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The recrystallization and thermal oxidation behavior of CP-titanium
Materials Science and Engineering C 26 (2006) 1367 – 1372 www.elsevier.com/locate/msec
The recrystallization and thermal oxidation behavior of CP-titanium F.M. Gu¨c¸lu¨ *, H. C ¸ imenog˘lu, E.S. KayalN Istanbul Technical University, Department of Metallurgy and Materials Engineering, 34469, Maslak, Istanbul, Turkey Available online 27 September 2005
Abstract In this study, the effect of cold working on the recrystallization and thermal oxidation behaviours of Grade 2 quality commercial purity titanium (CP-Ti) were examined. Recrystallization tests were carried out on the 23%, 35% and 50% cold worked samples at various temperatures in between 550 and 700 -C. Thermal oxidation of the cold worked samples was conducted at 600 and 650 -C. Recrystallization activation energy was calculated as 66 – 88 kJ/mol. Since thermal oxidation progresses by oxide layer growth and diffusion of oxygen into the substrate (oxygen diffusion zone formation), oxygen diffusion zone formation activation energies of 149 – 170 kJ/mol were calculated for the examined cold work ratios. These results indicate that when a cold worked CP-Ti was subjected to thermal oxidation treatment to improve its surface hardness and wear resistance, it loses its bulk hardness due to recrystallization. D 2005 Elsevier B.V. All rights reserved. Keywords: Titanium; Recrystallization; Thermal oxidation
1. Introduction Titanium and its alloys are important choices of biomaterials due to their high strength to weight ratio, improved corrosion resistance and excellent biocompatibility resulting in no allergic and/or thrombotic reaction with the surrounding tissue [1,2]. Ti6Al4V and commercial purity titanium (CP-Ti) grades are the most popular implantable biomaterials. Among the four CP-Ti grades, the main difference is oxygen content which improves the strength [3]. Ti6Al4V shows almost double load bearing capacity when compared to Grade 2 quality CP-Ti. Cold working is the main strengthening method for CP-Ti to improve its load bearing capacity [4]. However for many metals, hardening by cold working may not associate with a pronounced increase in friction and wear resistance [5]. Surface modification techniques are employed for titanium and its alloys to enhance the surface properties against friction and wear. The formations of metallic debris, originated from wear of these implants cause tissue blackening and metallosis that ends up with revision of implant in a short time after implantation [6]. Deposition and diffusion methods form a hard exterior surface layer. It is important to notice that in deposition techniques such as PVD, beneath the thin film coating, hardness sharply drops to * Corresponding author. Tel.: +90 212 285 68 95; fax: +90 212 285 34 27. E-mail address: [email protected] (F.M. Gu¨c¸lu¨). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.08.012
the original value of the substrate. Unlike deposition techniques, diffusion hardening methods result in gradual decrease of hardness beneath the exterior surface layer [1,8]. Design of dental implants, osteosynthesis plates, screws and total joint replacements requires chemically anodized surface to form a thin oxide layer [1,2]. The oxide film formed on the surface is quite thin and must be improved by utilizing diffusion hardening techniques. Recently, thermal oxidation appeared to be very promising surface modification method for producing hard surfaces on titanium and titanium alloys [9– 11]. This method provides useful mechanical support of external oxide layer (OL) by an oxygen diffusion zone (ODZ) beneath it and results in significant improvement in wear resistance. Laboratory tests showed more than 25-fold increase in wear resistance upon thermal oxidation after 60 h. Although the optimum temperature of thermal oxidation is reported as 600 and 650 -C [10], it may be associated with the loss of bulk hardness and load bearing capacity of cold worked CP-Ti due to recrystallization. In this paper, a systematic kinetic study has been carried out to evaluate the recrystallization and thermal oxidation behaviours of a cold worked CP-Ti. 2. Experimental In this investigation 4-mm-thick Grade 2 quality CP-Ti plate having hardness of 202 HV0.3 was used. Recrystallization and
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thermal oxidation experiments were conducted at temperatures where CP-Ti had hexagonal closed packed crystal structure, after rolling the plate to the thicknesses of 3.1 (23%), 2.6 (35%) and 2.0 (50%) mm at room temperature. Cold working ratios of 23%, 35% and 50% resulted in hardness values of 233, 263 and 276 HV0.3, respectively. Recrystallization studies were conducted by isothermal annealing of the cold worked samples (10 10 mm2) in salt bath at temperatures in between 550 and 700 -C. After grinding with 320 and 1200 mesh SiC papers, hardness measurements were made under indentation load of 300 g by utilizing a Vickers indenter. Thermal oxidation studies were made by holding the cold worked samples (10 10 mm2) at 600 -C and 650 -C in a laboratory type furnace for 36, 48, 60 and 72 h. The characterization of the oxidized samples were made by oxide thickness measurements on the cross-sections after polishing and etching (1% HF solution) according to Springer and Ahmed method [12] and hardness measurements under indentation load of 10 g in the unit of HV on the crosssections of the samples. Surface hardness measurements were conducted on the oxidized samples under indentation load of 50 g by utilizing a Vickers indenter. 3. Results and discussion 3.1. Recrystallization behavior of CP-Ti The results of recrystallization studies are presented in Fig. 1 in terms of ‘‘%Normalized Hardness—Annealing Time’’ graphs. Normalized hardness values were calculated by
(a)
dividing the hardness of the sample annealed for a certain time to that of the cold worked material. Fig. 1 illustrates that at the early stages of isothermal annealing, normalized hardness was sharply dropped (recrystallization region) and then reached to a steady state value (grain growth region) without showing any recovery region. Higher cold working ratios caused the material to be soften rapidly and leaded shorter time period to reach grain growth region. This behaviour can be explained in terms of the strain energy expended during cold working. Since strain energy is stored in the form of dislocations and other imperfections such as point defects in a cold worked metal, heavy deformation tends to increase the number of nucleation sites in the substructure, which accompanies with the accelaration of recrystallization process [13 – 15]. In the present study, Arrhenius equation [14 –16] 1=t r ¼ A r expð Q r =RT Þ
ð1Þ
was assumed to follow the recrystallization kinetics of the examined CP-Ti. In Eq. (1), t r is the time for 50% recrystallization, A r is a pre-exponential constant, Q r is the activation energy for recrystallization, R is the gas constant [8314 J/mol K] and T is the absolute annealing temperature. In this study, t r values were taken as the time corresponding to average of cold worked and fully annealed hardness values for each cold work ratio from Fig. 1. Fig. 2 illustrates the variation of ‘‘ln(t r) vs. (1/T)’’ graphs plotted according to Eq. (1) and the calculated Q r values are listed in Table 1. Activation energy for diffusion of Ti in hexagonal closed packed Ti was reported as 96 kJ/mol [17], which is close to the Q r values calculated (64 – 88 kJ/mol) in this study.
(b) 100
T = 550°C
Symbol - Cold Work 23 % 35 % 50 %
90 80 70 60
0
5
10
15
20
25
30
% Normalized Hardness
% Normalized Hardness
100
T = 600°C
Symbol - Cold Work 23 % 35 % 50 %
90 80 70 60
0
Annealing Time (minutes)
5
10
15
20
25
30
Annealing Time (minutes)
(d)
% Normalized Hardness
100
T = 650°C
Symbol - Cold Work 23 % 35 % 50 %
90 80 70 60 0
5
10
15
20
25
Annealing Time (minutes)
30
% Normalized Hardness
(c)
100
Symbol - Cold Work 23 % 35 % 50 %
T = 700°C
90 80 70 60
0
5
10
15
20
25
30
Annealing Time (minutes)
Fig. 1. The recrystallization curves of 23%, 35% and 50% cold worked CP-Ti held at (a) 550, (b) 600, (c) 650 and (d) 700 -C.
F.M. Gu¨c¸lu¨ et al. / Materials Science and Engineering C 26 (2006) 1367 – 1372
(b) 6
6
5
5
In t r (s)
In t r (s)
(a)
1369
4
3
4
3
In tr = - 3,358 + 6880,4 x (1/T)
In tr = -3,8487 + 7340,3 x (1/T) 2 0,0008
0,0010
0,0012
0,0014
2 0,0008
0,0010
(c)
0,0012
0,0014
1/T(1/K)
1/T(1/K)
6
In t r (s)
5
4
3
In tr = - 7,8351 + 10625 x (1/T) 2 0,0008
0,0010
0,0012
0,0014
1/T(1/K) Fig. 2. The Arrhenius plots for the CP-Ti cold working ratios of (a) 23%, (b) 35% and (c) 50%.
3.2. Thermal oxidation behavior of CP-titanium Cross-sectional optical micrographs of the oxidized samples are given in Fig. 3. Equiaxed CP-Ti grain structures in Fig. 3 indicate the complete recrystallization of the cold worked structures. The cold worked structure (elongated grains) were disappeared and replaced by equiaxed grains during thermal oxidation. Recrystallization of the substrate was accompanied by a dramatic reduction in hardness that was measured about 170 HV0.3. The high magnification optical micrograph of the oxidized sample shows the formation of two layers at the surface of the samples (Fig. 4). In the literature the external zone is called as ‘‘Oxide Layer’’ (OL), which is dense and integrated with the subsurface ‘‘Oxygen Diffusion Zone’’ (ODZ) [11]. It has been reported that the saturation limit of
Table 1 The calculated recrystallization activation energies for 23%, 35% and 50% deformation ratios Cold work ratio (%)
Recrystallization activation energy (kJ/mol)
23 35 50
64 66 88
34 at.% oxygen in the a-Ti is reached immediately at the OL/ substrate interface and then the dissolution of oxygen in the lattice gradually decreases [18,19]. Oxygen rich section of the ODZ appeared white in color in Fig. 4 which is sensitive to the etching reactant. Furthermore the dissolution of oxygen gradually decreases into the examined CP-Ti without formation of white layer. The variation of the surface hardness with oxidation time is illustrated in Fig. 5. Significant improvement in surface hardness is achieved by thermal oxidation in contrast to the core hardness. However, the different amounts of cold work applied prior to thermal oxidation did not cause any measurable difference on the surface hardness of the examined CP-Ti. Fig. 5 reveals that after thermal oxidation, higher hardness values were maintained for the higher oxidation temperatures and longer oxidation times. After 36 and 72 h oxidation thermal oxidation at 600 -C the average thicknesses of the formed OL were measured in between 0.7 Am and 1.0 Am, respectively. The average thicknesses of the OL formed after thermal oxidation at 650 -C were measured in between 1.26 Am and 2.0 Am after 36 and 72 oxidation hours, respectively. Relatively thin OL formation at 600 -C yielded lower surface hardness values especially at shorter oxidation times due to penetration of the indenter through the softer regions beneath the OL. The penetration depth of the indenter under indentation load of 50 g
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Cold Work (%)
23
35
50
Oxidation Time (hours)
Temperature (°C) 600
650
48 75 µm
75 µm
75 µm
75 µm
75 µm
75 µm
60
72
Fig. 3. The micrographs of thermally oxidized Grade 2 CP-Ti for different cold work ratios and different oxidation times at 600 and 650 -C.
was 2 Am after 36 h thermal oxidation at 600 -C, however it reduces to 1.7 Am after 72 h thermal oxidation at 600 -C. Oxidation at 650 -C promoted the formation of relatively thick OL resulted in surface hardness of about 900 HV0.05. The penetration depth of the indenter under the indentation load of 50 g was about 1.4 Am for the samples thermally oxidized at 650 -C. This result indicates that during surface hardness measurements on the samples oxidized at 650 -C, the penetration of the indenter stayed in the OL. The hardness measurements conducted on the cross-sections of the samples oxidized for 60 and 72 h are given in Fig. 6. After thermal oxidation, hardness values of 800 HV0.01 and 1000 HV0.01 were measured just beneath the oxide layer for 600 -C and 650 -C, respectively. The thickness of the OL formed at 600 -C was less than 1 Am and it was impossible to measure hardness from just beneath the OL. So that the hardness values were obtained from the intersection point of ‘‘Hardness-Oxygen Diffusion Depth’’ curve with ordinate to determine the hardness value just beneath the OL. The hardening beneath the OL arises from diffusion of oxygen into the inner regions. The oxygen diffusion and dissolution is
higher at 650 -C when compared to 600 -C. In Fig. 6, the hardness profiles are determined after 60 and 72 h thermal oxidation times because shorter oxidation times (< 60 h) produced relatively thin oxide layer. The thicknesses of ODZ beneath the OL were measured approximately 6– 7 Am and 12– 14 Am for 600 and 650 -C, respectively, from Fig. 6 where hardness values become at steady state value. Diffusion kinetics of oxygen into the examined CP titanium is analyzed by expressing the hardness profiles presented in Fig. 6 in the form of [19 – 23] h i ð H Hs Þ=ðHo Hs Þ ¼ erf z=2ð Dt Þ1=2 ð2Þ where H s is the hardness of the metal oxide interface, H o is the initial hardness of the material, H is the hardness at the distance z from surface, D is diffusion coefficient and t is time. The D value is calculated according to Eq. (2) where the left side of the equation is determined by hardness measurements. The hardness measurements are plotted and the change of the hardness from metal oxide interface through the lattice is modelled by a three-order polynom. After determination of this
Fig. 4. Cross-section micrograph of the 23% cold worked Grade 2 CP-Ti after thermal oxidation at 650 -C for 36 h.
F.M. Gu¨c¸lu¨ et al. / Materials Science and Engineering C 26 (2006) 1367 – 1372
1200 1000
10 % 35 % 50 % 10 % 35 % 50 %
650°C
D ¼ Do expð Qdif =RT Þ
800
600°C
600 400 200 30
45
60
75
Oxidation Time ( hours )
Fig. 5. The variation of surface hardness with oxidation time for 600 and 650 -C.
Oxidation Time (hours)
Cold work (%)
60
72
Hardness (HV0.01)
1200
23
1200
600°C
1000
650°C
800 600
R2 = 0,9964
400 200
R2 = 0,9978
Hardness (HV0.01)
1000
400 200
600
1200
650°C
1000
R2 = 0,9976
400 200
R2 = 0,9897
0
600°C 650°C
800 R2 = 0,995
600 400 200
5 10 15 20 25 Oxygen Diffusion Depth (µm)
1200
600°C
1000
Hardness (HV0.01)
5 10 15 20 25 Oxygen Diffusion Depth (µm)
R2 = 0,9959
0 0
50
R2 = 0,9977
0
600°C
800
R2 = 0,9976
600
5 10 15 20 25 Oxygen Diffusion Depth (µm)
1200
650°C
800
0
Hardness (HV0.01)
0
600°C
1000
0
35
ð3Þ
where D o is diffusion constant. For the examined CP-Ti after determination of D values for 600 and 650 -C from Eq. (2), oxygen diffusion activation energy values were calculated for oxidation times of 60 and 72 h separately from the ratio of Eq. (3) for temperatures of 600 and 650 -C. The averaged diffusion activation energy values calculated in the present study are listed in Table 2 with respect to cold work ratio. Q dif varied in between 149 and 170 kJ/mol depending on the amount of cold work applied prior to thermal oxidation. This variation of the calculated Q dif values may be
Hardness (HV0.01)
1400
three-order polynom, oxygen diffusion depth values were taken as z, the D value is calculated for each oxidation time and temperature by utilizing error function from Fig. 6. After determination of D, oxygen diffusion activation energy ( Q dif) can be calculated from [19 – 23]
Symbol - Cold Work
650°C
800 R2 = 0,9979
600 400 200
R2 = 0,9929
0 0
5 10 15 20 25 Oxygen Diffusion Depth (µm)
0
5 10 15 20 25 Oxygen Diffusion Depth (µm)
1200
600°C 650°C
1000
Hardness (HV0.01)
Surface hardness ( HV0,05 )
1600
1371
800 600
R2 = 0,995
400 200
R2 = 0,993
0 0
5 10 15 20 25 Oxygen Diffusion Depth (µm)
Fig. 6. The hardness profiles in the ODZ for 23%, 35% and 50% cold worked CP-Ti after thermal oxidation at 600 -C and 650 -C.
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Table 2 The oxygen diffusion activation energies of thermally oxidized CP-Ti at 600 and 650 -C for the examined cold work ratios Cold work (%)
23 35 50
Diffusion activation energy (kJ/mol) at 600 -C
at 650 -C
149 170 161
163 149 149
due to the accuracy of the hardness measurements used in this study. The hardness measurements were conducted under the indentation load of 10 g which was the lowest value of the hardness tester used in this study. The accuracy of this method is related to the precise determination of hardness change in ODZ which depends on the load used in the hardness measurements. Lower loads will allow us to determine hardness change more accurately due to more measurements in a thin layer. This will lead the decrease of the variation of the calculated Q dif values. As illustrated in Fig. 6, the sharp decrease of hardness in the ODZ could not be determined more accurately with the indentation load used in this study. In literature Q dif value was reported as 197 kJ/mol for undeformed a titanium in the temperature range between 650 and 875 -C [23], which is a little higher than the diffusion activation energies calculated in this study. 3.3. Evaluation of activation energies for CP-Ti The oxygen diffusion activation energy calculated from thermal oxidition studies were almost 2 times of recrystallization activation energy for the examined cold worked CP-Ti. These results indicate that, when a cold worked CP-Ti is subjected to a thermal oxidation treatment to improve its surface hardness and wear resistance, its core will be softened while is surface being hardened. Fig. 1 shows that when a cold worked CP-Ti is held at 600 and 650 -C, it will be recrystallized within 5 min. That means that at the end of thermal oxidation at long oxidation times (such as 60 –72 h) the recrystallization and grain growth will already take place in the bulk which causes softening. 4. Conclusion The results of the present study conducted on the various amounts of (23%, 35% and 50%) cold worked CP-Ti can be summarized as follows (1) Recovery region was not visible on the recrystallization curves obtained by isothermal annealing in between 550 and 700 -C after 15 s in a salt bath. Increase of cold working accelerated the softening of the material and reduced the annealing time to reach grain growth
region. Recrystallization activation energies were calculated as 64 – 88 kJ/mol for the applied cold working ratios. (2) Thermal oxidation of cold worked CP-Ti conducted at 600 and 650 -C resulted in significant surface hardening due to formation of oxide layer and oxygen diffusion zone beneath it. Oxygen diffusion activation energies of the different amounts of cold worked CP-Ti in this study were found in between 149 and 170 kJ/mol. Acknowledgement One of the authors (F.M.G.) would like to thank The State Planning Organization of Turkey for the support of his MSc study. References [1] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in medicine, Springer Verlag, Heidelberg, 2001, p. 26, 674, 772. [2] G. Lu¨tjering, J.C. Williams, Titanium, Springer-Verlag, Berlin, 2003, p. 345. [3] M.J. Donachie Jr., Titanium a Technical Guide, ASM International, Metals Park, OH, 1988, p. 28. [4] M.A. Imam, A.C. Fraker, in: S.A. Brown, J.E. Lemons (Eds.), Titanium Alloys as Implant Materials, in Medical Applications of Titanium and its Alloys, ASTM, Philedelphia, 1996, p. 3. [5] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, 1992, p. 210. [6] P.A. Dearnley, Proceedings of the Institution of Mechanical Engineers 213 (1999) (Part H). [8] P.A. Dearnley, Surface and Coatings Technology 198 (2005) 483. [9] H. Dong, T. Bell, Wear 238/2 (2002) 131. [10] H. Gu¨leryu¨z, H. C ¸ imenog˘lu, Biomaterials 25/16 (2004) 3325. [11] P.A. Dearnley, K.L. Dahm, H. C ¸ imenog˘lu, Wear 256-5 (2004) 469. [12] V.G. Voort, Metallographic preparation of titanium and its alloys, Buehler Tech-notes, 3, Issue 3. [13] R.E. Smallman, Modern Physical Metallurgy, Butterwort and Co. Ltd., London, 1980, p. 392. [14] R.E. Reed-Hill, Physical Metallurgy Principles, D.Van Nostrand Comp, New York, 1973, p. 271. [15] R.W.K. Honeycombe, The Plastic Deformation of Metals, Edward Arnold Ltd., London, 1994, p. 299. [16] F.J. Humpreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Elsevier Publications, Oxford, 1995, p. 95. [17] D.A. Askeland, The Science and Engineering of Materials, Chapman and Hall, London, 1990, p. 123. [18] P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, Essex, 1988, p. 289. [19] J. Unnam, R.N. Shenoy, R.K. Clark, Oxidation of Metals 26 (3/4) (1986) 231. [20] H.L. Du, P.K. Datta, D.B. Lewis, J.S. Burnell-Gray, Corrosion Science 36 (1994) 631. [21] S. Frangini, A. Mignone, F. De Riccardis, Corrosion Science 29 (1994) 714. [22] M. Go¨bel, V.A.C. Haannappel, M.F. Stroosnijder, Oxidation of Metals 55 (2001) 137. [23] A.M. Chaze, C. Coddet, Journal of Materials Science 22 (1987) 1206.
The recrystallization and thermal oxidation behavior of CP-titanium F.M. Gu¨c¸lu¨ *, H. C ¸ imenog˘lu, E.S. KayalN Istanbul Technical University, Department of Metallurgy and Materials Engineering, 34469, Maslak, Istanbul, Turkey Available online 27 September 2005
Abstract In this study, the effect of cold working on the recrystallization and thermal oxidation behaviours of Grade 2 quality commercial purity titanium (CP-Ti) were examined. Recrystallization tests were carried out on the 23%, 35% and 50% cold worked samples at various temperatures in between 550 and 700 -C. Thermal oxidation of the cold worked samples was conducted at 600 and 650 -C. Recrystallization activation energy was calculated as 66 – 88 kJ/mol. Since thermal oxidation progresses by oxide layer growth and diffusion of oxygen into the substrate (oxygen diffusion zone formation), oxygen diffusion zone formation activation energies of 149 – 170 kJ/mol were calculated for the examined cold work ratios. These results indicate that when a cold worked CP-Ti was subjected to thermal oxidation treatment to improve its surface hardness and wear resistance, it loses its bulk hardness due to recrystallization. D 2005 Elsevier B.V. All rights reserved. Keywords: Titanium; Recrystallization; Thermal oxidation
1. Introduction Titanium and its alloys are important choices of biomaterials due to their high strength to weight ratio, improved corrosion resistance and excellent biocompatibility resulting in no allergic and/or thrombotic reaction with the surrounding tissue [1,2]. Ti6Al4V and commercial purity titanium (CP-Ti) grades are the most popular implantable biomaterials. Among the four CP-Ti grades, the main difference is oxygen content which improves the strength [3]. Ti6Al4V shows almost double load bearing capacity when compared to Grade 2 quality CP-Ti. Cold working is the main strengthening method for CP-Ti to improve its load bearing capacity [4]. However for many metals, hardening by cold working may not associate with a pronounced increase in friction and wear resistance [5]. Surface modification techniques are employed for titanium and its alloys to enhance the surface properties against friction and wear. The formations of metallic debris, originated from wear of these implants cause tissue blackening and metallosis that ends up with revision of implant in a short time after implantation [6]. Deposition and diffusion methods form a hard exterior surface layer. It is important to notice that in deposition techniques such as PVD, beneath the thin film coating, hardness sharply drops to * Corresponding author. Tel.: +90 212 285 68 95; fax: +90 212 285 34 27. E-mail address: [email protected] (F.M. Gu¨c¸lu¨). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.08.012
the original value of the substrate. Unlike deposition techniques, diffusion hardening methods result in gradual decrease of hardness beneath the exterior surface layer [1,8]. Design of dental implants, osteosynthesis plates, screws and total joint replacements requires chemically anodized surface to form a thin oxide layer [1,2]. The oxide film formed on the surface is quite thin and must be improved by utilizing diffusion hardening techniques. Recently, thermal oxidation appeared to be very promising surface modification method for producing hard surfaces on titanium and titanium alloys [9– 11]. This method provides useful mechanical support of external oxide layer (OL) by an oxygen diffusion zone (ODZ) beneath it and results in significant improvement in wear resistance. Laboratory tests showed more than 25-fold increase in wear resistance upon thermal oxidation after 60 h. Although the optimum temperature of thermal oxidation is reported as 600 and 650 -C [10], it may be associated with the loss of bulk hardness and load bearing capacity of cold worked CP-Ti due to recrystallization. In this paper, a systematic kinetic study has been carried out to evaluate the recrystallization and thermal oxidation behaviours of a cold worked CP-Ti. 2. Experimental In this investigation 4-mm-thick Grade 2 quality CP-Ti plate having hardness of 202 HV0.3 was used. Recrystallization and
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F.M. Gu¨c¸lu¨ et al. / Materials Science and Engineering C 26 (2006) 1367 – 1372
thermal oxidation experiments were conducted at temperatures where CP-Ti had hexagonal closed packed crystal structure, after rolling the plate to the thicknesses of 3.1 (23%), 2.6 (35%) and 2.0 (50%) mm at room temperature. Cold working ratios of 23%, 35% and 50% resulted in hardness values of 233, 263 and 276 HV0.3, respectively. Recrystallization studies were conducted by isothermal annealing of the cold worked samples (10 10 mm2) in salt bath at temperatures in between 550 and 700 -C. After grinding with 320 and 1200 mesh SiC papers, hardness measurements were made under indentation load of 300 g by utilizing a Vickers indenter. Thermal oxidation studies were made by holding the cold worked samples (10 10 mm2) at 600 -C and 650 -C in a laboratory type furnace for 36, 48, 60 and 72 h. The characterization of the oxidized samples were made by oxide thickness measurements on the cross-sections after polishing and etching (1% HF solution) according to Springer and Ahmed method [12] and hardness measurements under indentation load of 10 g in the unit of HV on the crosssections of the samples. Surface hardness measurements were conducted on the oxidized samples under indentation load of 50 g by utilizing a Vickers indenter. 3. Results and discussion 3.1. Recrystallization behavior of CP-Ti The results of recrystallization studies are presented in Fig. 1 in terms of ‘‘%Normalized Hardness—Annealing Time’’ graphs. Normalized hardness values were calculated by
(a)
dividing the hardness of the sample annealed for a certain time to that of the cold worked material. Fig. 1 illustrates that at the early stages of isothermal annealing, normalized hardness was sharply dropped (recrystallization region) and then reached to a steady state value (grain growth region) without showing any recovery region. Higher cold working ratios caused the material to be soften rapidly and leaded shorter time period to reach grain growth region. This behaviour can be explained in terms of the strain energy expended during cold working. Since strain energy is stored in the form of dislocations and other imperfections such as point defects in a cold worked metal, heavy deformation tends to increase the number of nucleation sites in the substructure, which accompanies with the accelaration of recrystallization process [13 – 15]. In the present study, Arrhenius equation [14 –16] 1=t r ¼ A r expð Q r =RT Þ
ð1Þ
was assumed to follow the recrystallization kinetics of the examined CP-Ti. In Eq. (1), t r is the time for 50% recrystallization, A r is a pre-exponential constant, Q r is the activation energy for recrystallization, R is the gas constant [8314 J/mol K] and T is the absolute annealing temperature. In this study, t r values were taken as the time corresponding to average of cold worked and fully annealed hardness values for each cold work ratio from Fig. 1. Fig. 2 illustrates the variation of ‘‘ln(t r) vs. (1/T)’’ graphs plotted according to Eq. (1) and the calculated Q r values are listed in Table 1. Activation energy for diffusion of Ti in hexagonal closed packed Ti was reported as 96 kJ/mol [17], which is close to the Q r values calculated (64 – 88 kJ/mol) in this study.
(b) 100
T = 550°C
Symbol - Cold Work 23 % 35 % 50 %
90 80 70 60
0
5
10
15
20
25
30
% Normalized Hardness
% Normalized Hardness
100
T = 600°C
Symbol - Cold Work 23 % 35 % 50 %
90 80 70 60
0
Annealing Time (minutes)
5
10
15
20
25
30
Annealing Time (minutes)
(d)
% Normalized Hardness
100
T = 650°C
Symbol - Cold Work 23 % 35 % 50 %
90 80 70 60 0
5
10
15
20
25
Annealing Time (minutes)
30
% Normalized Hardness
(c)
100
Symbol - Cold Work 23 % 35 % 50 %
T = 700°C
90 80 70 60
0
5
10
15
20
25
30
Annealing Time (minutes)
Fig. 1. The recrystallization curves of 23%, 35% and 50% cold worked CP-Ti held at (a) 550, (b) 600, (c) 650 and (d) 700 -C.
F.M. Gu¨c¸lu¨ et al. / Materials Science and Engineering C 26 (2006) 1367 – 1372
(b) 6
6
5
5
In t r (s)
In t r (s)
(a)
1369
4
3
4
3
In tr = - 3,358 + 6880,4 x (1/T)
In tr = -3,8487 + 7340,3 x (1/T) 2 0,0008
0,0010
0,0012
0,0014
2 0,0008
0,0010
(c)
0,0012
0,0014
1/T(1/K)
1/T(1/K)
6
In t r (s)
5
4
3
In tr = - 7,8351 + 10625 x (1/T) 2 0,0008
0,0010
0,0012
0,0014
1/T(1/K) Fig. 2. The Arrhenius plots for the CP-Ti cold working ratios of (a) 23%, (b) 35% and (c) 50%.
3.2. Thermal oxidation behavior of CP-titanium Cross-sectional optical micrographs of the oxidized samples are given in Fig. 3. Equiaxed CP-Ti grain structures in Fig. 3 indicate the complete recrystallization of the cold worked structures. The cold worked structure (elongated grains) were disappeared and replaced by equiaxed grains during thermal oxidation. Recrystallization of the substrate was accompanied by a dramatic reduction in hardness that was measured about 170 HV0.3. The high magnification optical micrograph of the oxidized sample shows the formation of two layers at the surface of the samples (Fig. 4). In the literature the external zone is called as ‘‘Oxide Layer’’ (OL), which is dense and integrated with the subsurface ‘‘Oxygen Diffusion Zone’’ (ODZ) [11]. It has been reported that the saturation limit of
Table 1 The calculated recrystallization activation energies for 23%, 35% and 50% deformation ratios Cold work ratio (%)
Recrystallization activation energy (kJ/mol)
23 35 50
64 66 88
34 at.% oxygen in the a-Ti is reached immediately at the OL/ substrate interface and then the dissolution of oxygen in the lattice gradually decreases [18,19]. Oxygen rich section of the ODZ appeared white in color in Fig. 4 which is sensitive to the etching reactant. Furthermore the dissolution of oxygen gradually decreases into the examined CP-Ti without formation of white layer. The variation of the surface hardness with oxidation time is illustrated in Fig. 5. Significant improvement in surface hardness is achieved by thermal oxidation in contrast to the core hardness. However, the different amounts of cold work applied prior to thermal oxidation did not cause any measurable difference on the surface hardness of the examined CP-Ti. Fig. 5 reveals that after thermal oxidation, higher hardness values were maintained for the higher oxidation temperatures and longer oxidation times. After 36 and 72 h oxidation thermal oxidation at 600 -C the average thicknesses of the formed OL were measured in between 0.7 Am and 1.0 Am, respectively. The average thicknesses of the OL formed after thermal oxidation at 650 -C were measured in between 1.26 Am and 2.0 Am after 36 and 72 oxidation hours, respectively. Relatively thin OL formation at 600 -C yielded lower surface hardness values especially at shorter oxidation times due to penetration of the indenter through the softer regions beneath the OL. The penetration depth of the indenter under indentation load of 50 g
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Cold Work (%)
23
35
50
Oxidation Time (hours)
Temperature (°C) 600
650
48 75 µm
75 µm
75 µm
75 µm
75 µm
75 µm
60
72
Fig. 3. The micrographs of thermally oxidized Grade 2 CP-Ti for different cold work ratios and different oxidation times at 600 and 650 -C.
was 2 Am after 36 h thermal oxidation at 600 -C, however it reduces to 1.7 Am after 72 h thermal oxidation at 600 -C. Oxidation at 650 -C promoted the formation of relatively thick OL resulted in surface hardness of about 900 HV0.05. The penetration depth of the indenter under the indentation load of 50 g was about 1.4 Am for the samples thermally oxidized at 650 -C. This result indicates that during surface hardness measurements on the samples oxidized at 650 -C, the penetration of the indenter stayed in the OL. The hardness measurements conducted on the cross-sections of the samples oxidized for 60 and 72 h are given in Fig. 6. After thermal oxidation, hardness values of 800 HV0.01 and 1000 HV0.01 were measured just beneath the oxide layer for 600 -C and 650 -C, respectively. The thickness of the OL formed at 600 -C was less than 1 Am and it was impossible to measure hardness from just beneath the OL. So that the hardness values were obtained from the intersection point of ‘‘Hardness-Oxygen Diffusion Depth’’ curve with ordinate to determine the hardness value just beneath the OL. The hardening beneath the OL arises from diffusion of oxygen into the inner regions. The oxygen diffusion and dissolution is
higher at 650 -C when compared to 600 -C. In Fig. 6, the hardness profiles are determined after 60 and 72 h thermal oxidation times because shorter oxidation times (< 60 h) produced relatively thin oxide layer. The thicknesses of ODZ beneath the OL were measured approximately 6– 7 Am and 12– 14 Am for 600 and 650 -C, respectively, from Fig. 6 where hardness values become at steady state value. Diffusion kinetics of oxygen into the examined CP titanium is analyzed by expressing the hardness profiles presented in Fig. 6 in the form of [19 – 23] h i ð H Hs Þ=ðHo Hs Þ ¼ erf z=2ð Dt Þ1=2 ð2Þ where H s is the hardness of the metal oxide interface, H o is the initial hardness of the material, H is the hardness at the distance z from surface, D is diffusion coefficient and t is time. The D value is calculated according to Eq. (2) where the left side of the equation is determined by hardness measurements. The hardness measurements are plotted and the change of the hardness from metal oxide interface through the lattice is modelled by a three-order polynom. After determination of this
Fig. 4. Cross-section micrograph of the 23% cold worked Grade 2 CP-Ti after thermal oxidation at 650 -C for 36 h.
F.M. Gu¨c¸lu¨ et al. / Materials Science and Engineering C 26 (2006) 1367 – 1372
1200 1000
10 % 35 % 50 % 10 % 35 % 50 %
650°C
D ¼ Do expð Qdif =RT Þ
800
600°C
600 400 200 30
45
60
75
Oxidation Time ( hours )
Fig. 5. The variation of surface hardness with oxidation time for 600 and 650 -C.
Oxidation Time (hours)
Cold work (%)
60
72
Hardness (HV0.01)
1200
23
1200
600°C
1000
650°C
800 600
R2 = 0,9964
400 200
R2 = 0,9978
Hardness (HV0.01)
1000
400 200
600
1200
650°C
1000
R2 = 0,9976
400 200
R2 = 0,9897
0
600°C 650°C
800 R2 = 0,995
600 400 200
5 10 15 20 25 Oxygen Diffusion Depth (µm)
1200
600°C
1000
Hardness (HV0.01)
5 10 15 20 25 Oxygen Diffusion Depth (µm)
R2 = 0,9959
0 0
50
R2 = 0,9977
0
600°C
800
R2 = 0,9976
600
5 10 15 20 25 Oxygen Diffusion Depth (µm)
1200
650°C
800
0
Hardness (HV0.01)
0
600°C
1000
0
35
ð3Þ
where D o is diffusion constant. For the examined CP-Ti after determination of D values for 600 and 650 -C from Eq. (2), oxygen diffusion activation energy values were calculated for oxidation times of 60 and 72 h separately from the ratio of Eq. (3) for temperatures of 600 and 650 -C. The averaged diffusion activation energy values calculated in the present study are listed in Table 2 with respect to cold work ratio. Q dif varied in between 149 and 170 kJ/mol depending on the amount of cold work applied prior to thermal oxidation. This variation of the calculated Q dif values may be
Hardness (HV0.01)
1400
three-order polynom, oxygen diffusion depth values were taken as z, the D value is calculated for each oxidation time and temperature by utilizing error function from Fig. 6. After determination of D, oxygen diffusion activation energy ( Q dif) can be calculated from [19 – 23]
Symbol - Cold Work
650°C
800 R2 = 0,9979
600 400 200
R2 = 0,9929
0 0
5 10 15 20 25 Oxygen Diffusion Depth (µm)
0
5 10 15 20 25 Oxygen Diffusion Depth (µm)
1200
600°C 650°C
1000
Hardness (HV0.01)
Surface hardness ( HV0,05 )
1600
1371
800 600
R2 = 0,995
400 200
R2 = 0,993
0 0
5 10 15 20 25 Oxygen Diffusion Depth (µm)
Fig. 6. The hardness profiles in the ODZ for 23%, 35% and 50% cold worked CP-Ti after thermal oxidation at 600 -C and 650 -C.
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Table 2 The oxygen diffusion activation energies of thermally oxidized CP-Ti at 600 and 650 -C for the examined cold work ratios Cold work (%)
23 35 50
Diffusion activation energy (kJ/mol) at 600 -C
at 650 -C
149 170 161
163 149 149
due to the accuracy of the hardness measurements used in this study. The hardness measurements were conducted under the indentation load of 10 g which was the lowest value of the hardness tester used in this study. The accuracy of this method is related to the precise determination of hardness change in ODZ which depends on the load used in the hardness measurements. Lower loads will allow us to determine hardness change more accurately due to more measurements in a thin layer. This will lead the decrease of the variation of the calculated Q dif values. As illustrated in Fig. 6, the sharp decrease of hardness in the ODZ could not be determined more accurately with the indentation load used in this study. In literature Q dif value was reported as 197 kJ/mol for undeformed a titanium in the temperature range between 650 and 875 -C [23], which is a little higher than the diffusion activation energies calculated in this study. 3.3. Evaluation of activation energies for CP-Ti The oxygen diffusion activation energy calculated from thermal oxidition studies were almost 2 times of recrystallization activation energy for the examined cold worked CP-Ti. These results indicate that, when a cold worked CP-Ti is subjected to a thermal oxidation treatment to improve its surface hardness and wear resistance, its core will be softened while is surface being hardened. Fig. 1 shows that when a cold worked CP-Ti is held at 600 and 650 -C, it will be recrystallized within 5 min. That means that at the end of thermal oxidation at long oxidation times (such as 60 –72 h) the recrystallization and grain growth will already take place in the bulk which causes softening. 4. Conclusion The results of the present study conducted on the various amounts of (23%, 35% and 50%) cold worked CP-Ti can be summarized as follows (1) Recovery region was not visible on the recrystallization curves obtained by isothermal annealing in between 550 and 700 -C after 15 s in a salt bath. Increase of cold working accelerated the softening of the material and reduced the annealing time to reach grain growth
region. Recrystallization activation energies were calculated as 64 – 88 kJ/mol for the applied cold working ratios. (2) Thermal oxidation of cold worked CP-Ti conducted at 600 and 650 -C resulted in significant surface hardening due to formation of oxide layer and oxygen diffusion zone beneath it. Oxygen diffusion activation energies of the different amounts of cold worked CP-Ti in this study were found in between 149 and 170 kJ/mol. Acknowledgement One of the authors (F.M.G.) would like to thank The State Planning Organization of Turkey for the support of his MSc study. References [1] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in medicine, Springer Verlag, Heidelberg, 2001, p. 26, 674, 772. [2] G. Lu¨tjering, J.C. Williams, Titanium, Springer-Verlag, Berlin, 2003, p. 345. [3] M.J. Donachie Jr., Titanium a Technical Guide, ASM International, Metals Park, OH, 1988, p. 28. [4] M.A. Imam, A.C. Fraker, in: S.A. Brown, J.E. Lemons (Eds.), Titanium Alloys as Implant Materials, in Medical Applications of Titanium and its Alloys, ASTM, Philedelphia, 1996, p. 3. [5] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, 1992, p. 210. [6] P.A. Dearnley, Proceedings of the Institution of Mechanical Engineers 213 (1999) (Part H). [8] P.A. Dearnley, Surface and Coatings Technology 198 (2005) 483. [9] H. Dong, T. Bell, Wear 238/2 (2002) 131. [10] H. Gu¨leryu¨z, H. C ¸ imenog˘lu, Biomaterials 25/16 (2004) 3325. [11] P.A. Dearnley, K.L. Dahm, H. C ¸ imenog˘lu, Wear 256-5 (2004) 469. [12] V.G. Voort, Metallographic preparation of titanium and its alloys, Buehler Tech-notes, 3, Issue 3. [13] R.E. Smallman, Modern Physical Metallurgy, Butterwort and Co. Ltd., London, 1980, p. 392. [14] R.E. Reed-Hill, Physical Metallurgy Principles, D.Van Nostrand Comp, New York, 1973, p. 271. [15] R.W.K. Honeycombe, The Plastic Deformation of Metals, Edward Arnold Ltd., London, 1994, p. 299. [16] F.J. Humpreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, Elsevier Publications, Oxford, 1995, p. 95. [17] D.A. Askeland, The Science and Engineering of Materials, Chapman and Hall, London, 1990, p. 123. [18] P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, Essex, 1988, p. 289. [19] J. Unnam, R.N. Shenoy, R.K. Clark, Oxidation of Metals 26 (3/4) (1986) 231. [20] H.L. Du, P.K. Datta, D.B. Lewis, J.S. Burnell-Gray, Corrosion Science 36 (1994) 631. [21] S. Frangini, A. Mignone, F. De Riccardis, Corrosion Science 29 (1994) 714. [22] M. Go¨bel, V.A.C. Haannappel, M.F. Stroosnijder, Oxidation of Metals 55 (2001) 137. [23] A.M. Chaze, C. Coddet, Journal of Materials Science 22 (1987) 1206.