INVESTIGATION OF MECHANICAL TWINNING IN ...

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J.P. Cui, Y.L. Hao, S.J. Li, M.L. Sui, D.X. Li, R. Yang. "Reversible Movement of Homogenously Nucleated. Dislocations in a β-Titanium Alloy", Physical Review.
Proceedings of the 13th World Conference on Titanium Edited by: Vasisht Venkatesh, Adam L. Pilchak, John E. Allison, Sreeramamurthy Ankem, Rodney Boyer, Julie Christodoulou, Hamish L. Fraser, M. Ashraf Imam, Yoji Kosaka, Henry J. Rack, Amit Chatterjee, and Andy Woodfield TMS (The Minerals, Metals & Materials Society), 2016

INVESTIGATION OF MECHANICAL TWINNING IN THE Ti-24Nb-4Zr-8Sn β TITANIUM ALLOY Philippe Castany1, Yang Yang1, Emmanuel Bertrand2, Marilyne Cornen1, Thierry Gloriant1 INSA Rennes, Institut des Sciences Chimiques de Rennes (ISCR CNRS 6226), 20 avenue des Buttes de Coësmes, 35708 Rennes Cedex 7, France 2 Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, Rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France

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Keywords: Metastable β Titanium Alloy, Twinning, EBSD, TEM. Abstract

investigated. Metastable β titanium alloys are known to deform plastically via dislocation slip or twinning. Main observed slip systems consist of 1/2 Burgers vectors dislocations gliding in {110} or {112} planes [11, 12, 13], and, more rarely, in {123} planes [13]. Twinning is also frequently observed in this alloy family: The {332} twinning system is the most commonly detected system [14, 15, 16, 17, 18, 19], even if the {112} system is also occasionally identified in some alloy compositions [16, 18]. The purpose of the present paper is to investigate the formation and structure of twins in the solution-treated Ti2448 alloy after tensile deformation.

Mechanical twinning is investigated in the Ti-24Nb-4Zr-8Sn (wt. %) metastable β titanium alloy. Specimens deformed by tensile tests show deformation bands in most of grains. These bands are clearly observed by optical microscopy but cannot properly indexed and analyzed by Electron Back-Scattered Diffraction (EBSD) experiments, whereas this technic is commonly used to identify such deformation bands as twins in other metastable β titanium alloys. Transmission Electron Microscopy (TEM) analyses reveal that these bands are {332} twins containing other internal nano-sized {332} twins. The presence of such internal twinning was never observed in other metastable β titanium alloys and causes the failure of correct indexation by EBSD in the present Ti-24Nb-4Zr-8Sn alloy.

Material and Experimental Methods A hot-forged Ti2448 cylinder with diameter of 55 mm was used as raw material in this work. The Table 1 gives the chemical composition of the alloy. Slice samples were directly cut in the raw cylinder and then multi-pass cold rolled with a 94 % reduction rate. Cold rolled specimens were next solution treated under high vacuum at 900 °C for 30 minutes followed by water quenching (solution treated state, ST). After thermal treatments, all specimens were cleaned in an acid solution made of 50 % HF and 50 % HNO3 (in volume) to remove any potential oxidation layer.

Introduction The Ti-24Nb-4Zr-8Sn alloy (wt. %, abbreviated as Ti2448) is a multifunctional metastable β-type titanium alloy (bodycentered cubic structure) with low Young’s modulus, high strength, good corrosion resistance and high recoverable strain. Even if this alloy can be used in several domains, it was specifically designed with only highly biocompatible elements ensuring a superior biocompatibility. The mechanical properties, particularly its low Young’s modulus, combined with its very good biocompatibility, make this alloy suitable for biomedical applications such as prostheses, implants or various medical devices. The mechanical properties of the Ti2448 alloy, as for all metastable β titanium alloys, are very dependent on the processing route and the applied thermo-mechanical treatments due to the subsequent microstructural changes. As an example, Young’s modulus below 50 GPa with 3.3 % recoverable strain was obtained after hot-working [1, 2, 3, 4]. The microstructure and subsequent mechanical properties can also be improved by short thermal treatments after cold-working such as 6 min at 600 °C [5] or 3 min at 700 °C [6], that allow higher strength and better superelasticity due to a reduced grain size. Annealing in the α+β domain is also another route to improve the mechanical strength due to fine precipitation of α phase [7, 8].

Mechanical properties and superelasticity were estimated by conventional and cyclic tensile tests with a strain rate of 10-4 s-1. Cyclic tensile tests consist of strain increments of 0.5 % followed by stress release up to an elongation of 5 %. An extensometer was used to ensure the accuracy of strain. Normalized flat tensile specimens with 3mm×15mm×0.5mm gage dimensions were used. The tensile direction was chosen parallel to the rolling direction. Table 1. Chemical composition of hot forged Ti2448 (wt%) Nb Zr Sn O Ti 23.9 4.05 8.22 0.16 Balance Microstructure after deformation was investigated on tensile specimens after interrupted tensile test at 5 % of strain by optical microscopy, Electron Back-Scattered Diffraction (EBSD) in Scanning Electron Microscope (SEM) and Transmission Electron Microscopy (TEM). Prior to optical and EBSD observations, samples were first mechanically polished and next chemically etched with a 15 % HNO3, 8 % HF and 77 % H2O solution (vol.%). Samples for TEM observations were thinned using a twin-jet electropolishing system with a solution of 4 % perchloric acid and 96 % methanol (vol.%) at -20°C.

However, mechanisms of plastic deformation were not deeply investigated in the Ti2448 alloy whereas reversible mechanisms responsible of its superelasticity, such as reversible nucleation of dislocation loops [9] or stressinduced martensitic transformation [10], were more acutely

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Results and Discussion

indexed within each deformation band in this alloy. Indeed, in other similar metastable β titanium alloys, twins are unambiguously indexed and characterized with the same EBSD apparatus [11, 16, 17]. Consequently, in the present Ti2448 alloy, deformation bands cannot be identified as twins by EBSD. In order to identify the nature of these deformation bands, TEM analysis were next performed.

Previous investigations on this alloy in the ST state showed that the microstructure is only composed of equiaxed grains of β phase with an average grain size of 50 μm [6, 10]. Figure 1 displays conventional and cyclic tensile curves of the present alloy. From these stress-strain curves, elongation at rupture, ultimate tensile strength, incipient Young’s modulus and maximum recoverable strain were measured to be 13 %, 870 MPa, 58 GPa and 2.3 %, respectively [6]. On the Figure 1, the point at 5 % of strain is marked: This point corresponds to the end of the cyclic tensile test from which the subsequent observations of the microstructure after deformation were done. This point is clearly in the plastic domain of the curve and the microstructure should contain plastic deformation features.

Figure 3. Image Quality (a) and Inverse Pole Figure (b) maps of a 5 % deformed specimen. The Figure 4 (a) shows a typical bright field TEM image of deformation bands in the present Ti2448 alloy strained until 5 %. The bands are 1 μm wide and contain a lot of internal nano-sized bands with an average width of 100 nm. The presence of such nano-sized internal bands within the primary band explains why the indexation by EBSD was not possible (Figure 3). Indeed, the size of internal bands is small in comparison with the probe size used in SEM: The EBSD system is thus unable to index properly each crystal, probably due to a superimposition of Kikuchi patterns of both primary and internal bands.

Figure 1. Conventional and cyclic tensile strain-stress curves for the Ti2448 alloy after solution treatment.

In order to identify the nature of these deformation bands, Selected Area Electron Diffraction (SAED) and dark field imaging are used. An example is given in Figure 4 (b) with a diffraction pattern showing a zone axis of the parent grain (matrix) parallel to a zone axis of the band that is typical of a {332} twinning relationship. The Figure 4 (c) shows thus a dark field image made with a spot corresponding of the primary band. A careful analysis of several deformation bands demonstrates that all the observed bands are {332} twins for the primary bands as well as for internal bands.

Figure 2. Optical micrograph of a 5 % strained specimen showing numerous deformation bands. An optical micrograph of the microstructure after an interrupted tensile test at 5% of strain is shown in the Figure 2. Numerous deformation bands are visible in most grains and seem to be twins. In order to characterize unambiguously that these deformation bands are twins, EBSD analyses were performed (Figure 3). The Figure 3 (a) shows the Image Quality (IQ) map wherein the deformation bands are clearly visible, similarly to the optical micrograph of the Figure 2. The Inverse Pole Figure (IPF) map of the Figure 3 (b) should determine the twinning relationship between the bands and the surrounding matrix. However, the bands are not correctly indexed whereas the matrix is perfectly indexed. Surprisingly, several crystallographic orientations seem to be

In metastable β titanium alloys, the presence of {332} twins is common, but these twins are generally observed without any internal twinning [11, 14, 15, 16, 17]. To the authors’ knowledge, such internal twinning was only observed in a Ti5Mo-6V-3Cr-3Al alloy [20]. Occurrence of internal twinning inside primary twins is thus very rare in this kind of alloys and can be considered as a peculiarity of the present Ti2448 alloy.

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twin. Their widths are about 50 nm that is much smaller than the 1 μm of the primary bands in the specimen deformed at 5 % of strain. Therefore, it can be concluded that the primary twins are first nucleated and continue to expand when the strain increases. Next, when the stress is high enough, secondary twinning takes place inside primary bands and allows the plastic deformation of the material to continue. Conclusion The twinning microstructure of a Ti2248 metastable β titanium alloy deformed by tensile test has been investigated via EBSD and TEM experiments. Contrarily to other metastable β titanium alloys, the present alloy exhibits a peculiar twinning microstructure consisting with primary twins presenting internal nano-sized twins. TEM experiments show that all the twins are {332} twins. Due to their small size, these twins cannot be correctly indexed and identified by EBSD analyses. TEM observations of specimens deformed at lower strain show that primary twins are formed without any internal twins. The sequence of deformation is thus the following one: Nucleation and growth of primary twins and, next, formation of internal nano-sized twins inside primary twins. References 1. Y.L. Hao, S.J. Li, S.Y. Sun, C.Y. Zheng, Q.M. Hu, R. Yang. "Super-elastic titanium alloy with unstable plastic deformation", Applied Physics Letters, 87 (2005), 091906. 2. Y.L. Hao, S.J. Li, S.Y. Sun, C.Y. Zheng, R. Yang. "Elastic deformation behaviour of Ti-24Nb-4Zr-7.9Sn for biomedical applications", Acta Biomaterialia, 3 (2007), 277-286. 3. Y.L. Hao, Z.B. Zhang, S.J. Li, R. Yang. "Microstructure and mechanical behavior of a Ti–24Nb–4Zr–8Sn alloy processed by warm swaging and warm rolling", Acta Materialia, 60 (2012), 2169-2177. 4. Y.L. Hao, S.J. Li, B.B. Sun, M.L. Sui, R. Yang. "Ductile Titanium Alloy with Low Poisson's Ratio", Physical Review Letters, 98 (2007), 216405. 5. F. Sun, S. Nowak, T. Gloriant, P. Laheurte, A. Eberhardt, F. Prima. "Influence of a short thermal treatment on the superelastic properties of a titanium-based alloy", Scripta Materialia, 63 (2010), 1053-1056. 6. Y. Yang, P. Castany, M. Cornen, I. Thibon, F. Prima, T. Gloriant. "Texture investigation of the superelastic Ti–24Nb– 4Zr–8Sn alloy", Journal of Alloys and Compounds, 591 (2014), 85-90. 7. S.J. Li, M.T. Jia, F. Prima, Y.L. Hao, R. Yang. "Improvements in nonlinear elasticity and strength by grain refinement in a titanium alloy with high oxygen content", Scripta Materialia, 64 (2011), 1015-1018. 8. F. Sun, Y.L. Hao, J.Y. Zhang, F. Prima. "Contribution of nano-sized lamellar microstructure on recoverable strain of Ti–24Nb–4Zr–7.9Sn titanium alloy", Materials Science and Engineering: A, 528 (2011), 7811-7815. 9. J.P. Cui, Y.L. Hao, S.J. Li, M.L. Sui, D.X. Li, R. Yang. "Reversible Movement of Homogenously Nucleated Dislocations in a β-Titanium Alloy", Physical Review Letters, 102 (2009), 045503. 10. Y. Yang, P. Castany, M. Cornen, F. Prima, S.J. Li, Y.L. Hao, T. Gloriant. "Characterization of the martensitic

Figure 4. Bright field TEM micrograph of a specimen deformed at 5 % of strain showing a primary twin with internal nano-sized twins (a) and the corresponding selectedarea diffraction pattern showing a zone axis of the parent grain parallel to a zone axis of the twin (b); dark field TEM micrograph using a spot of the twin (c).

Figure 5. Microstructure after 2 % of deformation: Bright field TEM micrograph of a primary twin without any internal twin (a) and the corresponding selected-area diffraction pattern showing a zone axis of the parent grain parallel to a zone axis of the twin (b). Complementarily to previous observations, specimens were also tensile tested to 2 % strain in order to analyze the onset of the formation of twins and to determine if primary twins are formed with internal twins or not. The Figure 5 shows an example of a bright field TEM image of such a specimen (a) and the corresponding SAED pattern (b). The orientation relationship between the band and the matrix is the same than the Figure 4 (b) with a zone axis of the matrix parallel to a zone axis of the band. Consequently, the observed band corresponds also to a {332} twin. All deformation bands in this specimen deformed at 2 % of strain are similar to the one of the Figure 5 without any internal

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transformation in the superelastic Ti–24Nb–4Zr–8Sn alloy by in situ synchrotron X-ray diffraction and dynamic mechanical analysis", Acta Materialia, 88 (2015), 25-33. 11. M. Besse, P. Castany, T. Gloriant. "Mechanisms of deformation in gum metal TNTZ-O and TNTZ titanium alloys: A comparative study on the oxygen influence", Acta Materialia, 59 (2011), 5982-5988. 12. P. Castany, M. Besse, T. Gloriant. "Dislocation mobility in gum metal beta-titanium alloy studied via in situ transmission electron microscopy", Physical Review B, 84 (2011), 020201. 13. P. Castany, M. Besse, T. Gloriant. "In situ TEM study of dislocation slip in a metastable β titanium alloy", Scripta Materialia, 66 (2012), 371-373. 14. M. Oka, Y. Taniguchi. "{332} Deformation twins in a Ti-15.5 pct V alloy", Metallurgical and Materials Transactions A, 10 (1979), 651-653. 15. S. Hanada, O. Izumi. "Transmission Electron Microscopic Observations of Mechanical Twinning in Metastable Beta Titanium Alloys", Metallurgical Transactions A, 17 (1986), 1409-1420. 16. E. Bertrand, P. Castany, I. Péron, T. Gloriant. "Twinning system selection in a metastable -titanium alloy by Schmid factor analysis", Scripta Materialia, 64 (2011), 1110-1113. 17. A. Ramarolahy, P. Castany, F. Prima, P. Laheurte, I. Péron, T. Gloriant. "Microstructure and mechanical behavior of superelastic Ti-24Nb-0.5O and Ti-24Nb-0.5N biomedical alloys", Journal of the Mechanical Behavior of Biomedical Materials, 9 (2012), 83-90. 18. F. Sun, J.Y. Zhang, M. Marteleur, T. Gloriant, P. Vermaut, D. Laillé, P. Castany, C. Curfs, P.J. Jacques, F. Prima. "Investigation of early stage deformation mechanisms in a metastable titanium alloy showing combined twinninginduced plasticity and transformation-induced plasticity effects", Acta Materialia, 61 (2013), 6406-6417. 19. H. Tobe, H.Y. Kim, T. Inamura, H. Hosoda, S. Miyazaki. "Origin of {332} twinning in metastable -Ti alloys", Acta Materialia, 64 (2014), 345-355. 20. G. Rusakov, A. Litvinov, V. Litvinov. "Deformation twinning of titanium -alloys of transition class", Metal Science and Heat Treatment, 48 (2006), 244-251.

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