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Magnetic dipole and electric quadrupole interactions of 181Ta probe in Ni–Hf alloy
Journal of Alloys and Compounds 475 (2009) 38–41
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Magnetic dipole and electric quadrupole interactions of 181 Ta probe in Ni–Hf alloy ´ ´ V. Ivanovski, V. Koteski, A. Umicevi c´ ∗ , B. Cekic, ˇ ˇ ´ Cavor, ´ S. Pavlovic´ J. BeloˇsevicM. Siljegovi c, Institute of Nuclear Sciences Vinca, P.O. Box 522, 11001 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 27 June 2008 Received in revised form 25 July 2008 Accepted 27 July 2008 Available online 17 September 2008 Keywords: Intermetallics Hyperfine interactions PAC
a b s t r a c t The hyperfine interactions of 181 Ta probe in the nickel–2 at.% hafnium alloy have been studied by the perturbed angular correlation method in the temperature range 78–1131 K. The magnitude of the magnetic dipole interaction of 181 Hf/181 Ta substituting in ferromagnetic Ni(Hf) solid solution decreases with increasing temperature. As a result of the restricted solid solubility of Hf in Ni, a second phase (HfNi5 ) in the Ni–Hf sample was detected. 181 Hf/181 Ta which resides in HfNi5 senses weak electric quadrupole interaction. The observed anomalous temperature behavior was ascribed to distortions in the HfNi5 cubic phase. The third measured hyperfine interaction corresponds to the quadrupole interaction of 181 Hf/181 Ta positioned in HfO2 contamination originating from an annealing procedure above 900 K. © 2008 Elsevier B.V. All rights reserved.
1. Introduction On account of the significant technological importance and frequent employment in the hot stage of jet turbines, the nickel-base alloys are often subject to various and numerous investigations. Particularly, a small Hf addition (less than 1 at.%) to nickel alloys improves the oxidation resistance by enhancing the scale adhesion [1] and leads to a substantial grain refinement [2]. The strengthening of Ni-alloy with 2 at.% of Hf observed during the earliest period of ageing is attributed to the occurrence of HfNi5 precipitates [3]. The region of the Ni–Hf phase diagram with nickel content more than 83 at.% exhibits a restricted solid solubility and presence of a two-phase field below the eutectic temperature 1190 ◦ C [4,5]. The hafnium solvus curve is not well known [6]. The maximum Hf solid solubility at the eutectic temperature is estimated to be 1.3 at.% [7]. As the temperature is lowered, the solubility sharply decreases, going below 0.2 at.% at around 500 ◦ C. To our present knowledge, in most studies on Ni–Hf alloys [1,3,7,8,9] containing small atomic percent of hafnium, metallurgical as well as microstructural features were investigated and no attention has been devoted to their magnetic properties. So far, the only magnetization study of Hf–Ni alloys (below 300 K) is done around the paramagnetic–ferromagnetic transition and has showed that the critical concentration of Ni for the onset of ferromagnetism in these alloys is about 89.5 at.% Ni [10]. Since the
∗ Corresponding author. Tel.: +381 11 3408 549; fax: +381 11 3440 100. ´ ´ E-mail address: [email protected] (A. Umicevi c). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.107
perturbed angular correlation (PAC) method is highly sensitive to the nearest surroundings of the probe atoms, our work is focused more on the local behavior of the magnetic and electric properties around 181 Hf/181 Ta incorporated in different phases present in the nickel–2 at.% hafnium alloy. The parameters characterizing hyperfine interactions at various 181 Ta sites in Ni–Hf alloy have been determined below and above the magnetically ordering temperature. 2. Experimental procedure The nickel–2 at.% hafnium alloy was prepared from high purity Ni(5N) and Hf(2N) in a RF oven under Ar overpressure. The oven chamber had been cycled several times from 10−5 mbar to Ar overpressure to reduce the oxygen partial pressure to below 10−14 ppm. To achieve a homogenous distribution of Hf, the sample was turned over and remelted several times. Our previous X-ray diffraction and scanning electron microscopy measurements confirmed alloying and phase formation in the Ni–Hf sample according to the Ni–Hf phase diagram [11]. Hf atoms occupy two different sites: substitutional lattice positions in Ni and in the HfNi5 phase with defective lattice structure relative to that of the equilibrium phase. The PAC method is based on the interaction of an excited intermediate nuclear state of a ␥–␥ cascade with the magnetic hyperfine fields and/or electric field gradients (EFG) present at the probe site. A detailed description of the method is given in Ref. [12]. As the excited states of 181 Ta are populated in the  decay of the 42 d 181 Hf isotope, the 181 Ta is produced in the reactor by exposing Hf in the alloy to a thermal neutron flux higher than 1014 n cm−2 s−1 . Before the irradiation, the sample was sealed in high vacuum in a quartz ampoule. The radiation damage in the sample was minimized by the annealing procedure in several steps, starting with 1 h at 1343 K and reaching room temperature after 30 min stops at 1000, 800, 750, 700, and 650 K, respectively. The cooling time between the different temperatures was of the order of 10–15 min. The PAC measurements of the polycrystalline Ni–Hf alloy were performed with the 133–482 keV ␥–␥ cascade of 181 Ta ion probe. The PAC spectra, in temperature range of 78–1131 K, were collected with three BaF2 detectors
A. Umi´cevi´c et al. / Journal of Alloys and Compounds 475 (2009) 38–41 in a fast-slow coincidence set up. The time resolution (FWHM) for the mentioned ␥–␥ cascade was around 1 ns. For higher temperatures, the quartz ampoule was mounted in a furnace. The coincidence spectra N(,t) measured at angles = 180◦ and = 90◦ between the detectors formed an experimentally anisotropy R(t): R(t) = 2
◦
◦
N(t, 180 ) − N(t, 90 ) N(t, 180◦ ) + 2N(t, 90◦ )
(1)
The least-square fitting procedure to the experimental PAC spectra using the DEPACK program [13] assumed the presence of one magnetic dipole (MDI) and two electric quadrupole interactions (EQI) in the Ni–Hf alloy: R(t) = Aeff 22
i fi G22 (t)
(2)
i
is the experimental anisotropy factor and the sum runs over all sites i where Aeff 22 i (t) is containing having different hyperfine fields and the perturbation factor G22 information on the ith hyperfine interaction present in the sample. The MDI is related to the magnetic hyperfine field Hhf acting on 181 Ta probe according to the relation: ωL =
gN Hhf h ¯
(3)
and is characterized by the Larmor frequency ωL , a Lorentzian frequency distribution width parameter , the nuclear g-factor g = 1.316(12) [14] and the nuclear magneton N . The EQI at the 181 Ta site corresponds to the electric quadrupole frequency: ωQ =
eQVZZ 4I(2I − 1)¯h
(4)
with the electric quadrupole frequency distribution width ı. Vzz is the largest component of the EFG tensor and = (Vxx − Vyy )/Vzz is its asymmetry parameter, Q = 2.36(5)b is the quadrupole moment for the 181 Ta intermediate level I = 5/2 of the measured ␥–␥ cascade [15].
3. Results and discussion The representative PAC spectra of the polycrystalline Ni–Hf sample measured at different temperatures are shown in Fig. 1, together with the corresponding Fourier transforms. The solid curves seen in Fig. 1 represent the results of the fitting procedure, described above. We assumed the presence of three nonequivalent sites for
Table 1 Relevant MDI parameters for nickel–2 at.% hafnium alloy T (K) 78 291 362 469 289 a
(1)
ωL
(Mrad/s)
571(2) 536(2) 488(2) 338(3) 534(2)a
181
39
Ta probe in the Ni(Hf) solid solution phase in the
1 (%)
Hhf (T)
f1 (%)
3.7(5) 0.8(5) 1.5(5) 6.1(9) 1.6(5)
9.06(3) 8.53(4) 7.74(3) 5.36(4) 8.48(3)
31(2) 16(2) 21(1) 26(2) 17(1)
Remeasured at RT after the whole temperature range has been completed.
the 181 Ta probe-ion substituting in the ferromagnetic Ni(Hf) solid solution, the paramagnetic HfNi5 and the dielectric HfO2 phases. The interaction frequency of 536 Mrad/s was derived from the experimental spectra at room temperature (RT). This interaction was absent from the spectra above the Curie point of nickel (TC = 631 K). According to the results of previous PAC measurements on samples with very low Hf content [16,17,18], the corresponding interaction (given in Table 1) was assigned to the substitutional site of 181 Ta ion-probe in the ferromagnetic Ni lattice. The magnetic hyperfine field is decreasing with increasing temperature, as seen from Table 1. The sign of the internal field was not measured. Considering the fraction of ∼16% of probe atoms (Table 1) experiencing Hhf in the Ni–Hf sample at RT, it is indicative that the actual Hf concentration in the Ni(Hf) phase is much lower than the predicted maximum Hf solid solubility in the nickel matrix. In order to make an additional estimate of Hf content in the nickel–2 at.% hafnium alloy, we used the empirical equation of Ref. [7] which gave 0.33 at.% at RT. Moreover, we found that the same equation in the case of previously investigated nickel–0.2 at.% hafnium alloy also gave a good prediction of 0.22 at.%. The values of Hhf in the nickel–2 at.% hafnium alloy at higher temperatures are lower than the ones obtained for Hhf in the nickel–0.2 at.% hafnium system [18]. The fractions of probe atoms experiencing weak quadrupole frequency (3 Mrad/s at RT) with anomalous temperature dependence were significant (49–67%). The corresponding EQI parameters are
Fig. 1. PAC spectra with corresponding Fourier transforms for 181 Ta probe in the nickel–2 at.% hafnium alloy at the indicated measuring temperatures.
40
A. Umi´cevi´c et al. / Journal of Alloys and Compounds 475 (2009) 38–41
Fig. 3. Temperature variations of the quadrupole interaction parameters of 181 Ta in the HfO2 contamination in the nickel–2 at.% hafnium alloy; () remeasured at RT after the whole temperature range has been completed.
procedure above 900 K. A similar effect was observed for Co–Hf alloys after the same annealing procedure above 900 K [25]. (2)
Fig. 2. Temperature evolution of the quadrupole frequency ωQ and frequency distribution width ı2 for Ta nuclei in the defective HfNi5 phase present in the nickel–2 at.% hafnium alloy due to restricted solid solubility of Hf in Ni; () remeasured at RT after the whole temperature range has been completed. 181
presented in Fig. 2. The rather large frequency distribution parameter ı2 (18–51%) indicates a defect associated 181 Ta site. As seen from the previous experiments [11], distortions are present in the fcc HfNi5 phase. Therefore, this EQI could be ascribed to Ta probe substituting in irregular HfNi5 phase. The anomalous temperature evolution of the quadrupole frequency values (in Fig. 2) may depend on the defect mobility at certain temperatures. The temperature independent EFG asymmetry parameter 2 ≈ 1 fitted well the experimental spectra in the whole measured range. The assignment of the weak EQI to 181 Hf/181 Ta in the defective HfNi5 phase present in the sample, has not been undoubtedly clarified. The experimental observations argue against any interpretation involving Hf clustering, since it requires much stronger EQI than the fitted one [19,20,21]. Also, since there is considerable impurity/host size misfit, it is likely to expect lattice defects around the Hf impurity atom in nickel lattice. Up to date, as reviewed by Pleiter and Hohenemser [22], the studies on defects in nickel lattice were performed using 111 Cd PAC probe. The majority of the observed 111 Cd defect sites in nickel experienced the damped magnetic hyperfine field in regard to the one at 111 Cd substitutional site in Ni [22]. Our fitting procedure for the measured PAC spectra gave the best fit without an assumption of another Larmor frequency. However, we cannot completely exclude the contributions from 181 Ta sites experiencing various EQIs caused by the neighboring lattice defects present at grain boundaries of mixing phases. Unexpectedly, the analysis of PAC spectra above the magnetic ordering temperature required introduction of a third hyperfine interaction: a quadrupole frequency (128 Mrad/s at RT) was present in the experimental spectra up to 1131 K, as demonstrated in Fig. 1 for the experimental spectrum at 747 K. The temperature dependence of corresponding EQI parameters are shown in Fig. 3. After examined in the whole measured range, these values coincided with those previously known for the monoclinic HfO2 [23,24]. The quadrupole interaction data presented in Fig. 3 are unambiguous proof of an oxidation process in the sample due to the annealing
4. Summary We have studied the magnetic dipole and electric quadrupole interactions of the 181 Ta probe substituting in various phases present in the nickel–2 at.% hafnium alloy due to the restricted solid solubility of Hf in Ni. A decreasing of the magnetic hyperfine field acting on probe in the ferromagnetic Ni(Hf) solid solution with increasing temperature has been observed. The weak EQI with unusual temperature dependence is ascribed to 181 Ta residing in the defective HfNi5 precipitate, although alternative interpretations (probe sites at grain boundaries) cannot be completely disregarded. The annealing procedure above 900 K resulted in the oxidation process forming HfO2 precipitates. Acknowledgements This work has been supported by the grant No. 141022G from the Serbian Ministry of Science. We would like to thank Gerhard Schumacher and Werner Rönnfeldt for the preparation of the samples. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
H. Hindam, D.P. Whittle, Oxid. Met. 18 (5–6) (1982) 245–284. J. Stringer, Mater. Sci. Eng. A 120 (1989) 129–137. M. Hajjaji, J. Alloys Compd. 274 (1998) 189–192. M.E. Kirkpatrick, W.L. Larsen, Trans. ASM 54 (1961) 580. V.N. Svechnikov, A.K. Shurin, G.P. Dimitrieva, Izv. Akad. Nauk. SSSR Metall. 6 (1967) 176. P. Nash, A. Nash, in: T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 2, 2nd edition, ASM, 1992, p. 2094. M. Hajjaji, J. Alloys Compd. 274 (1998) 185–188. J. Tóth, I. Bakonyi, G. Hilscher, Europhys. Lett. 24 (5) (1993) 379–384. A. Chiraki, B. Fogarassy, G. Van Tendeloo, P. Lamparter, M. Tegze, I. Bakonyi, J. Alloys Compd. 210 (1994) 135–141. I. Bakonyi, L.F. Kiss, E. Varga, L.K. Varga, Phys. Rev. B 71 (2005), 014402(8). ˇ ´ ´ H.-E. Mahnke, B. Cekic, ´ J. Grbovic, ´ V. Koteski, J. Belosevic´ Cavor, A. Umicevi c, Mater. Sci. Forum 518 (2006) 325–330. H. Frauenfelder, R.M. Steffen, in: K. Siegbahn (Ed.), Alpha-, Beta- and GammaRay-Spectroscopy, North Holland Publishing Company, Amsterdam, 1965, pp. 1101–1188. B. Lindgren, Hyperfine Interact. C1 (1996) 613. P. Raghavan, At. Data Nucl. Data Tables 42 (1989) 189. T. Butz, A. Lerf, Phys. Lett. 97A (1983) 217–218.
A. Umi´cevi´c et al. / Journal of Alloys and Compounds 475 (2009) 38–41 [16] E. Gerdau, H. Winkler, W. Gebert, B. Giese, J. Braunsfurth, Hyperfine Interact. 1 (1976) 459–467. [17] J. Oddou, J. Berthier, P. Peretto, Phys. Rev. B 17 (1978) 222–240. ˇ ´ A.B. Umicevi ´ ´ J.N. Beloˇsevic´ Cavor, [18] B.Dj. Cekic, c, V.J. Koteski, V.N. Ivanovski, M.N. ´ Solid State Commun. 145 (2008) 465–468. Stojkovic, [19] R.W. Sommerfeldt, T.W. Cannon, L.W. Coleman, L. Schecter, Phys. Rev. 138 (1965) B763–B765.
41
[20] P. da, R. Andrade, A. Maciel, J.D. Rogers, Phys. Rev. 159 (1967) 196–200. [21] M. Salomon, L. Boström, T. Lindqvist, M. Perez, M. Zwanziger, Ark. Fysik 27 (7) (1964) 97–121. [22] F. Pleiter, C. Hohenemser, Phys. Rev. B 25 (1982) 106–126. ´ B. Cekic, ´ Hyperfine Interact. 14 (1983) 105–110. [23] S. Koiˇcki, M. Manasijevic, [24] A. Ayala, R. Alonso, A. López-García, Phys. Rev. B 50 (1994) 3547–3552. [25] S.C. Bedi, M. Forker, Phys. Rev. B 47 (1993) 14948–14960.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Magnetic dipole and electric quadrupole interactions of 181 Ta probe in Ni–Hf alloy ´ ´ V. Ivanovski, V. Koteski, A. Umicevi c´ ∗ , B. Cekic, ˇ ˇ ´ Cavor, ´ S. Pavlovic´ J. BeloˇsevicM. Siljegovi c, Institute of Nuclear Sciences Vinca, P.O. Box 522, 11001 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 27 June 2008 Received in revised form 25 July 2008 Accepted 27 July 2008 Available online 17 September 2008 Keywords: Intermetallics Hyperfine interactions PAC
a b s t r a c t The hyperfine interactions of 181 Ta probe in the nickel–2 at.% hafnium alloy have been studied by the perturbed angular correlation method in the temperature range 78–1131 K. The magnitude of the magnetic dipole interaction of 181 Hf/181 Ta substituting in ferromagnetic Ni(Hf) solid solution decreases with increasing temperature. As a result of the restricted solid solubility of Hf in Ni, a second phase (HfNi5 ) in the Ni–Hf sample was detected. 181 Hf/181 Ta which resides in HfNi5 senses weak electric quadrupole interaction. The observed anomalous temperature behavior was ascribed to distortions in the HfNi5 cubic phase. The third measured hyperfine interaction corresponds to the quadrupole interaction of 181 Hf/181 Ta positioned in HfO2 contamination originating from an annealing procedure above 900 K. © 2008 Elsevier B.V. All rights reserved.
1. Introduction On account of the significant technological importance and frequent employment in the hot stage of jet turbines, the nickel-base alloys are often subject to various and numerous investigations. Particularly, a small Hf addition (less than 1 at.%) to nickel alloys improves the oxidation resistance by enhancing the scale adhesion [1] and leads to a substantial grain refinement [2]. The strengthening of Ni-alloy with 2 at.% of Hf observed during the earliest period of ageing is attributed to the occurrence of HfNi5 precipitates [3]. The region of the Ni–Hf phase diagram with nickel content more than 83 at.% exhibits a restricted solid solubility and presence of a two-phase field below the eutectic temperature 1190 ◦ C [4,5]. The hafnium solvus curve is not well known [6]. The maximum Hf solid solubility at the eutectic temperature is estimated to be 1.3 at.% [7]. As the temperature is lowered, the solubility sharply decreases, going below 0.2 at.% at around 500 ◦ C. To our present knowledge, in most studies on Ni–Hf alloys [1,3,7,8,9] containing small atomic percent of hafnium, metallurgical as well as microstructural features were investigated and no attention has been devoted to their magnetic properties. So far, the only magnetization study of Hf–Ni alloys (below 300 K) is done around the paramagnetic–ferromagnetic transition and has showed that the critical concentration of Ni for the onset of ferromagnetism in these alloys is about 89.5 at.% Ni [10]. Since the
∗ Corresponding author. Tel.: +381 11 3408 549; fax: +381 11 3440 100. ´ ´ E-mail address: [email protected] (A. Umicevi c). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.107
perturbed angular correlation (PAC) method is highly sensitive to the nearest surroundings of the probe atoms, our work is focused more on the local behavior of the magnetic and electric properties around 181 Hf/181 Ta incorporated in different phases present in the nickel–2 at.% hafnium alloy. The parameters characterizing hyperfine interactions at various 181 Ta sites in Ni–Hf alloy have been determined below and above the magnetically ordering temperature. 2. Experimental procedure The nickel–2 at.% hafnium alloy was prepared from high purity Ni(5N) and Hf(2N) in a RF oven under Ar overpressure. The oven chamber had been cycled several times from 10−5 mbar to Ar overpressure to reduce the oxygen partial pressure to below 10−14 ppm. To achieve a homogenous distribution of Hf, the sample was turned over and remelted several times. Our previous X-ray diffraction and scanning electron microscopy measurements confirmed alloying and phase formation in the Ni–Hf sample according to the Ni–Hf phase diagram [11]. Hf atoms occupy two different sites: substitutional lattice positions in Ni and in the HfNi5 phase with defective lattice structure relative to that of the equilibrium phase. The PAC method is based on the interaction of an excited intermediate nuclear state of a ␥–␥ cascade with the magnetic hyperfine fields and/or electric field gradients (EFG) present at the probe site. A detailed description of the method is given in Ref. [12]. As the excited states of 181 Ta are populated in the  decay of the 42 d 181 Hf isotope, the 181 Ta is produced in the reactor by exposing Hf in the alloy to a thermal neutron flux higher than 1014 n cm−2 s−1 . Before the irradiation, the sample was sealed in high vacuum in a quartz ampoule. The radiation damage in the sample was minimized by the annealing procedure in several steps, starting with 1 h at 1343 K and reaching room temperature after 30 min stops at 1000, 800, 750, 700, and 650 K, respectively. The cooling time between the different temperatures was of the order of 10–15 min. The PAC measurements of the polycrystalline Ni–Hf alloy were performed with the 133–482 keV ␥–␥ cascade of 181 Ta ion probe. The PAC spectra, in temperature range of 78–1131 K, were collected with three BaF2 detectors
A. Umi´cevi´c et al. / Journal of Alloys and Compounds 475 (2009) 38–41 in a fast-slow coincidence set up. The time resolution (FWHM) for the mentioned ␥–␥ cascade was around 1 ns. For higher temperatures, the quartz ampoule was mounted in a furnace. The coincidence spectra N(,t) measured at angles = 180◦ and = 90◦ between the detectors formed an experimentally anisotropy R(t): R(t) = 2
◦
◦
N(t, 180 ) − N(t, 90 ) N(t, 180◦ ) + 2N(t, 90◦ )
(1)
The least-square fitting procedure to the experimental PAC spectra using the DEPACK program [13] assumed the presence of one magnetic dipole (MDI) and two electric quadrupole interactions (EQI) in the Ni–Hf alloy: R(t) = Aeff 22
i fi G22 (t)
(2)
i
is the experimental anisotropy factor and the sum runs over all sites i where Aeff 22 i (t) is containing having different hyperfine fields and the perturbation factor G22 information on the ith hyperfine interaction present in the sample. The MDI is related to the magnetic hyperfine field Hhf acting on 181 Ta probe according to the relation: ωL =
gN Hhf h ¯
(3)
and is characterized by the Larmor frequency ωL , a Lorentzian frequency distribution width parameter , the nuclear g-factor g = 1.316(12) [14] and the nuclear magneton N . The EQI at the 181 Ta site corresponds to the electric quadrupole frequency: ωQ =
eQVZZ 4I(2I − 1)¯h
(4)
with the electric quadrupole frequency distribution width ı. Vzz is the largest component of the EFG tensor and = (Vxx − Vyy )/Vzz is its asymmetry parameter, Q = 2.36(5)b is the quadrupole moment for the 181 Ta intermediate level I = 5/2 of the measured ␥–␥ cascade [15].
3. Results and discussion The representative PAC spectra of the polycrystalline Ni–Hf sample measured at different temperatures are shown in Fig. 1, together with the corresponding Fourier transforms. The solid curves seen in Fig. 1 represent the results of the fitting procedure, described above. We assumed the presence of three nonequivalent sites for
Table 1 Relevant MDI parameters for nickel–2 at.% hafnium alloy T (K) 78 291 362 469 289 a
(1)
ωL
(Mrad/s)
571(2) 536(2) 488(2) 338(3) 534(2)a
181
39
Ta probe in the Ni(Hf) solid solution phase in the
1 (%)
Hhf (T)
f1 (%)
3.7(5) 0.8(5) 1.5(5) 6.1(9) 1.6(5)
9.06(3) 8.53(4) 7.74(3) 5.36(4) 8.48(3)
31(2) 16(2) 21(1) 26(2) 17(1)
Remeasured at RT after the whole temperature range has been completed.
the 181 Ta probe-ion substituting in the ferromagnetic Ni(Hf) solid solution, the paramagnetic HfNi5 and the dielectric HfO2 phases. The interaction frequency of 536 Mrad/s was derived from the experimental spectra at room temperature (RT). This interaction was absent from the spectra above the Curie point of nickel (TC = 631 K). According to the results of previous PAC measurements on samples with very low Hf content [16,17,18], the corresponding interaction (given in Table 1) was assigned to the substitutional site of 181 Ta ion-probe in the ferromagnetic Ni lattice. The magnetic hyperfine field is decreasing with increasing temperature, as seen from Table 1. The sign of the internal field was not measured. Considering the fraction of ∼16% of probe atoms (Table 1) experiencing Hhf in the Ni–Hf sample at RT, it is indicative that the actual Hf concentration in the Ni(Hf) phase is much lower than the predicted maximum Hf solid solubility in the nickel matrix. In order to make an additional estimate of Hf content in the nickel–2 at.% hafnium alloy, we used the empirical equation of Ref. [7] which gave 0.33 at.% at RT. Moreover, we found that the same equation in the case of previously investigated nickel–0.2 at.% hafnium alloy also gave a good prediction of 0.22 at.%. The values of Hhf in the nickel–2 at.% hafnium alloy at higher temperatures are lower than the ones obtained for Hhf in the nickel–0.2 at.% hafnium system [18]. The fractions of probe atoms experiencing weak quadrupole frequency (3 Mrad/s at RT) with anomalous temperature dependence were significant (49–67%). The corresponding EQI parameters are
Fig. 1. PAC spectra with corresponding Fourier transforms for 181 Ta probe in the nickel–2 at.% hafnium alloy at the indicated measuring temperatures.
40
A. Umi´cevi´c et al. / Journal of Alloys and Compounds 475 (2009) 38–41
Fig. 3. Temperature variations of the quadrupole interaction parameters of 181 Ta in the HfO2 contamination in the nickel–2 at.% hafnium alloy; () remeasured at RT after the whole temperature range has been completed.
procedure above 900 K. A similar effect was observed for Co–Hf alloys after the same annealing procedure above 900 K [25]. (2)
Fig. 2. Temperature evolution of the quadrupole frequency ωQ and frequency distribution width ı2 for Ta nuclei in the defective HfNi5 phase present in the nickel–2 at.% hafnium alloy due to restricted solid solubility of Hf in Ni; () remeasured at RT after the whole temperature range has been completed. 181
presented in Fig. 2. The rather large frequency distribution parameter ı2 (18–51%) indicates a defect associated 181 Ta site. As seen from the previous experiments [11], distortions are present in the fcc HfNi5 phase. Therefore, this EQI could be ascribed to Ta probe substituting in irregular HfNi5 phase. The anomalous temperature evolution of the quadrupole frequency values (in Fig. 2) may depend on the defect mobility at certain temperatures. The temperature independent EFG asymmetry parameter 2 ≈ 1 fitted well the experimental spectra in the whole measured range. The assignment of the weak EQI to 181 Hf/181 Ta in the defective HfNi5 phase present in the sample, has not been undoubtedly clarified. The experimental observations argue against any interpretation involving Hf clustering, since it requires much stronger EQI than the fitted one [19,20,21]. Also, since there is considerable impurity/host size misfit, it is likely to expect lattice defects around the Hf impurity atom in nickel lattice. Up to date, as reviewed by Pleiter and Hohenemser [22], the studies on defects in nickel lattice were performed using 111 Cd PAC probe. The majority of the observed 111 Cd defect sites in nickel experienced the damped magnetic hyperfine field in regard to the one at 111 Cd substitutional site in Ni [22]. Our fitting procedure for the measured PAC spectra gave the best fit without an assumption of another Larmor frequency. However, we cannot completely exclude the contributions from 181 Ta sites experiencing various EQIs caused by the neighboring lattice defects present at grain boundaries of mixing phases. Unexpectedly, the analysis of PAC spectra above the magnetic ordering temperature required introduction of a third hyperfine interaction: a quadrupole frequency (128 Mrad/s at RT) was present in the experimental spectra up to 1131 K, as demonstrated in Fig. 1 for the experimental spectrum at 747 K. The temperature dependence of corresponding EQI parameters are shown in Fig. 3. After examined in the whole measured range, these values coincided with those previously known for the monoclinic HfO2 [23,24]. The quadrupole interaction data presented in Fig. 3 are unambiguous proof of an oxidation process in the sample due to the annealing
4. Summary We have studied the magnetic dipole and electric quadrupole interactions of the 181 Ta probe substituting in various phases present in the nickel–2 at.% hafnium alloy due to the restricted solid solubility of Hf in Ni. A decreasing of the magnetic hyperfine field acting on probe in the ferromagnetic Ni(Hf) solid solution with increasing temperature has been observed. The weak EQI with unusual temperature dependence is ascribed to 181 Ta residing in the defective HfNi5 precipitate, although alternative interpretations (probe sites at grain boundaries) cannot be completely disregarded. The annealing procedure above 900 K resulted in the oxidation process forming HfO2 precipitates. Acknowledgements This work has been supported by the grant No. 141022G from the Serbian Ministry of Science. We would like to thank Gerhard Schumacher and Werner Rönnfeldt for the preparation of the samples. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
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