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BNb2O5 formation and relevant phenomena observed during oxidation of a Nb-2.5 at.% Mo alloy
Journal of the Less-Common Metals, 37 (1974) 257-268 ((“IElsevier Sequoia S.A., Lausanne - Printed in The Netherlands
B-Nb20s DURING
MICHIO
257
FORMATION AND RELEVANT PHENOMENA OXIDATION OF A Nb2.5 AT.% MO ALLOY
YAMAWAKI.
MASAYOSHI
KANNO
and TAKASHI
OBSERVED
MUKAIBO
Department of Nuclear Engineering, The University of Tokyo, Tokyo (Japan) (Received
November
10, 1973; in revised form February
12, 1974)
SUMMARY
Nb2.5 at.% MO alloy shows an anomalous parabolic-to-plateau-like type weight-increase behavior during oxidation at around 835°C. For a further study of this behavior, X-ray and thermogravimetric methods have been used. The X-ray study showed that the allotropic transformation of Nb205 from the T-form to the B-form and, further, to the M-form occurs in the oxide scales during oxidation of the alloy. It has been shown that during the plateau-like stage of the reaction B-Nb205 is the dominant constituent of the outer part of the scale, indicating that the B-Nb205 crystals constitute a barrier for the diffusion of oxygen through the scale. The results of the measurement of the structural changes along the depth direction of the scales formed on the alloy suggested that the B-to-M transformation is probably accelerated by the stress. The formation of a large amount of B-Nb205 in the scale was accompanied by a change in the apparent reaction scheme from the NbO,-forming type to the NbO/NbOz-forming type.
INTRODUCTION
A previous study of the oxidation of Nb-MO alloys’ indicated an anomalous weight-increase behavior of Nb2.5 at.% MO alloy during oxidation at 835°C. The weight gained by the alloy initially increased parabolically, then halted, resulting in a weight-gain curve which may be described as a parabolic-to-plateau-like curve. X-ray diffraction study of the oxidized specimens showed that a considerable amount of B-Nb205 was formed on the alloy by oxidation at 835°C. Hence, the anomalous weight increasing behavior was tentatively attributed to the B-Nb205 crystals in the scale. The oxidation of NbMo alloys has been studied in some detail, since small amounts of molybdenum produce a significant reduction in the oxidation rateze6. Argent and Phelps’ studied the oxidation of several NbMo alloys including Nb-1.9 at.% MO and Nb-3.4 at.% MO alloys in the temperature range 400-9OOC, whilst Kolski’ studied the oxidation of Nb-10 wt.% MO alloy in the temperature range 400-1200°C. Taylor and Stringer’ studied the oxidation of Nb5.5 at.% MO single crystals and polycrystals, especially in relation to the morphological aspects,
258
M. YAMAWAKI,
M. KANNO, T. MUKAIBO
in the temperature range 530-1100°C. However, the anomalous kinetic behavior mentioned above, which is shown by Nb-2.5 at.% MO alloy at around 835°C and the formation of B-Nbz05 in the oxide scale were only recently reported by the present authors’. This study has been carried out to clarify in more detail the oxidation behavior of Nb-2.5 at.% MO alloy at around 835°C. Emphasis has been laid on the relationship between the allotropic transformation of NbzOs which occurs in the scale and the anomalous weight change of the specimen. EXPERIMENTAL
PROCEDURE
The material used for this investigation was the same Nb2.5 at.% MO alloy (nominal composition) as that used in the former study’. Analysis of this alloy by means of an electron-probe microanalyser gave a molybdenum content of 2.6 at.%. The electron-beam-melted button was cold-rolled to 0.2 mm in thickness. Rectangular coupons of 8 mm x 27 mm were cut from the sheet and heat-treated in vacuum at 1350°C. As a result, the specimens became polycrystalline, with an average grain diameter of about 0.1 mm. Subsequently, the specimens were electropolished in a solution of HzS04 + 10% HF, then thoroughly washed and degreased. Oxidation was carried out in a silica-glass system. The prepared sample was placed in a silica bucket, which was then suspended in a silica tube by a platinum wire connected to a quartz spring. The specimen was first held in a cool portion of the silica tube. The system was then evacuated and filled to the desired pressure of 0.20 atm with oxygen dried by passing over magnesium perchlorate. The specimen was then quickly lowered into the hot zone, at the operating temperature, by means of the glass cock and brass chain system. The weight gains were obtained by reading the extension of the calibrated silica spring with a cathetometer. After oxidation, X-ray patterns from the surfaces of the oxidized specimens were obtained for identification, using copper Kol radiation. Some specimens were mounted on a flat surface in polyester resin and ground back 10-20 pm on 240 mesh emery papers, the X-ray pattern then being taken from the ground surface. The surface grinding and the X-ray test were repeated until the substrate peaks became dominant in the X-ray pattern obtained. In this way, structural changes along the depth direction of the oxide scales formed at 835°C were determined. EXPERIMENTAL
Kinetic
RESULTS
study
The weight gained by a rectangular specimen of Nb-2.5 at.% MO alloy during oxidation at 835°C was measured over an extended period of time and the weight-gain curve obtained is shown in Fig. 1. As shown in the figure, the curve is separable into three stages: (1) an initial parabolic stage, (2) an intermediate plateau-like stage, and (3) a final sigmoidal weight-increasing stage. Metallographic examination carried out after the end of the run, revealed that there was no metal core left in the specimen, indicating that the whole alloy specimen had been consumed during the run.
OXIDATION OF A Nb-2.5 At.% MO ALLOY
0
20
10
259
40
30
Time fh) Fig. 1. Oxidation behavior of Nb2.5 at.% MO alloy at 835°C.
1
2
Time (h) Fig. 2. Oxidation behavior of Nb-2.5 at.% MO alloy between 735 and 985°C.
The weight-gain curves obtained for temperatures ranging from 735 to 985°C are shown in Fig. 2. At temperatures of 795, 810 and 835”C, a transition of the rate from parabolic to a level plateau occurred. At 863”C, even though the parabolic-to-plateau-like rate transition occurred, the plateau-like stage appeared with a definite positive slope.
260
M. YAMAWAKI,
M. KANNO,
T. MUKAIBO
All weight-gain data were re-plotted against tt. This parabolic plotting revealed that a transition from an initial parabolic to a second parabolic rate occurred at 885 and 910°C where the first parabolic rate constant, i.e., the parabolic rate constant calculated from the first parabolic part, was larger than the second. At 935°C or above, a breakaway occurred following upon the initial parabolic oxidation stage. At 735 or 785°C the weight gained by the specimen increased parabolically and no distinct rate transition was observed to occur within the reaction period. Parabolic rate constants were calculated from the parabolic parts of each weight-gain curve and are plotted against the reciprocal of the absolute temperature in Fig. 3. At 885 and 910°C both the first and the second parabolic rate constants are also plotted. The first parabolic rate constant increases with temperature up to about 850°C then decreases up to 935°C beyond which, up to 985°C it again increases. The points of the second parabolic rate constants for 885 and 910°C appear to be located on a smooth curve with the points of the first parabolic rate constants for temperatures above 935°C.
8
9
104/T(“K) Fig. 3. Arrhenius plot for parabolic oxidation 0, second parabolic rate.
10
rate of Nb2.5
at.% MO alloy.
0,
first parabolic
rate,
Curve I: K,= 1.72 x 10zl exp (- 105,OOO/RT) Curve II: K,=2.73 x lo3 exp (-21,40O/RT).
X-ray study
Typical X-ray patterns taken from surfaces of the specimens which were oxidized at 835°C for various lengths of time are shown in Fig. 4. Detailed results of the identification of the products formed at 835°C are given in Table I. As the X-ray patterns shown in Fig. 4 and Table I were those taken from the surfaces of the scales,
OXIDATION
OF A Nb2.5
261
At.% MO ALLOY
(a)
Fig. 4. X-ray patterns of Nb2.5 at.% MO alloy oxide scales various lengths of time. (a) 20 min, (b) 2 h, (c) 4 h 10 min.
formed
by oxidation
at 835°C
for
M. YAMAWAKI,
262
M. KANNO, T. MUKAIBO
TABLE I X-RAY PATTERNS OF LENGTHS OF TIME Time (min)
Phases found
10 20 30 40 60 120 250 870 1080 2370
T%M T$M>B T>B>M T>B>M TlB>M B>T$M B>M BrM B>M B%M
Nb-2.5
AT.%
MO ALLOY
OXIDIZED
AT
835°C
FOR
VARIOUS
they only reveal the composition of the outer part. From the results in Table I it seems evident that T-Nbz05 (notation by Schafer et aLlo) forms at first, then is transformed to B-Nb205 and subsequently to M-Nb205, neglecting a small amount of M-Nb205 which was formed in the initial stage of the reaction. The results of the identification of the products formed at various temperatures are given in Table II. These show that the rates of the transformations T-to-B and B-to-M are both accelerated by raising the temperature. X-ray patterns taken from the specimens oxidized at 885 or 910°C revealed that both T-Nb20s and M-Nbz05 existed in the scale in the initial stage (at 3 min), but that subsequently the T-Nb205 disappeared leaving only M-Nb205 in the scale. This result seems to indicate that at these temperatures, the T-Nb205 initially formed transforms directly to M-NbzOs, passing through the intermediate stage of B-Nb205. TABLE II X-RAY PATTERNS Temperature
OF Nb-2.5 AT.% MO ALLOY OXIDIZED Time (min)
Phases found
f”C) 735 735 785 795 810 835 863 885 885 910 910 935 935 985
30 120 120 140 120 120 270 3 120 3 60 3 120 110
T T>B T>B B>T B>T>M B>T+M M>B T”M M>T M>T M M M M
BETWEEN 735 AND 985°C
OXIDATION
OF A Nb--2.5 At.?,, MO ALLOY
263
m n
A
264
M. YAMAWAKI,
M. KANNO.
T. MUKAIBO
Structural changes along the depth direction of the oxide scales formed at 835°C are shown in Table III. The results suggest that the allotropic transformation of polymorphs of Nbz05 is affected by the location of the Nbz05 crystals in the scale. Taking into consideration that the scale is forming continuously as the reaction proceeds at the metal/oxide interface”, the B-to-M transformation appears to proceed much faster in the inner part of the scale than in the outer part, whilst the rate of the T-to-B transformation appears not to be greatly affected by the location of the Nb205 crystals in the scale. X-ray patterns from the scale/substrate interface region of the specimen oxidized for 20 min at 835°C contained some peaks of T-Nb205 and the substrate, but did not contain peaks of NbOz, NbO and NbO,. From this result it seems evident that T-Nb205 is formed directly from the alloy substrate at the initial stage of oxidation at 835°C. The X-ray pattern from the scale/substrate interface region of the specimen oxidized for 40 min, however, contained peaks of NbO,. This result shows that the reaction scheme changes between the reaction times of 20 and 40 min from (NbO solid solution+T-Nb20,) to (NbO solid solution+ NbO, platelets+T-NbzOs), where, for simplicity, molybdenum, the minor alloy constituent, is neglected. X-ray patterns from the scale/substrate interface region of the specimen oxidized for 2 h or longer contained peaks of NbOz and/or NbO instead of NbO,. This result seems to show that the reaction scheme changes between the reaction times of 40 and 120 min from the NbO,-forming type to (NbO solid solution-+NbO+Nb02-+NbZO~), where again molybdenum is. neglected for simplicity. Most of the X-ray patterns which were identified as those of T-Nb205 or B-Nb205 showed the existence of preferred orientation. In the case of T-Nb205, the disproportionately strong intensities of the monoclinic 200, 403, and 406 peaks* (28=28.3, 49.8 and 58.8, respectively) and the absence of the 040 peak (28=22.6), results identical with those of Sheasbyi3 for the oxidation of pure niobium indicate the existence of preferred orientation of the [ lOO]axis perpendicular to the plane parallel with the specimen surface. In the case of B-Nb205, the disproportionately strong intensities of the monoclinic 200, 202, and 222 peaks** (28=26.8, 35.7 and 47.5, respectively) and the absence of the 022 peak (20= 33.3) indicate the existence of preferred orientation of the [loll axis perpendicular to the plane parallel with the specimen surface. However, the degree of preferred orientation shown by B-Nb205 was seen to diminish with increasing reaction time, as shown typically by the pattern from the specimen oxidized for 4h 10 min (Fig. 4 (c)) where the presence of almost equiaxed B-Nb205 crystals is indicated. The M-Nbz05 crystals, on the other hand, showed no preferred orientation. The eight strong diffraction peaks, observed and attributed to nonstoichiometry of the oxide by Sheasby in the study of the oxidation of pure niobium13, were not observed in this study. Visual inspection A macrograph of the specimen reacted for 46.5 h at 835°C is shown in Fig. 5. * Indexing ** Indexing
by Terao”. by Terao14.
OXIDATION
OF A Nb-2.5
Fig. 5. Macrograph
At.% MO ALLOY
of a specimen
oxidized
265
for 46.5 h at 835°C.
The specimen split into two nearly equivalent parts from the center plane. This splitting appears to correspond with that of the metal core observed by Taylor and Stringer9 during the oxidation of Nb-5.5 at.% MO alloy. Each half split further into two parts, i.e., a curled, dense, outer scale of a dark gray color and a rough interior of a whitish-gray color. The latter was presumably formed as a result of the abrupt oxidation of the alloy core following scale detachment. DISCUSSION
From the comparison between the results of the kinetic study and the X-ray study it appears quite evident that the unusual kinetic behavior shown by Nb2.5 at.o/, MO alloy during oxidation at about 835°C is intimately related to the transformations of Nbz05 in scales formed on the alloy. The appearance of a plateau-like stage was always related to the T-to-B transformation of Nbz05 in the scale. In particular, in the temperature range 795-835”C, where a level plateau appeared, a very large portion of the oxide product in the outer part of the scale was identified as B-Nb205. Thus, it is concluded that the B-Nb20f crystals constitute an effective barrier against the diffusion of oxygen in the scale. At present, a direct comparison between the diffusion rate ‘of oxygen in the B-Nb205 layer and that in another polymorph of NbzOs is not possible on account of the lack of data concerning the properties of B-NbzOs. However, references in the literature support the suggestion that the diffusion of oxygen in B-Nb205 crystals is difftcult. Gruehn et ul.15*‘o report that the Nb4+ content of the blue B-crystals was hardly lowered on ignition at 800°C in air for several hours. Further, according to Laves et al . I6 1 B-Nb205 possesses the smallest molar volume of all known forms of NbzOs, with the anionic lattice consisting of a slightly deformed hexagonal-closepacking of O’-, contrasting with the more open structures of other forms of
266
M. YAMAWAKI,
M. KANNO,
NbOs 10*17. Thus, it seems reasonable to assume that B-Nb205
T. MUKAIBO
has the most compact lattice (through which the diffusion of oxygen is most difficult) of all known forms of Nbz05, and that, therefore, B-Nbz05 can constitute a barrier for the diffusion of oxygen in the scale. The results ofthemeasurement of structural changes along the depth direction of scales indicated that the B-to-M transformation proceeded faster in the inner part of the scale than in the outer. If the oxidation reaction proceeds by the inward migration of oxygen, and the subsequent reaction of oxygen with the metal at the metal/oxide interface forms a voluminous oxide, as in the present case, compressive stresses will arise in the newly-formed oxide at the interface’*. The compressive stresses will then be alleviated by plastic flow in the oxide or some other process I9 . Thus a stress gradient between the innermost part and the outermost part of the scale will be established during the oxidation reaction. On comparing this stress gradient with the observed depth dependence of the rate of the B-to-M transformation of Nbz05, the transformation may be presumed to be accelerated by the stress. This assumption appears to explain the reason for the transformation of T-Nb205 to M-Nb205, without the appearance of any B-Nb,O,, in the scale formed on pure niobium, so far as can be recognized by X-ray diffraction13. It would also explain the reason for T-Nb205, in powder form, transforming to B-Nb205 without immediate further transformation to M-Nb20512*20, As observed in this study, small amounts of molybdenum cause B-Nb205 to appear in an appreciable quantity in the oxide scale on the alloy. This remarkable effect of molybdenum alloying may be explained by the differences in size and valency of the two metal ions. If some of the niobium ions in the cation lattices of Nb205 are replaced by the smaller molybdenum ions, the molar volume of the oxide is probably reduced. Also, if some of the pentavalent niobium ions in the Nb205 lattice are replaced by hexavalent molybdenum ions, the number of oxygen vacancies in the Nb205 is probably reduced on account of the oxygen-deficient n-type semiconductivity of the NbZ05 ” . This may also cause a reduction in the molar volume of the oxide and, consequently, the reduction in the oxidation rate of the alloy according to Wagner’s theory. Then the compressive stresses in the scale may be reduced and, as a result, the stress-accelerated B-to-M transformation may be suppressed to a certain degree. At 885 and 910°C even on Nb-2.5 at.% MO alloy, B-Nb205 did not appear in the transformation of T-Nb205 to M-Nb205, as far as could be recognized by X-ray diffraction, The absence of B-Nbz05 may be attributed to the higher temperatures, but the detailed mechanism of the transformation of Nbz05 is not known at present. The transition from one parabolic rate to another observed at these temperatures, seems to be related to T-to-M direct transformation in the scale. The initial parabolic oxidation is probably due to the oxygen diffusion in a scale composed principally of T-Nb205, and the second parabolic oxidation to the oxygen diffusion in the M-Nb205 layer. Actually, the points indicating the second parabolic rate constants for 885 and 910°C in Fig. 3 fall on a smooth curve with the points indicating the first parabolic rate constants for the temperatures above 935°C where the oxygen diffusion in the M-Nb205 layer most probably determines the overall reaction rate. According to Sheasby l3 , T-Nb205 transformed directly to M-Nb205 in the oxide scale formed on the electropolished surface of pure
OXIDATION
OF A Nb-2.5
At.% Mo ALLOY
267
niobium and this transformation caused an abrupt reduction in the rate of the scale growth. This phenomenon appears to correspond with the above one parabolic to another parabolic rate transition. The change in the apparent reaction scheme from the NbO,-forming type to the NbO/NbOz-forming type, which was clarified by the X-ray study, appears to be intimately related to the T-to-B transformation which occurred in the outer part of the scale. The cessation of the oxygen supply from the environment, which was caused by the T-to-B transformation in the scale, probably forced the scalesubstrate system to become a simple diffusion couple between NbzOf and the Nb-2.5 at.% MO alloy. By annealing this diffusion couple, the thermodynamicallystable intermediate phases, NbOz and NbO, were probably allowed to form in layers between the NbZOJ and the alloy (saturated with oxygen). The disappearance of the NbO, platelets may be attributed to the relief of the tensile stresses imposed on the oxygen-saturated alloy surface which originally caused the formation of the NbO, platelets2’. In conclusion, it was found that the B-Nb205 crystals constitute a highly effective barrier for the migration of oxygen in the scale formed on Nb-2.5 at.% MO alloy, If a system is developed to form a highly-stabilized B-Nbz05 layer on the alloy, it may be useful in the production of a new niobium-based alloy with a remarkable oxidation resistance at elevated temperatures. ACKNOWLEDGEMENTS
The authors wish to thank Dr. T. Umeda for performing the electronprobe-microanalysis of the specimen. Thanks are due to Miss M. Yoshitoshi for typing the manuscript.
REFERENCES 1 M. Yamawaki, M. Kanno and T. Mukaibo, Denki Kagaku, 41(1973) 735. 2 H. P. Kling, in B. W. Gonser and E. M. Sherwood (eds.), The ~ee~~o~~g~ us ~io~i~~, Wiley, New York. 1958, p. 87. 3 F. J. Clams and C. A. Barrett, in B. W. Gonser and E. M. Sherwood (eds.), The Technology of Niobium, Wiley, New York, 1958, p. 92. 4 C. T. Sims, W. D. Klopp and R. I. Jaffee, Trans. Amer. sac. Metals, 51 (1959) 256. 5 W. D. Klopp, Battelle Mem. Inst.. DMIC Rept. 123, 1960. p. 48. 6 D. A. Prokoshkin, E. V. Vasilyeva and A. M. Ryabishev. Issled. PO. Zharoproch., Splaoam. Akad. Nauk SSSR. IFLK Met., 10 (1963) 233. 7 B. B. Argent and B. Phelps, J. ~ess-Co~~u~ Metals, 2 (1960) 181. 8 T. L. Kolski. Trans. Amer. Sot. Metals, 57 (1964) 690. 9 A. Taylor and J. Striqger, Corrosion Sk., 12 (1972) 349. 10 H. Schgfer, R. Gruehn and F. Schulte, Angew. Chem. Int. Ed. Engl., 5 (1966) 40. 11 P. Kofstad and H. KjBllesdal, Trans. AIME, 221 (1961) 285. 12 N. Terao. Japan J. Appl. Phys., 2 (1963) 156. 13 J. S. Sheasby, Oxidation of Metals, 1 (1969) 121. 14 N. Terao, Japatt J. Appl. Phys., 4 (1965) 8. 15 R. Gruehn, F. Shulte and H. Schlfer, Angew, Ctxm., 76 (1964) 685. 16 F. Laves. W. Petter and H. Wulf, Naturwiss., 51 (1964) 633. 17 W. Mertin, S. Andersson and R. Gruehn, J. Solid State C’hem., 1 (1970) 419. 18 P. Kofstad. High-Temperature Oxidation of Metals, Wiley. New York, 1966, p. 231.
268 19 20 21 22
J. F. P. L.
M. YAMAWAKI, Stringer, Corrosion Sci., 10 (1970) 513. Laves, R. Moser and W. Petter, Naturwiss., 51 (1964) 356. Kofstad, .I. Phys. Chem. Solids, 23 (1962) 1571. J. Weirick and W. L. Larsen, J. Electrochem. Sot., 119 (1972) 472.
M. KANNO,
T. MUKAIBO
B-Nb20s DURING
MICHIO
257
FORMATION AND RELEVANT PHENOMENA OXIDATION OF A Nb2.5 AT.% MO ALLOY
YAMAWAKI.
MASAYOSHI
KANNO
and TAKASHI
OBSERVED
MUKAIBO
Department of Nuclear Engineering, The University of Tokyo, Tokyo (Japan) (Received
November
10, 1973; in revised form February
12, 1974)
SUMMARY
Nb2.5 at.% MO alloy shows an anomalous parabolic-to-plateau-like type weight-increase behavior during oxidation at around 835°C. For a further study of this behavior, X-ray and thermogravimetric methods have been used. The X-ray study showed that the allotropic transformation of Nb205 from the T-form to the B-form and, further, to the M-form occurs in the oxide scales during oxidation of the alloy. It has been shown that during the plateau-like stage of the reaction B-Nb205 is the dominant constituent of the outer part of the scale, indicating that the B-Nb205 crystals constitute a barrier for the diffusion of oxygen through the scale. The results of the measurement of the structural changes along the depth direction of the scales formed on the alloy suggested that the B-to-M transformation is probably accelerated by the stress. The formation of a large amount of B-Nb205 in the scale was accompanied by a change in the apparent reaction scheme from the NbO,-forming type to the NbO/NbOz-forming type.
INTRODUCTION
A previous study of the oxidation of Nb-MO alloys’ indicated an anomalous weight-increase behavior of Nb2.5 at.% MO alloy during oxidation at 835°C. The weight gained by the alloy initially increased parabolically, then halted, resulting in a weight-gain curve which may be described as a parabolic-to-plateau-like curve. X-ray diffraction study of the oxidized specimens showed that a considerable amount of B-Nb205 was formed on the alloy by oxidation at 835°C. Hence, the anomalous weight increasing behavior was tentatively attributed to the B-Nb205 crystals in the scale. The oxidation of NbMo alloys has been studied in some detail, since small amounts of molybdenum produce a significant reduction in the oxidation rateze6. Argent and Phelps’ studied the oxidation of several NbMo alloys including Nb-1.9 at.% MO and Nb-3.4 at.% MO alloys in the temperature range 400-9OOC, whilst Kolski’ studied the oxidation of Nb-10 wt.% MO alloy in the temperature range 400-1200°C. Taylor and Stringer’ studied the oxidation of Nb5.5 at.% MO single crystals and polycrystals, especially in relation to the morphological aspects,
258
M. YAMAWAKI,
M. KANNO, T. MUKAIBO
in the temperature range 530-1100°C. However, the anomalous kinetic behavior mentioned above, which is shown by Nb-2.5 at.% MO alloy at around 835°C and the formation of B-Nbz05 in the oxide scale were only recently reported by the present authors’. This study has been carried out to clarify in more detail the oxidation behavior of Nb-2.5 at.% MO alloy at around 835°C. Emphasis has been laid on the relationship between the allotropic transformation of NbzOs which occurs in the scale and the anomalous weight change of the specimen. EXPERIMENTAL
PROCEDURE
The material used for this investigation was the same Nb2.5 at.% MO alloy (nominal composition) as that used in the former study’. Analysis of this alloy by means of an electron-probe microanalyser gave a molybdenum content of 2.6 at.%. The electron-beam-melted button was cold-rolled to 0.2 mm in thickness. Rectangular coupons of 8 mm x 27 mm were cut from the sheet and heat-treated in vacuum at 1350°C. As a result, the specimens became polycrystalline, with an average grain diameter of about 0.1 mm. Subsequently, the specimens were electropolished in a solution of HzS04 + 10% HF, then thoroughly washed and degreased. Oxidation was carried out in a silica-glass system. The prepared sample was placed in a silica bucket, which was then suspended in a silica tube by a platinum wire connected to a quartz spring. The specimen was first held in a cool portion of the silica tube. The system was then evacuated and filled to the desired pressure of 0.20 atm with oxygen dried by passing over magnesium perchlorate. The specimen was then quickly lowered into the hot zone, at the operating temperature, by means of the glass cock and brass chain system. The weight gains were obtained by reading the extension of the calibrated silica spring with a cathetometer. After oxidation, X-ray patterns from the surfaces of the oxidized specimens were obtained for identification, using copper Kol radiation. Some specimens were mounted on a flat surface in polyester resin and ground back 10-20 pm on 240 mesh emery papers, the X-ray pattern then being taken from the ground surface. The surface grinding and the X-ray test were repeated until the substrate peaks became dominant in the X-ray pattern obtained. In this way, structural changes along the depth direction of the oxide scales formed at 835°C were determined. EXPERIMENTAL
Kinetic
RESULTS
study
The weight gained by a rectangular specimen of Nb-2.5 at.% MO alloy during oxidation at 835°C was measured over an extended period of time and the weight-gain curve obtained is shown in Fig. 1. As shown in the figure, the curve is separable into three stages: (1) an initial parabolic stage, (2) an intermediate plateau-like stage, and (3) a final sigmoidal weight-increasing stage. Metallographic examination carried out after the end of the run, revealed that there was no metal core left in the specimen, indicating that the whole alloy specimen had been consumed during the run.
OXIDATION OF A Nb-2.5 At.% MO ALLOY
0
20
10
259
40
30
Time fh) Fig. 1. Oxidation behavior of Nb2.5 at.% MO alloy at 835°C.
1
2
Time (h) Fig. 2. Oxidation behavior of Nb-2.5 at.% MO alloy between 735 and 985°C.
The weight-gain curves obtained for temperatures ranging from 735 to 985°C are shown in Fig. 2. At temperatures of 795, 810 and 835”C, a transition of the rate from parabolic to a level plateau occurred. At 863”C, even though the parabolic-to-plateau-like rate transition occurred, the plateau-like stage appeared with a definite positive slope.
260
M. YAMAWAKI,
M. KANNO,
T. MUKAIBO
All weight-gain data were re-plotted against tt. This parabolic plotting revealed that a transition from an initial parabolic to a second parabolic rate occurred at 885 and 910°C where the first parabolic rate constant, i.e., the parabolic rate constant calculated from the first parabolic part, was larger than the second. At 935°C or above, a breakaway occurred following upon the initial parabolic oxidation stage. At 735 or 785°C the weight gained by the specimen increased parabolically and no distinct rate transition was observed to occur within the reaction period. Parabolic rate constants were calculated from the parabolic parts of each weight-gain curve and are plotted against the reciprocal of the absolute temperature in Fig. 3. At 885 and 910°C both the first and the second parabolic rate constants are also plotted. The first parabolic rate constant increases with temperature up to about 850°C then decreases up to 935°C beyond which, up to 985°C it again increases. The points of the second parabolic rate constants for 885 and 910°C appear to be located on a smooth curve with the points of the first parabolic rate constants for temperatures above 935°C.
8
9
104/T(“K) Fig. 3. Arrhenius plot for parabolic oxidation 0, second parabolic rate.
10
rate of Nb2.5
at.% MO alloy.
0,
first parabolic
rate,
Curve I: K,= 1.72 x 10zl exp (- 105,OOO/RT) Curve II: K,=2.73 x lo3 exp (-21,40O/RT).
X-ray study
Typical X-ray patterns taken from surfaces of the specimens which were oxidized at 835°C for various lengths of time are shown in Fig. 4. Detailed results of the identification of the products formed at 835°C are given in Table I. As the X-ray patterns shown in Fig. 4 and Table I were those taken from the surfaces of the scales,
OXIDATION
OF A Nb2.5
261
At.% MO ALLOY
(a)
Fig. 4. X-ray patterns of Nb2.5 at.% MO alloy oxide scales various lengths of time. (a) 20 min, (b) 2 h, (c) 4 h 10 min.
formed
by oxidation
at 835°C
for
M. YAMAWAKI,
262
M. KANNO, T. MUKAIBO
TABLE I X-RAY PATTERNS OF LENGTHS OF TIME Time (min)
Phases found
10 20 30 40 60 120 250 870 1080 2370
T%M T$M>B T>B>M T>B>M TlB>M B>T$M B>M BrM B>M B%M
Nb-2.5
AT.%
MO ALLOY
OXIDIZED
AT
835°C
FOR
VARIOUS
they only reveal the composition of the outer part. From the results in Table I it seems evident that T-Nbz05 (notation by Schafer et aLlo) forms at first, then is transformed to B-Nb205 and subsequently to M-Nb205, neglecting a small amount of M-Nb205 which was formed in the initial stage of the reaction. The results of the identification of the products formed at various temperatures are given in Table II. These show that the rates of the transformations T-to-B and B-to-M are both accelerated by raising the temperature. X-ray patterns taken from the specimens oxidized at 885 or 910°C revealed that both T-Nb20s and M-Nbz05 existed in the scale in the initial stage (at 3 min), but that subsequently the T-Nb205 disappeared leaving only M-Nb205 in the scale. This result seems to indicate that at these temperatures, the T-Nb205 initially formed transforms directly to M-NbzOs, passing through the intermediate stage of B-Nb205. TABLE II X-RAY PATTERNS Temperature
OF Nb-2.5 AT.% MO ALLOY OXIDIZED Time (min)
Phases found
f”C) 735 735 785 795 810 835 863 885 885 910 910 935 935 985
30 120 120 140 120 120 270 3 120 3 60 3 120 110
T T>B T>B B>T B>T>M B>T+M M>B T”M M>T M>T M M M M
BETWEEN 735 AND 985°C
OXIDATION
OF A Nb--2.5 At.?,, MO ALLOY
263
m n
A
264
M. YAMAWAKI,
M. KANNO.
T. MUKAIBO
Structural changes along the depth direction of the oxide scales formed at 835°C are shown in Table III. The results suggest that the allotropic transformation of polymorphs of Nbz05 is affected by the location of the Nbz05 crystals in the scale. Taking into consideration that the scale is forming continuously as the reaction proceeds at the metal/oxide interface”, the B-to-M transformation appears to proceed much faster in the inner part of the scale than in the outer part, whilst the rate of the T-to-B transformation appears not to be greatly affected by the location of the Nb205 crystals in the scale. X-ray patterns from the scale/substrate interface region of the specimen oxidized for 20 min at 835°C contained some peaks of T-Nb205 and the substrate, but did not contain peaks of NbOz, NbO and NbO,. From this result it seems evident that T-Nb205 is formed directly from the alloy substrate at the initial stage of oxidation at 835°C. The X-ray pattern from the scale/substrate interface region of the specimen oxidized for 40 min, however, contained peaks of NbO,. This result shows that the reaction scheme changes between the reaction times of 20 and 40 min from (NbO solid solution+T-Nb20,) to (NbO solid solution+ NbO, platelets+T-NbzOs), where, for simplicity, molybdenum, the minor alloy constituent, is neglected. X-ray patterns from the scale/substrate interface region of the specimen oxidized for 2 h or longer contained peaks of NbOz and/or NbO instead of NbO,. This result seems to show that the reaction scheme changes between the reaction times of 40 and 120 min from the NbO,-forming type to (NbO solid solution-+NbO+Nb02-+NbZO~), where again molybdenum is. neglected for simplicity. Most of the X-ray patterns which were identified as those of T-Nb205 or B-Nb205 showed the existence of preferred orientation. In the case of T-Nb205, the disproportionately strong intensities of the monoclinic 200, 403, and 406 peaks* (28=28.3, 49.8 and 58.8, respectively) and the absence of the 040 peak (28=22.6), results identical with those of Sheasbyi3 for the oxidation of pure niobium indicate the existence of preferred orientation of the [ lOO]axis perpendicular to the plane parallel with the specimen surface. In the case of B-Nb205, the disproportionately strong intensities of the monoclinic 200, 202, and 222 peaks** (28=26.8, 35.7 and 47.5, respectively) and the absence of the 022 peak (20= 33.3) indicate the existence of preferred orientation of the [loll axis perpendicular to the plane parallel with the specimen surface. However, the degree of preferred orientation shown by B-Nb205 was seen to diminish with increasing reaction time, as shown typically by the pattern from the specimen oxidized for 4h 10 min (Fig. 4 (c)) where the presence of almost equiaxed B-Nb205 crystals is indicated. The M-Nbz05 crystals, on the other hand, showed no preferred orientation. The eight strong diffraction peaks, observed and attributed to nonstoichiometry of the oxide by Sheasby in the study of the oxidation of pure niobium13, were not observed in this study. Visual inspection A macrograph of the specimen reacted for 46.5 h at 835°C is shown in Fig. 5. * Indexing ** Indexing
by Terao”. by Terao14.
OXIDATION
OF A Nb-2.5
Fig. 5. Macrograph
At.% MO ALLOY
of a specimen
oxidized
265
for 46.5 h at 835°C.
The specimen split into two nearly equivalent parts from the center plane. This splitting appears to correspond with that of the metal core observed by Taylor and Stringer9 during the oxidation of Nb-5.5 at.% MO alloy. Each half split further into two parts, i.e., a curled, dense, outer scale of a dark gray color and a rough interior of a whitish-gray color. The latter was presumably formed as a result of the abrupt oxidation of the alloy core following scale detachment. DISCUSSION
From the comparison between the results of the kinetic study and the X-ray study it appears quite evident that the unusual kinetic behavior shown by Nb2.5 at.o/, MO alloy during oxidation at about 835°C is intimately related to the transformations of Nbz05 in scales formed on the alloy. The appearance of a plateau-like stage was always related to the T-to-B transformation of Nbz05 in the scale. In particular, in the temperature range 795-835”C, where a level plateau appeared, a very large portion of the oxide product in the outer part of the scale was identified as B-Nb205. Thus, it is concluded that the B-Nb20f crystals constitute an effective barrier against the diffusion of oxygen in the scale. At present, a direct comparison between the diffusion rate ‘of oxygen in the B-Nb205 layer and that in another polymorph of NbzOs is not possible on account of the lack of data concerning the properties of B-NbzOs. However, references in the literature support the suggestion that the diffusion of oxygen in B-Nb205 crystals is difftcult. Gruehn et ul.15*‘o report that the Nb4+ content of the blue B-crystals was hardly lowered on ignition at 800°C in air for several hours. Further, according to Laves et al . I6 1 B-Nb205 possesses the smallest molar volume of all known forms of NbzOs, with the anionic lattice consisting of a slightly deformed hexagonal-closepacking of O’-, contrasting with the more open structures of other forms of
266
M. YAMAWAKI,
M. KANNO,
NbOs 10*17. Thus, it seems reasonable to assume that B-Nb205
T. MUKAIBO
has the most compact lattice (through which the diffusion of oxygen is most difficult) of all known forms of Nbz05, and that, therefore, B-Nbz05 can constitute a barrier for the diffusion of oxygen in the scale. The results ofthemeasurement of structural changes along the depth direction of scales indicated that the B-to-M transformation proceeded faster in the inner part of the scale than in the outer. If the oxidation reaction proceeds by the inward migration of oxygen, and the subsequent reaction of oxygen with the metal at the metal/oxide interface forms a voluminous oxide, as in the present case, compressive stresses will arise in the newly-formed oxide at the interface’*. The compressive stresses will then be alleviated by plastic flow in the oxide or some other process I9 . Thus a stress gradient between the innermost part and the outermost part of the scale will be established during the oxidation reaction. On comparing this stress gradient with the observed depth dependence of the rate of the B-to-M transformation of Nbz05, the transformation may be presumed to be accelerated by the stress. This assumption appears to explain the reason for the transformation of T-Nb205 to M-Nb205, without the appearance of any B-Nb,O,, in the scale formed on pure niobium, so far as can be recognized by X-ray diffraction13. It would also explain the reason for T-Nb205, in powder form, transforming to B-Nb205 without immediate further transformation to M-Nb20512*20, As observed in this study, small amounts of molybdenum cause B-Nb205 to appear in an appreciable quantity in the oxide scale on the alloy. This remarkable effect of molybdenum alloying may be explained by the differences in size and valency of the two metal ions. If some of the niobium ions in the cation lattices of Nb205 are replaced by the smaller molybdenum ions, the molar volume of the oxide is probably reduced. Also, if some of the pentavalent niobium ions in the Nb205 lattice are replaced by hexavalent molybdenum ions, the number of oxygen vacancies in the Nb205 is probably reduced on account of the oxygen-deficient n-type semiconductivity of the NbZ05 ” . This may also cause a reduction in the molar volume of the oxide and, consequently, the reduction in the oxidation rate of the alloy according to Wagner’s theory. Then the compressive stresses in the scale may be reduced and, as a result, the stress-accelerated B-to-M transformation may be suppressed to a certain degree. At 885 and 910°C even on Nb-2.5 at.% MO alloy, B-Nb205 did not appear in the transformation of T-Nb205 to M-Nb205, as far as could be recognized by X-ray diffraction, The absence of B-Nbz05 may be attributed to the higher temperatures, but the detailed mechanism of the transformation of Nbz05 is not known at present. The transition from one parabolic rate to another observed at these temperatures, seems to be related to T-to-M direct transformation in the scale. The initial parabolic oxidation is probably due to the oxygen diffusion in a scale composed principally of T-Nb205, and the second parabolic oxidation to the oxygen diffusion in the M-Nb205 layer. Actually, the points indicating the second parabolic rate constants for 885 and 910°C in Fig. 3 fall on a smooth curve with the points indicating the first parabolic rate constants for the temperatures above 935°C where the oxygen diffusion in the M-Nb205 layer most probably determines the overall reaction rate. According to Sheasby l3 , T-Nb205 transformed directly to M-Nb205 in the oxide scale formed on the electropolished surface of pure
OXIDATION
OF A Nb-2.5
At.% Mo ALLOY
267
niobium and this transformation caused an abrupt reduction in the rate of the scale growth. This phenomenon appears to correspond with the above one parabolic to another parabolic rate transition. The change in the apparent reaction scheme from the NbO,-forming type to the NbO/NbOz-forming type, which was clarified by the X-ray study, appears to be intimately related to the T-to-B transformation which occurred in the outer part of the scale. The cessation of the oxygen supply from the environment, which was caused by the T-to-B transformation in the scale, probably forced the scalesubstrate system to become a simple diffusion couple between NbzOf and the Nb-2.5 at.% MO alloy. By annealing this diffusion couple, the thermodynamicallystable intermediate phases, NbOz and NbO, were probably allowed to form in layers between the NbZOJ and the alloy (saturated with oxygen). The disappearance of the NbO, platelets may be attributed to the relief of the tensile stresses imposed on the oxygen-saturated alloy surface which originally caused the formation of the NbO, platelets2’. In conclusion, it was found that the B-Nb205 crystals constitute a highly effective barrier for the migration of oxygen in the scale formed on Nb-2.5 at.% MO alloy, If a system is developed to form a highly-stabilized B-Nbz05 layer on the alloy, it may be useful in the production of a new niobium-based alloy with a remarkable oxidation resistance at elevated temperatures. ACKNOWLEDGEMENTS
The authors wish to thank Dr. T. Umeda for performing the electronprobe-microanalysis of the specimen. Thanks are due to Miss M. Yoshitoshi for typing the manuscript.
REFERENCES 1 M. Yamawaki, M. Kanno and T. Mukaibo, Denki Kagaku, 41(1973) 735. 2 H. P. Kling, in B. W. Gonser and E. M. Sherwood (eds.), The ~ee~~o~~g~ us ~io~i~~, Wiley, New York. 1958, p. 87. 3 F. J. Clams and C. A. Barrett, in B. W. Gonser and E. M. Sherwood (eds.), The Technology of Niobium, Wiley, New York, 1958, p. 92. 4 C. T. Sims, W. D. Klopp and R. I. Jaffee, Trans. Amer. sac. Metals, 51 (1959) 256. 5 W. D. Klopp, Battelle Mem. Inst.. DMIC Rept. 123, 1960. p. 48. 6 D. A. Prokoshkin, E. V. Vasilyeva and A. M. Ryabishev. Issled. PO. Zharoproch., Splaoam. Akad. Nauk SSSR. IFLK Met., 10 (1963) 233. 7 B. B. Argent and B. Phelps, J. ~ess-Co~~u~ Metals, 2 (1960) 181. 8 T. L. Kolski. Trans. Amer. Sot. Metals, 57 (1964) 690. 9 A. Taylor and J. Striqger, Corrosion Sk., 12 (1972) 349. 10 H. Schgfer, R. Gruehn and F. Schulte, Angew. Chem. Int. Ed. Engl., 5 (1966) 40. 11 P. Kofstad and H. KjBllesdal, Trans. AIME, 221 (1961) 285. 12 N. Terao. Japan J. Appl. Phys., 2 (1963) 156. 13 J. S. Sheasby, Oxidation of Metals, 1 (1969) 121. 14 N. Terao, Japatt J. Appl. Phys., 4 (1965) 8. 15 R. Gruehn, F. Shulte and H. Schlfer, Angew, Ctxm., 76 (1964) 685. 16 F. Laves. W. Petter and H. Wulf, Naturwiss., 51 (1964) 633. 17 W. Mertin, S. Andersson and R. Gruehn, J. Solid State C’hem., 1 (1970) 419. 18 P. Kofstad. High-Temperature Oxidation of Metals, Wiley. New York, 1966, p. 231.
268 19 20 21 22
J. F. P. L.
M. YAMAWAKI, Stringer, Corrosion Sci., 10 (1970) 513. Laves, R. Moser and W. Petter, Naturwiss., 51 (1964) 356. Kofstad, .I. Phys. Chem. Solids, 23 (1962) 1571. J. Weirick and W. L. Larsen, J. Electrochem. Sot., 119 (1972) 472.
M. KANNO,
T. MUKAIBO