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Anomalous Effect of Pressure on the Curie Temperature in Mechanically Alloyed Fe-Ni Invar
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ScienceDirect
ACTA METALLURGICA SINICA (ENGLISH LETTERS)
Acta Metall. Sin. (Engl. Lett.) Vol. 20 No. 3 pp205-209 Jun. 2007
www.arns,org.cn
ANOMALOUS EFFECT OF PRESSURE ON THE CURIE TEMPERATURE IN MECHANICALLY ALLOYED Fe-Ni INVAR X . H . Wei* Department of Electronic Science, Huizhou University, Huizhou 516007, China
F. Ono Department of Physics, Okayama University, 3-1-1 Tsushima-Naka, Okayama 700-8530,Japan Manuscript received 21 September 2006; in revised form 6 December 2006
Measurements of magnetic susceptibility in mechanically alloyed Fe-Ni Invar alloys were taken under pressures up to 7.SGPa. The rate of decrease in the Curie temperature for 700°C annealed specimen was larger than that annealed at lOOO't7. This result can be explained by considering the fact that the width of the concentration fluctuation becomes larger in the specimen annealed at lower temperature. KEY WORDS invar alloy; Fe-Ni, mechanical alloying; Curie temperature
1. Introduction Fe-Ni alloys with Ni-concentration around 35at.% are called Invar alloys, which show anomalously small thermal expansion coefficients around room temperature, and hence, these have wide practical applications in various fields since the discovery more than a century ago"]. This anomaly is interpreted to exist because of the large positive value of the magnetovolume effect that cancels the normal positive part of the thermal expansion caused by unharmonic terms of the lattice vibration[241. Besides the anomalously low thermal expansion coefficient, Fe-Ni Invar alloysshow various other anomalies both in mechanical and magnetic properties. Anomalies in magnetic properties are unusual decrease of the magnetic moment from the Slater-Paulingcurve with the decrease of Ni-concentration and increase of the high field susceptibility. These anomalies are interpreted as a result of the instability of the 3d-band ferromagnetism in fcc metals and alloys. Owing to the existence of a large sharp peak at the top of the 3d-electron band of the fcc phase, the ferromagnetic state becomes energetically unstable when the number of outer electrons is decreased by the decrease in the Ni-concentration beyond the Invar regiod5]. According to this model, the ferromagnetic state becomes unstable at a critical concentration of Ni just 'Corresponding author. Tel.: +86 752 2359966; fax: +86 752 2527271. E-mail address: [email protected] (X.H. Wei)
.
206 .
below 30at.%, and therefore, the spontaneous magnetization decreases abruptly to zero at this point. However, the observed decrease of the magnetic moment from the Slater-Paulingcurve is not as sharp as expected. To explain the actual curvature of the decrease of the magnetic moment from the Slater-Pauling curve, several models have been proposed taking into account the existence of the fluctuation of alloy concentration. Kachi et $. [61 adopted a Gaussian distribution of the concentration fluctuation and explained the experimental curve. Komura and TakedaL7counted an effective local concentration around an Fe atom and also succeeded in explaining their results of small angle neutron scattering experiments. It has been frequently pointed out that the Fe-concentration in Fe-Ni Invar alloy is close to the critical value where martensitic transformation takes place, and inclusion of the bcc-phase through concentration fluctuation will significantly affect its magnetic properties. However, no direct evidence for the existence of the bcc-phase has been observed in carehlly annealed Fe-Ni Invar alloys. With the use of an X-ray diffuse scattering technique, the existence of tiny scale lattice distortion, which was extremely small to be observed by an electron microscope,was detected in a carefilly annealed single crystal disc of Fe-35.4at.% Ni Invar alloy[*].The lattice distortion, which was observed only in the low temperature region below IOOK, was a local shear stress along the 01 1-plane. This stress can be generated around a small local part in a specimen where the Ni-concentration is lower than the matrix. Thus the concentration fluctuation is expected to play an important role in both the mechanical and magnetic properties of Fe-Ni Invar alloys. One of the good candidates for introducing concentration fluctuation is to adopt a ball milling technique introduced by Kuhrt and Schultz['l, who synthesized Fe-Ni alloys by ball milling and found that the kinetics of the martensitic transformation are significantly moditied when compared with coarse grain as-cast alloys. Hong and Fultz"'] showed that the bcc-phase destabilized with ite increase of milling intensity. This ball milling technique has been utilized to introduce concentration fluctuation into Fe-Ni Invar alloys['1~121.
2. Synthesis of the Specimen Using Mechanical Alloying Elemental Fe and Ni powders of particle size of 100 mesh and purity of 99.9% were milled for 75h in a planetary ball mill in argon atmosphere[""z1using stainless steel balls and containers. The ball-to-powder weight ratio was 10 : 1. This ratio was two times larger than that used by Hong and Fultz[''], and was considered to be large enough to make perfect alloys. The intensity of the milling expressed by the ball velocity hitting was 5 d s . This value was again stronger than the value of 3 m / s adopted by them, and was again enough to obtain perfect alloys. The products were annealed at 700 and 1000°C for Ih in vacuum shielded quartz tubes and were then quenched down to room temperature. The specimen annealed at 1000°C can be considered as a bulk material of which the physical characteristics are almost equal to those made by usual melting. Comparing the Curie temperature of the 1000°C annealed specimen with the standard curve established for the Fe-Ni alloy system, the Ni-concentration of the present alloys is determined to be 30.9at.%.
3. Measurements of AC Susceptibility Under High Pressure Measurements of AC susceptibility for specimen annealed at 700 "C were taken under hydrostatic pressures up to 7.5GPa using a cubic anvil press operated by a 250t hydraulic press. The specimen was put into a cylinder and was then placed in the high pressure medium of flolinate with a pick-up coil system wound around it. Measurement for 1000°C annealed specimen was taken up to 1.4GPa using a gaspressure operated high pressure cell made of nonmagnetic Cu-Be alloy.
.
207 .
4. ResultsandDiscussion The observed AC susceptibilityus. temperature curves under hydrostatic pressures up to 7.5GPa for Fe-30.9at.%Ni Invar alloy annealed at 700"Cafter mechanical alloying are shown in Fig. 1. Since the highest attainable temperature was limited to 400K, it was difficult to obtain the full susceptibility-temperature curve. However, the curves seen in Fig.1 are almost parallel to each other, and it is possible to obtain a relative change in Tc from the initial decrease of the start points of curves. The Curie temperatures determined for mechanical Invar alloys annealed at 700 and 1000°C are plotted in Fig.2 against pressure; it is seen in Fig.2 that the rate of decrease in 1000°C annealed specimen is lesser than that of the 700°C annealed one, which has a wider concentration fluctuation. This tendency was also observed for an Fe-Ni Invar alloy with Ni concentration of 3 1.9at.%, which was slightly larger than the present specimen[l2]. According to the experimental results obtained by Hong and Fultz"'], most physical properties in mechanically alloyed Fe-Ni Invar alloys are saturated aRer annealing above 600°C.Therefore, it seems very strange that in the present experiments, the pressure coefficient of the Curie temperature still varies after annealing at temperatures higher than 700°C. 15GPa This tendency can be explained by considering the effect of concentration fluctuation. As seen in Fig.3, the pressure coefficient of the Curie temperature in bulk Fe-Ni alloy series['] has a sharp peak at the Ni concentration of 30at.%. The average concentration of the present specimen is very close to this peak. Therefore, when the concentration fluctuation be100 150 200 250 300 350 400 Temperature, K comes wider as in a mechanically alloyed specimen, the average pressure coefficient of the Fig. 1 AC (alternating current) susceptibility-temperature Curie temperature decreases because of the curves for Fe-30.9at.%Ni mechanical alloy annealed at 700°C under hydrostatic pressures up to 7.5GPa. smaller contribution from both sides of the
300
o
-
Annealed at 1000°C Annealed at 700°C
5 -
h
250.
9z
Y
c-"
200
4 -
3 -
6
.
?
2 -
c" 1 -
150.
or 0
1
2 3 Pressure, GPa
4
-
5
Fig.2 Pressure dependence of the Curie temperarures Tc in Fe-30.9at.%Ni alloy annealed at 700 and 1000°Cafter mechanical alloying.
Fe,o&xNix, at %
Fig.3 Concentration dependence of the pressure coefficient of the Curie temperature in bulk Fe-Ni Invar alloys[*'.
. 208 . Flunctuation width u 6 4
8
I
5.0 -
h
s-a (3
4.5
-
4.0
-
3.5
-
. t-" u
?
2.5 -
1
9 Experimental -*--Calculated
./'
3.0 -
.
2
'
i
0
/--
/*
/ /-
J'
/' 500
600
700
800
900
1000
Annealed temperature, 'C
Fig.4 The pressure coefficient of Tc against annealed temperature. The full curve is calculated assuming a Gaussian function with the correspondingvalues for u.
peak. The average pressure coeficient of the Curie temperature for a mechanically alloyed specimen can be calculated by multiplying a Gaussian distribution function to the well established pressure coefficient of Tc us. Ni-concentration curve for bulk Fe-Ni alloys shown in Fig.3. In the previous works["], the relationship between the concentration fluctuation width (+ and the annealed temperature was established by comparing the experimentally observed Curie temperature with that calculated by assuming a Gaussian distribution for the concentration fluctuation. Using the parameter a, thus determined, the pressure coefficient of Tc was calculated for 700 and 1000°Cannealed specimens. The results were compared with the experimental values in Fig.4; it is seen in Fig.4 that the calculated curve agrees well with the experimental points.
4. Conclusions It was found from the present experiments that the absolute value of the pressure coefficient of the Curie temperature in Fe-30.9at.YoNi Invar mechanical alloy becomes smaller for alloys with wider concentration fluctuations. This fact can be explained by assuming a Gaussian distribution function for the concentration fluctuation.
Acknnwledgements-This work was supported by the Grant-in-Aid for Scientijk Research ( C ) of the Ministiy of Education, Culture, Sports and Technology of Japan. (No. 11650681).
REFERENCES 1 C.E. Guillaume and C.R. Acad, Science 10 (1897) 235. 2 E.F. Wasserrnan, Ferromagnetic Materials, eds. K.H.J. Buschow and E.P. Wohlfarth (Vo1.5, North-Holland, Amsterdam, 1990) p.238. 3 F. Ono, H. Maeta, and L. Bang, The Inuar Efect: A Centennial Symposium, ed. J. Wittenauer (The Minerals, Metals & Materials SOC.,Tai Bei, 1997) p.197.
. 209
.
F. Ono, H. Maeta, and T. Kittaka, Physica B 119 (1983) 78. T. Mizoguchi, J. Phys. SOC.Jpn. 25 (1968) 904. S. Kachi, H. Asano, and N. Nakanishi, J. Phys. Soc. Jpn. 25 (1968) 285. S. Komura and T. Takeda, J. Magn. Magn. Mater. 10 (1979) 191. F. Ono, H. Maeta, and L. Bang, The Invar Effect: A Centenid Symposium, ed. J. Wittenauer (The Minerals, Metals and Materials SOC.,Tai Bei, 1997) p.197. 9 C. Kuhrt and L. Schultz,J. Appl. Phys. 73 (1993) 1975. 10 L.B. Hong and B. Fultz, J. Appl. Phys. 79 (1996) 3946. 11 S. Wei, K. Hayashi, and F. Ono, J. Magn. Soc. Jpn. 23 (1999) 391. 12 S. Wei, R. Duraj, R. Zach, M. Matsushita, A. Takahashi, H. Inoue, F. Ono, H. Maeta, A. Iwase, and S. Endo, J. Phys. Condenced Matter. 14 (2002) 11081.
4 5 6 7 8
ScienceDirect
ACTA METALLURGICA SINICA (ENGLISH LETTERS)
Acta Metall. Sin. (Engl. Lett.) Vol. 20 No. 3 pp205-209 Jun. 2007
www.arns,org.cn
ANOMALOUS EFFECT OF PRESSURE ON THE CURIE TEMPERATURE IN MECHANICALLY ALLOYED Fe-Ni INVAR X . H . Wei* Department of Electronic Science, Huizhou University, Huizhou 516007, China
F. Ono Department of Physics, Okayama University, 3-1-1 Tsushima-Naka, Okayama 700-8530,Japan Manuscript received 21 September 2006; in revised form 6 December 2006
Measurements of magnetic susceptibility in mechanically alloyed Fe-Ni Invar alloys were taken under pressures up to 7.SGPa. The rate of decrease in the Curie temperature for 700°C annealed specimen was larger than that annealed at lOOO't7. This result can be explained by considering the fact that the width of the concentration fluctuation becomes larger in the specimen annealed at lower temperature. KEY WORDS invar alloy; Fe-Ni, mechanical alloying; Curie temperature
1. Introduction Fe-Ni alloys with Ni-concentration around 35at.% are called Invar alloys, which show anomalously small thermal expansion coefficients around room temperature, and hence, these have wide practical applications in various fields since the discovery more than a century ago"]. This anomaly is interpreted to exist because of the large positive value of the magnetovolume effect that cancels the normal positive part of the thermal expansion caused by unharmonic terms of the lattice vibration[241. Besides the anomalously low thermal expansion coefficient, Fe-Ni Invar alloysshow various other anomalies both in mechanical and magnetic properties. Anomalies in magnetic properties are unusual decrease of the magnetic moment from the Slater-Paulingcurve with the decrease of Ni-concentration and increase of the high field susceptibility. These anomalies are interpreted as a result of the instability of the 3d-band ferromagnetism in fcc metals and alloys. Owing to the existence of a large sharp peak at the top of the 3d-electron band of the fcc phase, the ferromagnetic state becomes energetically unstable when the number of outer electrons is decreased by the decrease in the Ni-concentration beyond the Invar regiod5]. According to this model, the ferromagnetic state becomes unstable at a critical concentration of Ni just 'Corresponding author. Tel.: +86 752 2359966; fax: +86 752 2527271. E-mail address: [email protected] (X.H. Wei)
.
206 .
below 30at.%, and therefore, the spontaneous magnetization decreases abruptly to zero at this point. However, the observed decrease of the magnetic moment from the Slater-Paulingcurve is not as sharp as expected. To explain the actual curvature of the decrease of the magnetic moment from the Slater-Pauling curve, several models have been proposed taking into account the existence of the fluctuation of alloy concentration. Kachi et $. [61 adopted a Gaussian distribution of the concentration fluctuation and explained the experimental curve. Komura and TakedaL7counted an effective local concentration around an Fe atom and also succeeded in explaining their results of small angle neutron scattering experiments. It has been frequently pointed out that the Fe-concentration in Fe-Ni Invar alloy is close to the critical value where martensitic transformation takes place, and inclusion of the bcc-phase through concentration fluctuation will significantly affect its magnetic properties. However, no direct evidence for the existence of the bcc-phase has been observed in carehlly annealed Fe-Ni Invar alloys. With the use of an X-ray diffuse scattering technique, the existence of tiny scale lattice distortion, which was extremely small to be observed by an electron microscope,was detected in a carefilly annealed single crystal disc of Fe-35.4at.% Ni Invar alloy[*].The lattice distortion, which was observed only in the low temperature region below IOOK, was a local shear stress along the 01 1-plane. This stress can be generated around a small local part in a specimen where the Ni-concentration is lower than the matrix. Thus the concentration fluctuation is expected to play an important role in both the mechanical and magnetic properties of Fe-Ni Invar alloys. One of the good candidates for introducing concentration fluctuation is to adopt a ball milling technique introduced by Kuhrt and Schultz['l, who synthesized Fe-Ni alloys by ball milling and found that the kinetics of the martensitic transformation are significantly moditied when compared with coarse grain as-cast alloys. Hong and Fultz"'] showed that the bcc-phase destabilized with ite increase of milling intensity. This ball milling technique has been utilized to introduce concentration fluctuation into Fe-Ni Invar alloys['1~121.
2. Synthesis of the Specimen Using Mechanical Alloying Elemental Fe and Ni powders of particle size of 100 mesh and purity of 99.9% were milled for 75h in a planetary ball mill in argon atmosphere[""z1using stainless steel balls and containers. The ball-to-powder weight ratio was 10 : 1. This ratio was two times larger than that used by Hong and Fultz[''], and was considered to be large enough to make perfect alloys. The intensity of the milling expressed by the ball velocity hitting was 5 d s . This value was again stronger than the value of 3 m / s adopted by them, and was again enough to obtain perfect alloys. The products were annealed at 700 and 1000°C for Ih in vacuum shielded quartz tubes and were then quenched down to room temperature. The specimen annealed at 1000°C can be considered as a bulk material of which the physical characteristics are almost equal to those made by usual melting. Comparing the Curie temperature of the 1000°C annealed specimen with the standard curve established for the Fe-Ni alloy system, the Ni-concentration of the present alloys is determined to be 30.9at.%.
3. Measurements of AC Susceptibility Under High Pressure Measurements of AC susceptibility for specimen annealed at 700 "C were taken under hydrostatic pressures up to 7.5GPa using a cubic anvil press operated by a 250t hydraulic press. The specimen was put into a cylinder and was then placed in the high pressure medium of flolinate with a pick-up coil system wound around it. Measurement for 1000°C annealed specimen was taken up to 1.4GPa using a gaspressure operated high pressure cell made of nonmagnetic Cu-Be alloy.
.
207 .
4. ResultsandDiscussion The observed AC susceptibilityus. temperature curves under hydrostatic pressures up to 7.5GPa for Fe-30.9at.%Ni Invar alloy annealed at 700"Cafter mechanical alloying are shown in Fig. 1. Since the highest attainable temperature was limited to 400K, it was difficult to obtain the full susceptibility-temperature curve. However, the curves seen in Fig.1 are almost parallel to each other, and it is possible to obtain a relative change in Tc from the initial decrease of the start points of curves. The Curie temperatures determined for mechanical Invar alloys annealed at 700 and 1000°C are plotted in Fig.2 against pressure; it is seen in Fig.2 that the rate of decrease in 1000°C annealed specimen is lesser than that of the 700°C annealed one, which has a wider concentration fluctuation. This tendency was also observed for an Fe-Ni Invar alloy with Ni concentration of 3 1.9at.%, which was slightly larger than the present specimen[l2]. According to the experimental results obtained by Hong and Fultz"'], most physical properties in mechanically alloyed Fe-Ni Invar alloys are saturated aRer annealing above 600°C.Therefore, it seems very strange that in the present experiments, the pressure coefficient of the Curie temperature still varies after annealing at temperatures higher than 700°C. 15GPa This tendency can be explained by considering the effect of concentration fluctuation. As seen in Fig.3, the pressure coefficient of the Curie temperature in bulk Fe-Ni alloy series['] has a sharp peak at the Ni concentration of 30at.%. The average concentration of the present specimen is very close to this peak. Therefore, when the concentration fluctuation be100 150 200 250 300 350 400 Temperature, K comes wider as in a mechanically alloyed specimen, the average pressure coefficient of the Fig. 1 AC (alternating current) susceptibility-temperature Curie temperature decreases because of the curves for Fe-30.9at.%Ni mechanical alloy annealed at 700°C under hydrostatic pressures up to 7.5GPa. smaller contribution from both sides of the
300
o
-
Annealed at 1000°C Annealed at 700°C
5 -
h
250.
9z
Y
c-"
200
4 -
3 -
6
.
?
2 -
c" 1 -
150.
or 0
1
2 3 Pressure, GPa
4
-
5
Fig.2 Pressure dependence of the Curie temperarures Tc in Fe-30.9at.%Ni alloy annealed at 700 and 1000°Cafter mechanical alloying.
Fe,o&xNix, at %
Fig.3 Concentration dependence of the pressure coefficient of the Curie temperature in bulk Fe-Ni Invar alloys[*'.
. 208 . Flunctuation width u 6 4
8
I
5.0 -
h
s-a (3
4.5
-
4.0
-
3.5
-
. t-" u
?
2.5 -
1
9 Experimental -*--Calculated
./'
3.0 -
.
2
'
i
0
/--
/*
/ /-
J'
/' 500
600
700
800
900
1000
Annealed temperature, 'C
Fig.4 The pressure coefficient of Tc against annealed temperature. The full curve is calculated assuming a Gaussian function with the correspondingvalues for u.
peak. The average pressure coeficient of the Curie temperature for a mechanically alloyed specimen can be calculated by multiplying a Gaussian distribution function to the well established pressure coefficient of Tc us. Ni-concentration curve for bulk Fe-Ni alloys shown in Fig.3. In the previous works["], the relationship between the concentration fluctuation width (+ and the annealed temperature was established by comparing the experimentally observed Curie temperature with that calculated by assuming a Gaussian distribution for the concentration fluctuation. Using the parameter a, thus determined, the pressure coefficient of Tc was calculated for 700 and 1000°Cannealed specimens. The results were compared with the experimental values in Fig.4; it is seen in Fig.4 that the calculated curve agrees well with the experimental points.
4. Conclusions It was found from the present experiments that the absolute value of the pressure coefficient of the Curie temperature in Fe-30.9at.YoNi Invar mechanical alloy becomes smaller for alloys with wider concentration fluctuations. This fact can be explained by assuming a Gaussian distribution function for the concentration fluctuation.
Acknnwledgements-This work was supported by the Grant-in-Aid for Scientijk Research ( C ) of the Ministiy of Education, Culture, Sports and Technology of Japan. (No. 11650681).
REFERENCES 1 C.E. Guillaume and C.R. Acad, Science 10 (1897) 235. 2 E.F. Wasserrnan, Ferromagnetic Materials, eds. K.H.J. Buschow and E.P. Wohlfarth (Vo1.5, North-Holland, Amsterdam, 1990) p.238. 3 F. Ono, H. Maeta, and L. Bang, The Inuar Efect: A Centennial Symposium, ed. J. Wittenauer (The Minerals, Metals & Materials SOC.,Tai Bei, 1997) p.197.
. 209
.
F. Ono, H. Maeta, and T. Kittaka, Physica B 119 (1983) 78. T. Mizoguchi, J. Phys. SOC.Jpn. 25 (1968) 904. S. Kachi, H. Asano, and N. Nakanishi, J. Phys. Soc. Jpn. 25 (1968) 285. S. Komura and T. Takeda, J. Magn. Magn. Mater. 10 (1979) 191. F. Ono, H. Maeta, and L. Bang, The Invar Effect: A Centenid Symposium, ed. J. Wittenauer (The Minerals, Metals and Materials SOC.,Tai Bei, 1997) p.197. 9 C. Kuhrt and L. Schultz,J. Appl. Phys. 73 (1993) 1975. 10 L.B. Hong and B. Fultz, J. Appl. Phys. 79 (1996) 3946. 11 S. Wei, K. Hayashi, and F. Ono, J. Magn. Soc. Jpn. 23 (1999) 391. 12 S. Wei, R. Duraj, R. Zach, M. Matsushita, A. Takahashi, H. Inoue, F. Ono, H. Maeta, A. Iwase, and S. Endo, J. Phys. Condenced Matter. 14 (2002) 11081.
4 5 6 7 8