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The role of nitrogen in the oxidation behaviour of titanium and some binary alloys
Journal
of the Less-Common
Metals,
124 (1986) 73 - 84
73
THE ROLE OF NITROGEN IN THE OXIDATION BEHAVIOUR OF TITANIUM AND SOME BINARY ALLOYS A. M. CHAZE and C. CODDET Laboratoire de M~tallu~gie et Ph~sic~~himie des Mat~~au~, Unitt! associke au CNRS 849, ~ni~e~~t~ de ~omp~~gne, B.P. 233, 6~2~6 CompiSgne C&den (France) (Received February 7,1986)
Summary The oxidation behaviour of titanium and of several binary alloys with aluminium (1.65, 3, 5, lO’wt.%),with chromium(1,4,11, 19 wt.%)and with silicon (0.25, 0.5, 1 wt.%), has been studied between 500 and 700 “C in oxygen (100 Torr) and in air (atmospheric pressure); oxidation times up to several thousand hours have allowed the role of nitrogen in the oxidation process to be examined. Kinetics results show, in all cases, better oxidation resistance in air than in oxygen. Morphological examinations and analytical results also show the marked influence of nitrogen on the structure, composition and mechanical properties of the oxide layers. A mechanism is proposed to explain the role of nitrogen in the oxidation process.
1. Introduction There have been few studies published on the oxidation of titanium and its alloys in the presence of nitrogen, i.e. mainly in air [ 1 - 4] ; some work has been reported on the oxidation in air of binary alloys of titanium with aluminium, chromium or silicon [5 - 93, but the authors have not determined the effect of nitrogen on the oxidation kinetics. As far as the oxide layer is concerned, after oxidation in air, several cases have been reported: (1) For alloys of the type Ti-Al, only oxides of titanium have been observed [ 561. (2) For Ti-Cr alloys, in addition to oxides of tit~ium and chromium, some authors have identified nitrides of these metals [7 - 91. (3) For Ti-Si alloys, a recent study [lo] has allowed the oxidation behaviour in air of the eutectic alloy Ti-Si (8.5 wt.% silicon), to be characterized. According to this author a layer of i-utile covers the entire specimen while islands of titanium nitride TiN appear at the oxide-alloy interface. Most of these studies have been performed at relatively high temperatures (T> 700 “C) and for quite short times (t < 100 h). In addition, no mechanism has been proposed to date to explain the role of nitrogen in the oxidation process; we were therefore interested in clarifying this role. With 0022-5088/861$3.50
0 Elsevier Sequoia/Printed
in Tbe Netherlands
74
this objective the oxidation behaviour in air and in oxygen of titanium and several of its alloys with different amounts of aluminium, chromium and silicon has been studied; temperatures between 500 and 700 “C for times of a few hundred to several thousand hours were employed, conditions similar to those encountered by titanium alloys in certain industrial applications [ll-131.
2. Materials and experimental
methods
The starting material in this study is titanium Ugine-Aciers. Its composition is given in Table 1.
type
T35 supplied
by
TABLE 1 Titanium T35 composition Impurity
Wt.%
at.%
02
H2
N2
c
Al
Cr
Si
Fe
Mn
0.073 0.209
0.0009 0.043
0.014 0.047
0.016 0.063
0.007 0.012
0.05 0.046
0.038 0.06
0.052 0.045
0.007 0.006
A range of binary alloys containing aluminium (1.65, 3, 5 and 10 wt.%), chromium (1, 4, 11 and 19 wt.%) and silicon (0.25, 0.5 and 1 wt.%) have been made up from titanium T35. Alloy and sample preparation, oxidation procedure, and the techniques used to examine and analyze oxide layers, have been described in detail elsewhere [ 11 - 131 and will therefore only be described here briefly. In summary, titanium and its alloys were prepared in the form of small parallelepipeds polished under water on silicon carbide paper to grade 1000 (18 pm). The oxidation reactions were followed either by continuous thermogravimetry in pure oxygen (100 Torr) for durations of about 500 h, or by daily weighing of samples oxidized in air (at atmospheric pressure) for durations up to several thousand hours.
3. Kinetic
behaviour
The oxidation behaviour of unalloyed titanium was studied in air and in oxygen following the experimental procedure described in Section 2. The thermogravimetric results obtained under these conditions are presented in Figs. 1 and 2. The general form of the kinetic curves does not appear to be influenced significantly either by the technique used, i.e. isothermal measurements or discontinuous weighing, or by the nature of the oxidizing atmosphere, oxygen or air. It should be noted nevertheless that the weight increases recorded for specimens oxidized in oxygen are in all cases
300
Fig. 1. Weight gains vs. time for the oxidation ings): sample surface area, 6.2 cm2.
of titanium
Fig. 2. Weight gains us. time for the oxidation of titanium in pure oxygen (continuous thermogravimetry): sample surface area, 1.8 cm2.
(Paz,
tlhl
400
in air (discontinuous
weigh-
100
Torr)
larger than those recorded for samples oxidized in air at identical temperatures. It is important to specify, however, that during oxidation in oxygen the thermogravimetric measurements are isothermal, whereas in air the samples undergo regular temperature variations as discontinuous weight measurements are carried out after cooling. The oxidation time for samples oxidized in air is therefore reduced by approximately 2% in comparison with oxidation times in oxygen, but the difference recorded between weight increases for samples in air and in oxygen is considerable; this slight decrease in oxidation time cannot therefore explain the difference in the behaviour of the metal in the two environments. For the case of oxidation in oxygen, the kinetics laws are parabolic only at 550 and 600 “C (Fig. 3); beyond this, the general form of the curve tends to become paralinear. For oxidation in air, the kinetics laws are quasiparabolic over the whole range of temperatures studied (550 - 700 “C) (Fig. 4). This result is in agreement with the results of Morton and Baldwin [2],
5
Fig. 3. Parabolic Fig. 4. Parabolic cm2) (cf. Fig. 1).
10
15
20
plot of the oxidation plot of the oxidation
5
t”‘lwl
kinetics kinetics
of titanium of titanium
10
in oxygen
15
20
t’?th”
(cf. Fig. 2).
in air (sample
surface
area,
6.2
76
who observed that the transition from parabolic to linear kinetics was considerably delayed in air compared with that observed in oxygen, although the curves had the same form. However, the application, for a range of oxidation times, of a parabolic regression treatment to the kinetics results, which allows a better appreciation of the differences with respect to a theoretical parabolic law, reveals that the rate constant associated with this law changes with time: more specifically, the rate constant decreases with time for a temperature of 550 “C (Fig. 5a) and increases at temperatures of 600 - 650 “C and 700 “C (Fig. 5b). The stabilization of this constant is only observed for oxidation times greater than about 500 h (Table 2).
(b) Fig. 5. Evolution of the thermogravimetric curve, with oxidation time, for titanium oxidized in air at: (a) 550 “C (1, 100 h; 2, 375 h; 3, 800 h; 4,1000 h), (b) 700 “C (1, 70 h; 2, 144 h; 3, 240 h; 4, 370 h; 5, 435 h). + is the mathematical origin of the curves after a parabolic regression treatment.
77 TABLE 2 Parabolic rate constants calculated for two oxidation times in air h, (mg2 crnm4 h-l) x lo4
T (“‘3
Oxidation duration (h)
550
40 1000
8.25 5.53
600
40 1000
33.46 36.9
650
40 1000
120.34 169.92
700
40 1000
403.33 578.4
It is therefore not surprising to note a large scatter in published values of the rate constants and activation energies, given the variation observed as a function of oxidation time and the very long time required for stabilization. Our experimental results thus indicate that the activation energies (Fig. 6) associated with the oxidation reaction of titanium in oxygen and in air are equal to 181 kJ mol-’ and 214 kJ mol-’ respectively, for rate constants stabilized as a function of time. However, if the short oxidation time values are examined, it is observed that the activation energies in the two environments are virtually identical, around 181 kJ mol-‘; it is only in air and for long oxidation times that an evolution of the activation energy towards higher values is observed, up to a stabilization after about 500 h. In fact the stabilized value of the activation energy for oxidation in air (214 kJ mol-‘) is seen to be similar to that found by Strafford and Towel [I41 for the nitrid-
Fig. 6. Parabolic rate constants for oxidation of titanium in 9, oxygen and in air (0, 40 h; a, 1000 h) as a function of the reciprocal absolute temperature.
78
ing of titanium (219 kJ mol-l). Even if one considers that traces of water vapour could play a role [ 15, 161, this suggests that nitrogen influences the oxidation mechanism. As far as the binary alloys are concerned, these have been the subject of a detailed study, the results of which have been published elsewhere [ll - 131. The kinetics results are not repeated here, as they show that the differences in behaviour related to the nature of the atmosphere are very similar to those observed for pure titanium: (a) faster kinetics in oxygen than in air; (b) evolution of the parabolic rate constants as a function of time for oxidation in air; (c) activation energies higher in air than in oxygen (Table 3).
TABLE 3 Activation energies (kJ mol-‘) corresponding to the oxidation of unalloyed titanium and titanium binary alloys in oxygen and in air Reaction condition3
Ti35 TiAl 1.65 TiSi 0.25 TiSi 0.5 TiSi 1 aAtmosphere,
Air, 4Ok
Air, more than 500 h
Owzen, 500 h
184
214 276 221 230 242
181 181
188 192 201
191 200
oxidation duration.
4. Morphology and composition of oxide layers The examination of the crystallization features on the outer part of the oxide layers shows a relatively uniform appearance, with no marked differences as a function of the nature of the oxidizing atmosphere. With regard to the internal architecture of the oxide layers, in general, for the temperature range considered, a duplex type layer is observed with an external sublayer of basaltic appearance and a microcrystalline internal sublayer. However, from 700 “C in oxygen a stratification of this layer is seen (Fig. 7). The nature of the identified phases is specified in Table 4. For all cases, the major phase is TiOZ r-utile. This phase, as well as alumina Al,Os, chromium oxide CrZ03 and chromium nitride Cr,N, have been revealed by X-ray diffraction performed only on the surface of oxidized samples [ 11, 121. Titanium nitride TiN was not detected on the surface, but was found in transverse sections. Silica SiOZ in cristobalite form was only revealed by electron ~crodiffraction in ~ansmission, using thin slices prepared by ionic th~n~g V33.
79
(a)
(b) Fig. 7. Cross-sections of oxide layers formed on the TiSi0.5 alloy oxidized at 700 “C in (a) air, (b) oxygen.
Diffusion of oxygen into the metal region just under the oxide, particularly for pure titanium, has been extensively studied and reported in a number of papers, particularly those of Garcia [ 171. We have attempted to verify TABLE 4 Composition
of the oxide layers
T35
Ti-Al(l.65 - 10%) Ti-Cr (1 - 19%) Ti-Si (0.25 - 1%)
Air/oxygen
Air/oxygen
TiOz TiOz TiOz TiOz
Al203 Cr203
SiO,
Air
Q-zN
TiN
80
the effect of the atmosphere on this phase of the oxidation process. From microhardness profiles a marked difference in the penetration of oxygen into the metal, between pure oxygen and ambient air atmospheres, has been noted. From the microhardness profiles shown in Fig. 8, a simple approximate assessment of the amount of dissolved oxygen in the metal has been made. In order to do this, the data established by Garcia [17] and Dubertet [18], relating microhardness to the oxygen content of the Ti-0 solid solution, are available; the data of Dubertet [18] are used here, assuming that the oxygen concentration at the metal-oxide interface is 25 at.%. The results obtained are presented in Table 5.
. . I 50
100
150
200
dl,,ml
Fig. 8. Microhardness of the metallic core, vs. depth, for titanium oxidized in 0, oxygen and in A, air (700 “C, 435 h). TABLE 5 Comparison of amounts of oxygen dissolved in the metallic substrate O(ss) and fixed in the aggregate O(tot) during the oxidation reactions in air and in oxygen (700 “C, 435 h) of titanium ( f 0.05 mg cmP2) Atmosvhere
0( ss) (mg cmP2) O(tot) (mg cmP2) C(ss)/C(tot)
Oxygen
Air
3.00 6.33 0.48
1.99 4.67 0.43
The influence of nitrogen at the inner interface seems therefore to be confirmed, a decrease in the oxygen proportion entering the solid solution being observed in the case of oxidation in air. For the alloys a reduction in the amount of oxygen dissolved in the substrate for oxidation in air compared with that in oxygen is also observed. For alloys containing aluminium and chromium the effects of the alloying element and those of nitrogen appear to be additive in a simple manner, whereas for the alloy containing silicon a marked synergistic effect of the two elements is observed [ 191.
81
5. Adhesion of oxide layers The residual adhesion of the oxide layers to the metal substrate has also been the subject of a detailed study. Measurements were made after cooling, on oxidized samples for identical weight increases at a given temperature. Some of the results presented in Table 6 show that the adhesion of oxide layers is always greater for samples oxidized in air than for those oxidized in oxygen.
TABLE 6 Comparison of the oxide-to-metal in air and in oxygen Oxidation
residual adhesive force (MPa 5 5) for titanium oxidized
conditions
Atmosphere
Am (mg cm-2)
Air
02
600
0.9
.75
56
650
1.7
54
37
650
2.5
43
28
700
0.9
64
49
700
1.4
39
20
Temp.
(“C)
6. Discussion of results Overall, the results obtained reveal a significant effect of nitrogen on the oxidation resistance of titanium and its alloys. It is generally accepted that the oxidation mechanism of titanium at temperatures lower than 750 “C consists of a mixed regime involving principally the diffusion of oxygen into the oxide and into the metal [17,20 - 223. The growth of the oxide at the internal interface is accompanied by large stresses, which may lead to the periodic fracture of the oxide, as a result of the considerable differences between the specific volumes of the metal and the oxide [23, 241; this phenomenon is particularly important at temperatures greater than 700 “C. The oxidation of the metal in air, i.e. principally in the presence of nitrogen and oxygen (if the influence of water vapour is neglected) reveals, in comparison with oxidation in pure oxygen, the following phenomena. (a) The oxidation rate is reduced. This deceleration involves both the dissolution of oxygen into the metal and the formation of the oxide. However, it may be seen that the dissolution of oxygen into the metal phase is more affected since the ratio of oxygen fixed in the layer to oxygen in solid solution is higher in air than in oxygen.
82
(b) The temperature range over which the kinetics are quasiparabolic is increased towards higher temperatures when nitrogen is present. In addition an evolution of the parabolic rate constant as a function of time is noted, the length of time necessary to achieve stabilization being around 500 h. (c) The activation energy associated with the oxidation process, which is of the same order of magnitude for the two environments considered, evolves for the case of oxidation in air to reach a value of about 214 kJ mol-‘, a value close to that found for the nitriding reaction of titanium [ 141. (d) For equal thicknesses, the layers formed in air show better adhesion to the metal substrate than those formed in oxygen. In addition, the stratification phenomenon is considerably reduced. (e) For oxidation of alloys, nitrides have been identified at the metaloxide interface, Cr,N for Ti-Cr alloys and TiN for Ti-Si alloys. If one considers the thermodynamic and kinetic data available the following points may be noted. (1) The diffusion coefficient for nitrogen in titanium is about ten times smaller than that of oxygen [ 25, 261. (2) It seems the diffusion coefficient for nitrogen in rutile is higher than that of oxygen [lo]. (3) The thermodynamic stability of oxides is greater than that of nitrides for titanium. However, the situation is more complex for alloys: examination of the thermodynamic stability diagrams for the systems involved, Ti-AI-O-N, Ti-Cr-O-N and Ti-Si-O-N (Fig. 9), reveals that the stability domain of Cr2N is represented by a relatively small zone of compositions; therefore this zone gives valuable information about the probable values of partial pressures (proportional to the activities) of oxygen and nitrogen at the internal interface. CrzN is liable to form for 10-lo
Fig. 9. Thermodynamic O-N systems.
equilibrium
diagrams
of Ti-Al-O-N,
Ti-Cr-O-N,
and
Ti-Si-
83
thermodynamically stable. The presence of an element liable to modify very slightly this ratio, in lowering the activity of oxygen for example, could thus lead to the formation of titanium nitride TiN. This is indeed what is observed for silicon, for which the role of the nitrogen is, in all likelihood, amplified following the formation of titanium nitride at the internal interface. In the light of these points, nitrogen in the air may be considered to modify the oxidation of titanium and its alloys in the following manner. At the start of the oxidation reaction the superior thermodynamic stability of the oxides relative to the nitrides leads naturally to the formation of an oxide layer little different from those formed in pure oxygen (similar kinetics and apparent activation energies). Gradually, as the oxidation reaction proceeds, the atmospheric nitrogen is incorporated in small amounts in the rutile layer, probably in the anionic vacancies, although this cannot be verified as we were unable to observe the nitrogen in the rutile. The diffusion coefficient of nitrogen in rutile, higher than that of oxygen, leads to a progressive accumulation of this element near the metal-oxide interface, its dissolution in the metal being slower than that of oxygen. This accumulation of nitrogen at the internal interface, clearly revealed in certain alloys by the identification of the nitrides Cr,N and TIN, will have the following results. (1) A reduction in the oxygen concentration at the interface and consequently a decrease in the diffusion rate of oxygen in the Ti-0 solid solution. (2) A reduction in the rate of building up of the TiOz lattice at the internal interface, in other words the concentration in anionic vacancies at this interface, which will also reduce the diffusion rate of oxygen in the r-utile and thus the growth rate of the layer. (3) A modification of the stress accumulation and/or relaxation modes in the layer, perhaps simply due to the reduction in oxidation rate, which is revealed as a more compact layer in which the influence of short-circuit diffusion is reduced (the oxidation law remains parabolic).
7. Conclusions This study has enabled us to extend the range of the investigation of the oxidation behaviour of titanium and its alloys in air to lower temperatures and longer times. Our results show in particular the important influence of test duration on the reliable determination of the parameters governing oxidation kinetics and thus allow us to explain some of the disagreements between published numerical values (rate constants, activation energies). In addition, these results show the marked effect of nitrogen in reducing the oxidation rate of titanium and its alloys, this effect being observed for both the Ti-0 solid solution and the oxide layer, the mechanism involving the region of the metal-oxide interface.
84
Acknowledgments This work was carried out in collaboration with the Cezus (Compagnie Europeenne du Zirconium) research center at Ugine which provided the specimens and was initiated during a joint investigation program assisted by the “Delegation g la recherche scientifique et technique” (Contracts 78.7.1051 and 78.7.1052).
References 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
P. Kofstad, High Temperature Oxidation of Metals, Wiley, New York, 1966. P. H. Morton and W. M. Baldwin, Trans. AZME, 44 (1952) 1004. T. Hurlen, Acta Chem. Stand., 13 (2) (1959) 365. B. Champin. L. Graff, M. Armand, G. Beranger and C. Coddet, J. Less-Common Met., 69 (1980) 163 - 183. M. J. Anissimov, D. M. Vasielev, N. J. Lochakova, A. J. Melieker, V. V. Trofimov and S. J. Tchouriev, Leningradskij Ordena Lenina Politekhnicheskij Znstitut Zmeni M. I. Kalinina, 305 (1970) 29. H. W. Maynor Jr. and R. E. Swift, Corrosion (Houston), 12 (1956) 49. D. J. McPherson and M. G. Fontana, Trans. Am. Sot. Met., 43 (1951) 1098. D. J. McPherson and M. G. Fontana, Met. Prog., 55 (1949) 366. A. M. Chaze, Thesis, Compibgne, 1985. M. Raffy, Thesis, Ecole Nationale Superieure de Chimie de Paris, 1981. A. M. Chaze, C. Coddet and G. Beranger, J. Less-Common Met., 83 (1982) 49 - 70. A. M. Chaze and C. Coddet, Oxid. Met., 21 (3/4) (1984) 205. A. M. Chaze and C. Coddet, Oxid. Met., to be published. K. N. Strafford and J. M. Towel, Oxid. Met., 10 (1) (1976) 41. F. Motte, C. Coddet, P. Sarrazin, M. Azzopardi and J. Besson, Oxid. Met., 10 (1976) 113. P. Sarrazin, F. Motte, J. Besson and C. Coddet, J. Less-Common Met., 59 (1978) 111. E. A. Garcia, Thesis, Orsay, 1974; Met. Corros. Znd., (1978) 638 - 640 and (1979) 641. A. Dubertret, Thesis, Paris, 1970. A. M. Chaze and C. Coddet, J. Mater. Sci., to be published. C. Coddet, Thesis, Universite Scientifique et Medicale et Institut National Polytechnique de Grenoble, 1977. J. E. Lopes Gomez and A. M. Huntz, Oxid. Met., 14 (3) (1980) 249. D. David, P. Cremery, C. Coddet and G. Beranger, J. Less-Common Met., 69 (1980) 81 - 92. G. Beranger and C. Coddet, J. Microsc. Spectrosc. Electron., 5 (1980) 793. C. Coddet, J. F. Chretien and G. Beranger, in J. C. Williams and A. F. Belov (eds.), Titanium and Titanium Alloys: Scientific and Technological Aspects, Vol. 2, Plenum, New York, 1982, p. 1097. D. David, G. Beranger and E. A. Garcia, J. Electrochem. Sot., 130 (6) (1983) 1423. A. Anttila, J. Raisanen and J. Keinonen, Appl. Phys. Lett., 42 (6) (1983) 498.
of the Less-Common
Metals,
124 (1986) 73 - 84
73
THE ROLE OF NITROGEN IN THE OXIDATION BEHAVIOUR OF TITANIUM AND SOME BINARY ALLOYS A. M. CHAZE and C. CODDET Laboratoire de M~tallu~gie et Ph~sic~~himie des Mat~~au~, Unitt! associke au CNRS 849, ~ni~e~~t~ de ~omp~~gne, B.P. 233, 6~2~6 CompiSgne C&den (France) (Received February 7,1986)
Summary The oxidation behaviour of titanium and of several binary alloys with aluminium (1.65, 3, 5, lO’wt.%),with chromium(1,4,11, 19 wt.%)and with silicon (0.25, 0.5, 1 wt.%), has been studied between 500 and 700 “C in oxygen (100 Torr) and in air (atmospheric pressure); oxidation times up to several thousand hours have allowed the role of nitrogen in the oxidation process to be examined. Kinetics results show, in all cases, better oxidation resistance in air than in oxygen. Morphological examinations and analytical results also show the marked influence of nitrogen on the structure, composition and mechanical properties of the oxide layers. A mechanism is proposed to explain the role of nitrogen in the oxidation process.
1. Introduction There have been few studies published on the oxidation of titanium and its alloys in the presence of nitrogen, i.e. mainly in air [ 1 - 4] ; some work has been reported on the oxidation in air of binary alloys of titanium with aluminium, chromium or silicon [5 - 93, but the authors have not determined the effect of nitrogen on the oxidation kinetics. As far as the oxide layer is concerned, after oxidation in air, several cases have been reported: (1) For alloys of the type Ti-Al, only oxides of titanium have been observed [ 561. (2) For Ti-Cr alloys, in addition to oxides of tit~ium and chromium, some authors have identified nitrides of these metals [7 - 91. (3) For Ti-Si alloys, a recent study [lo] has allowed the oxidation behaviour in air of the eutectic alloy Ti-Si (8.5 wt.% silicon), to be characterized. According to this author a layer of i-utile covers the entire specimen while islands of titanium nitride TiN appear at the oxide-alloy interface. Most of these studies have been performed at relatively high temperatures (T> 700 “C) and for quite short times (t < 100 h). In addition, no mechanism has been proposed to date to explain the role of nitrogen in the oxidation process; we were therefore interested in clarifying this role. With 0022-5088/861$3.50
0 Elsevier Sequoia/Printed
in Tbe Netherlands
74
this objective the oxidation behaviour in air and in oxygen of titanium and several of its alloys with different amounts of aluminium, chromium and silicon has been studied; temperatures between 500 and 700 “C for times of a few hundred to several thousand hours were employed, conditions similar to those encountered by titanium alloys in certain industrial applications [ll-131.
2. Materials and experimental
methods
The starting material in this study is titanium Ugine-Aciers. Its composition is given in Table 1.
type
T35 supplied
by
TABLE 1 Titanium T35 composition Impurity
Wt.%
at.%
02
H2
N2
c
Al
Cr
Si
Fe
Mn
0.073 0.209
0.0009 0.043
0.014 0.047
0.016 0.063
0.007 0.012
0.05 0.046
0.038 0.06
0.052 0.045
0.007 0.006
A range of binary alloys containing aluminium (1.65, 3, 5 and 10 wt.%), chromium (1, 4, 11 and 19 wt.%) and silicon (0.25, 0.5 and 1 wt.%) have been made up from titanium T35. Alloy and sample preparation, oxidation procedure, and the techniques used to examine and analyze oxide layers, have been described in detail elsewhere [ 11 - 131 and will therefore only be described here briefly. In summary, titanium and its alloys were prepared in the form of small parallelepipeds polished under water on silicon carbide paper to grade 1000 (18 pm). The oxidation reactions were followed either by continuous thermogravimetry in pure oxygen (100 Torr) for durations of about 500 h, or by daily weighing of samples oxidized in air (at atmospheric pressure) for durations up to several thousand hours.
3. Kinetic
behaviour
The oxidation behaviour of unalloyed titanium was studied in air and in oxygen following the experimental procedure described in Section 2. The thermogravimetric results obtained under these conditions are presented in Figs. 1 and 2. The general form of the kinetic curves does not appear to be influenced significantly either by the technique used, i.e. isothermal measurements or discontinuous weighing, or by the nature of the oxidizing atmosphere, oxygen or air. It should be noted nevertheless that the weight increases recorded for specimens oxidized in oxygen are in all cases
300
Fig. 1. Weight gains vs. time for the oxidation ings): sample surface area, 6.2 cm2.
of titanium
Fig. 2. Weight gains us. time for the oxidation of titanium in pure oxygen (continuous thermogravimetry): sample surface area, 1.8 cm2.
(Paz,
tlhl
400
in air (discontinuous
weigh-
100
Torr)
larger than those recorded for samples oxidized in air at identical temperatures. It is important to specify, however, that during oxidation in oxygen the thermogravimetric measurements are isothermal, whereas in air the samples undergo regular temperature variations as discontinuous weight measurements are carried out after cooling. The oxidation time for samples oxidized in air is therefore reduced by approximately 2% in comparison with oxidation times in oxygen, but the difference recorded between weight increases for samples in air and in oxygen is considerable; this slight decrease in oxidation time cannot therefore explain the difference in the behaviour of the metal in the two environments. For the case of oxidation in oxygen, the kinetics laws are parabolic only at 550 and 600 “C (Fig. 3); beyond this, the general form of the curve tends to become paralinear. For oxidation in air, the kinetics laws are quasiparabolic over the whole range of temperatures studied (550 - 700 “C) (Fig. 4). This result is in agreement with the results of Morton and Baldwin [2],
5
Fig. 3. Parabolic Fig. 4. Parabolic cm2) (cf. Fig. 1).
10
15
20
plot of the oxidation plot of the oxidation
5
t”‘lwl
kinetics kinetics
of titanium of titanium
10
in oxygen
15
20
t’?th”
(cf. Fig. 2).
in air (sample
surface
area,
6.2
76
who observed that the transition from parabolic to linear kinetics was considerably delayed in air compared with that observed in oxygen, although the curves had the same form. However, the application, for a range of oxidation times, of a parabolic regression treatment to the kinetics results, which allows a better appreciation of the differences with respect to a theoretical parabolic law, reveals that the rate constant associated with this law changes with time: more specifically, the rate constant decreases with time for a temperature of 550 “C (Fig. 5a) and increases at temperatures of 600 - 650 “C and 700 “C (Fig. 5b). The stabilization of this constant is only observed for oxidation times greater than about 500 h (Table 2).
(b) Fig. 5. Evolution of the thermogravimetric curve, with oxidation time, for titanium oxidized in air at: (a) 550 “C (1, 100 h; 2, 375 h; 3, 800 h; 4,1000 h), (b) 700 “C (1, 70 h; 2, 144 h; 3, 240 h; 4, 370 h; 5, 435 h). + is the mathematical origin of the curves after a parabolic regression treatment.
77 TABLE 2 Parabolic rate constants calculated for two oxidation times in air h, (mg2 crnm4 h-l) x lo4
T (“‘3
Oxidation duration (h)
550
40 1000
8.25 5.53
600
40 1000
33.46 36.9
650
40 1000
120.34 169.92
700
40 1000
403.33 578.4
It is therefore not surprising to note a large scatter in published values of the rate constants and activation energies, given the variation observed as a function of oxidation time and the very long time required for stabilization. Our experimental results thus indicate that the activation energies (Fig. 6) associated with the oxidation reaction of titanium in oxygen and in air are equal to 181 kJ mol-’ and 214 kJ mol-’ respectively, for rate constants stabilized as a function of time. However, if the short oxidation time values are examined, it is observed that the activation energies in the two environments are virtually identical, around 181 kJ mol-‘; it is only in air and for long oxidation times that an evolution of the activation energy towards higher values is observed, up to a stabilization after about 500 h. In fact the stabilized value of the activation energy for oxidation in air (214 kJ mol-‘) is seen to be similar to that found by Strafford and Towel [I41 for the nitrid-
Fig. 6. Parabolic rate constants for oxidation of titanium in 9, oxygen and in air (0, 40 h; a, 1000 h) as a function of the reciprocal absolute temperature.
78
ing of titanium (219 kJ mol-l). Even if one considers that traces of water vapour could play a role [ 15, 161, this suggests that nitrogen influences the oxidation mechanism. As far as the binary alloys are concerned, these have been the subject of a detailed study, the results of which have been published elsewhere [ll - 131. The kinetics results are not repeated here, as they show that the differences in behaviour related to the nature of the atmosphere are very similar to those observed for pure titanium: (a) faster kinetics in oxygen than in air; (b) evolution of the parabolic rate constants as a function of time for oxidation in air; (c) activation energies higher in air than in oxygen (Table 3).
TABLE 3 Activation energies (kJ mol-‘) corresponding to the oxidation of unalloyed titanium and titanium binary alloys in oxygen and in air Reaction condition3
Ti35 TiAl 1.65 TiSi 0.25 TiSi 0.5 TiSi 1 aAtmosphere,
Air, 4Ok
Air, more than 500 h
Owzen, 500 h
184
214 276 221 230 242
181 181
188 192 201
191 200
oxidation duration.
4. Morphology and composition of oxide layers The examination of the crystallization features on the outer part of the oxide layers shows a relatively uniform appearance, with no marked differences as a function of the nature of the oxidizing atmosphere. With regard to the internal architecture of the oxide layers, in general, for the temperature range considered, a duplex type layer is observed with an external sublayer of basaltic appearance and a microcrystalline internal sublayer. However, from 700 “C in oxygen a stratification of this layer is seen (Fig. 7). The nature of the identified phases is specified in Table 4. For all cases, the major phase is TiOZ r-utile. This phase, as well as alumina Al,Os, chromium oxide CrZ03 and chromium nitride Cr,N, have been revealed by X-ray diffraction performed only on the surface of oxidized samples [ 11, 121. Titanium nitride TiN was not detected on the surface, but was found in transverse sections. Silica SiOZ in cristobalite form was only revealed by electron ~crodiffraction in ~ansmission, using thin slices prepared by ionic th~n~g V33.
79
(a)
(b) Fig. 7. Cross-sections of oxide layers formed on the TiSi0.5 alloy oxidized at 700 “C in (a) air, (b) oxygen.
Diffusion of oxygen into the metal region just under the oxide, particularly for pure titanium, has been extensively studied and reported in a number of papers, particularly those of Garcia [ 171. We have attempted to verify TABLE 4 Composition
of the oxide layers
T35
Ti-Al(l.65 - 10%) Ti-Cr (1 - 19%) Ti-Si (0.25 - 1%)
Air/oxygen
Air/oxygen
TiOz TiOz TiOz TiOz
Al203 Cr203
SiO,
Air
Q-zN
TiN
80
the effect of the atmosphere on this phase of the oxidation process. From microhardness profiles a marked difference in the penetration of oxygen into the metal, between pure oxygen and ambient air atmospheres, has been noted. From the microhardness profiles shown in Fig. 8, a simple approximate assessment of the amount of dissolved oxygen in the metal has been made. In order to do this, the data established by Garcia [17] and Dubertet [18], relating microhardness to the oxygen content of the Ti-0 solid solution, are available; the data of Dubertet [18] are used here, assuming that the oxygen concentration at the metal-oxide interface is 25 at.%. The results obtained are presented in Table 5.
. . I 50
100
150
200
dl,,ml
Fig. 8. Microhardness of the metallic core, vs. depth, for titanium oxidized in 0, oxygen and in A, air (700 “C, 435 h). TABLE 5 Comparison of amounts of oxygen dissolved in the metallic substrate O(ss) and fixed in the aggregate O(tot) during the oxidation reactions in air and in oxygen (700 “C, 435 h) of titanium ( f 0.05 mg cmP2) Atmosvhere
0( ss) (mg cmP2) O(tot) (mg cmP2) C(ss)/C(tot)
Oxygen
Air
3.00 6.33 0.48
1.99 4.67 0.43
The influence of nitrogen at the inner interface seems therefore to be confirmed, a decrease in the oxygen proportion entering the solid solution being observed in the case of oxidation in air. For the alloys a reduction in the amount of oxygen dissolved in the substrate for oxidation in air compared with that in oxygen is also observed. For alloys containing aluminium and chromium the effects of the alloying element and those of nitrogen appear to be additive in a simple manner, whereas for the alloy containing silicon a marked synergistic effect of the two elements is observed [ 191.
81
5. Adhesion of oxide layers The residual adhesion of the oxide layers to the metal substrate has also been the subject of a detailed study. Measurements were made after cooling, on oxidized samples for identical weight increases at a given temperature. Some of the results presented in Table 6 show that the adhesion of oxide layers is always greater for samples oxidized in air than for those oxidized in oxygen.
TABLE 6 Comparison of the oxide-to-metal in air and in oxygen Oxidation
residual adhesive force (MPa 5 5) for titanium oxidized
conditions
Atmosphere
Am (mg cm-2)
Air
02
600
0.9
.75
56
650
1.7
54
37
650
2.5
43
28
700
0.9
64
49
700
1.4
39
20
Temp.
(“C)
6. Discussion of results Overall, the results obtained reveal a significant effect of nitrogen on the oxidation resistance of titanium and its alloys. It is generally accepted that the oxidation mechanism of titanium at temperatures lower than 750 “C consists of a mixed regime involving principally the diffusion of oxygen into the oxide and into the metal [17,20 - 223. The growth of the oxide at the internal interface is accompanied by large stresses, which may lead to the periodic fracture of the oxide, as a result of the considerable differences between the specific volumes of the metal and the oxide [23, 241; this phenomenon is particularly important at temperatures greater than 700 “C. The oxidation of the metal in air, i.e. principally in the presence of nitrogen and oxygen (if the influence of water vapour is neglected) reveals, in comparison with oxidation in pure oxygen, the following phenomena. (a) The oxidation rate is reduced. This deceleration involves both the dissolution of oxygen into the metal and the formation of the oxide. However, it may be seen that the dissolution of oxygen into the metal phase is more affected since the ratio of oxygen fixed in the layer to oxygen in solid solution is higher in air than in oxygen.
82
(b) The temperature range over which the kinetics are quasiparabolic is increased towards higher temperatures when nitrogen is present. In addition an evolution of the parabolic rate constant as a function of time is noted, the length of time necessary to achieve stabilization being around 500 h. (c) The activation energy associated with the oxidation process, which is of the same order of magnitude for the two environments considered, evolves for the case of oxidation in air to reach a value of about 214 kJ mol-‘, a value close to that found for the nitriding reaction of titanium [ 141. (d) For equal thicknesses, the layers formed in air show better adhesion to the metal substrate than those formed in oxygen. In addition, the stratification phenomenon is considerably reduced. (e) For oxidation of alloys, nitrides have been identified at the metaloxide interface, Cr,N for Ti-Cr alloys and TiN for Ti-Si alloys. If one considers the thermodynamic and kinetic data available the following points may be noted. (1) The diffusion coefficient for nitrogen in titanium is about ten times smaller than that of oxygen [ 25, 261. (2) It seems the diffusion coefficient for nitrogen in rutile is higher than that of oxygen [lo]. (3) The thermodynamic stability of oxides is greater than that of nitrides for titanium. However, the situation is more complex for alloys: examination of the thermodynamic stability diagrams for the systems involved, Ti-AI-O-N, Ti-Cr-O-N and Ti-Si-O-N (Fig. 9), reveals that the stability domain of Cr2N is represented by a relatively small zone of compositions; therefore this zone gives valuable information about the probable values of partial pressures (proportional to the activities) of oxygen and nitrogen at the internal interface. CrzN is liable to form for 10-lo
Fig. 9. Thermodynamic O-N systems.
equilibrium
diagrams
of Ti-Al-O-N,
Ti-Cr-O-N,
and
Ti-Si-
83
thermodynamically stable. The presence of an element liable to modify very slightly this ratio, in lowering the activity of oxygen for example, could thus lead to the formation of titanium nitride TiN. This is indeed what is observed for silicon, for which the role of the nitrogen is, in all likelihood, amplified following the formation of titanium nitride at the internal interface. In the light of these points, nitrogen in the air may be considered to modify the oxidation of titanium and its alloys in the following manner. At the start of the oxidation reaction the superior thermodynamic stability of the oxides relative to the nitrides leads naturally to the formation of an oxide layer little different from those formed in pure oxygen (similar kinetics and apparent activation energies). Gradually, as the oxidation reaction proceeds, the atmospheric nitrogen is incorporated in small amounts in the rutile layer, probably in the anionic vacancies, although this cannot be verified as we were unable to observe the nitrogen in the rutile. The diffusion coefficient of nitrogen in rutile, higher than that of oxygen, leads to a progressive accumulation of this element near the metal-oxide interface, its dissolution in the metal being slower than that of oxygen. This accumulation of nitrogen at the internal interface, clearly revealed in certain alloys by the identification of the nitrides Cr,N and TIN, will have the following results. (1) A reduction in the oxygen concentration at the interface and consequently a decrease in the diffusion rate of oxygen in the Ti-0 solid solution. (2) A reduction in the rate of building up of the TiOz lattice at the internal interface, in other words the concentration in anionic vacancies at this interface, which will also reduce the diffusion rate of oxygen in the r-utile and thus the growth rate of the layer. (3) A modification of the stress accumulation and/or relaxation modes in the layer, perhaps simply due to the reduction in oxidation rate, which is revealed as a more compact layer in which the influence of short-circuit diffusion is reduced (the oxidation law remains parabolic).
7. Conclusions This study has enabled us to extend the range of the investigation of the oxidation behaviour of titanium and its alloys in air to lower temperatures and longer times. Our results show in particular the important influence of test duration on the reliable determination of the parameters governing oxidation kinetics and thus allow us to explain some of the disagreements between published numerical values (rate constants, activation energies). In addition, these results show the marked effect of nitrogen in reducing the oxidation rate of titanium and its alloys, this effect being observed for both the Ti-0 solid solution and the oxide layer, the mechanism involving the region of the metal-oxide interface.
84
Acknowledgments This work was carried out in collaboration with the Cezus (Compagnie Europeenne du Zirconium) research center at Ugine which provided the specimens and was initiated during a joint investigation program assisted by the “Delegation g la recherche scientifique et technique” (Contracts 78.7.1051 and 78.7.1052).
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