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Thermal stability of pure bcc Zr fabricated by high pressure torsion
Materials Letters 64 (2010) 211–214
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Thermal stability of pure bcc Zr fabricated by high pressure torsion M.T. Pérez-Prado a,⁎, A. Sharafutdinov b, A.P. Zhilyaev c,d a
IMDEA Materials, 28040 Madrid, Spain Innovation Scientific Technology Center “Iskra,” 450000, Ufa, Russia c Centro Nacional de Investigaciones Metalúrgicas (CENIM), CSIC, 28040 Madrid, Spain d Institute for Metals Superplasticity Problems, RAS, 450001 Ufa, Russia b
a r t i c l e
i n f o
Article history: Received 1 September 2009 Accepted 20 October 2009 Available online 25 October 2009 Keywords: Omega Zr Beta Zr Thermal stability Nanostructure High pressure torsion
a b s t r a c t Recently pure omega plus bcc Zr was fabricated for the first time through the simultaneous application of compression and shear to pure alpha Zr by high pressure torsion. This phase was found to be stable under ambient conditions after processing. Here the thermal stability of the pure bcc Zr thus fabricated is analyzed using differential scanning calorimetry (DSC), in-situ X-ray diffraction at high temperature and transmission electron microscopy (TEM). Our results show that the temperature of the reverse transformation of the bcc phase is close to that of the omega phase. The presence of a mixed structure formed by alternating nanolaminates of the omega and the bcc phases might play a key role in the retention of these two phases at ambient pressure and temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction High pressure torsion (HPT) is a mechanical processing technique that consists on applying simultaneously compression and shear stresses to a disk-shaped sample placed between two anvils [1]. For the past decade this technique has been profusely utilized to fabricate bulk nanocrystalline metals, with average grain sizes smaller than 100 nm [1]. Moreover, recent investigations have demonstrated that high pressure torsion can be further used to develop new routes for the fabrication of advanced materials, since it allows stabilizing at room temperature and 1 atm, metallic phases that were so far only stable under extreme conditions of pressure and/or temperature [1–5]. The enormous potential of high pressure torsion to induce phase transformations in metals has still to be exploited. Alloys of the Group IV transition elements such as Zr and Ti have a wide range of key applications in the aerospace, biomedical and nuclear industries [6–8]. Both Zr and Ti are considered model materials for the study of phase transitions due to the wide range of transformations they experience [6]. At room temperature and 1 atm pure Zr crystallizes with an hcp structure (α phase: a = 3.232 Å, c = 5.147 Å). When heated at T > 1135 K the α phase transforms into a bcc (β) structure before melting (beta phase: a =3.568 Å). With increasing hydrostatic pressure the sequence α >ω (simple hexagonal) > β takes place [6] as the d-band occupation increases by s–d electron transfer (omega phase: a = 5.039 Å, c = 3.136 Å). The transition pressures are Pα > ω = 2–6 GPa and Pω > β = 30 GPa. Pure Ti also undergoes the same sequence of phase transformations, albeit at different transition temperatures and pres⁎ Corresponding author. E-mail address: [email protected] (M.T. Pérez-Prado). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.049
sures. The ω and β phases have attractive properties, such as a higher superconducting temperature [9–11] and lower elastic modulus (β-Ti) [12]. However, so far these properties could not be exploited since, when decreasing the temperature or releasing the pressure, the reverse transformations take place [9]. Overall, the properties of the high pressure phases are still widely unknown. It has been recently observed for the first time that bulk pure ω + bcc Zr may be stabilized at 25 °C and 1 atm using HPT [3]. The retention of these two high pressure phases under ambient conditions is currently not understood. The aim of this paper is to investigate the thermal stability of pure bcc Zr fabricated by HPT, in order to explore the temperature range of applicability of this material and to gain further understanding in shear induced phase transformations. 2. Experimental procedure The material studied here is commercially pure (99.98%) alpha Zr with an equiaxed grain size of 13 μm [2,3]. Pure Zr disks of 10 mm in diameter and 2 mm in thickness were processed by high pressure torsion (HPT) using an unconstrained press. A pressure of 6 GPa and 5 turns were used, at an approximate speed of 1 rpm. The thermal stability of the phases present was analyzed using differential scanning calorimetry in a Perkin Elmer DSC 7 under pure argon flow. Heating rates ranging from 20 to 60 K/min were utilized. The activation energy of the transformation is calculated using the Kissinger method, i.e., by analyzing the shift in the transition temperature with increasing heating rates. Additionally, the evolution of the crystalline phases present in the processed samples was tracked by in-situ X-ray diffraction at high temperature using Cu Kα radiation in an Xpert-Pro Panalytical diffractometer. 2θ angles ranging from 25° to 140° were
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M.T. Pérez-Prado et al. / Materials Letters 64 (2010) 211–214
scanned at steps of 0.017°. A first X-ray diffraction pattern was recorded at 25 °C and then subsequent diffractograms, up to 5 more, were measured at temperatures ranging from 50 °C to 300 °C at 24 min. intervals. The heating time was approximately 5 min. The total annealing time was approximately 2 h and 24 min. The microstructure of selected as-processed samples was analyzed by transmission electron microscopy (TEM) using a 200 kV JEM-2000 FX TEM. Sample preparation for TEM is described in [2]. High resolution TEM was performed in a Philips CM200-FEG TEM. 3. Results and discussion Fig. 1a illustrates the DSC plot corresponding to pure ω + β Zr processed by unconstrained HPT using 6 GPa and 5 turns and cycled thermally between room temperature and 600 °C at several heating rates (20, 40 and 60 K/min) under an argon flow. The presence of only one major exothermic peak suggests that the reverse transformation of both phases, resulting ultimately in the formation of the α phase, takes place at the same or very similar temperatures. The transition temperatures, calculated for the three heating rates utilized are 198.6 °C (20 K/min), 206.3 °C (40 K/min), and 210.4 °C (60 K/min). Due to the inertia associated with constant heating rate cycles, these data do not allow us to determine the exact transition temperature. However, since inertia increases with increasing rates, it can be concluded that the transformation must take place at temperatures near 198.6 °C (~471 K) (which correspond to the lowest rate used). This value is close to that reported for the reverse transformation of metastable ω particles, formed in Zr alloys under hydrostatic pressure and retained after unloading (470 K) [6]. The activation energy of the transformation (Q t) was calculated using the Kissinger method (Fig. 1b), and its value came out to be 181.7 ±18.2 kJ/mol. The parameters for the Kissinger plot are included in Table 1. Qt is close to the activation energy for self diffusion (Q SD) of Zr (Q SD = 216 kJ/mol). The activation energy for grain boundary diffusion in Zr, Q GB, is significantly lower (Q GB = 101 kJ/mol). The change in the crystalline structure of ω + β pure Zr during annealing was also examined by in-situ X-ray diffraction. Fig. 2a illustrates a sequence of diffraction patterns measured during annealing at 150 °C. The starting material is formed by a large volume fraction of transformed phases (ω + β), although a small amount of α phase remains present. The volume fraction of α phase increases already after an annealing time of 48 min at 150 °C, as evidenced by the appearance and heightening of some α peaks. Phase identification is not trivial, as most ω and β peaks overlap. Out of the peaks that are
Table 1 Data from DSC studies corresponding to pure Zr processed by HPT. b, K/min
T, °C
T, K
1/T, K− 1
ln(b/T2)
Error
ΔE, J/g
20 40 60
198.5 206.3 210.4
471.65 479.45 483.55
0.00212 0.00209 0.00207
15.30821 16.03416 16.45665
0.38271 0.40085 0.41142
0.671 1.284 1.451
b represents the heating rate.
visible, only those at 46.2° and 96.8° (highlighted with arrows in Fig. 2a) belong exclusively to the ω and β phases, respectively. The evolution of the intensity of these two peaks during annealing at 150 °C is illustrated in Fig. 2b and c. The intensity of the two peaks decreases clearly with annealing time. The change in the shape of the (222) beta peak between 24′ and 48′ in Fig. 2c is due to the gradual disappearance of this peak and to the overlap with the left tail of the nearby (211) alpha peak at 96°. In order to evaluate further whether the omega and beta phases disappear simultaneously, the change in the height of the two peaks highlighted with arrows (which correspond exclusively to one of the two phases) was monitored during annealing at temperatures ranging from 50 °C to 300 °C (Fig. 3). The heights of both peaks remain constant during annealing at 50 °C and 100 °C and they start decreasing right after the beginning of the 150 °C heat treatment at approximately the same rate. At temperatures higher than 200 °C both peaks disappear immediately after the start of the treatment, their intensity reaching the background level after 20 min of annealing. This data, again, suggest, that the kinetics of the reverse transformation is similar in both phases. The above results prove that the stability of the ω and β phases is intimately related. Both are retained at ambient conditions after processing by HPT and both appear to transform back during annealing at very similar conditions of time and temperature. These observations might shed some light on the origin of the stability of these high pressure phases under ambient conditions. In particular, it appears that the presence of the two phases might be necessary to create the local stresses required to stabilize pure ω and β Zr at 25 °C and 1 atm. Caspersen et al. [13] have predicted the formation of mixed phase structures in pure Fe (alternating lamella of the bcc and hcp phases) as a consequence of the application of shear during hydrostatic loading. Each layer would undergo uniform deformation. Lamellar structures are indeed often seen in Zr martensitic structures [6]. According to Caspersen et al. [13] larger amounts of shear would lead to the
Fig. 1. (a) DSC plot corresponding to pure ω + β Zr processed by unconstrained HPT using 6 GPa and 5 turns and cycled thermally between room temperature and 600 °C at 20 K/min, 40 K/min, and 60 K/min; (b) corresponding Kissinger plot (b represents the heating rate).
M.T. Pérez-Prado et al. / Materials Letters 64 (2010) 211–214
213
Fig. 2. (a) X-ray diffraction patterns corresponding to pure Zr processed by HPT using 6 GPa and 5 turns and annealed at 150 °C during different time intervals. (b) Enlarged view of the evolution with annealing at 150 °C of the (111) ω peak at 46.2°; (c) enlarged view of the evolution with annealing at 150 °C of the (222) β peak at 96.8°.
Fig. 3. Change in the maximum intensity of the (11–21)ω and (222)β peaks with time at different annealing temperatures. The background level is indicated using a wide gray transparent band.
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M.T. Pérez-Prado et al. / Materials Letters 64 (2010) 211–214
Fig. 4. (a) TEM micrograph showing the laminate structure present in pure Zr processed by HPT using 6 GPa and 5 turns; (b) high resolution TEM micrograph of the laminates; (c) SAD pattern illustrating showing the rings belonging to the omega and beta phases.
formation of increasingly finer laminates, to lower transition pressures (TP) and to larger hysteresis (i.e., to a retention of the transformed phases at pressures below TP upon unloading). In particular, their model predicts that shear strains (εf) from 0.02 to 0.06 cause variations in TP between 11 and 25 GPa. The present results in HPTed pure Zr seem to constitute an extreme case of this behavior. Fig. 4a shows a TEM micrograph of pure Zr processed by HPT using a pressure of 6 GPa and 5 turns. It can be clearly seen that the nanostructure is formed by laminates with alternating contrast. The width of the lamella varies between 5 and 20 nm. A high resolution TEM image of one lamella is depicted in Fig. 4b and the corresponding SAD pattern are plotted in Fig. 4c. The large shear strain imposed, approximately εf = 5 at the edge of the HPT disk, presumably gives rise to the formation of nanolaminates of alternating ω and β phases, leading to a high hysteresis and therefore a retention of the mixed structure under ambient conditions once the pressure is released. 4. Conclusions In summary, the thermal stability of pure bcc Zr fabricated by high pressure torsion has been investigated. DSC measurements show that the transition temperature is approximately 198 °C and in-situ X-ray diffraction reveals that this phase starts to disappear immediately upon heating at 150 °C. The thermal stability of pure bcc Zr seems thus very similar to that of pure ω Zr. The presence of alternating nanolamella of ω and β phases, originated as a consequence of the application of shear
under pressure, might give rise to the adequate local stress conditions necessary for the retention of both phases under ambient temperature and pressure. Acknowledgments Funding from under projects MAT 2006-11202, CCG07-CSIC/MAT2270, and MAT 2006-11202 is acknowledged. The authors are grateful to Prof. S. Suriñach (UAB, Spain) and to the CAI Difracción de Rayos X (UCM). References [1] Zhilyaev AP, Langdon TG. Prog Mater Sci 2008;53:893. [2] Pérez-Prado MT, Gimazov AA, Ruano OA, Kassner ME, Zhilyaev AP. Scr Mater 2008;58:219. [3] Pérez-Prado MT, Zhilyaev AP. Phys Rev Lett 2009;102:175,504. [4] Todaza Y, Sasaki J, Moto T, Umemoto M. Scr Mater 2008;59:615. [5] Edalati K, Horita Z, Yagi S, Matsubara E. Mater. Sci. Eng. A 2009;523:277. [6] Banerjee S, Mukhopadhyay P. Phase transformations: examples from titanium and zirconium alloys. Pergamon Materials Series, 1st EditionAmsterdam: Elsevier; 2007. [7] Tewari R, Srivastava D, Dey GK, Chakravarti JK, Banerjee S. J Nucl Mater 2008; 383:153. [8] Saldaña L, Méndez-Vilas A, Jiang L, Multigner M, González-Carrasco JL, PérezPrado MT, et al. Biomaterials 2007;28:4343. [9] Sikka SK, Vohra YK, Chidambaram R. Prog Mater Sci 1982;27:245. [10] Bashkin IO, Tissen VG, Nefedova MV, Ponyatovsky EG. Physica C 2006;434:191. [11] Buzea C, Robbie K. Supercond Sci Technol 2005;18:R1. [12] Geetha M, Singh AK, Asokamani R, Gogia AK. Prog Mater Sci 2009;54:397. [13] Caspersen KJ, Lew A, Ortiz M, Carter EA. Phys Rev Lett 2004;93:115,501.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Thermal stability of pure bcc Zr fabricated by high pressure torsion M.T. Pérez-Prado a,⁎, A. Sharafutdinov b, A.P. Zhilyaev c,d a
IMDEA Materials, 28040 Madrid, Spain Innovation Scientific Technology Center “Iskra,” 450000, Ufa, Russia c Centro Nacional de Investigaciones Metalúrgicas (CENIM), CSIC, 28040 Madrid, Spain d Institute for Metals Superplasticity Problems, RAS, 450001 Ufa, Russia b
a r t i c l e
i n f o
Article history: Received 1 September 2009 Accepted 20 October 2009 Available online 25 October 2009 Keywords: Omega Zr Beta Zr Thermal stability Nanostructure High pressure torsion
a b s t r a c t Recently pure omega plus bcc Zr was fabricated for the first time through the simultaneous application of compression and shear to pure alpha Zr by high pressure torsion. This phase was found to be stable under ambient conditions after processing. Here the thermal stability of the pure bcc Zr thus fabricated is analyzed using differential scanning calorimetry (DSC), in-situ X-ray diffraction at high temperature and transmission electron microscopy (TEM). Our results show that the temperature of the reverse transformation of the bcc phase is close to that of the omega phase. The presence of a mixed structure formed by alternating nanolaminates of the omega and the bcc phases might play a key role in the retention of these two phases at ambient pressure and temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction High pressure torsion (HPT) is a mechanical processing technique that consists on applying simultaneously compression and shear stresses to a disk-shaped sample placed between two anvils [1]. For the past decade this technique has been profusely utilized to fabricate bulk nanocrystalline metals, with average grain sizes smaller than 100 nm [1]. Moreover, recent investigations have demonstrated that high pressure torsion can be further used to develop new routes for the fabrication of advanced materials, since it allows stabilizing at room temperature and 1 atm, metallic phases that were so far only stable under extreme conditions of pressure and/or temperature [1–5]. The enormous potential of high pressure torsion to induce phase transformations in metals has still to be exploited. Alloys of the Group IV transition elements such as Zr and Ti have a wide range of key applications in the aerospace, biomedical and nuclear industries [6–8]. Both Zr and Ti are considered model materials for the study of phase transitions due to the wide range of transformations they experience [6]. At room temperature and 1 atm pure Zr crystallizes with an hcp structure (α phase: a = 3.232 Å, c = 5.147 Å). When heated at T > 1135 K the α phase transforms into a bcc (β) structure before melting (beta phase: a =3.568 Å). With increasing hydrostatic pressure the sequence α >ω (simple hexagonal) > β takes place [6] as the d-band occupation increases by s–d electron transfer (omega phase: a = 5.039 Å, c = 3.136 Å). The transition pressures are Pα > ω = 2–6 GPa and Pω > β = 30 GPa. Pure Ti also undergoes the same sequence of phase transformations, albeit at different transition temperatures and pres⁎ Corresponding author. E-mail address: [email protected] (M.T. Pérez-Prado). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.049
sures. The ω and β phases have attractive properties, such as a higher superconducting temperature [9–11] and lower elastic modulus (β-Ti) [12]. However, so far these properties could not be exploited since, when decreasing the temperature or releasing the pressure, the reverse transformations take place [9]. Overall, the properties of the high pressure phases are still widely unknown. It has been recently observed for the first time that bulk pure ω + bcc Zr may be stabilized at 25 °C and 1 atm using HPT [3]. The retention of these two high pressure phases under ambient conditions is currently not understood. The aim of this paper is to investigate the thermal stability of pure bcc Zr fabricated by HPT, in order to explore the temperature range of applicability of this material and to gain further understanding in shear induced phase transformations. 2. Experimental procedure The material studied here is commercially pure (99.98%) alpha Zr with an equiaxed grain size of 13 μm [2,3]. Pure Zr disks of 10 mm in diameter and 2 mm in thickness were processed by high pressure torsion (HPT) using an unconstrained press. A pressure of 6 GPa and 5 turns were used, at an approximate speed of 1 rpm. The thermal stability of the phases present was analyzed using differential scanning calorimetry in a Perkin Elmer DSC 7 under pure argon flow. Heating rates ranging from 20 to 60 K/min were utilized. The activation energy of the transformation is calculated using the Kissinger method, i.e., by analyzing the shift in the transition temperature with increasing heating rates. Additionally, the evolution of the crystalline phases present in the processed samples was tracked by in-situ X-ray diffraction at high temperature using Cu Kα radiation in an Xpert-Pro Panalytical diffractometer. 2θ angles ranging from 25° to 140° were
212
M.T. Pérez-Prado et al. / Materials Letters 64 (2010) 211–214
scanned at steps of 0.017°. A first X-ray diffraction pattern was recorded at 25 °C and then subsequent diffractograms, up to 5 more, were measured at temperatures ranging from 50 °C to 300 °C at 24 min. intervals. The heating time was approximately 5 min. The total annealing time was approximately 2 h and 24 min. The microstructure of selected as-processed samples was analyzed by transmission electron microscopy (TEM) using a 200 kV JEM-2000 FX TEM. Sample preparation for TEM is described in [2]. High resolution TEM was performed in a Philips CM200-FEG TEM. 3. Results and discussion Fig. 1a illustrates the DSC plot corresponding to pure ω + β Zr processed by unconstrained HPT using 6 GPa and 5 turns and cycled thermally between room temperature and 600 °C at several heating rates (20, 40 and 60 K/min) under an argon flow. The presence of only one major exothermic peak suggests that the reverse transformation of both phases, resulting ultimately in the formation of the α phase, takes place at the same or very similar temperatures. The transition temperatures, calculated for the three heating rates utilized are 198.6 °C (20 K/min), 206.3 °C (40 K/min), and 210.4 °C (60 K/min). Due to the inertia associated with constant heating rate cycles, these data do not allow us to determine the exact transition temperature. However, since inertia increases with increasing rates, it can be concluded that the transformation must take place at temperatures near 198.6 °C (~471 K) (which correspond to the lowest rate used). This value is close to that reported for the reverse transformation of metastable ω particles, formed in Zr alloys under hydrostatic pressure and retained after unloading (470 K) [6]. The activation energy of the transformation (Q t) was calculated using the Kissinger method (Fig. 1b), and its value came out to be 181.7 ±18.2 kJ/mol. The parameters for the Kissinger plot are included in Table 1. Qt is close to the activation energy for self diffusion (Q SD) of Zr (Q SD = 216 kJ/mol). The activation energy for grain boundary diffusion in Zr, Q GB, is significantly lower (Q GB = 101 kJ/mol). The change in the crystalline structure of ω + β pure Zr during annealing was also examined by in-situ X-ray diffraction. Fig. 2a illustrates a sequence of diffraction patterns measured during annealing at 150 °C. The starting material is formed by a large volume fraction of transformed phases (ω + β), although a small amount of α phase remains present. The volume fraction of α phase increases already after an annealing time of 48 min at 150 °C, as evidenced by the appearance and heightening of some α peaks. Phase identification is not trivial, as most ω and β peaks overlap. Out of the peaks that are
Table 1 Data from DSC studies corresponding to pure Zr processed by HPT. b, K/min
T, °C
T, K
1/T, K− 1
ln(b/T2)
Error
ΔE, J/g
20 40 60
198.5 206.3 210.4
471.65 479.45 483.55
0.00212 0.00209 0.00207
15.30821 16.03416 16.45665
0.38271 0.40085 0.41142
0.671 1.284 1.451
b represents the heating rate.
visible, only those at 46.2° and 96.8° (highlighted with arrows in Fig. 2a) belong exclusively to the ω and β phases, respectively. The evolution of the intensity of these two peaks during annealing at 150 °C is illustrated in Fig. 2b and c. The intensity of the two peaks decreases clearly with annealing time. The change in the shape of the (222) beta peak between 24′ and 48′ in Fig. 2c is due to the gradual disappearance of this peak and to the overlap with the left tail of the nearby (211) alpha peak at 96°. In order to evaluate further whether the omega and beta phases disappear simultaneously, the change in the height of the two peaks highlighted with arrows (which correspond exclusively to one of the two phases) was monitored during annealing at temperatures ranging from 50 °C to 300 °C (Fig. 3). The heights of both peaks remain constant during annealing at 50 °C and 100 °C and they start decreasing right after the beginning of the 150 °C heat treatment at approximately the same rate. At temperatures higher than 200 °C both peaks disappear immediately after the start of the treatment, their intensity reaching the background level after 20 min of annealing. This data, again, suggest, that the kinetics of the reverse transformation is similar in both phases. The above results prove that the stability of the ω and β phases is intimately related. Both are retained at ambient conditions after processing by HPT and both appear to transform back during annealing at very similar conditions of time and temperature. These observations might shed some light on the origin of the stability of these high pressure phases under ambient conditions. In particular, it appears that the presence of the two phases might be necessary to create the local stresses required to stabilize pure ω and β Zr at 25 °C and 1 atm. Caspersen et al. [13] have predicted the formation of mixed phase structures in pure Fe (alternating lamella of the bcc and hcp phases) as a consequence of the application of shear during hydrostatic loading. Each layer would undergo uniform deformation. Lamellar structures are indeed often seen in Zr martensitic structures [6]. According to Caspersen et al. [13] larger amounts of shear would lead to the
Fig. 1. (a) DSC plot corresponding to pure ω + β Zr processed by unconstrained HPT using 6 GPa and 5 turns and cycled thermally between room temperature and 600 °C at 20 K/min, 40 K/min, and 60 K/min; (b) corresponding Kissinger plot (b represents the heating rate).
M.T. Pérez-Prado et al. / Materials Letters 64 (2010) 211–214
213
Fig. 2. (a) X-ray diffraction patterns corresponding to pure Zr processed by HPT using 6 GPa and 5 turns and annealed at 150 °C during different time intervals. (b) Enlarged view of the evolution with annealing at 150 °C of the (111) ω peak at 46.2°; (c) enlarged view of the evolution with annealing at 150 °C of the (222) β peak at 96.8°.
Fig. 3. Change in the maximum intensity of the (11–21)ω and (222)β peaks with time at different annealing temperatures. The background level is indicated using a wide gray transparent band.
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Fig. 4. (a) TEM micrograph showing the laminate structure present in pure Zr processed by HPT using 6 GPa and 5 turns; (b) high resolution TEM micrograph of the laminates; (c) SAD pattern illustrating showing the rings belonging to the omega and beta phases.
formation of increasingly finer laminates, to lower transition pressures (TP) and to larger hysteresis (i.e., to a retention of the transformed phases at pressures below TP upon unloading). In particular, their model predicts that shear strains (εf) from 0.02 to 0.06 cause variations in TP between 11 and 25 GPa. The present results in HPTed pure Zr seem to constitute an extreme case of this behavior. Fig. 4a shows a TEM micrograph of pure Zr processed by HPT using a pressure of 6 GPa and 5 turns. It can be clearly seen that the nanostructure is formed by laminates with alternating contrast. The width of the lamella varies between 5 and 20 nm. A high resolution TEM image of one lamella is depicted in Fig. 4b and the corresponding SAD pattern are plotted in Fig. 4c. The large shear strain imposed, approximately εf = 5 at the edge of the HPT disk, presumably gives rise to the formation of nanolaminates of alternating ω and β phases, leading to a high hysteresis and therefore a retention of the mixed structure under ambient conditions once the pressure is released. 4. Conclusions In summary, the thermal stability of pure bcc Zr fabricated by high pressure torsion has been investigated. DSC measurements show that the transition temperature is approximately 198 °C and in-situ X-ray diffraction reveals that this phase starts to disappear immediately upon heating at 150 °C. The thermal stability of pure bcc Zr seems thus very similar to that of pure ω Zr. The presence of alternating nanolamella of ω and β phases, originated as a consequence of the application of shear
under pressure, might give rise to the adequate local stress conditions necessary for the retention of both phases under ambient temperature and pressure. Acknowledgments Funding from under projects MAT 2006-11202, CCG07-CSIC/MAT2270, and MAT 2006-11202 is acknowledged. The authors are grateful to Prof. S. Suriñach (UAB, Spain) and to the CAI Difracción de Rayos X (UCM). References [1] Zhilyaev AP, Langdon TG. Prog Mater Sci 2008;53:893. [2] Pérez-Prado MT, Gimazov AA, Ruano OA, Kassner ME, Zhilyaev AP. Scr Mater 2008;58:219. [3] Pérez-Prado MT, Zhilyaev AP. Phys Rev Lett 2009;102:175,504. [4] Todaza Y, Sasaki J, Moto T, Umemoto M. Scr Mater 2008;59:615. [5] Edalati K, Horita Z, Yagi S, Matsubara E. Mater. Sci. Eng. A 2009;523:277. [6] Banerjee S, Mukhopadhyay P. Phase transformations: examples from titanium and zirconium alloys. Pergamon Materials Series, 1st EditionAmsterdam: Elsevier; 2007. [7] Tewari R, Srivastava D, Dey GK, Chakravarti JK, Banerjee S. J Nucl Mater 2008; 383:153. [8] Saldaña L, Méndez-Vilas A, Jiang L, Multigner M, González-Carrasco JL, PérezPrado MT, et al. Biomaterials 2007;28:4343. [9] Sikka SK, Vohra YK, Chidambaram R. Prog Mater Sci 1982;27:245. [10] Bashkin IO, Tissen VG, Nefedova MV, Ponyatovsky EG. Physica C 2006;434:191. [11] Buzea C, Robbie K. Supercond Sci Technol 2005;18:R1. [12] Geetha M, Singh AK, Asokamani R, Gogia AK. Prog Mater Sci 2009;54:397. [13] Caspersen KJ, Lew A, Ortiz M, Carter EA. Phys Rev Lett 2004;93:115,501.