-4 s -1 2х1-5 s -1 2х1-3 s -1,,5 1, 1,5 2, 2,5 True strin, Figure 4 () Mcrogrphs of test specimens of UFG Ti-6Al-4V fter SP deformtion t different tempertures nd strin rte of s -1 nd () true stress- true strin curves for UFG Ti-6Al-4V otined t Т=55 C nd vrious strin rtes. Apprent Activtion Energy of Plstic Deformtion. The ctivtion energy for specific deformtion process cn provide insight into the rte-controlling mechnism. Under the optiml prmeters of superplsticity, Q=149 kj/mol t n=2 ws otined. In erlier works [4, 1] for the sme lloy, similr vlues of 176 kj/mol for region II of superplsticity t T=6 С nd 16 kj/mol t tempertures etween 65 nd 955 C were reported. Microstructure Evolution. Microstructure evolution of the UFG lloy t T=55 C s function of strin nd strin rte ws quntified. Strin rte cn gretly influence microstructure evolution; decrese in strin rte often results in grin growth. For strin = 1%, the men grin size ws found to e 29 nm t =5 1-3 s -1, 49 nm t =2 1-4 s -1, nd 53 nm t =2 1-5 s - 1. An increse in strin using the optiml strin rte for superplsticity lso resulted in considerle grin growth such tht mesurle frction of grins with size of the order of 14 nm ws oserved (Fig. 5). The plot in Fig 5 further quntifies strin-induced grin growth t s -1 nd Т=55 С s compred to tht during nneling t 55 C for equivlent times Frction, % 1 5 d-d, nm Grin size, nm , h Figure 5 () Grin-size distriution fter = 5% nd () strin-induced grin growth t s -1 nd Т=55 С in comprison with tht during nneling for the sme time s tht needed for given strin. In () d denotes the men grin size fter given strin nd d is the men grin size fter nneling for the equivlent time. Under optiml SP conditions, the strin rte sensitivity ws determined to e m=.52,.62 nd.65 for elongtions of 1, 2, nd 5%, respectively. Consequently, despite considerle grin growth fter lrge plstic strin, SP flow ws still oserved. TEM investigtion of sustructure evolution during SP deformtion of the UFG lloy to n elongtion of 1% t Т=55 C nd vrious strin rtes indicted the presence of disloctions in the phse even fter slowest strin rte of s -1. The disloction density tended to decrese with decrese in strin rte. However, the solute chnge in comprison to the initil condition ws smll. The sence of porosity in the specimens deformed to frcture ( =1%) t s -1 ws slo noted.

5 Texture Evolution. The evolution of the crystllogrphic texture during nneling nd SP deformtion ws quite typicl [6]. In the initil condition (Fig. 6), the (2) lph-phse pole figure consisted of shrp split texture with sl poles tilted ~3 from the ND towrd the TD nd slightly split towrd the RD direction. Anneling chnged neither the intensity nor the loction of the poles considerly (Fig. 6). During SP deformtion, the sl poles moved towrd the RD direction (Fig. 6c). The intensity of poles decresed slightly with decresing SP strin rte. An increse in strin from 1 to 5% under the optiml strin rte of =2 1-4 s -1 resulted in the formtion of diffuse pole figure with much lower intensity (Fig. 6d). The Kern's fctor clculted from the pole density [11] for the norml plne of tensile specimens decresed from.55 in the initil condition to.42 fter 5% elongtion t s -1. c d Figure 6 (2) pole figures tken from the norml plne of the plte produced y MF nd rolling: () In the initil condition nd () fter nneling t 55 C for.5 h. (2) pole figures mesured on tension-specimen gge sections (on the surfce corresponding to the norml plne of the initil sheet) fter testing t =2 1-4 s -1 to n elongtion of (c) 1% or (d) 5%. Discussion UFG Ti-6Al-4V exhiited SP ehvior t the extremely low temperture of 55 C. This temperture flls into the intervl typiclly used to ge the lloy (i.e., С [12]), thus implying resonle diffusivity. However, decrese in temperture to 45 C (elow the lower ound of this intervl) resulted in n increse in flow stress to 76 MP, vlue more thn 4 times tht t 55 C (17 MP) (Fig. 3). The decrese in grin size to 135 nm mde possile SP deformtion with high vlues of strin rte sensitivity, uniform nd totl elongtion, nd flow hrdening s result of dynmic grin growth/lph-prticle corsening under the optiml conditions. However, some fetures of the SP ehvior re noteworthy. It is usully thought tht SP ehvior in Ti-6Al-4V cn e expected when the rtio of the / volume frctions is close to 1 [6, 12]. This is ecuse (i) grin oundry sliding occurs more redily long interphse oundries nd (ii) microstructure stility is enhnced. A reduction of temperture to 55 C leds to mrked decrese in the frction of the phse to ~1%, therey decresing the length of the interphse oundries nd incresing microstructurl instility. Indeed, the kinetics of strin-induced grin growth re much fster thn those during sttic nneling (Fig. 5). In ddition, the fct tht the tested specimens did not exhiit porosity ner the frcture surfce t low tempertures ppers very unusul, nd my e prtilly rtionlized on the sis of grin-corsening enhnced relxtion of intergrnulr stresses during SP deformtion. The mechnism of LTSP in UFG Ti-6Al-4V cn e deduced sed on the vlue of the pprent ctivtion energy. Although the vlue otined here (149 kj/mol) is slightly lower thn tht reported for higher-temperture SP (176 nd 16 kj/mol [4, 1]), the difference is not lrge. Also, the vlue of ctivtion energy otined in the present work is similr to tht reported in [13] for grin-oundry diffusion in the SP regime from 8 to 95 C for conventionl Ti-6Al-4V (189 kj/mole). Thus, it my e hypothesized tht the deformtion mechnism for UFG Ti-6Al-4V t lower tempertures is grinoundry sliding ccommodted y grin-oundry diffusion. The reduction in texture intensity during SP deformtion cn lso e scried to grin-oundry sliding nd the concomitnt rndomiztion effect ssocited with the rottion of lph prticles. The

6 phenomenon is similr to tht oserved previously in Ti-6Al-4V [9] nd Zn-22%Al [6]. The oserved decrese in the Kern s fctor from.52 to.44 lso suggests wekening of crystllogrphic texture towrd rndom one, which would exhiit Kern s fctor of.361 mesured for the norml plne of tested specimens (,333 in perfectly-rndom mteril). Summry nd Conclusions 1. Superplstic deformtion of Ti-6Al-4V with men grin size of 135 nm t the extremely low temperture of 55 C ws investigted. Using the optiml strin rte of s -1, totl elongtion of 1% ws otined with strin rte sensitivity nd flow stress of.47 nd 13 MP, respectively. 2. Microstructure oservtions showed extensive deformtion-induced grin growth during SP deformtion. The specimens did not hve porosity ner the frcture surfce. 3. The texture wekened considerly during low-temperture SP deformtion. Such wekening ws mirrored in the vlues of the Kern s fctor which decresed from.52 to.44, the ltter vlue pproching tht of rndom mteril (i.e.,.361) 4. The pprent ctivtion energy under the optiml SP conditions ws found to e of 149 kj/mole t n=2 (m=.5) indicting tht the dominnt deformtion mechnism ws grin-oundry sliding ccommodted y grin-oundry diffusion nd possily mtrix-disloction ctivity. Acknowledgements This work ws supported y Grnt P725 from the Russin Ministry of Science nd Eduction. References [1] H. Ingki, Enhnced superplsticity in high strength Ti lloys, Z. Metllk. 86 (1995) [2] G.A. Slishchev et l., Development of Ti-6Al-4V sheet with low temperture superplstic properties, J. Mter. Proc. Technol. 116 (21) [3] S.V. Zheretsov et l., Production of sumicrocrystlline structure in lrge-scle Ti-6Al-4V illet y wrm severe deformtion processing, Script Mter. 51 (24) [4] Y.G. Ko et l., Low-temperture superplsticity of ultr-fine-grined Ti-6AI-4V processed y equl-chnnel ngulr pressing, Metll. Mter. Trns. 37A (26) [5] A.V. Sergueev et l., Superplstic ehviour of ultrfine-grined Ti-6Al-4V lloys, Mter. Sci. Eng. A323 (22) [6] O.A. Kiyshev: Superplsticity of Alloys, Intermetllides nd Cermics (Springer-Verlg, Berlin 1992). [7] G.A. Slishchev et l., Sumicrocrystlline nd nnocrystlline structure formtion in mterils nd serch for outstnding superplstic properties, Mter. Sci. Forum (1994) [8] Y. Mishin, C. Herzig, Diffusion in the Ti-Al system, Act Mter. 48 (2) [9] S.L. Semitin et l., Plstic flow nd microstructure evolution during low-temperture superplsticity of ultrfine Ti-6Al-4V sheet mteril, Metll. Mter. Trns. 41A (21) [1] G.A. Srgent et l., Low-temperture corsening nd plstic flow ehvior of n / titnium illet mteril with n ultrfine microstructure, Metll. Mter. Trns. 39A (28) [11] G. Wssermn nd I. Greven: Texturen metllischer Werkstoffe (Springer-Verlg, Berlin 1962). [12] M.J. Donchie, Jr.: Titmium: Technicl Guide (ASM Interntionl, USA 2). [13] A. Arieli, A. Rosen, Superplstic deformtion of Ti-6Al-4V, Metll. Trns. 8A (1977)