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Structure and mechanical properties of commercial purity titanium processed by ECAP at room temperature
Materials Science and Engineering A 528 (2011) 7708–7714
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Structure and mechanical properties of commercial purity titanium processed by ECAP at room temperature Yue Zhang a , Roberto B. Figueiredo b,c , Saleh N. Alhajeri b,d , Jing Tao Wang a , Nong Gao b , Terence G. Langdon b,e,∗ a
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK c Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Belo Horizonte, MG 31270-901, Brazil d Department of Manufacturing Engineering, College of Technological Studies, PAAET, Shuwaikh 70654, Kuwait e Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA b
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 17 June 2011 Accepted 17 June 2011 Available online 24 June 2011 Keywords: Equal-channel angular pressing Hardness Strength Titanium Twinning
a b s t r a c t Experiments were conducted on commercial purity titanium to evaluate the microstructure and mechanical properties after processing by equal-channel angular pressing (ECAP) at room temperature. Samples were successfully processed through 1 and 2 passes at two different pressing speeds using an ECAP die with a channel angle of 135◦ . Following ECAP, a microstructural evaluation showed evidence for the occurrence of basal, prismatic and pyramidal slip together with {1 0 1¯ 2}1¯ 0 1 1 twinning. Hardness measurements confirmed the occurrence of homogeneous strengthening throughout the samples. Tensile testing revealed a high ultimate tensile strength of ∼750 MPa after 2 passes of ECAP but with a reduction in the measured elongation to failure. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Processing by equal-channel angular pressing (ECAP) has become an established procedure for introducing significant grain refinement into bulk metals [1]. However, processing by ECAP is difficult in some materials, such as hexagonal close-packed metals, where the number of independent slip systems is limited. This difficulty led to an early conclusion, in experiments conducted on commercial purity (CP) titanium using an ECAP die with a channel angle of ˚ = 90◦ , that it was not possible to process this material at room temperature because of the occurrence of significant segmentation in which the billets become sheared into discrete but inter-connected segments during the pressing operation [2]. The significance of unstable flow and segmentation has now become a critical issue in the processing of bulk solids by ECAP [3–5].
∗ Corresponding author at: Department of Aerospace & Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453, USA. Tel.: +1 213 740 0491; fax: +1 213 740 8071. E-mail address: [email protected] (T.G. Langdon). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.06.054
Detailed finite element calculations showed these so-called “difficult-to-work” alloys may be processed more easily by using an ECAP die having an increased channel angle [6]. Furthermore, this approach was clearly demonstrated with a two-phase Mg8% Li alloy where it was shown that the alloy may be pressed without cracking through only a single pass at room temperature when using a die with ˚ = 90◦ whereas the same alloy may be pressed without cracking up to 10 passes at room temperature when the channel angle is increased to 135◦ [7]. A similar approach was used later for the successful pressing of CP Ti through a single pass at room temperature using a die with ˚ = 120◦ [8] and more recently these experiments were continued to produce samples of CP Ti pressed through 8 passes with ˚ = 120◦ [9]. Titanium is currently receiving much interest because it exhibits excellent biocompatibility [10] and has a very significant potential for use as a dental implant material [11]. For these implants, CP Ti is currently processed by ECAP through four passes at 723 K using a die with a channel angle of 90◦ , it is forge drawn with a cumulative deformation of 80%, and finally it is annealed at 573–623 K to produce long rods having diameters of 7 mm and lengths up to ∼3 m [12]. The grain size in these as-processed rods was reported
Y. Zhang et al. / Materials Science and Engineering A 528 (2011) 7708–7714
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Fig. 1. Schematic illustrations of (a) the cross-sectional and (b) the longitudinal planes showing dashed lines to illustrate the traverses followed for recording individual measurements of the Vickers microhardness and (c) the procedure for recording the hardness values by taking microhardness values around each selected point with these measurements separated from the selected position by a distance of 0.15 mm. The distances between the selected points for the individual hardness measurements were 1.0 mm on the cross-sectional plane and 3.0 mm on the longitudinal plane, respectively.
as ∼150 nm. However, it is readily apparent there would be a significant advantage, in terms of both speed and experimental simplicity, if the processing of the CP Ti was conducted by ECAP at room temperature. In practice, this is difficult because no detailed information is currently available describing the microstructural characteristics and the mechanical properties of CP Ti when processed by ECAP at room temperature. Accordingly, the present investigation was initiated to provide this information. 2. Experimental material and procedures Commercial purity (CP) titanium of grade 2 (99.5% purity) was received in the form of rods with diameters of 10 mm from Global Metal Trading (UK) Ltd. These rods were cut into billets having lengths of 65 mm for processing by ECAP and then the rods were annealed in air at 993 K for 2 h. The microstructure after annealing was characterized by equiaxed grains with an average grain size of ∼10 m. The billets were processed by ECAP for either 1 or 2 passes at room temperature using a solid die with an internal channel angle of ˚ = 135◦ and an outer arc of curvature of ≈ 20◦ at the point of intersection of the two parts of the channel. It can be shown that these values of ˚ and produce an equivalent strain, ε, of ∼0.46 in each separate pass through the die [13]. The processing was conducted using a 200 ton DMG hydraulic press operating at constant rates of displacement of either 0.5 or 0.05 mm s−1 and the billets were rotated by 90◦ between the first and second pass using processing route BC [14]. A lubricant of an MoS2 suspension in a mineral oil was used between the billets and the die walls in order to reduce frictional effects. After processing by ECAP, each billet was cut along the central line, perpendicular to the upper surface, using a spark-erosion facility and the surfaces were then carefully polished using a series of abrasive papers containing SiC to a final level of 4000 grit equivalent to a particle size of 3 m. These polished surfaces were used for an evaluation of the microhardness levels at selected points along each surface. Similar sets of measurements were also taken on crosssectional planes by cutting the billets perpendicular to the pressing direction. Fig. 1 illustrates (a) a cross-sectional and (b) a longitudinal section through the billet where the dashed lines denote the locations for the microhardness measurements. The lines in Fig. 1(a) are located perpendicular to each other and pass through the center of the billet on the cross-sectional plane as viewed from the front of the billet: thus, the top to bottom of this plane is denoted a vertical traverse and the left to right of this plane is denoted a horizontal traverse. The lines in Fig. 1(b) are located along the longitudinal central axis and at distances of 1.0 mm from the top and bottom surfaces on the longitudinal section. The values of the Vickers microhardness, Hv, were recorded along each dashed line shown in Fig. 1(a) and (b) at positions separated by distances of 1.0 mm on the cross-sectional planes and 3.0 mm on the longitudinal planes. At each selected position, a cluster of four equally spaced
indentations was made around the selected point with the indentations separated from these positions by a distance of 0.15 mm as illustrated in Fig. 1(c). These four values were used to estimate the average hardness and the relevant error bars. An important limitation in conventional ECAP is that gross distortions are introduced immediately adjacent to the front and rear edges of the billets and these distortions may be avoided only by performing the ECAP using a die with parallel channels [15]. To avoid these regions of gross distortion, and following procedures introduced earlier [16,17], the longitudinal hardness measurements were taken over total lengths of ∼40 mm within the central portions of each billet. For each microhardness measurement, the hardness was recorded using a Matsuzawa Seiki MHT-1 microhardness tester equipped with a Vickers indenter using a load of 300 gf and a dwell time of 10 s. The microstructure of the materials was evaluated after ECAP using transmission electron microscopy (TEM). Disks having diameters of 3 mm and thicknesses of 0.68 mm were cut from the longitudinal plane of each sample and mechanical polishing was used to reduce the thickness to ∼60 m. The samples were polished to perforation using a twin-jet polisher with a solution of 5% perchloric acid, 35% butanol and 60% methanol at a temperature of 243 K with a voltage of 50–60 V and a current of ∼50 mA. A JEM2010 electron microscope operating at 105 kV was used to examine the foils with a diaphragm aperture having a diameter of ∼1 m for selected area electron diffraction (SAED). For testing in tension, dog-bone specimens were machined from the as-received material and the billets processed at 0.05 mm s−1 . These specimens had gauge lengths of 25 mm parallel to the longitudinal axial directions and gauge cross-sections of 2.5 mm × 1.2 mm. Tensile tests were performed using an Instron testing machine equipped with an electronic extensometer. All tensile tests were conducted at room temperature (296 K) using a constant rate of cross-head displacement which imposed an initial strain rate, ε˙ 0 , of 1.0 × 10−3 s−1 . 3. Experimental results 3.1. Appearance of the billets and microstructural characterization The appearance of the four samples of CP Ti is shown in Fig. 2 after processing by ECAP. It is readily apparent that the surfaces of the CP Ti samples are smooth and there is an absence of any segmentation after pressing at both speeds. Nevertheless, close inspection of the billet processed by 2 passes at the faster speed revealed the presence of some cracking in this sample. The microstructures of the samples processed by 1 and 2 passes of ECAP at 0.05 mm s−1 , and the associated SAED patterns, are shown in Figs. 3 and 4 respectively. Traces of the slip planes parallel to (1 1 2 3) are visible in the material processed through 1 pass of ECAP, thereby producing a lamellar pattern in the microstructure.
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Fig. 2. Appearance of the four billets after pressing through 1 or 2 ECAP passes at two different pressing speeds.
Measurements gave an average lamellar spacing of ∼300 nm. Fig. 4 shows the presence of a high density of dislocations in the interior of a coarse grain after processing through 2 passes of ECAP and there are visible traces of slip planes parallel to the basal (0 0 0 1) and prismatic (0 1 1¯ 0) planes. Despite the high density of dislocations in this sample, dislocation cell boundaries or subgrain boundaries are not readily distinguished thereby suggesting that the total imposed strain of ∼0.92 is not sufficient to produce a well-defined grain refinement. Fig. 5 shows a series of TEM images of a region containing twins in the material processed by 1 pass of ECAP at 0.05 mm s−1 together with the relevant SAED patterns which are used to distinguish between the matrix and the twinned regions. It is observed that the twins have different thicknesses and they appear bent. An analysis of the SAED pattern showed the twins are of the {1 0 1¯ 2}1¯ 0 1 1 type. 3.2. The hardness and mechanical properties after processing by ECAP The distributions of hardness along the selected lines on the cross-sectional planes and the longitudinal planes of the billets are
shown in Fig. 6 after processing by ECAP at 0.5 mm s−1 . Fig. 6(a) gives the distributions of hardness along the billet cross-sections after processing through 1 and 2 passes (1p and 2p), Fig. 6(b) gives the distribution of hardness along the longitudinal section after processing by 1 pass and Fig. 6(c) gives the distribution of hardness along the longitudinal section after processing by 2 passes of ECAP: in each plot, the average hardness of the unprocessed material (∼170 Hv) is shown by the lower dashed line. It is observed that there is a high level of homogeneity in the hardness distributions on the cross-sectional plane of the billet after one pass of ECAP but there is a tendency for some heterogeneity of hardness on the cross-sectional plane of the billet after the second pass. These observations are confirmed by the distributions of hardness recorded along the longitudinal sections in Fig. 6(b) and (c). Thus, the billet processed to a single pass of ECAP exhibits a high degree of homogeneity while the billet processed by 2 passes exhibits some heterogeneity. The occurrence of some heterogeneity is consistent with the cracking observed in this billet. The distributions of hardness along selected lines on the longitudinal planes of the billets processed at the slower pressing speed of 0.05 mm s−1 are shown in Fig. 7 for samples processed by (a) 1
Fig. 3. (a) Microstructure by TEM of CP Ti processed by 1 pass of ECAP showing lamellar boundaries parallel to the (1 1 2¯ 3) slip plane and (b) the corresponding selected area electron diffraction pattern for a [7 2¯ 5¯ 3] zone axis.
Y. Zhang et al. / Materials Science and Engineering A 528 (2011) 7708–7714
Fig. 4. (a) Bright field TEM micrograph of CP Ti processed by 2 passes of ECAP showing substructure boundaries parallel to the (0 0 0 1) and (0 1 1¯ 0) slip planes, respectively, and (b) the corresponding selected area electron diffraction pattern for a [2 1¯ 1¯ 0] zone axis.
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and (b) 2 passes of ECAP, respectively: again, the average hardness of the unprocessed annealed material is shown by the lower dashed lines. Inspection of these data demonstrates there is a very significant increase in the hardness after 1 pass of ECAP, to ∼210–240 Hv, although the imposed strain is only ∼0.46. Furthermore, there is a homogeneous increase in hardness throughout the central length of ∼40 mm in the billet after 1 pass. There is an additional increase in the hardness after 2 passes, to ∼245 Hv and, unlike the billet pressed at the faster speed, there is good homogeneity except only that some limited heterogeneity is visible at the outer edges of the 40 mm central portion of the sample. After 2 passes, almost all of the individual datum points have hardness values larger than 230 Hv. A comparison of Figs. 6 and 7 shows that a higher level of homogeneity is attained when using a lower pressing speed and this is consistent with the absence of any cracking after pressing at the slower speed. Furthermore, it is also consistent with earlier experiments investigating the effect of the pressing speed on samples of pure aluminum [18]. For the tensile testing, Fig. 8 shows plots of stress vs. strain for the annealed and unprocessed material and for the material processed by 1 and 2 passes of ECAP. It is apparent that the annealed material exhibits a low yield stress, a high rate of strain hardening and a reasonable ductility. Processing by ECAP significantly increases the yield stress but reduces both the strain hardening capability and the ductility. The measured elongations to failure are ∼20% in the annealed material but only ∼6% after 2 passes of ECAP. There is a maximum stress of ∼600 MPa in the annealed condition but this is increased to ∼750 MPa after 2 passes of ECAP.
Fig. 5. Microstructure by TEM of CP Ti processed by 1 pass of ECAP showing a serial of {1 0 1¯ 2}1¯ 0 1 1 mechanical twins according to the corresponding selected area electron diffraction pattern for a [2 1¯ 1¯ 0] zone axis.
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Fig. 7. Individual values of the Vickers microhardness Hv recorded on the longitudinal plane after (a) 1 pass and (b) 2 passes of ECAP.
An analysis of the dislocation structures and SAED patterns of the CP-Ti after 1 and 2 passes, as shown in Figs. 3 and 4, reveals the occurrence of basal {0 0 0 1}1 1 2¯ 0, prismatic {0 1 1¯ 0}1 1 2¯ 0 and ¯ 1 2¯ 3 slip during ECAP at room temperpyramidal c+a {1 1 2¯ 2}1 ature. Furthermore, and as shown in Fig. 5, deformation twinning after 1 pass was identified as {1 0 1¯ 2}1¯ 0 1 1 type where this is con-
Fig. 6. Individual values of the Vickers microhardness Hv recorded after pressing at 0.5 mm s−1 on (a) the cross-sectional plane after 1 and 2 passes, (b) the longitudinal planes after 1 pass and (c) the longitudinal plane after 2 passes.
4. Discussion 4.1. Microstructural characteristics This investigation shows that it is feasible to process grade 2 titanium by ECAP at room temperature for 2 passes when using a die with a channel angle of 135◦ and after processing the material exhibits features that are characteristic of deformation on different slip planes combined with the occurrence of some twinning.
Fig. 8. Flow stress as a function of strain in tensile testing for an annealed sample and specimens processed through 1 and 2 passes of ECAP at a pressing speed of 0.05 mm s−1 .
Y. Zhang et al. / Materials Science and Engineering A 528 (2011) 7708–7714
sistent with earlier observations on the processing of CP Ti and Ti alloys by ECAP at elevated temperatures [19–22], with an investigation of the processing of single crystal Ti at room temperature [23] and with an earlier observation of twinning in CP Ti after ECAP at room temperature [8]. It is noted also that this type of twinning is usually coarse and has a bending configuration in observations by optical microscopy [24] which is consistent with the present observations. The analysis of the structure of the deformed samples shows also that the dislocation density increases significantly in ECAP processing. Despite this increase, the coarser initial grain structure remains evident even after 2 passes of ECAP, where this is attributed to the low number of passes and the consequent relatively low strain when using an ECAP die with a channel angle of 135◦ . Therefore, the mechanism of hardening which is visible in Figs. 6 and 7 after a single pass is due to the increase in the dislocation density, the introduction of twinning and the associated strain hardening. 4.2. Behavior in mechanical testing This study shows that a high level of homogeneity is achieved after 1 pass of ECAP when pressing is carried out at 0.5 mm s−1 but there is less homogeneity after 2 passes. As noted earlier, there was evidence for cracking in the billet processed by two passes at the faster pressing speed. Therefore, it is concluded that this speed is not suitable for the processing of materials for use as structural components. Accordingly, it is necessary to concentrate on the samples processed at the slower speed. An important conclusion from this study is that the microhardness measurements reveal a high level of homogeneity along the longitudinal axes of the CP Ti billets processed by ECAP at 0.05 mm s−1 where this is consistent with earlier longitudinal hardness measurements on commercial purity Al [17] and an Al-6061 alloy [16]. This longitudinal homogeneity confirms the possibility of making use of the ECAP processing of CP Ti at room temperature for the fabrication of structural components for use in medical and dental implants. It follows from Fig. 7 that the increase in hardness, by comparison with the annealed material, is ∼35% after 1 pass of ECAP and ∼40% after 2 passes. There are smaller increases in the ultimate tensile strengths in Fig. 8 of ∼20% after 1 pass and ∼23% after 2 passes, respectively, and there is also an associated pronounced reduction in the measured elongations to failure. It is difficult to make a direct comparison between the present mechanical data and an earlier report for the same material, also processed by ECAP at room temperature, showing different values for the tensile properties including the elongations [24]. This difficulty arises because of the well-established effect that differences in the specimen sizes will influence the results in the two sets of data [25]. Thus, the present investigation used specimens with a ratio of gauge length/cross section of ∼8.3 whereas the specimens in the earlier report used specimens having a ratio of only ∼2.2 [24]. It is reasonable to anticipate that necking effects will play a key role in the elongation of specimens with low gauge length/cross section ratios and this explains the significantly larger elongations observed in the earlier report: for example, there was an earlier elongation of ∼16% for CP Ti processed by 2 passes of ECAP with a die having ˚ = 135◦ [24] whereas the present results show an elongation of ∼6% in Fig. 8 for CP Ti processed under the same conditions. Despite these experimental difficulties, it is instructive to note that the value of the ultimate tensile strength, UTS , of ∼750 MPa after 2 passes of ECAP at room temperature is comparable with the strengths of ∼645 and 790 MPa achieved after 2 and 8 passes of ECAP at room temperature when using a die with a channel angle of
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120◦ [9]. Furthermore, it is an improvement over the values of UTS reported when processing by ECAP at elevated temperatures using a die with ˚ = 90◦ : for example, these values are ∼540 MPa after 7 passes when processing in the temperature range of 723–773 K [26] and ∼710 MPa when processing in the range of 673–723 K [27,28]. Thus, the present results confirm the potential for achieving high strength CP Ti through processing by ECAP at room temperature. 5. Summary and conclusions 1. Commercial purity grade 2 titanium was successfully processed through 1 and 2 passes of ECAP at room temperature using a die with a channel angle of 135◦ and two pressing speeds of 0.5 and 0.05 mm s−1 . There was some cracking in the billet pressed through 2 passes at the faster speed. 2. There was evidence for basal, prismatic and pyramidal slip activity and {1 0 1¯ 2}1¯ 0 1 1 twinning after processing by ECAP but there was only a limited formation of dislocation cell boundaries. 3. The strengthening of the material was homogeneous on both the cross-sectional and longitudinal planes except for some heterogeneity after pressing through 2 passes at the faster pressing speed of 0.5 mm s−1 . This heterogeneity was associated with the presence of cracking. 4. There were increases of ∼40% in hardness and ∼23% in the ultimate tensile strength after pressing through 2 passes of ECAP at the slower speed. The elongation to failure decreased from ∼20% in the annealed condition to ∼6% after 2 passes of ECAP. Acknowledgements This work was supported in part by ICUK (Innovation China-UK) under project Soton 15, in part by the Natural Science Foundation of China under Grant No. 51041003 and in part by the National Science Foundation of the United States under Grant No. DMR0855009. References [1] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881. [2] S.L. Semiatin, V.M. Segal, R.E. Goforth, N.D. Frey, D.P. DeLo, Metall. Mater. Trans. 30A (1999) 1425. [3] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Mater. Sci. Eng. A518 (2009) 124. [4] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Metall. Mater. Trans. 41A (2010) 778. [5] P.R. Cetlin, M.T.P. Aguilar, R.B. Figueiredo, T.G. Langdon, J. Mater. Sci. 45 (2010) 4561. [6] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Acta Mater. 55 (2007) 4769. [7] M. Furui, H. Kitamura, H. Anada, T.G. Langdon, Acta Mater. 55 (2007) 1083. [8] X. Zhao, W. Fu, X. Yang, T.G. Langdon, Scripta Mater. 59 (2008) 542. [9] X. Zhao, X. Yang, X. Liu, X. Wang, T.G. Langdon, Mater. Sci. Eng. A527 (2010) 6335. [10] Y. Estrin, C. Kasper, S. Diederichs, R. Lapovok, J. Biomed. Mater. Res. 90A (2009) 1239. [11] R.Z. Valiev, I.P. Semenova, V.V. Latysh, H. Rack, T.C. Lowe, J. Petruzelka, L. Dluhos, D. Hrusak, J. Sochova, Adv. Eng. Mater. 10 (2008) B15. [12] R.Z. Valiev, I.P. Semenova, E. Jakushina, V.V. Latysh, H. Rack, T.C. Lowe, J. Petruˇzelka, L. Dluhoˇs, D. Hruˇsák, J. Sochová, Mater. Sci. Forum. 584–586 (2008) 49. [13] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [14] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A257 (1998) 328. [15] G.I. Raab, Mater. Sci. Eng. A410–411 (2005) 230. [16] M. Prell, C. Xu, T.G. Langdon, Mater. Sci. Eng. A480 (2008) 449. [17] S.N. Alhajeri, N. Gao, T.G. Langdon, Mater. Sci. Eng. A528 (2011) 3833. [18] P.B. Berbon, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. 30A (1999) 1989. [19] D.H. Shin, I. Kim, J. Kim, Y.S. Kim, S.L. Semiatin, Acta Mater. 51 (2003) 983. [20] G.G. Yapici, I. Karaman, Z.P. Luo, Acta Mater. 54 (2006) 3755. [21] Y.J. Li, Y.J. Chen, J.C. Walmsley, R.H. Mathiesen, S. Dumoulin, H.J. Roven, Scripta Mater. 62 (2010) 443. [22] Y.J. Chen, Y.J. Li, J.C. Walmsley, S. Dumoulin, H.J. Roven, Metall. Mater. Trans. 41A (2010) 787. [23] X. Tan, H. Gu, S. Zhang, C. Laird, Mater. Sci. Eng. A189 (1994) 77.
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[24] Y. Zhang, S.N. Alhajeri, R.B. Figueiredo, N. Gao, J.T. Wang, T.G. Langdon, Mater. Sci. Forum. 667–669 (2011) 719. [25] Y.H. Zhao, Y.Z. Guo, Q. Wei, A.M. Dangelewicz, C. Xu, Y.T. Zhu, T.G. Langdon, Y.Z. Zhou, E.J. Lavernia, Scripta Mater. 59 (2008) 627. [26] V.V. Stolyarov, Y.T. Zhu, T.C. Lowe, R.K. Islamgaliev, R.Z. Valiev, Nanostruct. Mater. 11 (1999) 947.
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Structure and mechanical properties of commercial purity titanium processed by ECAP at room temperature Yue Zhang a , Roberto B. Figueiredo b,c , Saleh N. Alhajeri b,d , Jing Tao Wang a , Nong Gao b , Terence G. Langdon b,e,∗ a
Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK c Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Belo Horizonte, MG 31270-901, Brazil d Department of Manufacturing Engineering, College of Technological Studies, PAAET, Shuwaikh 70654, Kuwait e Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA b
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 17 June 2011 Accepted 17 June 2011 Available online 24 June 2011 Keywords: Equal-channel angular pressing Hardness Strength Titanium Twinning
a b s t r a c t Experiments were conducted on commercial purity titanium to evaluate the microstructure and mechanical properties after processing by equal-channel angular pressing (ECAP) at room temperature. Samples were successfully processed through 1 and 2 passes at two different pressing speeds using an ECAP die with a channel angle of 135◦ . Following ECAP, a microstructural evaluation showed evidence for the occurrence of basal, prismatic and pyramidal slip together with {1 0 1¯ 2}1¯ 0 1 1 twinning. Hardness measurements confirmed the occurrence of homogeneous strengthening throughout the samples. Tensile testing revealed a high ultimate tensile strength of ∼750 MPa after 2 passes of ECAP but with a reduction in the measured elongation to failure. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Processing by equal-channel angular pressing (ECAP) has become an established procedure for introducing significant grain refinement into bulk metals [1]. However, processing by ECAP is difficult in some materials, such as hexagonal close-packed metals, where the number of independent slip systems is limited. This difficulty led to an early conclusion, in experiments conducted on commercial purity (CP) titanium using an ECAP die with a channel angle of ˚ = 90◦ , that it was not possible to process this material at room temperature because of the occurrence of significant segmentation in which the billets become sheared into discrete but inter-connected segments during the pressing operation [2]. The significance of unstable flow and segmentation has now become a critical issue in the processing of bulk solids by ECAP [3–5].
∗ Corresponding author at: Department of Aerospace & Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453, USA. Tel.: +1 213 740 0491; fax: +1 213 740 8071. E-mail address: [email protected] (T.G. Langdon). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.06.054
Detailed finite element calculations showed these so-called “difficult-to-work” alloys may be processed more easily by using an ECAP die having an increased channel angle [6]. Furthermore, this approach was clearly demonstrated with a two-phase Mg8% Li alloy where it was shown that the alloy may be pressed without cracking through only a single pass at room temperature when using a die with ˚ = 90◦ whereas the same alloy may be pressed without cracking up to 10 passes at room temperature when the channel angle is increased to 135◦ [7]. A similar approach was used later for the successful pressing of CP Ti through a single pass at room temperature using a die with ˚ = 120◦ [8] and more recently these experiments were continued to produce samples of CP Ti pressed through 8 passes with ˚ = 120◦ [9]. Titanium is currently receiving much interest because it exhibits excellent biocompatibility [10] and has a very significant potential for use as a dental implant material [11]. For these implants, CP Ti is currently processed by ECAP through four passes at 723 K using a die with a channel angle of 90◦ , it is forge drawn with a cumulative deformation of 80%, and finally it is annealed at 573–623 K to produce long rods having diameters of 7 mm and lengths up to ∼3 m [12]. The grain size in these as-processed rods was reported
Y. Zhang et al. / Materials Science and Engineering A 528 (2011) 7708–7714
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Fig. 1. Schematic illustrations of (a) the cross-sectional and (b) the longitudinal planes showing dashed lines to illustrate the traverses followed for recording individual measurements of the Vickers microhardness and (c) the procedure for recording the hardness values by taking microhardness values around each selected point with these measurements separated from the selected position by a distance of 0.15 mm. The distances between the selected points for the individual hardness measurements were 1.0 mm on the cross-sectional plane and 3.0 mm on the longitudinal plane, respectively.
as ∼150 nm. However, it is readily apparent there would be a significant advantage, in terms of both speed and experimental simplicity, if the processing of the CP Ti was conducted by ECAP at room temperature. In practice, this is difficult because no detailed information is currently available describing the microstructural characteristics and the mechanical properties of CP Ti when processed by ECAP at room temperature. Accordingly, the present investigation was initiated to provide this information. 2. Experimental material and procedures Commercial purity (CP) titanium of grade 2 (99.5% purity) was received in the form of rods with diameters of 10 mm from Global Metal Trading (UK) Ltd. These rods were cut into billets having lengths of 65 mm for processing by ECAP and then the rods were annealed in air at 993 K for 2 h. The microstructure after annealing was characterized by equiaxed grains with an average grain size of ∼10 m. The billets were processed by ECAP for either 1 or 2 passes at room temperature using a solid die with an internal channel angle of ˚ = 135◦ and an outer arc of curvature of ≈ 20◦ at the point of intersection of the two parts of the channel. It can be shown that these values of ˚ and produce an equivalent strain, ε, of ∼0.46 in each separate pass through the die [13]. The processing was conducted using a 200 ton DMG hydraulic press operating at constant rates of displacement of either 0.5 or 0.05 mm s−1 and the billets were rotated by 90◦ between the first and second pass using processing route BC [14]. A lubricant of an MoS2 suspension in a mineral oil was used between the billets and the die walls in order to reduce frictional effects. After processing by ECAP, each billet was cut along the central line, perpendicular to the upper surface, using a spark-erosion facility and the surfaces were then carefully polished using a series of abrasive papers containing SiC to a final level of 4000 grit equivalent to a particle size of 3 m. These polished surfaces were used for an evaluation of the microhardness levels at selected points along each surface. Similar sets of measurements were also taken on crosssectional planes by cutting the billets perpendicular to the pressing direction. Fig. 1 illustrates (a) a cross-sectional and (b) a longitudinal section through the billet where the dashed lines denote the locations for the microhardness measurements. The lines in Fig. 1(a) are located perpendicular to each other and pass through the center of the billet on the cross-sectional plane as viewed from the front of the billet: thus, the top to bottom of this plane is denoted a vertical traverse and the left to right of this plane is denoted a horizontal traverse. The lines in Fig. 1(b) are located along the longitudinal central axis and at distances of 1.0 mm from the top and bottom surfaces on the longitudinal section. The values of the Vickers microhardness, Hv, were recorded along each dashed line shown in Fig. 1(a) and (b) at positions separated by distances of 1.0 mm on the cross-sectional planes and 3.0 mm on the longitudinal planes. At each selected position, a cluster of four equally spaced
indentations was made around the selected point with the indentations separated from these positions by a distance of 0.15 mm as illustrated in Fig. 1(c). These four values were used to estimate the average hardness and the relevant error bars. An important limitation in conventional ECAP is that gross distortions are introduced immediately adjacent to the front and rear edges of the billets and these distortions may be avoided only by performing the ECAP using a die with parallel channels [15]. To avoid these regions of gross distortion, and following procedures introduced earlier [16,17], the longitudinal hardness measurements were taken over total lengths of ∼40 mm within the central portions of each billet. For each microhardness measurement, the hardness was recorded using a Matsuzawa Seiki MHT-1 microhardness tester equipped with a Vickers indenter using a load of 300 gf and a dwell time of 10 s. The microstructure of the materials was evaluated after ECAP using transmission electron microscopy (TEM). Disks having diameters of 3 mm and thicknesses of 0.68 mm were cut from the longitudinal plane of each sample and mechanical polishing was used to reduce the thickness to ∼60 m. The samples were polished to perforation using a twin-jet polisher with a solution of 5% perchloric acid, 35% butanol and 60% methanol at a temperature of 243 K with a voltage of 50–60 V and a current of ∼50 mA. A JEM2010 electron microscope operating at 105 kV was used to examine the foils with a diaphragm aperture having a diameter of ∼1 m for selected area electron diffraction (SAED). For testing in tension, dog-bone specimens were machined from the as-received material and the billets processed at 0.05 mm s−1 . These specimens had gauge lengths of 25 mm parallel to the longitudinal axial directions and gauge cross-sections of 2.5 mm × 1.2 mm. Tensile tests were performed using an Instron testing machine equipped with an electronic extensometer. All tensile tests were conducted at room temperature (296 K) using a constant rate of cross-head displacement which imposed an initial strain rate, ε˙ 0 , of 1.0 × 10−3 s−1 . 3. Experimental results 3.1. Appearance of the billets and microstructural characterization The appearance of the four samples of CP Ti is shown in Fig. 2 after processing by ECAP. It is readily apparent that the surfaces of the CP Ti samples are smooth and there is an absence of any segmentation after pressing at both speeds. Nevertheless, close inspection of the billet processed by 2 passes at the faster speed revealed the presence of some cracking in this sample. The microstructures of the samples processed by 1 and 2 passes of ECAP at 0.05 mm s−1 , and the associated SAED patterns, are shown in Figs. 3 and 4 respectively. Traces of the slip planes parallel to (1 1 2 3) are visible in the material processed through 1 pass of ECAP, thereby producing a lamellar pattern in the microstructure.
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Fig. 2. Appearance of the four billets after pressing through 1 or 2 ECAP passes at two different pressing speeds.
Measurements gave an average lamellar spacing of ∼300 nm. Fig. 4 shows the presence of a high density of dislocations in the interior of a coarse grain after processing through 2 passes of ECAP and there are visible traces of slip planes parallel to the basal (0 0 0 1) and prismatic (0 1 1¯ 0) planes. Despite the high density of dislocations in this sample, dislocation cell boundaries or subgrain boundaries are not readily distinguished thereby suggesting that the total imposed strain of ∼0.92 is not sufficient to produce a well-defined grain refinement. Fig. 5 shows a series of TEM images of a region containing twins in the material processed by 1 pass of ECAP at 0.05 mm s−1 together with the relevant SAED patterns which are used to distinguish between the matrix and the twinned regions. It is observed that the twins have different thicknesses and they appear bent. An analysis of the SAED pattern showed the twins are of the {1 0 1¯ 2}1¯ 0 1 1 type. 3.2. The hardness and mechanical properties after processing by ECAP The distributions of hardness along the selected lines on the cross-sectional planes and the longitudinal planes of the billets are
shown in Fig. 6 after processing by ECAP at 0.5 mm s−1 . Fig. 6(a) gives the distributions of hardness along the billet cross-sections after processing through 1 and 2 passes (1p and 2p), Fig. 6(b) gives the distribution of hardness along the longitudinal section after processing by 1 pass and Fig. 6(c) gives the distribution of hardness along the longitudinal section after processing by 2 passes of ECAP: in each plot, the average hardness of the unprocessed material (∼170 Hv) is shown by the lower dashed line. It is observed that there is a high level of homogeneity in the hardness distributions on the cross-sectional plane of the billet after one pass of ECAP but there is a tendency for some heterogeneity of hardness on the cross-sectional plane of the billet after the second pass. These observations are confirmed by the distributions of hardness recorded along the longitudinal sections in Fig. 6(b) and (c). Thus, the billet processed to a single pass of ECAP exhibits a high degree of homogeneity while the billet processed by 2 passes exhibits some heterogeneity. The occurrence of some heterogeneity is consistent with the cracking observed in this billet. The distributions of hardness along selected lines on the longitudinal planes of the billets processed at the slower pressing speed of 0.05 mm s−1 are shown in Fig. 7 for samples processed by (a) 1
Fig. 3. (a) Microstructure by TEM of CP Ti processed by 1 pass of ECAP showing lamellar boundaries parallel to the (1 1 2¯ 3) slip plane and (b) the corresponding selected area electron diffraction pattern for a [7 2¯ 5¯ 3] zone axis.
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Fig. 4. (a) Bright field TEM micrograph of CP Ti processed by 2 passes of ECAP showing substructure boundaries parallel to the (0 0 0 1) and (0 1 1¯ 0) slip planes, respectively, and (b) the corresponding selected area electron diffraction pattern for a [2 1¯ 1¯ 0] zone axis.
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and (b) 2 passes of ECAP, respectively: again, the average hardness of the unprocessed annealed material is shown by the lower dashed lines. Inspection of these data demonstrates there is a very significant increase in the hardness after 1 pass of ECAP, to ∼210–240 Hv, although the imposed strain is only ∼0.46. Furthermore, there is a homogeneous increase in hardness throughout the central length of ∼40 mm in the billet after 1 pass. There is an additional increase in the hardness after 2 passes, to ∼245 Hv and, unlike the billet pressed at the faster speed, there is good homogeneity except only that some limited heterogeneity is visible at the outer edges of the 40 mm central portion of the sample. After 2 passes, almost all of the individual datum points have hardness values larger than 230 Hv. A comparison of Figs. 6 and 7 shows that a higher level of homogeneity is attained when using a lower pressing speed and this is consistent with the absence of any cracking after pressing at the slower speed. Furthermore, it is also consistent with earlier experiments investigating the effect of the pressing speed on samples of pure aluminum [18]. For the tensile testing, Fig. 8 shows plots of stress vs. strain for the annealed and unprocessed material and for the material processed by 1 and 2 passes of ECAP. It is apparent that the annealed material exhibits a low yield stress, a high rate of strain hardening and a reasonable ductility. Processing by ECAP significantly increases the yield stress but reduces both the strain hardening capability and the ductility. The measured elongations to failure are ∼20% in the annealed material but only ∼6% after 2 passes of ECAP. There is a maximum stress of ∼600 MPa in the annealed condition but this is increased to ∼750 MPa after 2 passes of ECAP.
Fig. 5. Microstructure by TEM of CP Ti processed by 1 pass of ECAP showing a serial of {1 0 1¯ 2}1¯ 0 1 1 mechanical twins according to the corresponding selected area electron diffraction pattern for a [2 1¯ 1¯ 0] zone axis.
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Fig. 7. Individual values of the Vickers microhardness Hv recorded on the longitudinal plane after (a) 1 pass and (b) 2 passes of ECAP.
An analysis of the dislocation structures and SAED patterns of the CP-Ti after 1 and 2 passes, as shown in Figs. 3 and 4, reveals the occurrence of basal {0 0 0 1}1 1 2¯ 0, prismatic {0 1 1¯ 0}1 1 2¯ 0 and ¯ 1 2¯ 3 slip during ECAP at room temperpyramidal c+a {1 1 2¯ 2}1 ature. Furthermore, and as shown in Fig. 5, deformation twinning after 1 pass was identified as {1 0 1¯ 2}1¯ 0 1 1 type where this is con-
Fig. 6. Individual values of the Vickers microhardness Hv recorded after pressing at 0.5 mm s−1 on (a) the cross-sectional plane after 1 and 2 passes, (b) the longitudinal planes after 1 pass and (c) the longitudinal plane after 2 passes.
4. Discussion 4.1. Microstructural characteristics This investigation shows that it is feasible to process grade 2 titanium by ECAP at room temperature for 2 passes when using a die with a channel angle of 135◦ and after processing the material exhibits features that are characteristic of deformation on different slip planes combined with the occurrence of some twinning.
Fig. 8. Flow stress as a function of strain in tensile testing for an annealed sample and specimens processed through 1 and 2 passes of ECAP at a pressing speed of 0.05 mm s−1 .
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sistent with earlier observations on the processing of CP Ti and Ti alloys by ECAP at elevated temperatures [19–22], with an investigation of the processing of single crystal Ti at room temperature [23] and with an earlier observation of twinning in CP Ti after ECAP at room temperature [8]. It is noted also that this type of twinning is usually coarse and has a bending configuration in observations by optical microscopy [24] which is consistent with the present observations. The analysis of the structure of the deformed samples shows also that the dislocation density increases significantly in ECAP processing. Despite this increase, the coarser initial grain structure remains evident even after 2 passes of ECAP, where this is attributed to the low number of passes and the consequent relatively low strain when using an ECAP die with a channel angle of 135◦ . Therefore, the mechanism of hardening which is visible in Figs. 6 and 7 after a single pass is due to the increase in the dislocation density, the introduction of twinning and the associated strain hardening. 4.2. Behavior in mechanical testing This study shows that a high level of homogeneity is achieved after 1 pass of ECAP when pressing is carried out at 0.5 mm s−1 but there is less homogeneity after 2 passes. As noted earlier, there was evidence for cracking in the billet processed by two passes at the faster pressing speed. Therefore, it is concluded that this speed is not suitable for the processing of materials for use as structural components. Accordingly, it is necessary to concentrate on the samples processed at the slower speed. An important conclusion from this study is that the microhardness measurements reveal a high level of homogeneity along the longitudinal axes of the CP Ti billets processed by ECAP at 0.05 mm s−1 where this is consistent with earlier longitudinal hardness measurements on commercial purity Al [17] and an Al-6061 alloy [16]. This longitudinal homogeneity confirms the possibility of making use of the ECAP processing of CP Ti at room temperature for the fabrication of structural components for use in medical and dental implants. It follows from Fig. 7 that the increase in hardness, by comparison with the annealed material, is ∼35% after 1 pass of ECAP and ∼40% after 2 passes. There are smaller increases in the ultimate tensile strengths in Fig. 8 of ∼20% after 1 pass and ∼23% after 2 passes, respectively, and there is also an associated pronounced reduction in the measured elongations to failure. It is difficult to make a direct comparison between the present mechanical data and an earlier report for the same material, also processed by ECAP at room temperature, showing different values for the tensile properties including the elongations [24]. This difficulty arises because of the well-established effect that differences in the specimen sizes will influence the results in the two sets of data [25]. Thus, the present investigation used specimens with a ratio of gauge length/cross section of ∼8.3 whereas the specimens in the earlier report used specimens having a ratio of only ∼2.2 [24]. It is reasonable to anticipate that necking effects will play a key role in the elongation of specimens with low gauge length/cross section ratios and this explains the significantly larger elongations observed in the earlier report: for example, there was an earlier elongation of ∼16% for CP Ti processed by 2 passes of ECAP with a die having ˚ = 135◦ [24] whereas the present results show an elongation of ∼6% in Fig. 8 for CP Ti processed under the same conditions. Despite these experimental difficulties, it is instructive to note that the value of the ultimate tensile strength, UTS , of ∼750 MPa after 2 passes of ECAP at room temperature is comparable with the strengths of ∼645 and 790 MPa achieved after 2 and 8 passes of ECAP at room temperature when using a die with a channel angle of
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120◦ [9]. Furthermore, it is an improvement over the values of UTS reported when processing by ECAP at elevated temperatures using a die with ˚ = 90◦ : for example, these values are ∼540 MPa after 7 passes when processing in the temperature range of 723–773 K [26] and ∼710 MPa when processing in the range of 673–723 K [27,28]. Thus, the present results confirm the potential for achieving high strength CP Ti through processing by ECAP at room temperature. 5. Summary and conclusions 1. Commercial purity grade 2 titanium was successfully processed through 1 and 2 passes of ECAP at room temperature using a die with a channel angle of 135◦ and two pressing speeds of 0.5 and 0.05 mm s−1 . There was some cracking in the billet pressed through 2 passes at the faster speed. 2. There was evidence for basal, prismatic and pyramidal slip activity and {1 0 1¯ 2}1¯ 0 1 1 twinning after processing by ECAP but there was only a limited formation of dislocation cell boundaries. 3. The strengthening of the material was homogeneous on both the cross-sectional and longitudinal planes except for some heterogeneity after pressing through 2 passes at the faster pressing speed of 0.5 mm s−1 . This heterogeneity was associated with the presence of cracking. 4. There were increases of ∼40% in hardness and ∼23% in the ultimate tensile strength after pressing through 2 passes of ECAP at the slower speed. The elongation to failure decreased from ∼20% in the annealed condition to ∼6% after 2 passes of ECAP. Acknowledgements This work was supported in part by ICUK (Innovation China-UK) under project Soton 15, in part by the Natural Science Foundation of China under Grant No. 51041003 and in part by the National Science Foundation of the United States under Grant No. DMR0855009. References [1] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881. [2] S.L. Semiatin, V.M. Segal, R.E. Goforth, N.D. Frey, D.P. DeLo, Metall. Mater. Trans. 30A (1999) 1425. [3] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Mater. Sci. Eng. A518 (2009) 124. [4] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Metall. Mater. Trans. 41A (2010) 778. [5] P.R. Cetlin, M.T.P. Aguilar, R.B. Figueiredo, T.G. Langdon, J. Mater. Sci. 45 (2010) 4561. [6] R.B. Figueiredo, P.R. Cetlin, T.G. Langdon, Acta Mater. 55 (2007) 4769. [7] M. Furui, H. Kitamura, H. Anada, T.G. Langdon, Acta Mater. 55 (2007) 1083. [8] X. Zhao, W. Fu, X. Yang, T.G. Langdon, Scripta Mater. 59 (2008) 542. [9] X. Zhao, X. Yang, X. Liu, X. Wang, T.G. Langdon, Mater. Sci. Eng. A527 (2010) 6335. [10] Y. Estrin, C. Kasper, S. Diederichs, R. Lapovok, J. Biomed. Mater. Res. 90A (2009) 1239. [11] R.Z. Valiev, I.P. Semenova, V.V. Latysh, H. Rack, T.C. Lowe, J. Petruzelka, L. Dluhos, D. Hrusak, J. Sochova, Adv. Eng. Mater. 10 (2008) B15. [12] R.Z. Valiev, I.P. Semenova, E. Jakushina, V.V. Latysh, H. Rack, T.C. Lowe, J. Petruˇzelka, L. Dluhoˇs, D. Hruˇsák, J. Sochová, Mater. Sci. Forum. 584–586 (2008) 49. [13] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [14] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A257 (1998) 328. [15] G.I. Raab, Mater. Sci. Eng. A410–411 (2005) 230. [16] M. Prell, C. Xu, T.G. Langdon, Mater. Sci. Eng. A480 (2008) 449. [17] S.N. Alhajeri, N. Gao, T.G. Langdon, Mater. Sci. Eng. A528 (2011) 3833. [18] P.B. Berbon, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. 30A (1999) 1989. [19] D.H. Shin, I. Kim, J. Kim, Y.S. Kim, S.L. Semiatin, Acta Mater. 51 (2003) 983. [20] G.G. Yapici, I. Karaman, Z.P. Luo, Acta Mater. 54 (2006) 3755. [21] Y.J. Li, Y.J. Chen, J.C. Walmsley, R.H. Mathiesen, S. Dumoulin, H.J. Roven, Scripta Mater. 62 (2010) 443. [22] Y.J. Chen, Y.J. Li, J.C. Walmsley, S. Dumoulin, H.J. Roven, Metall. Mater. Trans. 41A (2010) 787. [23] X. Tan, H. Gu, S. Zhang, C. Laird, Mater. Sci. Eng. A189 (1994) 77.
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