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Effect of boron on microstructure and mechanical properties of multicomponent titanium alloys
Materials Letters 158 (2015) 111–114
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of boron on microstructure and mechanical properties of multicomponent titanium alloys I.V. Okulov a,b,n, M.F. Sarmanova c, A.S. Volegov a,d, A. Okulov e, U. Kühn a, W. Skrotzki f, J. Eckert a,b a
IFW Dresden, Institut für Komplexe Materialien, Helmholtzstr. 20, D-01069 Dresden, Germany TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany c Leibniz-Institut für Oberflächenmodifizierung e. V. Permoserstraße 15, 04318 Leipzig, Germany d Institute of Natural Sciences, Ural Federal University, 620000 Ekaterinburg, Russia e Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia f Technische Universität Dresden, Institut für Strukturphysik, D-01062 Dresden, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 1 June 2015 Accepted 3 June 2015 Available online 6 June 2015
Boron refinement is an established method to refine the grain structure of titanium alloys. The effect of boron on microstructure of multicomponent Ti–Nb–Cu–Co–Al–B alloys was studied. These as-cast alloys exhibit a composite-like microstructure consisting of about 92 vol% of β-Ti dendrites surrounded by an ultrafine-structured eutectic composed of β-Ti and B2 TiCo. It was found that significant additions (up to 1 at%) of boron do not result in a pronounced refinement of microstructure in these alloys. However, noticeable strengthening and stiffening effects are observed for the alloys with increasing boron concentration. In particular, the yield strength of Ti–13.6Nb–6Co–5.1Cu–6.5Al (at%) improves from 1110 730 MPa to 1200 730 MPa with addition of 1 at% of boron. Additionally, increasing boron content affects the morphology of the eutectic structure causing its degradation. & 2015 Elsevier B.V. All rights reserved.
Keywords: Titanium alloys Microstructure Boron refinement Electron microscopy Mechanical properties
In recent years, considerable research has been performed on studying the effect of boron additions on the microstructure and mechanical properties of titanium alloys. It was reported that, in a variety of titanium alloys [1–5], minor boron additions (typically, below 1 at%) are effective to refine the microstructure of cast specimens. This typically results in an improvement of the mechanical properties of titanium alloys, in particular, in an increase of tensile strength and stiffness [2,5]. Different mechanisms of boron-induced grain refinement have been proposed for cast titanium alloys. One mechanism supposes that boride particles, which are either not dissolved upon melting (in case borides were added to the alloy) or primarily formed upon solidification, act as nucleants [1]. Another approach hypothesizes that the refinement is related to constitutional supercooling at the solid–liquid interface caused by boron rejection from primarily solidified grains into the liquid ahead of the solidification front [2,3]. The mechanism of the microstructural refinement is even more complicated in multicomponent titanium alloys because of mutual n Correspondence to: P.O. Box 270016, D-01171 Fax: þ49 351 4659 452. E-mail address: [email protected] (I.V. Okulov).
http://dx.doi.org/10.1016/j.matlet.2015.06.017 0167-577X/& 2015 Elsevier B.V. All rights reserved.
Dresden,
Germany.
contributions of different alloying elements [4,6–8]. In particular, it was found that some transition metals (e.g. Ni, Cu, Co, Cr and Fe) have a significant effect on grain refinement in titanium alloys [7]. The current work aims to clarify the boron refinement effect in multicomponent alloys containing several transition metals, which also contribute to refinement of microstructure. Therefore, two alloys with different boron content, namely, Ti–13.6Nb–6Co– 5.1Cu–6.5Al–0.5B (at%) and Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B (at%), were derived from the parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al (at%) exhibiting significant ultimate tensile strength of 1265 740 MPa and tensile ductility of 13.5 70.5% already in the as-cast state [9]. The parent alloy belongs to a group of titanium alloys consisting of primary beta titanium dendrites surrounded by an ultrafine-structured eutectic [9–11]. The samples in cylindrical shape (100 mm length and 12 mm diameter) were prepared by inductive melting and casting into a water-cooled Cu-die under pure argon atmosphere (99,999%). The details of preparation, composition and structure characterization methods have been already described elsewhere [9]. The mechanical properties were measured under compressive loading in an Instron 8562 testing machine at a constant strain rate of 1 10 4 s 1 at room temperature. The strain was measured by a laser extensometer (Fiedler Optoelektronik). The compressive test
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Fig. 1. XRD patterns of Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B (a) and Ti–13.6Nb–6Co– 5.1Cu–6.5Al–1B (b) (Mo-Kα1 radiation).
samples were prepared by electro-erosive machining according to the ASTM E9 standard from the as-cast rods. According to the X-Ray diffraction (XRD) patterns (Fig. 1) the studied alloys consist of predominant β-Ti (space group Im 3m) and a minor amount of TiCo (space group Pm 3m). For both alloys the lattice parameters of the same phases are very similar. The lattice parameter of the β-Ti phase is 0.3237 70.0001 nm, which is lower than the value a0 ¼0.3282 nm for pure β-Ti at 25 °C (as obtained by extrapolation) [12]. This may indicate that the β-Ti phase is a solid solution. The lattice parameter of the TiCo phase is 0.3040 70.0001 nm. These crystallographic parameters are very close to those of the parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al [9]. It has to be noted that the peaks of the β-Ti phase exhibit a pronounced shouldering (Fig. 1) indicating an additional bcc phase with a lattice parameter of 0.32157 0.0001 nm. The origin of this phase is probably caused by a microsegregation in the β-Ti dendrites similar to that observed for the parent Ti–13.6Nb–6Co– 5.1Cu–6.5Al alloy [9]. According to scanning electron microscopy (SEM) analysis three annulus-shaped microstructural zones were identified on the cross sections of the rods: a fine-grained surface-near zone (about 750 7250 mm in width), a dendritic zone (about 4 70.5 mm in width) (Fig. 2a and b) and a central dendritic zone with interdendritic porosity. For further analysis, the dendritic zone was microstructurally and mechanically characterized. The primary β-Ti grains in these alloys exhibit a dendritic morphology and are surrounded by an ultrafine-structured eutectic network (Figs. 2 and 3). The volume fraction of the β-Ti dendrites is 92 72 vol% for both alloys what is similar to the parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al. The secondary arm spacing deduced from the SEM images is 6.9 70.5 mm and 6.2 70.5 mm for Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B and Ti–13.6Nb– 6Co–5.1Cu–6.5Al–1B, respectively. Fine dark needles are found in the interior of the eutectic structures (Fig. 3b and c) and along the grain boundaries of the β-Ti dendrites (Fig. 2c). According to the previous studies [2,5,13] these needles seems to be TiB. Comparing the microstructural parameters of the studied alloys with the boron-free parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al [9], it can be deduced that boron additions do not have a significant effect on shape and size of the β-Ti dendrites with an exception of a slight decrease of its secondary arm spacing. However, increasing boron concentration affects the shape of the eutectic microconstituent causing its degradation (Fig. 3). The detailed microstructural analysis allows proposing a solidification mechanism for the studied alloys. As the temperature of the alloy melt decreases below the liquidus temperature, β-Ti dendrites start to grow. According to the elemental distribution maps (not shown here, but very similar to that for the parent alloy
Fig. 2. Microstructure of the studied alloys: (a) the dendritic zone of Ti–13.6Nb– 6Co–5.1Cu–6.5Al–0.5B; (b) the dendritic zone of Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B and (c) the TiB precipitates distributed along grain boundary in Ti–13.6Nb–6Co– 5.1Cu–6.5Al–1B.
[9]), solidification of the β-Ti dendrites is accompanied by diffusion of Cu and Co from the dendrites to the residual melt as well as by back diffusion of Ti and Nb. Once a sufficient concentration of Cu and Co in the residual melt is achieved, it solidifies into an ultrafine-structured eutectic consisting of β-Ti and B2 TiCo. According to previous results [2] boron typically segregates to the interdendritic regions. This is in agreement with the observation of the TiB needle-shape particles along the boundaries of the β-Ti dendrites (Fig. 2c) and into the interior of the eutectic structure (Fig. 3b and c). The degradation of the eutectic with increasing concentration of boron indicates that the TiB particles precipitate during solidification of the eutectic causing irregularities in the solidification front of the eutectic and, eventually, its degradation. Despite the fact that boron is a proven effective refinement agent for numerous titanium alloys [1–5,13] its effectiveness for the multicomponent titanium alloys containing transition metals such as Cu and Co is not very pronounced. However, this fact seems to be in line with a theory stating that boron refinement is related to constitutional supercooling at the solid–liquid interface caused by boron [2,3]. Since the microstructure of the alloys exhibits a strong segregation of Cu and Co to the periphery of the βTi dendrites these elements also induce an undercooling at the solid–liquid interface and, thus, come into competition with boron. Due to much lower boron concentrations compared to Cu and Co in the studied alloys the effect of boron on the overall refinement of their microstructure is minor. The result of the compressive tests shows that the yield strength of the developed alloys increases with increasing concentration of boron from 1150 730 MPa to 1200 730 MPa for Ti– 13.6Nb–6Co–5.1Cu–6.5Al–0.5B and Ti–13.6Nb–6Co–5.1Cu–6.5Al– 1B, respectively (Table 1). Additionally, the boron-containing alloys exhibit higher values of yield strength compared to that of the parent alloy, which is 1110 730 MPa (Table 1 and Fig. 4). The following possible reasons for the increase of alloys strength with
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Fig. 3. Microstructure of the studied alloys indicating degradation of the eutectic with increasing concentration of boron: (a) the parent Ti–13.6Nb–6Co–5.1Cu–6.5Al alloy, (b) Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B and (c) Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B (Note: short yellow arrows indicate the TiB precipitates) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Table 1 Room temperature compressive properties of Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B, Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B and Ti–13.6Nb–6Co–5.1Cu–6.5Al. Alloy
Young’s modulus (GPa)
Ti–13.6Nb–6Co– 927 3 5.1Cu–6.5Al 977 3 Ti–13.6Nb–6Co– 5.1Cu–6.5Al– 0.5B Ti–13.6Nb–6Co– 1007 3 5.1Cu–6.5Al– 1B
Yield stress (MPa)
Maximum strength (MPa)
Fracture strain (%)
1110 730
1550 750
367 3
1150 730
1570 750
347 3
1200 730
1680 750
347 3
Fig. 4. Room temperature compressive stress–strain curves of Ti–13.6Nb–6Co– 5.1Cu–6.5Al–0.5B and Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B and of the parent Ti– 13.6Nb–6Co–5.1Cu–6.5Al alloy.
increasing boron concentration are refining of secondary arm spacing of the β-Ti dendrites [2] and dispersion strengthening due to the TiB particles [2,13]. The compressive fracture strain is similar for all boron-free and boron containing alloys (Table 1). However, due to the inherent brittleness of TiB as well as relatively easy crack initiation and propagation at the TiB particle/matrix interface [5,13] the higher volume fraction of the TiB particles typically leads to a lower tensile ductility in titanium alloys [2,5,13]. Additionally, the
needle-shape TiB particles precipitated in the interior of the eutectic structure may prevent interfacial slip transfer between the dendritic and the eutectic phases, which has been reported to be a key mechanism responsible for the high tensile ductility (13.5 70.5%) of the parent Ti–13.6Nb–6Co–5.1Cu–6.5Al alloy [9]. It has to be noted that the Young’s modulus of the studied alloys increases with increasing boron concentration (Table 1). The similar effect was previously observed for several commercial titanium alloys [2,13,14]. The higher concentration of boron leads to a higher volume fraction of the TiB particles (Fig. 3), which provide an efficient reinforcement toward the stiffness [2,13,14]. In summary, two Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B (at%) and T–13.6Nb–6Co–5.1Cu–6.5Al–1B (at%) were developed to study the effect of minor boron additions on the microstructure development, the grain refinement and the deformation behavior of multicomponent titanium alloys. Both alloys consist of the primary β-Ti dendrites surrounded by the ultrafine-structured eutectic and the needle-shape TiB precipitates. It was found that boron has only small but distinct refinement effect in the multicomponent Ti–Nb–Co–Cu–Al–B alloys. In particular, the addition of boron induces the refinement of the secondary dendrite arm spacing of the β-Ti phase. The reduced refinement effect of boron attributes to the presence of transition metals such as Cu and Ni. These elements refine the microstructure by the mechanism similar to boron and, therefore, come into a competition with the latter one. Since the relative concentration of boron is much lower compared to Cu and Co the refinement effect of boron is not very pronounced. However, noticeable strengthening and stiffening effects of boron additions were found for the studied alloys.
Acknowledgments The authors are grateful to S. Donath, F. Ebert and M. Frey for technical assistance. Funding by the EU and the Free State Saxony (Contract no. 100111842) within the European Centre for Emerging Materials and Processes Dresden (ECEMP) is gratefully acknowledged.
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References [1] T.T. Cheng, The mechanism of grain refinement in TiAl alloys by boron addition-an alternative hypothesis, Intermetallics 8 (2000) 29–37. [2] A. Helth, U. Siegel, U. Kühn, T. Gemming, W. Gruner, S. Oswald, et al., Influence of boron and oxygen on the microstructure and mechanical properties of highstrength Ti66Nb13Cu8Ni6.8Al6.2 alloys, Acta Mater. 61 (2013) 3324–3334. [3] S. Tamirisakandala, R.B. Bhat, J.S. Tiley, Miracle DB. Grain refinement of cast titanium alloys via trace boron addition, Scr. Mater. 53 (2005) 1421–1426. [4] M.J. Bermingham, S.D. McDonald, K. Nogita St., D.H. John, M.S. Dargusch, Effects of boron on microstructure in cast titanium alloys, Scr. Mater. 59 (2008) 538–541. [5] R. Sarkar, P. Ghosal, T.K. Nandy, K.K. Ray, Structure–property correlation of a boron and carbon modified as cast β titanium alloy, Philos. Mag. 93 (2013) 1936–1957. [6] M.J. Bermingham, S.D. Mcdonald, D.H. Stjohn, M.S. Dargusch, Segregation and grain refinement in cast titanium alloys, J. Mater. Res. (2009) 1529–1535. [7] G.V. Samsonov, V.A. Kashchuk, A.I. Cherkashin, Effect of transition metals on the grain size of titanium, Met. Term. Obrab. Met. (1971) 30–31.
[8] I.V. Okulov, U. Kühn, T. Marr, J. Freudenberger, I.V. Soldatov, L. Schultz, et al., Microstructure and mechanical properties of new composite structured Ti–V– Al–Cu–Ni alloys for spring applications, Mater. Sci. Eng. A 603 (2014) 76–83. [9] I.V. Okulov, M. Bönisch, U. Kühn, W. Skrotzki, J. Eckert, Significant tensile ductility and toughness in an ultrafine-structured Ti68.8Nb13.6Co6Cu5.1Al6.5 bimodal alloy, Mater. Sci. Eng. A 615 (2014) 457–463. [10] G. He, J. Eckert, W. Löser, L. Schultz, Novel Ti-base nanostructure-dendrite composite with enhanced plasticity, Nat. Mater. 2 (2003) 33–37. [11] G. He, J. Eckert, W. Löser, M. Hagiwara, Composition dependence of the microstructure and the mechanical properties of nano/ultrafine-structured Ti– Cu–Ni–Sn–Nb alloys, Acta Mater. 52 (2004) 3035–3046. [12] V.N. Moiseyev, Titanium Alloys Russian Aircraft and Aerospace Applications, CRC Press Taylor & Francis Group, Boca Raton, 2006. [13] J. Zhu, A. Kamiya, T. Yamada, W. Shi, K. Naganuma, Influence of boron addition on microstructure and mechanical properties of dental cast titanium alloys, Mater. Sci. Eng. A 339 (2003) 53–62. [14] I. Sen, Mechanical properties of B-modified Ti–6Al–4V alloys, J. Phys.: Conf. Ser. 240 (2010) 012039.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of boron on microstructure and mechanical properties of multicomponent titanium alloys I.V. Okulov a,b,n, M.F. Sarmanova c, A.S. Volegov a,d, A. Okulov e, U. Kühn a, W. Skrotzki f, J. Eckert a,b a
IFW Dresden, Institut für Komplexe Materialien, Helmholtzstr. 20, D-01069 Dresden, Germany TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany c Leibniz-Institut für Oberflächenmodifizierung e. V. Permoserstraße 15, 04318 Leipzig, Germany d Institute of Natural Sciences, Ural Federal University, 620000 Ekaterinburg, Russia e Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia f Technische Universität Dresden, Institut für Strukturphysik, D-01062 Dresden, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 1 June 2015 Accepted 3 June 2015 Available online 6 June 2015
Boron refinement is an established method to refine the grain structure of titanium alloys. The effect of boron on microstructure of multicomponent Ti–Nb–Cu–Co–Al–B alloys was studied. These as-cast alloys exhibit a composite-like microstructure consisting of about 92 vol% of β-Ti dendrites surrounded by an ultrafine-structured eutectic composed of β-Ti and B2 TiCo. It was found that significant additions (up to 1 at%) of boron do not result in a pronounced refinement of microstructure in these alloys. However, noticeable strengthening and stiffening effects are observed for the alloys with increasing boron concentration. In particular, the yield strength of Ti–13.6Nb–6Co–5.1Cu–6.5Al (at%) improves from 1110 730 MPa to 1200 730 MPa with addition of 1 at% of boron. Additionally, increasing boron content affects the morphology of the eutectic structure causing its degradation. & 2015 Elsevier B.V. All rights reserved.
Keywords: Titanium alloys Microstructure Boron refinement Electron microscopy Mechanical properties
In recent years, considerable research has been performed on studying the effect of boron additions on the microstructure and mechanical properties of titanium alloys. It was reported that, in a variety of titanium alloys [1–5], minor boron additions (typically, below 1 at%) are effective to refine the microstructure of cast specimens. This typically results in an improvement of the mechanical properties of titanium alloys, in particular, in an increase of tensile strength and stiffness [2,5]. Different mechanisms of boron-induced grain refinement have been proposed for cast titanium alloys. One mechanism supposes that boride particles, which are either not dissolved upon melting (in case borides were added to the alloy) or primarily formed upon solidification, act as nucleants [1]. Another approach hypothesizes that the refinement is related to constitutional supercooling at the solid–liquid interface caused by boron rejection from primarily solidified grains into the liquid ahead of the solidification front [2,3]. The mechanism of the microstructural refinement is even more complicated in multicomponent titanium alloys because of mutual n Correspondence to: P.O. Box 270016, D-01171 Fax: þ49 351 4659 452. E-mail address: [email protected] (I.V. Okulov).
http://dx.doi.org/10.1016/j.matlet.2015.06.017 0167-577X/& 2015 Elsevier B.V. All rights reserved.
Dresden,
Germany.
contributions of different alloying elements [4,6–8]. In particular, it was found that some transition metals (e.g. Ni, Cu, Co, Cr and Fe) have a significant effect on grain refinement in titanium alloys [7]. The current work aims to clarify the boron refinement effect in multicomponent alloys containing several transition metals, which also contribute to refinement of microstructure. Therefore, two alloys with different boron content, namely, Ti–13.6Nb–6Co– 5.1Cu–6.5Al–0.5B (at%) and Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B (at%), were derived from the parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al (at%) exhibiting significant ultimate tensile strength of 1265 740 MPa and tensile ductility of 13.5 70.5% already in the as-cast state [9]. The parent alloy belongs to a group of titanium alloys consisting of primary beta titanium dendrites surrounded by an ultrafine-structured eutectic [9–11]. The samples in cylindrical shape (100 mm length and 12 mm diameter) were prepared by inductive melting and casting into a water-cooled Cu-die under pure argon atmosphere (99,999%). The details of preparation, composition and structure characterization methods have been already described elsewhere [9]. The mechanical properties were measured under compressive loading in an Instron 8562 testing machine at a constant strain rate of 1 10 4 s 1 at room temperature. The strain was measured by a laser extensometer (Fiedler Optoelektronik). The compressive test
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Fig. 1. XRD patterns of Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B (a) and Ti–13.6Nb–6Co– 5.1Cu–6.5Al–1B (b) (Mo-Kα1 radiation).
samples were prepared by electro-erosive machining according to the ASTM E9 standard from the as-cast rods. According to the X-Ray diffraction (XRD) patterns (Fig. 1) the studied alloys consist of predominant β-Ti (space group Im 3m) and a minor amount of TiCo (space group Pm 3m). For both alloys the lattice parameters of the same phases are very similar. The lattice parameter of the β-Ti phase is 0.3237 70.0001 nm, which is lower than the value a0 ¼0.3282 nm for pure β-Ti at 25 °C (as obtained by extrapolation) [12]. This may indicate that the β-Ti phase is a solid solution. The lattice parameter of the TiCo phase is 0.3040 70.0001 nm. These crystallographic parameters are very close to those of the parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al [9]. It has to be noted that the peaks of the β-Ti phase exhibit a pronounced shouldering (Fig. 1) indicating an additional bcc phase with a lattice parameter of 0.32157 0.0001 nm. The origin of this phase is probably caused by a microsegregation in the β-Ti dendrites similar to that observed for the parent Ti–13.6Nb–6Co– 5.1Cu–6.5Al alloy [9]. According to scanning electron microscopy (SEM) analysis three annulus-shaped microstructural zones were identified on the cross sections of the rods: a fine-grained surface-near zone (about 750 7250 mm in width), a dendritic zone (about 4 70.5 mm in width) (Fig. 2a and b) and a central dendritic zone with interdendritic porosity. For further analysis, the dendritic zone was microstructurally and mechanically characterized. The primary β-Ti grains in these alloys exhibit a dendritic morphology and are surrounded by an ultrafine-structured eutectic network (Figs. 2 and 3). The volume fraction of the β-Ti dendrites is 92 72 vol% for both alloys what is similar to the parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al. The secondary arm spacing deduced from the SEM images is 6.9 70.5 mm and 6.2 70.5 mm for Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B and Ti–13.6Nb– 6Co–5.1Cu–6.5Al–1B, respectively. Fine dark needles are found in the interior of the eutectic structures (Fig. 3b and c) and along the grain boundaries of the β-Ti dendrites (Fig. 2c). According to the previous studies [2,5,13] these needles seems to be TiB. Comparing the microstructural parameters of the studied alloys with the boron-free parent alloy Ti–13.6Nb–6Co–5.1Cu–6.5Al [9], it can be deduced that boron additions do not have a significant effect on shape and size of the β-Ti dendrites with an exception of a slight decrease of its secondary arm spacing. However, increasing boron concentration affects the shape of the eutectic microconstituent causing its degradation (Fig. 3). The detailed microstructural analysis allows proposing a solidification mechanism for the studied alloys. As the temperature of the alloy melt decreases below the liquidus temperature, β-Ti dendrites start to grow. According to the elemental distribution maps (not shown here, but very similar to that for the parent alloy
Fig. 2. Microstructure of the studied alloys: (a) the dendritic zone of Ti–13.6Nb– 6Co–5.1Cu–6.5Al–0.5B; (b) the dendritic zone of Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B and (c) the TiB precipitates distributed along grain boundary in Ti–13.6Nb–6Co– 5.1Cu–6.5Al–1B.
[9]), solidification of the β-Ti dendrites is accompanied by diffusion of Cu and Co from the dendrites to the residual melt as well as by back diffusion of Ti and Nb. Once a sufficient concentration of Cu and Co in the residual melt is achieved, it solidifies into an ultrafine-structured eutectic consisting of β-Ti and B2 TiCo. According to previous results [2] boron typically segregates to the interdendritic regions. This is in agreement with the observation of the TiB needle-shape particles along the boundaries of the β-Ti dendrites (Fig. 2c) and into the interior of the eutectic structure (Fig. 3b and c). The degradation of the eutectic with increasing concentration of boron indicates that the TiB particles precipitate during solidification of the eutectic causing irregularities in the solidification front of the eutectic and, eventually, its degradation. Despite the fact that boron is a proven effective refinement agent for numerous titanium alloys [1–5,13] its effectiveness for the multicomponent titanium alloys containing transition metals such as Cu and Co is not very pronounced. However, this fact seems to be in line with a theory stating that boron refinement is related to constitutional supercooling at the solid–liquid interface caused by boron [2,3]. Since the microstructure of the alloys exhibits a strong segregation of Cu and Co to the periphery of the βTi dendrites these elements also induce an undercooling at the solid–liquid interface and, thus, come into competition with boron. Due to much lower boron concentrations compared to Cu and Co in the studied alloys the effect of boron on the overall refinement of their microstructure is minor. The result of the compressive tests shows that the yield strength of the developed alloys increases with increasing concentration of boron from 1150 730 MPa to 1200 730 MPa for Ti– 13.6Nb–6Co–5.1Cu–6.5Al–0.5B and Ti–13.6Nb–6Co–5.1Cu–6.5Al– 1B, respectively (Table 1). Additionally, the boron-containing alloys exhibit higher values of yield strength compared to that of the parent alloy, which is 1110 730 MPa (Table 1 and Fig. 4). The following possible reasons for the increase of alloys strength with
I.V. Okulov et al. / Materials Letters 158 (2015) 111–114
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Fig. 3. Microstructure of the studied alloys indicating degradation of the eutectic with increasing concentration of boron: (a) the parent Ti–13.6Nb–6Co–5.1Cu–6.5Al alloy, (b) Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B and (c) Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B (Note: short yellow arrows indicate the TiB precipitates) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Table 1 Room temperature compressive properties of Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B, Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B and Ti–13.6Nb–6Co–5.1Cu–6.5Al. Alloy
Young’s modulus (GPa)
Ti–13.6Nb–6Co– 927 3 5.1Cu–6.5Al 977 3 Ti–13.6Nb–6Co– 5.1Cu–6.5Al– 0.5B Ti–13.6Nb–6Co– 1007 3 5.1Cu–6.5Al– 1B
Yield stress (MPa)
Maximum strength (MPa)
Fracture strain (%)
1110 730
1550 750
367 3
1150 730
1570 750
347 3
1200 730
1680 750
347 3
Fig. 4. Room temperature compressive stress–strain curves of Ti–13.6Nb–6Co– 5.1Cu–6.5Al–0.5B and Ti–13.6Nb–6Co–5.1Cu–6.5Al–1B and of the parent Ti– 13.6Nb–6Co–5.1Cu–6.5Al alloy.
increasing boron concentration are refining of secondary arm spacing of the β-Ti dendrites [2] and dispersion strengthening due to the TiB particles [2,13]. The compressive fracture strain is similar for all boron-free and boron containing alloys (Table 1). However, due to the inherent brittleness of TiB as well as relatively easy crack initiation and propagation at the TiB particle/matrix interface [5,13] the higher volume fraction of the TiB particles typically leads to a lower tensile ductility in titanium alloys [2,5,13]. Additionally, the
needle-shape TiB particles precipitated in the interior of the eutectic structure may prevent interfacial slip transfer between the dendritic and the eutectic phases, which has been reported to be a key mechanism responsible for the high tensile ductility (13.5 70.5%) of the parent Ti–13.6Nb–6Co–5.1Cu–6.5Al alloy [9]. It has to be noted that the Young’s modulus of the studied alloys increases with increasing boron concentration (Table 1). The similar effect was previously observed for several commercial titanium alloys [2,13,14]. The higher concentration of boron leads to a higher volume fraction of the TiB particles (Fig. 3), which provide an efficient reinforcement toward the stiffness [2,13,14]. In summary, two Ti–13.6Nb–6Co–5.1Cu–6.5Al–0.5B (at%) and T–13.6Nb–6Co–5.1Cu–6.5Al–1B (at%) were developed to study the effect of minor boron additions on the microstructure development, the grain refinement and the deformation behavior of multicomponent titanium alloys. Both alloys consist of the primary β-Ti dendrites surrounded by the ultrafine-structured eutectic and the needle-shape TiB precipitates. It was found that boron has only small but distinct refinement effect in the multicomponent Ti–Nb–Co–Cu–Al–B alloys. In particular, the addition of boron induces the refinement of the secondary dendrite arm spacing of the β-Ti phase. The reduced refinement effect of boron attributes to the presence of transition metals such as Cu and Ni. These elements refine the microstructure by the mechanism similar to boron and, therefore, come into a competition with the latter one. Since the relative concentration of boron is much lower compared to Cu and Co the refinement effect of boron is not very pronounced. However, noticeable strengthening and stiffening effects of boron additions were found for the studied alloys.
Acknowledgments The authors are grateful to S. Donath, F. Ebert and M. Frey for technical assistance. Funding by the EU and the Free State Saxony (Contract no. 100111842) within the European Centre for Emerging Materials and Processes Dresden (ECEMP) is gratefully acknowledged.
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