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An experimental study on the mechanism of texture evolution during hot-rolling process in a β titanium alloy
Journal of Alloys and Compounds 603 (2014) 23–27
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
An experimental study on the mechanism of texture evolution during hot-rolling process in a b titanium alloy Hongchao Kou a,⇑, Yi Chen a,b, Bin Tang a, Yuwen Cui b,⇑, Feng Sun a, Jinshan Li a, Xiangyi Xue a a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China Computational Alloy Design Group, IMDEA Materials Institute, Madrid 28040, Spain
a r t i c l e
i n f o
Article history: Received 26 September 2013 Received in revised form 12 February 2014 Accepted 12 March 2014 Available online 24 March 2014 Keywords: b-Titanium Hot rolling Texture Dynamic recrystallization
a b s t r a c t The formation of crystallographic texture in a b-titanium alloy (Ti–15Mo–3Al–2.7Nb–0.2Si, wt.%) during the dynamic recrystallization (DRX) in hot rolling process was studied. The results reveal that the DRX occurs during the hot rolling process, which in turn results in the weakening of the deformation texture. The EBSD and TEM analyses further confirm that the weakening is associated with the randomly-distributed orientation relations of newly formed DRX grains, as well as the large misorientation remained between the deformed matrix and the DRX grains. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hot plastic deformation accompanied by dynamic recrystallization (DRX) sustains one of the most promising methods for the grain refinement in a variety of metals and alloys [1–9]. The dynamic recrystallization can be classified as: (i) discontinuous dynamic recrystallization (DDRX), during which the nucleation and growth of new grains operate through a bulging mechanism with relatively low stacking fault energy [1]; (ii) continuous dynamic recrystallization (CDRX), which involves a transformation of low angle boundaries into high angle boundaries of the subgrains with relatively high stacking fault energy [3]. It is well known that the texture evolution can be profoundly complicated by the DRX because the deformation enhances the deformation texture while the recrystallization will subsequently change it during the hot deformation process [10,11]. Different texture evolution phenomena were investigated in a variety of metals and alloys [4,8,9], including such interesting examples that a sharp texture component inevitably results from the DRX in the c-TiAl [4] and Mg alloys [9], and no any change of the texture was noticed with the DRX in nickel and copper [4]. However, the texture evolution mechanism under the DRX condition in b titanium alloys has been received little attention.
⇑ Corresponding authors. Tel.: +86 29 88460568 (H. Kou), +34 917871876 (Y. Cui). E-mail addresses: [email protected] (H. Kou), [email protected] (Y. Cui). http://dx.doi.org/10.1016/j.jallcom.2014.03.070 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
The present work is to address a b titanium alloy Ti–15Mo–3Al– 2.7Nb–0.2Si, that is widely being used in aerospace and bio-implant industries [12,13] with a focus on studying the evolution of crystallographic texture and the orientation dependence characteristics of the DRX grains, as well as the variation of the texture with the DRX during the hot deformation process.
2. Experimental Several £45 mm Ti–15Mo–3Al–2.7Nb–0.2Si (chemical composition: Ti– 14.9Mo–3.03Al–2.64Nb–0.18Si–0.009C–0.031Fe–0.13O–0.013N–0.001H, wt.%) bars were made by vacuum self-consuming arc melting followed with gradual forgings at 1373 K, 1273 K, 1174 K, 1073 K in an open die forging equipment. The a/b phase transformation temperature of Ti–15Mo–3Al–2.7Nb–0.2Si alloy was measured to be 988 K by the metallographic technique. In order to obtain a fully b single-phase microstructure, all the Ti–15Mo–3Al–2.7Nb–0.2Si bars with a diameter of 45 mm were solid-solutioned at 1173 K for 4 h followed by water quenching, as shown in Fig. 1. The bar rolling process was conducted on an industry bar mill at the Western Superconducting Technologies Co., Ltd. (Xi’an, China). As shown in Fig. 2, the Ti–15Mo–3Al–2.7Nb–0.2Si bars were firstly heated up to 1133 K and held for 30 min, and then were pass-by-pass hot rolled respectively to £25 mm (with a total deformation of 56%), £18 mm (84% deformation), and £8 mm (97% deformation) in the atmospheric condition. A cross rolling, the most typical rolling technique for a bar, was performed, i.e., the specimens were rotated about rolling direction by 90° after each consecutive pass. After the rolling, the bars were water quenched. The texture of the center parts taken from the specimens was measured by using a Siemens D5000 X-ray diffractometer based on the Schulz reflection technique with Mo Ka radiation. The texture of the b phase was achieved by measuring three incomplete pole figures, namely, (1 1 0), (2 0 0) and (2 1 1), from which its complete orientation distribution function (ODF) was then calculated. Microstructures were characterized by OLYMPUS/PMG3 optical microscope (OM) and FEI Tecnai
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H. Kou et al. / Journal of Alloys and Compounds 603 (2014) 23–27
Fig. 1. Microstructure of the Ti–15Mo–3Al–2.7Nb–0.2Si alloy after solutioning at 1173 K for 4 h.
deformation, a complete DRX microstructure was attained as shown in Fig. 3(e). The close-up microstructure of the hot rolled specimens was characterized by TEM. As shown in Fig. 4, 56% deformation by hot-rolling triggered the activation of the slip system and the movement of the dislocation, which in turn resulted in the formation of evident slip bands and dislocation boundaries (see Fig. 4(a)). Fig. 4(b) shows that an increase of the deformation to 84% enhances the dislocation density and brings about some DRX grains along the elongated grain boundaries, whereas 97% deformation leads to a cellular microstructure consisting of fine DRX grains with a high dislocation density, see Fig. 4(c). The diffuse diffraction spots revealed by the selected area diffraction (SAD) analysis show these fine DRX grains having the random orientations. The orientation distribution function (ODF), a measure of the texture of the rolled samples, can be calculated from the measured incomplete pole figures by X-ray diffractometer based on the Schulz reflection technique, represent the intensity of each orientation in the whole 3D space [19]. In the ODF section, the intensity of each orientation expressed by the contour line while the Euler angles g(u1, U, u2) of each point indicate the texture components, which can be transformed to the Miller index (h k l)hu v wi by using Eq. (1).
0
1 2 3 cos u1 cos u2 sin u1 sin u2 cos U sin u2 sin U u h B C 6 7 @ v k A ¼ 4 cos u1 sin u2 sin u1 cos u2 cos U cos u2 sin U 5 sin u1 sin U cos U w l ð1Þ
Fig. 2. Schematic diagram of rolling process in the present work.
G2 F30 transmission electron microscope (TEM), while the orientation characteristics of the DRX grains and deformed matrix were investigated by Electron Back Scattered Diffraction (EBSD) with a HKL-Channel 5 system in JSM-6700F FEG Scanning Electron Microscope (SEM). The samples for OM observation were prepared following the conventional metallographic method with etching in a solution composed of 10 vol.% HF, 10 vol.% HNO3 and 80 vol.% H2O. The samples for the TEM observation were ground and polished using a twin jet electropolishing at the temperature about 250 K in a solution of 5 vol.% HClO4, 35 vol.% CH3(CH2)3OH and 60 vol.% CH3OH. The sample for the EBSD test was electropolished in a solution of 5 vol.% HClO4, and 95 vol.% C2H5OH. The EBSD patterns were acquired under the following conditions: an acceleration voltage of 20 kV, a specimen tilt of 70°, and a scan step size of 1 lm on a rectangular scan grid.
3. Results and discussion Fig. 3 shows the microstructure and texture evolution of the Ti–15Mo–3Al–2.7Nb–0.2Si alloy after a hot-rolling at 1133 K. It is apparent that the fiber-like microstructure becomes clearer along the longitudinal direction as the deformation increases from 56% to 97%. Closer inspection reveals that the prior b grains were elongated along the rolling direction after 56% deformation (Fig. 3(a)), while some small DRX grains exhibited at the elongated b grain boundaries in the 84% samples (Fig. 3(c)), whereas after 97%
The texture of the hot rolled specimens was obtained by analyzing the u2 = 45° ODF section, as shown in Fig. 3(b, d and f). The main texture components derived from the ODF for the 56% deformation sample are (1 1 2)h1 1 0i, (0 0 1)h1 0 0i, and (1 1 1)h1 1 0i, as well as a weak Goss texture (1 1 0)h0 0 1i (see Fig. 3(b)). When increasing the strain to 84%, the cube (0 0 1)h1 0 0i and Goss (1 1 0)h0 0 1i textures disappear and the (1 1 2)h1 1 0i and (1 1 1) h1 1 0i components weaken (see Fig. 3(d)); in case the rolling is up to 97%, Fig. 3(f) reveals that there remains a weak Goss texture (1 1 0)h0 0 1i only. Regarding the intensity of the texture, it significantly decreases from 10.8 to 4.0 as the strain increases from 56% to 97%. The (1 1 2)h1 1 0i, (1 1 1)h1 1 0i, (0 0 1)h1 0 0i and (1 1 0)h0 0 1i texture components formed after 56% deformation are the commonest components for the hot-rolled bcc metals [14–16]. The former two components are produced when the rolling direction is parallel to the h1 1 0i crystallographic axis while the latter two form when it is to the h0 0 1i axis. It is in Fig. 3(c) that a DRX is observed after 84% deformation accompanied with a weak texture distribution indicated by Fig. 3(d). The relationship between DRX and texture has been widely addressed [4–9]. It is well known that, when new DRX grains form by the grain boundary bulging, they inherit the orientations from the neighboring matrices during the subsequent deformation, therefore leading to an increase of the intensity of texture [4]. In case when the deformed grain boundaries exhibit a wavy morphology prior to the formation of new grains, like in nickel [17], there is an enhanced deformation inhomogeneity that in turn results in a remarkable local lattice rotation in various directions in the vicinities of grain boundaries, as such, new grains predominantly form with random orientation without increasing the texture. The above mechanisms explain either the enhancement or the weak dependence of the texture intensity with DRX, however, are not helpful in interpreting the weakening of the texture by the occurrence of DRX in the hot-rolling process of Ti–15Mo–3Al–2.7Nb–0.2Si alloy as shown in Fig. 1. In order to unveil the mechanism underlying the texture evolution during hot rolling process in Ti–15Mo–3Al–2.7Nb–0.2Si alloy,
H. Kou et al. / Journal of Alloys and Compounds 603 (2014) 23–27
25
Fig. 3. OM and texture of the Ti–15Mo–3Al–2.7Nb–0.2Si alloy after hot-rolling at 1133 K: (a and b) 56% deformation; (c and d) 84% deformation; (e and f) 97% deformation.
the EBSD was performed for the 84% deformation samples. Representative OIM image of microstructures evolved at the 84% deformation at 1133 K in Fig. 5 indicates that the formation of new grains takes place along original grain boundaries and at triple junctions, resulting in the development of the so-called necklace microstructure. It is apparent from the areas marked by the red1 rectangles in Fig. 5 that the less-deformed DRX grains primarily present near or at these grain boundaries, whereas the similar colors of the DRX grains and the deformed matrix imply a low misorientation between them. On the contrary, as revealed by the areas marked by the black rectangles in Fig. 5, a large misorientation between them was observed at the large deformation areas, the pole figures of which reveal these DRX grains with a random orientation distribution, as such, leading to the weakening of the texture intensity. To this end, the full perspective of the hot deformation Ti–15Mo–3Al–2.7Nb–0.2Si alloy is as follow: at the early stage of the hot deformation process, the DRX grains with similar orientations prefer to occurring along grain boundaries and at triple junctions. When increasing the deformation, the formed DRX grains start to rotate towards the preferred slip systems [18,20,21]. Due to multiple slip systems in b titanium and large misorientations 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
between them [18], these DRX grains inherent the large misorientations after large deformation, thereby providing these DRX grains with a random orientation, which in turn weakens the intensity of the deformation texture in the Ti–15Mo–3Al–2.7Nb–0.2Si alloy. The random orientation distribution of the DRX grains can be confirmed by the TEM observation in Fig. 4(c).
4. Conclusion Microstructure and texture formation in the Ti–15Mo–3Al– 2.7Nb–0.2Si alloy during hot rolling process were investigated. The following conclusions were drawn: The prior b grains were elongated along the rolling direction and the fiber-like microstructure formed during the hot rolling process. Some small DRX grains exhibited at the elongated b grain boundaries in the 84% samples, whereas after 97% deformation, a completely DRX microstructure was attained. There are (1 1 2)h1 1 0i, (0 0 1)h1 0 0i, (1 1 1)h1 1 0i and (1 1 0) h0 0 1i deformation texture components formed after 57% deformation. By increasing the deformation to 84%, the DRX grains occur while the texture weakens, when the deformation is up to 97%, there only remains a weak Goss texture (1 1 0)h0 0 1i.
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H. Kou et al. / Journal of Alloys and Compounds 603 (2014) 23–27
Fig. 4. Bright field TEM observation of Ti–15Mo–3Al–2.7Nb–0.2Si alloy after hot rolling at 1133 K: (a) 56% deformation; (b) 84% deformation; (c) 97% deformation.
Fig. 5. EBSD observation of the 84% deformation samples.
The EBSD and TEM results indicate that the weakening of the texture originates from the large misorientation between the deformed matrix and the new DRX grains, as well as the random orientation among these new DRX grains.
Acknowledgment The authors would like to thank the Program of Introducing Talents of Discipline to Universities (B08040) for financial support.
H. Kou et al. / Journal of Alloys and Compounds 603 (2014) 23–27
References [1] D.G. Cram, H.S. Zurob, Y.J.M. Brechet, C.R. Hutchinson, Modelling discontinuous dynamic recrystallization using a physically based model for nucleation, Acta Mater. 57 (2009) 5218–5228. [2] N. Dudova, A. Belyakov, T. Sakai, R. Kaibyshev, Dynamic recrystallization mechanisms operating in a Ni–20%Cr alloy under hot-to-warm working, Acta Mater. 58 (2010) 3624–3632. [3] S. Gourdet, F. Montheillet, A model of continuous dynamic recrystallization, Acta Mater. 51 (2003) 2685–2699. [4] M. Hasegawa, M. Yamamoto, H. Fukutomi, Formation mechanism of texture during dynamic recrystallization in c-TiAl, nickel and copper examined by microstructure observation and grain boundary analysis based on local orientation measurements, Acta Mater. 51 (2003) 3939–3950. [5] R. Cottam, J. Robson, G. Lorimer, B. Davis, Dynamic recrystallization of Mg and Mg–Y alloys: crystallographic texture development, Mater. Sci. Eng., A 485 (2008) 375–382. [6] S. Gourdet, F. Montheillet, An experimental study of the recrystallization mechanism during hot deformation of aluminium, Mater. Sci. Eng., A 283 (2000) 274–288. [7] A.M. Wusatowska-Sarnek, H. Miura, T. Sakai, Nucleation and microtexture development under dynamic recrystallization of copper, Mater. Sci. Eng., A 323 (2002) 177–186. [8] H. Beladi, P. Cizek, P.D. Hodgson, Texture and substructure characteristics of dynamic recrystallization in a Ni–30%Fe austenitic model alloy, Scripta Mater. 61 (2009) 528–531. [9] Q. Jin, S.-Y. Shim, S.-G. Lim, Correlation of microstructural evolution and formation of basal texture in a coarse grained Mg–Al alloy during hot rolling, Scripta Mater. 55 (2006) 843–846.
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[10] X.Y. Kong, W.C. Liu, J. Li, H. Yuan, Deformation and recrystallization textures in straight-rolled and pseudo cross-rolled AA 3105 aluminum alloy, J. Alloys Comp. 491 (2010) 301–307. [11] H. Borkar, R. Gauvin, M. Pekguleryuz, Effect of extrusion temperature on texture evolution and recrystallization in extruded Mg–1%Mn and Mg–1%Mn– 1.6%Sr alloys, J. Alloys Comp. 555 (2013) 219–224. [12] S. Malinov, W. Sha, P. Markovsky, Experimental study and computer modelling of the b ) a + b phase transformation in b21s alloy at isothermal conditions, J. Alloys Comp. 348 (2003) 110–118. [13] T. Bo, T. Bin, H. Fengbo, Y. Guanjun, L. Jinshan, Influence of strain rate on stress induced martensitic transformation in b solution treated TB8 alloy, J. Alloys Comp. 565 (2013) 1–5. [14] B. Sander, D. Raabe, Texture inhomogeneity in a Ti–Nb-based b-titanium alloy after warm rolling and recrystallization, Mater. Sci. Eng., A 479 (2008) 236– 247. [15] V.D. Cojocaru, D. Raducanu, D.M. Gordin, I. Cinca, Texture in ultra-strength Ti– 25Ta–25Nb alloy strips, J. Alloys Comp. 576 (2013) 170–176. [16] S. Banumathy, R.K. Mandal, A.K. Singh, Texture and anisotropy of a hot rolled Ti–16Nb alloy, J. Alloys Comp. 500 (2010) L26–L30. [17] M. Hasegawa, H. Fukutomi, Microstructural study on dynamic recrystallization and texture formation in pure nickel, Mater. Trans. 43 (2002) 1183–1190. [18] Y. Chen, J. Li, B. Tang, H. Kou, F. Zhang, H. Chang, L. Zhou, Grain boundary character distribution and texture evolution in cold-drawn Ti–45Nb wires, Mater. Lett. 98 (2013) 254–257. [19] S. Matthies, G.W. Vinel, On the reproduction of the orientation distribution function of texturized samples from reduced pole figures using the conception of a conditional ghost correction, Phys. Status Solidi (B) 112 (1982) 111–114. [20] D.A. Hughes, N. Hansen, High angle boundaries and orientation distributions at large strains, Scripta Metall. Mater. 33 (1995) 315–321. [21] D.A. Hughes, N. Hansen, High angle boundaries formed by grain subdivision mechanisms, Acta Scripta Mater. 45 (1997) 3871–3886.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
An experimental study on the mechanism of texture evolution during hot-rolling process in a b titanium alloy Hongchao Kou a,⇑, Yi Chen a,b, Bin Tang a, Yuwen Cui b,⇑, Feng Sun a, Jinshan Li a, Xiangyi Xue a a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, PR China Computational Alloy Design Group, IMDEA Materials Institute, Madrid 28040, Spain
a r t i c l e
i n f o
Article history: Received 26 September 2013 Received in revised form 12 February 2014 Accepted 12 March 2014 Available online 24 March 2014 Keywords: b-Titanium Hot rolling Texture Dynamic recrystallization
a b s t r a c t The formation of crystallographic texture in a b-titanium alloy (Ti–15Mo–3Al–2.7Nb–0.2Si, wt.%) during the dynamic recrystallization (DRX) in hot rolling process was studied. The results reveal that the DRX occurs during the hot rolling process, which in turn results in the weakening of the deformation texture. The EBSD and TEM analyses further confirm that the weakening is associated with the randomly-distributed orientation relations of newly formed DRX grains, as well as the large misorientation remained between the deformed matrix and the DRX grains. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hot plastic deformation accompanied by dynamic recrystallization (DRX) sustains one of the most promising methods for the grain refinement in a variety of metals and alloys [1–9]. The dynamic recrystallization can be classified as: (i) discontinuous dynamic recrystallization (DDRX), during which the nucleation and growth of new grains operate through a bulging mechanism with relatively low stacking fault energy [1]; (ii) continuous dynamic recrystallization (CDRX), which involves a transformation of low angle boundaries into high angle boundaries of the subgrains with relatively high stacking fault energy [3]. It is well known that the texture evolution can be profoundly complicated by the DRX because the deformation enhances the deformation texture while the recrystallization will subsequently change it during the hot deformation process [10,11]. Different texture evolution phenomena were investigated in a variety of metals and alloys [4,8,9], including such interesting examples that a sharp texture component inevitably results from the DRX in the c-TiAl [4] and Mg alloys [9], and no any change of the texture was noticed with the DRX in nickel and copper [4]. However, the texture evolution mechanism under the DRX condition in b titanium alloys has been received little attention.
⇑ Corresponding authors. Tel.: +86 29 88460568 (H. Kou), +34 917871876 (Y. Cui). E-mail addresses: [email protected] (H. Kou), [email protected] (Y. Cui). http://dx.doi.org/10.1016/j.jallcom.2014.03.070 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
The present work is to address a b titanium alloy Ti–15Mo–3Al– 2.7Nb–0.2Si, that is widely being used in aerospace and bio-implant industries [12,13] with a focus on studying the evolution of crystallographic texture and the orientation dependence characteristics of the DRX grains, as well as the variation of the texture with the DRX during the hot deformation process.
2. Experimental Several £45 mm Ti–15Mo–3Al–2.7Nb–0.2Si (chemical composition: Ti– 14.9Mo–3.03Al–2.64Nb–0.18Si–0.009C–0.031Fe–0.13O–0.013N–0.001H, wt.%) bars were made by vacuum self-consuming arc melting followed with gradual forgings at 1373 K, 1273 K, 1174 K, 1073 K in an open die forging equipment. The a/b phase transformation temperature of Ti–15Mo–3Al–2.7Nb–0.2Si alloy was measured to be 988 K by the metallographic technique. In order to obtain a fully b single-phase microstructure, all the Ti–15Mo–3Al–2.7Nb–0.2Si bars with a diameter of 45 mm were solid-solutioned at 1173 K for 4 h followed by water quenching, as shown in Fig. 1. The bar rolling process was conducted on an industry bar mill at the Western Superconducting Technologies Co., Ltd. (Xi’an, China). As shown in Fig. 2, the Ti–15Mo–3Al–2.7Nb–0.2Si bars were firstly heated up to 1133 K and held for 30 min, and then were pass-by-pass hot rolled respectively to £25 mm (with a total deformation of 56%), £18 mm (84% deformation), and £8 mm (97% deformation) in the atmospheric condition. A cross rolling, the most typical rolling technique for a bar, was performed, i.e., the specimens were rotated about rolling direction by 90° after each consecutive pass. After the rolling, the bars were water quenched. The texture of the center parts taken from the specimens was measured by using a Siemens D5000 X-ray diffractometer based on the Schulz reflection technique with Mo Ka radiation. The texture of the b phase was achieved by measuring three incomplete pole figures, namely, (1 1 0), (2 0 0) and (2 1 1), from which its complete orientation distribution function (ODF) was then calculated. Microstructures were characterized by OLYMPUS/PMG3 optical microscope (OM) and FEI Tecnai
24
H. Kou et al. / Journal of Alloys and Compounds 603 (2014) 23–27
Fig. 1. Microstructure of the Ti–15Mo–3Al–2.7Nb–0.2Si alloy after solutioning at 1173 K for 4 h.
deformation, a complete DRX microstructure was attained as shown in Fig. 3(e). The close-up microstructure of the hot rolled specimens was characterized by TEM. As shown in Fig. 4, 56% deformation by hot-rolling triggered the activation of the slip system and the movement of the dislocation, which in turn resulted in the formation of evident slip bands and dislocation boundaries (see Fig. 4(a)). Fig. 4(b) shows that an increase of the deformation to 84% enhances the dislocation density and brings about some DRX grains along the elongated grain boundaries, whereas 97% deformation leads to a cellular microstructure consisting of fine DRX grains with a high dislocation density, see Fig. 4(c). The diffuse diffraction spots revealed by the selected area diffraction (SAD) analysis show these fine DRX grains having the random orientations. The orientation distribution function (ODF), a measure of the texture of the rolled samples, can be calculated from the measured incomplete pole figures by X-ray diffractometer based on the Schulz reflection technique, represent the intensity of each orientation in the whole 3D space [19]. In the ODF section, the intensity of each orientation expressed by the contour line while the Euler angles g(u1, U, u2) of each point indicate the texture components, which can be transformed to the Miller index (h k l)hu v wi by using Eq. (1).
0
1 2 3 cos u1 cos u2 sin u1 sin u2 cos U sin u2 sin U u h B C 6 7 @ v k A ¼ 4 cos u1 sin u2 sin u1 cos u2 cos U cos u2 sin U 5 sin u1 sin U cos U w l ð1Þ
Fig. 2. Schematic diagram of rolling process in the present work.
G2 F30 transmission electron microscope (TEM), while the orientation characteristics of the DRX grains and deformed matrix were investigated by Electron Back Scattered Diffraction (EBSD) with a HKL-Channel 5 system in JSM-6700F FEG Scanning Electron Microscope (SEM). The samples for OM observation were prepared following the conventional metallographic method with etching in a solution composed of 10 vol.% HF, 10 vol.% HNO3 and 80 vol.% H2O. The samples for the TEM observation were ground and polished using a twin jet electropolishing at the temperature about 250 K in a solution of 5 vol.% HClO4, 35 vol.% CH3(CH2)3OH and 60 vol.% CH3OH. The sample for the EBSD test was electropolished in a solution of 5 vol.% HClO4, and 95 vol.% C2H5OH. The EBSD patterns were acquired under the following conditions: an acceleration voltage of 20 kV, a specimen tilt of 70°, and a scan step size of 1 lm on a rectangular scan grid.
3. Results and discussion Fig. 3 shows the microstructure and texture evolution of the Ti–15Mo–3Al–2.7Nb–0.2Si alloy after a hot-rolling at 1133 K. It is apparent that the fiber-like microstructure becomes clearer along the longitudinal direction as the deformation increases from 56% to 97%. Closer inspection reveals that the prior b grains were elongated along the rolling direction after 56% deformation (Fig. 3(a)), while some small DRX grains exhibited at the elongated b grain boundaries in the 84% samples (Fig. 3(c)), whereas after 97%
The texture of the hot rolled specimens was obtained by analyzing the u2 = 45° ODF section, as shown in Fig. 3(b, d and f). The main texture components derived from the ODF for the 56% deformation sample are (1 1 2)h1 1 0i, (0 0 1)h1 0 0i, and (1 1 1)h1 1 0i, as well as a weak Goss texture (1 1 0)h0 0 1i (see Fig. 3(b)). When increasing the strain to 84%, the cube (0 0 1)h1 0 0i and Goss (1 1 0)h0 0 1i textures disappear and the (1 1 2)h1 1 0i and (1 1 1) h1 1 0i components weaken (see Fig. 3(d)); in case the rolling is up to 97%, Fig. 3(f) reveals that there remains a weak Goss texture (1 1 0)h0 0 1i only. Regarding the intensity of the texture, it significantly decreases from 10.8 to 4.0 as the strain increases from 56% to 97%. The (1 1 2)h1 1 0i, (1 1 1)h1 1 0i, (0 0 1)h1 0 0i and (1 1 0)h0 0 1i texture components formed after 56% deformation are the commonest components for the hot-rolled bcc metals [14–16]. The former two components are produced when the rolling direction is parallel to the h1 1 0i crystallographic axis while the latter two form when it is to the h0 0 1i axis. It is in Fig. 3(c) that a DRX is observed after 84% deformation accompanied with a weak texture distribution indicated by Fig. 3(d). The relationship between DRX and texture has been widely addressed [4–9]. It is well known that, when new DRX grains form by the grain boundary bulging, they inherit the orientations from the neighboring matrices during the subsequent deformation, therefore leading to an increase of the intensity of texture [4]. In case when the deformed grain boundaries exhibit a wavy morphology prior to the formation of new grains, like in nickel [17], there is an enhanced deformation inhomogeneity that in turn results in a remarkable local lattice rotation in various directions in the vicinities of grain boundaries, as such, new grains predominantly form with random orientation without increasing the texture. The above mechanisms explain either the enhancement or the weak dependence of the texture intensity with DRX, however, are not helpful in interpreting the weakening of the texture by the occurrence of DRX in the hot-rolling process of Ti–15Mo–3Al–2.7Nb–0.2Si alloy as shown in Fig. 1. In order to unveil the mechanism underlying the texture evolution during hot rolling process in Ti–15Mo–3Al–2.7Nb–0.2Si alloy,
H. Kou et al. / Journal of Alloys and Compounds 603 (2014) 23–27
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Fig. 3. OM and texture of the Ti–15Mo–3Al–2.7Nb–0.2Si alloy after hot-rolling at 1133 K: (a and b) 56% deformation; (c and d) 84% deformation; (e and f) 97% deformation.
the EBSD was performed for the 84% deformation samples. Representative OIM image of microstructures evolved at the 84% deformation at 1133 K in Fig. 5 indicates that the formation of new grains takes place along original grain boundaries and at triple junctions, resulting in the development of the so-called necklace microstructure. It is apparent from the areas marked by the red1 rectangles in Fig. 5 that the less-deformed DRX grains primarily present near or at these grain boundaries, whereas the similar colors of the DRX grains and the deformed matrix imply a low misorientation between them. On the contrary, as revealed by the areas marked by the black rectangles in Fig. 5, a large misorientation between them was observed at the large deformation areas, the pole figures of which reveal these DRX grains with a random orientation distribution, as such, leading to the weakening of the texture intensity. To this end, the full perspective of the hot deformation Ti–15Mo–3Al–2.7Nb–0.2Si alloy is as follow: at the early stage of the hot deformation process, the DRX grains with similar orientations prefer to occurring along grain boundaries and at triple junctions. When increasing the deformation, the formed DRX grains start to rotate towards the preferred slip systems [18,20,21]. Due to multiple slip systems in b titanium and large misorientations 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
between them [18], these DRX grains inherent the large misorientations after large deformation, thereby providing these DRX grains with a random orientation, which in turn weakens the intensity of the deformation texture in the Ti–15Mo–3Al–2.7Nb–0.2Si alloy. The random orientation distribution of the DRX grains can be confirmed by the TEM observation in Fig. 4(c).
4. Conclusion Microstructure and texture formation in the Ti–15Mo–3Al– 2.7Nb–0.2Si alloy during hot rolling process were investigated. The following conclusions were drawn: The prior b grains were elongated along the rolling direction and the fiber-like microstructure formed during the hot rolling process. Some small DRX grains exhibited at the elongated b grain boundaries in the 84% samples, whereas after 97% deformation, a completely DRX microstructure was attained. There are (1 1 2)h1 1 0i, (0 0 1)h1 0 0i, (1 1 1)h1 1 0i and (1 1 0) h0 0 1i deformation texture components formed after 57% deformation. By increasing the deformation to 84%, the DRX grains occur while the texture weakens, when the deformation is up to 97%, there only remains a weak Goss texture (1 1 0)h0 0 1i.
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Fig. 4. Bright field TEM observation of Ti–15Mo–3Al–2.7Nb–0.2Si alloy after hot rolling at 1133 K: (a) 56% deformation; (b) 84% deformation; (c) 97% deformation.
Fig. 5. EBSD observation of the 84% deformation samples.
The EBSD and TEM results indicate that the weakening of the texture originates from the large misorientation between the deformed matrix and the new DRX grains, as well as the random orientation among these new DRX grains.
Acknowledgment The authors would like to thank the Program of Introducing Talents of Discipline to Universities (B08040) for financial support.
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