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Electrical resistivity of an oxide dispersion strengthened niobium alloy (Nb-0.5 wt%TiO2-ODS)
REFRACTORY METAls
&HARDMATERhl.s ELSEVIER
International
Journal
of Refractory
Metals & Hard
Materials
16 (1998) 143-147
Electrical resistivity of an oxide dispersion strengthened alloy (Nb-0.5 wt%TiOz-ODS)
niobium
H. R. Z. Sandim”“, C. A. M. Santosa, P.A. Suzuki”, M.P. Oteroa, A.F. Padilhab ‘Departamento
aDepatiamento de Engenharia de Materiais, FAENQUIL, PO. Box 116, 12600-000, Lorena-SE: Brazil de Engenharia MetalriTgica e de Materiais, Escola Politknica, Universidade de SCo Paulo, 05508-900, Sio Paulo-SE: Brazil Received
26 January
1998; accepted
12 May 1998
Abstract The precipitation of Ti02 during aging of supersaturated Nb-Ti-0 alloys was investigated using low-temperature electrical resistivity measurements. Aging temperatures were in the range comprised between 900 and 1100°C. The four-probe method was used to determine the transport properties up to 5 K and the respective transition temperatures (T,) in several Nb-Ti02 alloys with different oxygen contents. Transmission electron microscopy was also used to characterize these alloys before and after aging. It is believed that the decrease of the interstitial oxygen content in solid solution owing to the TiOz precipitation is the main responsible for the drop in the electrical resistivity in the normal state and for the slight changes on T,. 0 1998 Elsevier Science Ltd. All rights reserved. 1. Introduction The Nb-0.5 wt%TiOz alloy, strengthened by oxide dispersion (ODS), has been used for the manufacture of surgical implants [l]. Besides its biocompatibility, comparable to the commercial purity (CP) niobium [2], this alloy presents a much higher tensile strength (close to 1 GPa). Usually, this material is processed by a powder metallurgy technique, which includes the mixing of powders, compaction and high-vacuum sintering (T > 2000°C). Dissolution of TiO, particles takes place simultaneously with sintering. Afterwards, cold swaging and a suitable aging treatment are carried out to achieve the reprecipitation of fine TiOz particles in the niobium matrix [l]. Gennari et al. [2] reported that the optimal reprecipitation temperature to obtain a fine and homogeneous distribution of TiOz particles is ca 1000°C. Electrical resistivity has been used as a suitable technique to investigate microstructural changes in metals and alloys. In general, the electrical resistivity can be expressed according to eqn (1):
p(R) = P(T)+p(i)+p(def)+p(s)
(I)
where p(R) is the total electrical resistivity and p(7) and p(i) are, respectively, phonon and impurity *Corresponding
author.
0263-4368/98/$ - see front matter PZI: SO263-4368(98)00027-4
0 1998 Elsevier
scattering contributions. For practical purposes, the surface resistance contribution, p(s), is especially important only in the study of films. The term p(def) comprises the influence of dislocations and vacancies on the electrical resistivity. Several studies discussing the influence of oxygen on the resistivity of pure niobium were previously reported in the literature [3-51. The chosen system is very interesting to study TiOp precipitation using resistometric techniques at low temperatures. It is known that the influence of substitutional impurities, like titanium, on the electrical resistivity of niobium is very low. On the other hand, interstitial elements such as oxygen, nitrogen and carbon have a strong influence on the resistivity of niobium [6]. For practical purposes, the contribution of dislocations and vacancies to the niobium resistivity can be disregarded. Meyerhoff [7] has presented a relation to express the influence of the dislocations on the resistivity of some metals. According to this author, p(dis1) = 5.0 x lo-l9 N
(2)
where N is the dislocation density in cm/cm3. It means that even in severely work-hardened niobium, where N could reach - 10” cm/cm3, the contribution of the dislocations to the resistivity would be very low, for example, p(dis1) z 10m3PR cm. In comparison, this
Science Ltd. All rights reserved.
H. R. Z. Sandim et al./International Journal qf Refractory Metals & Hard Materials 16 (1998) 143-147
144
Table 1 Processing
parameters
and typical composition
Material
A B C D E
Nb-0.S%Ti02 Nb-0.5%TiOZ Nb-0.5%TiOZ Nb Nb
of the samples
investigated
Processing route
Condition
Sintering Sintering Sintering EBM EBM
Cold swaged, Cold swaged, Cold swaged, Cold drawn, Single crystal
0
58% R.A. 23% R.A. 55% R.A. z 99% R.A.
I)wt-ppm)
W-mm)
3365 2883 2501
(-) 884 <70
365
R.A., Reduction in area. EBM, Electron beam melting. (-), Not determined.
value is much lower than 0.5% of the resistivity found for the cold-swaged Nb-0.5 wt%TiO, alloy at room temperature. Regarding the vacancies, Meyerhoff also pointed out that their contribution is certainly lower than that of the dislocations and it becomes appreciable only near the melting point of the metal (2468”Q in the range of the complete dissolution of the precipitates. Assuming that the contribution of microstructural defects to the electrical resistivity of pure niobium is not significant, when compared with p(T) and p(i), the Nordheim’s rule for dilute solutions can be applied, in such a way that: P(i) K x,
(3)
where X, is the mole fraction of the impurity. This approximation is valid only for low temperatures (T< 20 K), where the phonon scatttering contribution is much less expressive. Assuming that the lowering of the interstitial oxygen content during aging is the main responsible for the decrease in the resistivity of the niobium matrix, such decrease could be expected to occur owing to the TiOZ precipitation. An investigation in the change of the electrical resistivity due to the variation in the oxygen content would allow a rough study of the precipitation kinetics. This work aims to study the changes in the electrical transport properties of Nb-TiOz alloys, with different oxygen contents, associating them with the precipitation of Ti02 in the range comprised between 900 and 1100°C.
2. Experimental Samples containing different amounts of oxygen were prepared and then aged under high vacuum in times ranging from 1 to 6 h. Heat treatments were performed under high vacuum ( = 10-O mbar) in order to minimize interstitial pick-up. The high vacuum was provided by a rotary mechanical pump connected to a turbomolecular pump to ensure a clean vacuum. Table 1 shows the processing parameters and the
respective oxygen and nitrogen contents of the samples herein investigated. Further details on the preparation of the samples were previously reported [8]. The electrical resistivity of Nb-TiO, alloys and pure niobium was measured using the four-probe method. The samples were cooled from room temperature to 77 K using liquid nitrogen. Lower temperatures (close to 5 K) were attained by means of a cool stream of helium gas inside the cryostat. The Nb-TiOz and the niobium single crystal rods were ca 3.7 mm in diameter and 25 mm in length. The CP niobium wire was 1.0 mm in diameter and 30 mm in length. The electrical current was provided by a Keithley programmable source (model 220). The current applied to all samples was 100 mA. The voltage between the contacts was measured with a Keithley nanovoltimeter (model 181). Temperatures between 77 and 300 K were measured using a platinum sensor. Temperatures below 77 K were measured using a germanium thermometer (both from Lake Shore Instruments). Voltage and temperature data were recorded using a PC-microcomputer. Transition temperatures (T,-values) were determined as the onset of the voltage drop during cooling down. The microstructure of the cold-worked and aged Nb-TiOz alloys was analyzed by transmission electron microscopy (TEM) in a JEOL-JC-100 microscope with 100 kV of accelerating voltage. TEM-specimens were thinned using a 2%HF-3%HzS04-methanol solution (in ~01%) at -44°C.
3. Results and discussion The cold-worked microstructure of Nb-TiOz specimens is characterized by the presence of dislocation tangles without a clear definition of a cellular substructure, as shown in Fig. l(a). The microstructure of the same specimen, aged at 1000°C for 15 min, presents fine precipitates of Ti02, homogeneously distributed, as observed in Fig. l(b). The average interparticle spacing measured in this sample is ca 100 nm. The mean particle diameter is ca 10 nm. It was also observed that the dislocations introduced during plastic deformation
H. R. 2. Sandim et al.JIntemational
145
Journal of Refractory Metals & Hard Materials 16 (1998) 143-147
are a favorable site for the precipitation of TiOzparticles. It is believed that mass transport along dislocations could be accelerated by pipe diffusion in this system. A similar microstructure is expected for the other Nb-ODS samples herein investigated (B and C). Figure 2 shows the changes on the electrical resistivity of sample B aged at different temperatures. All curves show a linear behavior, as expected for metallic materials. The resistivity values decrease with the lowering of the temperature. After cold swaging, the solubilized sample presents the highest resistivity values. By increasing the aging temperature from 900 to 1100°C for 1 h, a most pronounced decrease in resistivity is observed. This can be explained by the increase in the species diffusivity during precipitation and by the decrease in the TiOz solubility when the temperature decreases. On the other hand, the resistivity of the sample aged at 900°C for 6 h is even lower, suggesting
(4
1
25
I
I
1
Sample B
T
c!
20-
c: 2 :5 E
15-
.z 8 I-
lo-
z ._ b 8 E w
5I
I
I
50
100
150
I
I
250
200
I 300
Temperature (K) Fig. 2. Curves of the electrical different conditions.
resisitivity
of sample
B aged
under
that the l-h aging was not enough to promote a complete precipitation. The plot of the electrical resistivity vs temperature of sample (C) is shown in Fig. 3. In this figure, the aging temperature was fixed at 900°C while the time was changed from 1 to 4 h. All curves presented a similar behavior when compared with sample B. Sample C is less resistive than sample B owing to its initial lower oxygen content. It can be observed some deviations from linearity in the resistivity curves of samples B and C (Figs 2 and 3). These deviations are particularly noticeable in the coldswaged and aged conditions (9OOWl h). One could suggest an electrical resistivity dependence on the clustering process. For longer heat treating times, this anomaly disappears, probably owing to the accomplishment of the precipitation. The magnitude of these deviations is quite small since the expected volume fraction of TiOz is very low (- 1~01%). Several
------I $6”. $14
-
.f12
-
:i
IO-
’
8-
I 0’) Fig. 1. Transmission electron micrographs showing the microstructure of sample A after cold swaging (a) and after aging at 1000°C for 15 min, dark field (h).
A
9G@U4h
Terrperature (K) Fig. 3. Curves of the electrical for different aging times.
resistivity
of sample
C, aged at 900°C
146
H. R. 2. Sandim et al.iInternational Journal of Refractoy Metals & Hard Materials 16 (1998) 143-147
authors, using high temperature resistivity measurements in order to investigate in situ precipitation in supersaturated Al-base alloys, have reported the occurrence of noticeable resistivity maxima. They have been associated to the small size of Guinier-Preston zones and to the misfit between the zones or precipitates and the matrix near these clusters, however, the resistivity maximum is not yet completely understood [9]. Further investigation should be carried out in order to understand this uncommon behavior. We cannot also explain the unusual temperature dependence for the sample heat treated at 900°C for 4 h (Fig. 3). It is also known that oxygen degassing does not take place at temperatures ca 1100°C. It becomes significant at temperatures > 17OO”C, with the formation of volatile species such as NbO and NbOz, depending on the initial oxygen content of the sample [lo]. It must be emphasized that a significant amount of oxygen remains in solid solution after precipitation. This remaining oxygen is calculated by means of the difference of the total oxygen present in the TiOz particles (calculated as - 1730 wt-ppm) and the total amount of oxygen. In this case, if we take sample B, for example, the remaining oxygen in solid solution is close to 1153 wt-ppm. According to Tottle [3], for this amount of oxygen, the electrical resistivity of the Nb-0 solid solution is ca 18 @ cm at room temperature, which is in good agreement with the experimental data at same temperature (Sample B, 9OO”C/6 h). The decrease of the oxygen content during precipitation could be indicated by RRR-values, however, the remaining high oxygen+nitrogen contents in solid solution leave this effect out. Indeed, if we compare the RRR-values for sample C in the solubilized and aged conditions (9OO”C/4 h), both values are close to 2.3 (resistivity curves are almost parallel). In addition, RRR is a much more sensitive technique to detect compositional changes in materials with very low interstitial contents. Figure 4 shows the electrical resistivity curves of the Nb-0.5%Ti02 alloy before and after aging at 900°C (sample C) and pure niobium samples (D and E) at temperatures below 35 K. The difference between the electrical resistivity of CP cold-drawn niobium, with a reduction in area > 99% (sample D), and the highpurity single crystal niobium (sample E), is much smaller than that observed for the Nb-ODS sample alloy before and after aging. This difference between the resistivity levels of sample C in the resistive state could be explained neither by oxygen degassing nor by recovery. A superconducting transition takes place in pure niobium when cooled at temperatures below 10 K. The electrical resistivity of Nb-0.5%TiOz samples was measured and the results were compared with those
Table 2 T,-values Fig. 4
(onset)
for Nb-TiO?
and pure
niobium
samples
found
in
Sample
Condition
7” (K)
C C D E
Solubilized, cold swaged Aged at 9OOW4 h Cold drawn, > 99% R.A. Single crystal
8.5 8.9 9.0 9.2
obtained from pure niobium samples. The superconducting transition temperatures (T,.) obtained from Fig. 4 are also shown in Table 2. The sample aged at 9Oo”C/4 h presents a slightly higher T, value when compared with the cold-worked/annealed sample. This value could be associated with the higher oxygen content present in the solubilized condition. This behavior is in agreement with the data reported in the literature. DeSorbo [4] reported a decrease of ca 0.93 K/at%-0 in the transition temperature of Nb-0 solid solutions. Fhikiger et al. [ll] measured T, vs the Vickers hardness for several types of cold-worked niobium powders. They also concluded that T, decreases with the increase of the oxygen content. Another investigation, emphasizing changes on the magnetic properties, was previously carried out in colddrawn Nb-A&O3 wires [12]. It was shown that small additions of AlZ03, up to 2 wt%, have influenced the values of the residual resistivity, the critical current density and the magnetization behavior of the samples. On the other hand, T, remained nearly unaffected. Thermodynamics analysis shows that AlTO3 is much more stable than TiOz. It means that the amount of oxygen released due to the dissociation of A&O3 is much lower than the expected for the Nb-TiO,? system and thus, its influence on T,. In summary, the results show that the depletion of oxygen in the metallic matrix during precipitation of TiOz is the main reason for the decrease in resistivity. After precipitation, the material is composed of a metallic matrix and fine TiO* precipitates, as confirmed by TEM observations. Owing to the low volume fraction of this second phase (cl%), its contribution to the electrical resistivity is very low and can, therefore, be neglected for practical purposes.
4. Conclusions The decrease of the oxygen content in solid solution owing to the TiOz precipitation is the main responsible for the drop in the electrical resistivity in the normal state and for the slight changes on the transition temperature (Tc) of Nb-Ti-0 supersaturated alloys. The determination of the low-temperature electrical resistivity changes in the normal state can be used as a
H. R. Z. Sandim et al./Intemational Journal of Refractory Metals & Hard Materials 16 (1998) 143-147
very sensitive method for the study of the precipitation kinetics in the Nb-Ti-0 system.
Acknowledgements
The authors wish to thank Dr D.G. Pinatti for his valuable comments.
References
[II
VI
Schider S, Plenk H, Pfliiger G, Otte H, Ennemoser K. Tantal und Niob, zwei neue Werkstoffe fur Implantate im Vergleich mit anderen Implantatwerkstoffen. In: Ortner HM, editor, Proceedings of the 10th International Plansee Seminar, Reutte, May 1981, vol. 2, Verlagsanstalt Tyrolia, 1981; 131-160. Gennari U, Kny E, Gartner T. Niobium-titanium oxide alloy a novel approach to oxide dispersion strengthening. In: Bildstein H, Ortner HM, editor, Proceedings of the 12nd International Plansee Seminar, Reutte, May 1989, vol. 3, Verlagsanstalt Tyrolia, 1989; 587-614.
147
[3] Tottle CR. The physical and mechanical properties of niobium. J Inst Metals 1956-57;85:375-378. [4] DeSorbo W. Effect of dissolved gases on some superconducting properties of niobium. Phys Rev 1963;132:107-121. [5] Koch CC, Scarbrough JO, Kroeger DM. Effects of interstitial oxygen on the superconductivity of niobium. Phys Rev B 1974;9:888-897. [6] Hormann M. Production of high thermal conductivity niobium on a technical scale for high-frequency superconductivity. J Less-Common Metals 1988;139:1-14. [7] Meyerhoff RW. Preparation and electrical resistivity of ultrahigh purity niobium. .I Electrochemical Sot 1971;118:997-1001. [8] Sandim HRZ. PhD Thesis, Department of Metallurgical and Materials Engineering, University of SLo Paulo, 1996. [9] Kelly A, Nicholson RB. Precipitation hardening. In: Chalmers B, editor, Progress in Materials Science, 10. Oxford: Pergamon Press, 1966: 151-391. [lo] Schulze K, Jehn HA, Horz G. High-temperature interactions of refractory metals with gases. J Metals 1988;40:25-31. [ll] Fliikiger R, Foner S, McNiff EJ, Schwartz BB. High critical currents in cold-powder-metallurgy-processed superconducting Cu-Nb-Sn composites. Appl Phys Lett 1979;34:763-766. [12] Best KJ, Pfeiffer I. Uber die Supraleitfahigkeit van gesintertem Niob mit und ohne Zusatz von A&O?. Z Metallkde 1972;63:152-155.
&HARDMATERhl.s ELSEVIER
International
Journal
of Refractory
Metals & Hard
Materials
16 (1998) 143-147
Electrical resistivity of an oxide dispersion strengthened alloy (Nb-0.5 wt%TiOz-ODS)
niobium
H. R. Z. Sandim”“, C. A. M. Santosa, P.A. Suzuki”, M.P. Oteroa, A.F. Padilhab ‘Departamento
aDepatiamento de Engenharia de Materiais, FAENQUIL, PO. Box 116, 12600-000, Lorena-SE: Brazil de Engenharia MetalriTgica e de Materiais, Escola Politknica, Universidade de SCo Paulo, 05508-900, Sio Paulo-SE: Brazil Received
26 January
1998; accepted
12 May 1998
Abstract The precipitation of Ti02 during aging of supersaturated Nb-Ti-0 alloys was investigated using low-temperature electrical resistivity measurements. Aging temperatures were in the range comprised between 900 and 1100°C. The four-probe method was used to determine the transport properties up to 5 K and the respective transition temperatures (T,) in several Nb-Ti02 alloys with different oxygen contents. Transmission electron microscopy was also used to characterize these alloys before and after aging. It is believed that the decrease of the interstitial oxygen content in solid solution owing to the TiOz precipitation is the main responsible for the drop in the electrical resistivity in the normal state and for the slight changes on T,. 0 1998 Elsevier Science Ltd. All rights reserved. 1. Introduction The Nb-0.5 wt%TiOz alloy, strengthened by oxide dispersion (ODS), has been used for the manufacture of surgical implants [l]. Besides its biocompatibility, comparable to the commercial purity (CP) niobium [2], this alloy presents a much higher tensile strength (close to 1 GPa). Usually, this material is processed by a powder metallurgy technique, which includes the mixing of powders, compaction and high-vacuum sintering (T > 2000°C). Dissolution of TiO, particles takes place simultaneously with sintering. Afterwards, cold swaging and a suitable aging treatment are carried out to achieve the reprecipitation of fine TiOz particles in the niobium matrix [l]. Gennari et al. [2] reported that the optimal reprecipitation temperature to obtain a fine and homogeneous distribution of TiOz particles is ca 1000°C. Electrical resistivity has been used as a suitable technique to investigate microstructural changes in metals and alloys. In general, the electrical resistivity can be expressed according to eqn (1):
p(R) = P(T)+p(i)+p(def)+p(s)
(I)
where p(R) is the total electrical resistivity and p(7) and p(i) are, respectively, phonon and impurity *Corresponding
author.
0263-4368/98/$ - see front matter PZI: SO263-4368(98)00027-4
0 1998 Elsevier
scattering contributions. For practical purposes, the surface resistance contribution, p(s), is especially important only in the study of films. The term p(def) comprises the influence of dislocations and vacancies on the electrical resistivity. Several studies discussing the influence of oxygen on the resistivity of pure niobium were previously reported in the literature [3-51. The chosen system is very interesting to study TiOp precipitation using resistometric techniques at low temperatures. It is known that the influence of substitutional impurities, like titanium, on the electrical resistivity of niobium is very low. On the other hand, interstitial elements such as oxygen, nitrogen and carbon have a strong influence on the resistivity of niobium [6]. For practical purposes, the contribution of dislocations and vacancies to the niobium resistivity can be disregarded. Meyerhoff [7] has presented a relation to express the influence of the dislocations on the resistivity of some metals. According to this author, p(dis1) = 5.0 x lo-l9 N
(2)
where N is the dislocation density in cm/cm3. It means that even in severely work-hardened niobium, where N could reach - 10” cm/cm3, the contribution of the dislocations to the resistivity would be very low, for example, p(dis1) z 10m3PR cm. In comparison, this
Science Ltd. All rights reserved.
H. R. Z. Sandim et al./International Journal qf Refractory Metals & Hard Materials 16 (1998) 143-147
144
Table 1 Processing
parameters
and typical composition
Material
A B C D E
Nb-0.S%Ti02 Nb-0.5%TiOZ Nb-0.5%TiOZ Nb Nb
of the samples
investigated
Processing route
Condition
Sintering Sintering Sintering EBM EBM
Cold swaged, Cold swaged, Cold swaged, Cold drawn, Single crystal
0
58% R.A. 23% R.A. 55% R.A. z 99% R.A.
I)wt-ppm)
W-mm)
3365 2883 2501
(-) 884 <70
365
R.A., Reduction in area. EBM, Electron beam melting. (-), Not determined.
value is much lower than 0.5% of the resistivity found for the cold-swaged Nb-0.5 wt%TiO, alloy at room temperature. Regarding the vacancies, Meyerhoff also pointed out that their contribution is certainly lower than that of the dislocations and it becomes appreciable only near the melting point of the metal (2468”Q in the range of the complete dissolution of the precipitates. Assuming that the contribution of microstructural defects to the electrical resistivity of pure niobium is not significant, when compared with p(T) and p(i), the Nordheim’s rule for dilute solutions can be applied, in such a way that: P(i) K x,
(3)
where X, is the mole fraction of the impurity. This approximation is valid only for low temperatures (T< 20 K), where the phonon scatttering contribution is much less expressive. Assuming that the lowering of the interstitial oxygen content during aging is the main responsible for the decrease in the resistivity of the niobium matrix, such decrease could be expected to occur owing to the TiOZ precipitation. An investigation in the change of the electrical resistivity due to the variation in the oxygen content would allow a rough study of the precipitation kinetics. This work aims to study the changes in the electrical transport properties of Nb-TiOz alloys, with different oxygen contents, associating them with the precipitation of Ti02 in the range comprised between 900 and 1100°C.
2. Experimental Samples containing different amounts of oxygen were prepared and then aged under high vacuum in times ranging from 1 to 6 h. Heat treatments were performed under high vacuum ( = 10-O mbar) in order to minimize interstitial pick-up. The high vacuum was provided by a rotary mechanical pump connected to a turbomolecular pump to ensure a clean vacuum. Table 1 shows the processing parameters and the
respective oxygen and nitrogen contents of the samples herein investigated. Further details on the preparation of the samples were previously reported [8]. The electrical resistivity of Nb-TiO, alloys and pure niobium was measured using the four-probe method. The samples were cooled from room temperature to 77 K using liquid nitrogen. Lower temperatures (close to 5 K) were attained by means of a cool stream of helium gas inside the cryostat. The Nb-TiOz and the niobium single crystal rods were ca 3.7 mm in diameter and 25 mm in length. The CP niobium wire was 1.0 mm in diameter and 30 mm in length. The electrical current was provided by a Keithley programmable source (model 220). The current applied to all samples was 100 mA. The voltage between the contacts was measured with a Keithley nanovoltimeter (model 181). Temperatures between 77 and 300 K were measured using a platinum sensor. Temperatures below 77 K were measured using a germanium thermometer (both from Lake Shore Instruments). Voltage and temperature data were recorded using a PC-microcomputer. Transition temperatures (T,-values) were determined as the onset of the voltage drop during cooling down. The microstructure of the cold-worked and aged Nb-TiOz alloys was analyzed by transmission electron microscopy (TEM) in a JEOL-JC-100 microscope with 100 kV of accelerating voltage. TEM-specimens were thinned using a 2%HF-3%HzS04-methanol solution (in ~01%) at -44°C.
3. Results and discussion The cold-worked microstructure of Nb-TiOz specimens is characterized by the presence of dislocation tangles without a clear definition of a cellular substructure, as shown in Fig. l(a). The microstructure of the same specimen, aged at 1000°C for 15 min, presents fine precipitates of Ti02, homogeneously distributed, as observed in Fig. l(b). The average interparticle spacing measured in this sample is ca 100 nm. The mean particle diameter is ca 10 nm. It was also observed that the dislocations introduced during plastic deformation
H. R. 2. Sandim et al.JIntemational
145
Journal of Refractory Metals & Hard Materials 16 (1998) 143-147
are a favorable site for the precipitation of TiOzparticles. It is believed that mass transport along dislocations could be accelerated by pipe diffusion in this system. A similar microstructure is expected for the other Nb-ODS samples herein investigated (B and C). Figure 2 shows the changes on the electrical resistivity of sample B aged at different temperatures. All curves show a linear behavior, as expected for metallic materials. The resistivity values decrease with the lowering of the temperature. After cold swaging, the solubilized sample presents the highest resistivity values. By increasing the aging temperature from 900 to 1100°C for 1 h, a most pronounced decrease in resistivity is observed. This can be explained by the increase in the species diffusivity during precipitation and by the decrease in the TiOz solubility when the temperature decreases. On the other hand, the resistivity of the sample aged at 900°C for 6 h is even lower, suggesting
(4
1
25
I
I
1
Sample B
T
c!
20-
c: 2 :5 E
15-
.z 8 I-
lo-
z ._ b 8 E w
5I
I
I
50
100
150
I
I
250
200
I 300
Temperature (K) Fig. 2. Curves of the electrical different conditions.
resisitivity
of sample
B aged
under
that the l-h aging was not enough to promote a complete precipitation. The plot of the electrical resistivity vs temperature of sample (C) is shown in Fig. 3. In this figure, the aging temperature was fixed at 900°C while the time was changed from 1 to 4 h. All curves presented a similar behavior when compared with sample B. Sample C is less resistive than sample B owing to its initial lower oxygen content. It can be observed some deviations from linearity in the resistivity curves of samples B and C (Figs 2 and 3). These deviations are particularly noticeable in the coldswaged and aged conditions (9OOWl h). One could suggest an electrical resistivity dependence on the clustering process. For longer heat treating times, this anomaly disappears, probably owing to the accomplishment of the precipitation. The magnitude of these deviations is quite small since the expected volume fraction of TiOz is very low (- 1~01%). Several
------I $6”. $14
-
.f12
-
:i
IO-
’
8-
I 0’) Fig. 1. Transmission electron micrographs showing the microstructure of sample A after cold swaging (a) and after aging at 1000°C for 15 min, dark field (h).
A
9G@U4h
Terrperature (K) Fig. 3. Curves of the electrical for different aging times.
resistivity
of sample
C, aged at 900°C
146
H. R. 2. Sandim et al.iInternational Journal of Refractoy Metals & Hard Materials 16 (1998) 143-147
authors, using high temperature resistivity measurements in order to investigate in situ precipitation in supersaturated Al-base alloys, have reported the occurrence of noticeable resistivity maxima. They have been associated to the small size of Guinier-Preston zones and to the misfit between the zones or precipitates and the matrix near these clusters, however, the resistivity maximum is not yet completely understood [9]. Further investigation should be carried out in order to understand this uncommon behavior. We cannot also explain the unusual temperature dependence for the sample heat treated at 900°C for 4 h (Fig. 3). It is also known that oxygen degassing does not take place at temperatures ca 1100°C. It becomes significant at temperatures > 17OO”C, with the formation of volatile species such as NbO and NbOz, depending on the initial oxygen content of the sample [lo]. It must be emphasized that a significant amount of oxygen remains in solid solution after precipitation. This remaining oxygen is calculated by means of the difference of the total oxygen present in the TiOz particles (calculated as - 1730 wt-ppm) and the total amount of oxygen. In this case, if we take sample B, for example, the remaining oxygen in solid solution is close to 1153 wt-ppm. According to Tottle [3], for this amount of oxygen, the electrical resistivity of the Nb-0 solid solution is ca 18 @ cm at room temperature, which is in good agreement with the experimental data at same temperature (Sample B, 9OO”C/6 h). The decrease of the oxygen content during precipitation could be indicated by RRR-values, however, the remaining high oxygen+nitrogen contents in solid solution leave this effect out. Indeed, if we compare the RRR-values for sample C in the solubilized and aged conditions (9OO”C/4 h), both values are close to 2.3 (resistivity curves are almost parallel). In addition, RRR is a much more sensitive technique to detect compositional changes in materials with very low interstitial contents. Figure 4 shows the electrical resistivity curves of the Nb-0.5%Ti02 alloy before and after aging at 900°C (sample C) and pure niobium samples (D and E) at temperatures below 35 K. The difference between the electrical resistivity of CP cold-drawn niobium, with a reduction in area > 99% (sample D), and the highpurity single crystal niobium (sample E), is much smaller than that observed for the Nb-ODS sample alloy before and after aging. This difference between the resistivity levels of sample C in the resistive state could be explained neither by oxygen degassing nor by recovery. A superconducting transition takes place in pure niobium when cooled at temperatures below 10 K. The electrical resistivity of Nb-0.5%TiOz samples was measured and the results were compared with those
Table 2 T,-values Fig. 4
(onset)
for Nb-TiO?
and pure
niobium
samples
found
in
Sample
Condition
7” (K)
C C D E
Solubilized, cold swaged Aged at 9OOW4 h Cold drawn, > 99% R.A. Single crystal
8.5 8.9 9.0 9.2
obtained from pure niobium samples. The superconducting transition temperatures (T,.) obtained from Fig. 4 are also shown in Table 2. The sample aged at 9Oo”C/4 h presents a slightly higher T, value when compared with the cold-worked/annealed sample. This value could be associated with the higher oxygen content present in the solubilized condition. This behavior is in agreement with the data reported in the literature. DeSorbo [4] reported a decrease of ca 0.93 K/at%-0 in the transition temperature of Nb-0 solid solutions. Fhikiger et al. [ll] measured T, vs the Vickers hardness for several types of cold-worked niobium powders. They also concluded that T, decreases with the increase of the oxygen content. Another investigation, emphasizing changes on the magnetic properties, was previously carried out in colddrawn Nb-A&O3 wires [12]. It was shown that small additions of AlZ03, up to 2 wt%, have influenced the values of the residual resistivity, the critical current density and the magnetization behavior of the samples. On the other hand, T, remained nearly unaffected. Thermodynamics analysis shows that AlTO3 is much more stable than TiOz. It means that the amount of oxygen released due to the dissociation of A&O3 is much lower than the expected for the Nb-TiO,? system and thus, its influence on T,. In summary, the results show that the depletion of oxygen in the metallic matrix during precipitation of TiOz is the main reason for the decrease in resistivity. After precipitation, the material is composed of a metallic matrix and fine TiO* precipitates, as confirmed by TEM observations. Owing to the low volume fraction of this second phase (cl%), its contribution to the electrical resistivity is very low and can, therefore, be neglected for practical purposes.
4. Conclusions The decrease of the oxygen content in solid solution owing to the TiOz precipitation is the main responsible for the drop in the electrical resistivity in the normal state and for the slight changes on the transition temperature (Tc) of Nb-Ti-0 supersaturated alloys. The determination of the low-temperature electrical resistivity changes in the normal state can be used as a
H. R. Z. Sandim et al./Intemational Journal of Refractory Metals & Hard Materials 16 (1998) 143-147
very sensitive method for the study of the precipitation kinetics in the Nb-Ti-0 system.
Acknowledgements
The authors wish to thank Dr D.G. Pinatti for his valuable comments.
References
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