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Mössbauer analysis of submicrometer grained iron
Scripta METALLURGICA et MATERIALIA
Vol. 25, pp. 2717-2722, 1 9 9 1 Printed in the U.S.A.
Pergamon Press plc All rights reserved
MOSSBAUER ANALYSIS OF SUBMICROMETER GRAINED IRON
R. Z. Valiev',, R. R. Mulyukov', V. V. Ovchinnikov", V. A. Shabashov'" Institute of Metals Superplasticity Problems, •, U.S.S.R. Academy of Sciences, Khalturina, 39, Ufa 450001, U.S.S.R. Ural Department, U.S.S.R. Academy of Sciences, Sverdlovsk 620019, U.S.S.R. (Received April 2, 1991) (Revised September 26, 1991) Introductio~ Much attention has been devoted lately to the development of methods for producing submicrometer grained (SMG) materials and the study of their properties /i-6/. These meterials are polycrystals with very small, about O.i ~m, crystallites (grains). Their electric, magnetic, thermal, and diffusional characteristlcs and mechanical properties differ considerably from those of materials with conventional (coarse crystalline) structures, which is due to the large area of intercrystalline boundaries (IB). The latter fact leads to the conclusion that submicrometer grained materials are similar to so called nanocrystalline materials /7-9/ with the grain size ranging from 5 to 10 nm. However, nanocrystalline materials are produced by means of powder metallurgy and they have a residual porosity of about 5 to 7~. This i~ not the case in SMG samples produced by the strain-heating method /2-6/. Hence, they can be looked at as objects convenient for studing intercrystalline boundaries using conventional methods of solid state physics. Quite recently, in /i0/, a study using M6ssbauer spectroscopy indicated that in SMG iron there are atoms in states, distinguished by the parameters of their hyperfine electric a n d magnetic structures. One of the states corresponds to the grain phase (GP) and the other one is associated with the grain boundary phase (GBP).The fact that the hyperfine structure (HFS) parameters of the grain boundary phase atoms are strictly enough fixed enables us to speak about the grain boundary phase characteristic thickness which is an order of magnitude larger than the intercrystalline boundary thickness deduced from crystallographic observations The difference of GP atoms from GBP ones in manifested first of all in their dynamic behaviour. Thus, at present, there are a number of facts that provide convincing evidence for the existence of GBP in SMG metals. At the same time, many features of the GBP nature are still vague. The present paper is concerned with performing Mossbauer investigations on SMG iron samples with different mean grain sizes and at different temperatures in order to clarify the nature of the grain boundary phase. Experimental Procedure This paper as well as the earlier one /10/ deals with a study of SMG iron samples (99.97~ pure) using the method of Mossbauer spectroscopy. To produce a SMG structure, the samples (as in [2,3]) were subjected to intensive plastic torsional straining under pressure until a true logarithmic deformation of e=6 was achieved (the time of straining was less than 5 min ), followed by annealing at different temperatures. As a result, SMG samples with different mean grain sizes were obtained. For comparison, coarse grained samples with a mean grain size of 15 ~m were studied. Structures were studied in an electron microscope JEM-2000 EX.
2717 0036-9748/91 $3.00 + .00 Copyright (c) 1991 Pergamon Press plc
2718
SUBMICROMETER GRAINED Fe
The Mossbauer analysis of the samples at room temperature mathematical processing of experimental spectra were performed as The sample with d=0.23 ~m was studied at 77 K.
Vol. 25, No. 12
and the in [i0].
Results and Discussion Figure I shows the SMG iron structure produced by annealing after intensive plastic deformation. One can see that anneallng at 400°C for 1 hour has led to the formation of a granular structure with random high-angle boundaries. A mean grain size as defined by means of the linear intercept method was 0.12 ~m. The density of dislocations was about 108 cm -2. When the temperature of annealing rose to 400°C , the grains size became 0.22 ~m and the density of dislocations in the grains decreased. Annealing at 500 and 600 ° C for i hour yielded samples with mean grain sizes of 0.26 and 0.65 ~m respectively. The Mossbauer spectroscopy of the SMG iron samples with different mean grain sizes performed at room temperature has indicated that each experimental spectrum is a superposition of two subspectra (i and 2)(Fig. 2), referring to the grain and the grain boundary phase, respectively. Mathematically processed results presented in Table i indicate that for each mean grain size value, subspectra i and 2 have very similar widths for the respective peaks at i/2 of the peak height, but they are considerably different in their HFS parameters (efficient magnetic field Her r on the iron atom nuclei and isomeric shift ~) as well as in the values of the intensities I i and I2 The presence of the second subspectrum cannot be explained by the contamination of the sample. Firstly, sufficiently pure iron was studied. Secondly, a dependence of the second spectrum intensity I2on grain size has been observed in the experiment: with an increase of the grain size the value of 12decreases and practically disappears at d>2 ~m. Thirdly, the presence of interstitial impurities in Fe usually results in a positive isomeric shift [ii], while a negative shift is obtained in the experiment. All this confirms data reported in /i0/ on the existence of two different states of iron atoms in the SMG material under study which refer to the grain and grain boundary phase, respectively. The values of effective field, isomeric shift and the width of all the six peaks for the more intensive subspectrum number i corresponding to t h e grain phase, ~n fact coincide with the parameters of the calibrating (coarse grained) ~-Fe. The above listed parameters for subspectrum 2, referring to the GBP, are practically constant in spite of the changes of the grain size of the SMG iron. This fact indicates that the grain boundary phase h a s a physical nature which does not vary despite the difference in the graln sizes. The relative intensity of subspectrum 2 decreases with an increase of the grain size. To evaluate the thickness of the GBP (i.e. the physical width of IB) Ad and to find out how this value is related to the grain size, the relationship of the experimental values of the GBP atom fractlon c in the structure to the inverse quantity of the corresponding mean grain size, was analyzed (Fig. 3).The experimental fractional size was established by the relative integral intensity of subspectra c=IJ(I1+I2). It turned out that the data satisfactorily match the straight line, which indicates that the Ad value in studied range of grain size is approximately constant, because c ~.Ad/d, when Ad { d. Calculations indicated that the thickness of GBP (physical width of IB) was nd=(8.4±0.8)nm. This result agrees with values obtained through the measurement of specific heat and saturation magnetization and by the earlier
Vol. 25, No. 12
SUBMICROMETER GRAINED Fe
2719
M6ssbauer analyses of SMG materials [4, 5, i0]. It is important to note that very similar values of the peak widths of subspectra 1 and 2 indicate [Ii] that the GBP and GP crystalline structures are very much alike. This agrees with the HREM observatlons of atomic planes near the intercrystalline boundary and within the grains of nanocrystalline palladium [12]. The reduction of a hyperfine field Her f observed in the GBP can be assgciated with a number of peculiarities of the GBP electron structure /ii, 13/-: I) a decrease in the magnetic moment of the iron, atoms as compared with the grain size; 2) a reduction of the concentration of conduction electrons; 3) weakening of the direct and/or indirect (through conduction electrons) exchange of iron atoms in GBP and a respective decrease in the z-projection of the iron atom magnetic moments. The contribution of each of these effects must be evaluated additionally. A negative shift of the GBP subspectrum as computed to the GP subspectrum cannot be assigned to a trivial effect of the increasing lattice parameters in the vicinity of the crystallographic intercrystalllne boundary, which could result in the reduction of the density p of GBP, because according to the relationship 8:-l.4Ap/p mm/c /ii/, this would lead to a positive shift. Proceeding from the existing theoretical postulates, two possible explanations of the nature of the negative shift of the GBP subspectrum can be offered. One of them is associated with a possibility of increasing the density of s- electrons on the STFe nuclei due to the transition of %ome of the 4s conduction electrons into states localized on the iron atoms , i.e. by reducing "metallic" features in the interatomic bonds in GBP. The other explanation is a possible softening of the phonon spectrum (a decrease in the Debye temperature eD). To evaluate the Debye temperature of GBP, M6ssbauer spectra of samples with a mean grain size 0.23 um obtained at room temperature and at 77 K were used. The experiments confirmed that GBP also exists at 77 K. Computer data for the experimental spectra are given in Table 2. As the intensity of the Mossbauer spectrum lines is proportional to probability f of the phononless absorption of N-quanta, the diminishing ratio of the subspectrum 2 intensity relative to that of subspectrum 1 accompanying the rise of temperature indicates that the decrease of probability f with a rise in temperature is faster in the GBP than in the GP. This fact can be explained by a lower Debye temperature of the GBP as compared to the GP. The f value for the GBP was calculated using the experimgntal data obtained and the Debye approximation for the probability f [ii]:
3ER f
:
exp ~
T •
+
~
2 •
0
d
(i)
where E R is the free atom recoil energy,and k is the Boltzmann constant. In the calculation we made the natural assumption that the GP Debye temperature is equal to the eDvalue for monocrystalline or coarse grained Iron at 467K /14/.It was also assumed that the fraction of the GBP atoms in the structure was the same both at 77 K and room temperature (otherwise, if according to /5/ we assume that the GBP fraction grows with a rise of the temperature, the difference obtained below in the e D values for the GBP and the GP will increase).Calculations made with these assumptions indicated that the Debye " individually or in combination "" The transition of 3d-electrons shielding s-electrons from the iron atoms is hardly possible.
2720
SUBMICROMETER GRAINED Fe
Vol. 25, No. 12
temperature of the GBP is (240±50)K, i.e. lower by about 200K than that of the GP. The reduction of the efficient Debye temperature in nanocrystalline iron was also observed in [9]. Returning to the analysis of the shift of the GBP subspectrum against the GP subspectrum, we can see that a rather high difference in the Debye temperatures of GBP and GP is unable to provide an unequivocal explanation of the shift of the subspectra ~=-(0.3+0.4) mm/s observed in the experiment. The value of this shift calculated using the experimentally obtained difference in the temperatures e and the following formula /11/:
a= - 2 . 7 1 5 . 1 0 -4
D + 8T - ~
0
d
(2)
is -0.02 mm/s a t room temperature and - . 0 5 mm/s a t 77 K, which in a b s o l u t e f i g u r e s i s an o r d e r of magnitude s m a l l e r than t h a t o b t a i n e d e x p e r i m e n t a l l y . Hence, to provide a d e f i n i t i v e e x p l a n a t i o n of the s u b s p e c t r a s h i f t we should assume t h a t the l o c a l i z a t i o n of conduction e l e c t r o n s takes place in the GBP too. Results presented in this paper are in good agreement with results obtained during the study of other magnetic and thermal properties of SMG metals performed earlier /4, 5/. The observed increase in the specific heat of SMG metal is consistent with the specific heat of the GBP induced by a drop in the GBP Debye temperature. A decrease in the saturation magnetization of a ferromagnetic SMG nickel agrees with a possible decrease in the GBP atom magnetic moment. A decrease in the magnetic Curie Temperature T c of a submicrometer grained nickel as compared to T c of a coarse grained metal can be associated with the presence of GBP in the SMG metal, this GBP having a lower value of efficient magnetic field Her r and a higher (see Table 2) rate of the H e f f decrease with temperature as compared to the GP. Conclusions i. Mossbauer analysis of iron with mean grain sizes ranging from 0.12 to 0.65 ~m have conflrmed the existence of a specific state of atoms in a SMG metal identified with the existence of a specific.grain boundary phase. The GBP thickness (physical width of intercrystalline boundaries) in the studied range of grain sizes is approximatally constant and amounts to (8.4±0.8) rim. 2. No difference in the crystalline structures of the grain boundary and grain phases have been detected. The observed difference in the hyperfine structure parameter of GBP and GP testifies to the difference in their electron structures and phonon spectra, that apparently determines the difference in dynamic properties of their atoms. 3. A growth of the ratio of the GBP and GP intensity subspectra with the temperature decreasing from 300 down to 77 K has been observed. The difference in the ratios indicated that the Debye temperature of GBP was by 200 K lower than that of GP. Acknowledgements The authors would like to express their sincere thanks to A. Yu. Arkhipenko and I. M. Safarov for their help in carrying out the experiments.
i.
2.
References Morokhov I.D., Trusov L.I., and Lapovok V.I Physical Phenomena in Ultra d i s p e r s e Media.(Nauka p u b l . , Moscow, 1984) i i n R u s s i a n ) . Smirnova N.A., L e v i t V . I . , P i l y u g i n V.P., Kuznetsov R . I . , Degtyarov M.V.
Vol. 25, No. 12
SUBMICROMETER GRAINED Fe
2721
Metal.Phys.Metallogr. 62, 566(1986); Valiev R.Z., Kaibyshev O.A., KuzneCsov R.I. et a l . Proc.Acad. Sci.USSR. 301 t 864 ( 1 9 8 8 ) . 4. V a l l e y R . Z . , Mulyukov R.R., Mulyukov Kh.Ya., Novikov V.I. and Trusov L.I J . T e c h . P h y s . L e t t . 15,78 (1989). 5. V a l i e v R . Z . , Vishny-~oy Ya.D., Mulyukov R.R. and F a i n s h e i n G.S. Phys. S t a t . S o l . ( a ) 117,549 (1990). 6. I s l a m ~ a l i e v R.K7-., Akhmadeev N.A. Mulyukov R.R. and V a l i e v R.Z. Phys. S t a t . ~ o l . ( a ) 118, K27 (1990). ' 7. B i r r i n g e r R. a---n-~ G l e i t e r H., in Encyclopedia of M a t e r i a l s Science and Engineering, S u p p l . 1 , ed. by Cahn (Pergamon T r e s s , Oxford, 1988) p.339. 8. Rupp J. and B i r r i n g e r R. Phys.Rev. 36 , 788 (1987). 9. Herr U . , . J i n ~ J . , B i r r i n g e r R., Gon-~-v U. and G l e i t e r H. Appl.Phys. L e t t . 50, 472 (1987). 10. V a l i e v R . Z . , Mulyukov R.R. Ovchinnikov V.V. Phyl.Mag. L e t t . 6 2 , 253 (1990 11. L i t v i n o v V., Karakishev 0 D. and Ovchinnikov V.V. "Nuclear-G~-mma-Resonance Spectroscopy of Alloys". (Metallurgia, Moscow. 1982) (in Russian). 12. Thomas G.F., Siegel R.W. and Eastman F.A. Scr.Met.24,201 (1990): 13 Ovchinnikov V.V. and Geld P.V. Metal.Phys.Metallggr.6-S]-23 (1988) 14 American Institute of Physics Handbook. 3rd ed (Ame-ffYcan Institute of P h y s i c s , New York, 19720, p.4.
3.
Hyperfine IThe sample mean grain size, ~m
parameters Subspectrum
0.12 0.23 0.23* 0.26 0.65
of subspectra
Efficient magnetic field, Heff, kOe
TABLE 1 1 and 2 of different size
sample
mean grain
Isomeric shift against the model ~-Fe, mm/s
Width of the external peaks of spectrum at i/2 of the height mm/s
0.00 ± 0.06 -0.36 _ 0.06
0 . 3 2 __. 0 . 0 5 0.32 ± 0.05
0.00 i 0.06 -0.35 + 0.06
0 . 3 3 __. 0 . 0 5 0 . 5 4 _+ 0 . 0 5
0.00 ± 0.06 -0.25 5 0.06
0 . 3 3 -,- 0 . 0 5 0.35 5 0.05
0.00 5 0.06 -0.38 ± 0.06
0.34 + 0.05 0.38 5 0.05
1
5 5 5 5 5 5 5 5 330 5 5
2
302 5 5
0.00 ± 0.06 -0.30
0 . 3 0 ± 0.05 0.40 5 0 . 0 5
1 2 1 2 1 2 1 2
331 301 331 301 329 229 331 299
± ± ± 5 5 5 ± 5
_+ 0 . 0 6
* The data are taken from an earlier paper [i0]. TABLE 2 Hyperfine parameters of subspectra 1 and 2 of sample different temperature
with
Sub- Efficient mag- Isomeric shift Width of the exspec- netic field, against the mo- ternal peaks of del ~-Fe, mm/s spectrum at 1/2 of trum Heff(kOe) the heigth, mm/s
77K
300K
77K
300K
77K
300K
340±5
330±5
0.0
0.0
0.32±0.05 0.28±0.05
32055
30055
-0.34
-0.38
0.40±0.05 0.23±0.05
d=0.23
~m
for
Ratio of integral intensities (areas) of subspectra, I2/I l
77K
300K
0.14
0.09
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SUBMICROMETER GRAINED Fe
Vol. 25, No. 12
2 4
1.00
=o 0.96
0.g2
I 0
-5
i 5
Velocity (mm/s) Fig.1 The SMG from structure (d=0.12 ~m)" light field TEM image and microdiffraction pattern of an area of 0.75 um in diameter.
25
Fig.2 The SMG iron Mossbauer spectrum (d = 0.23 #m) at room temperature (isotope 57Co in Cr). The continuous lines indicate the division of the experimental spectrum into two subspectra.
!Y
20 .~.15 o10 5
I/d (1/jU... rn)
13
Fig.3 The dependence of the GBP atom fraction in the sample on the mean grain size. The line coresponds to the GBP thickness physical width of IB) which is 8.4 nm.
[
Vol. 25, pp. 2717-2722, 1 9 9 1 Printed in the U.S.A.
Pergamon Press plc All rights reserved
MOSSBAUER ANALYSIS OF SUBMICROMETER GRAINED IRON
R. Z. Valiev',, R. R. Mulyukov', V. V. Ovchinnikov", V. A. Shabashov'" Institute of Metals Superplasticity Problems, •, U.S.S.R. Academy of Sciences, Khalturina, 39, Ufa 450001, U.S.S.R. Ural Department, U.S.S.R. Academy of Sciences, Sverdlovsk 620019, U.S.S.R. (Received April 2, 1991) (Revised September 26, 1991) Introductio~ Much attention has been devoted lately to the development of methods for producing submicrometer grained (SMG) materials and the study of their properties /i-6/. These meterials are polycrystals with very small, about O.i ~m, crystallites (grains). Their electric, magnetic, thermal, and diffusional characteristlcs and mechanical properties differ considerably from those of materials with conventional (coarse crystalline) structures, which is due to the large area of intercrystalline boundaries (IB). The latter fact leads to the conclusion that submicrometer grained materials are similar to so called nanocrystalline materials /7-9/ with the grain size ranging from 5 to 10 nm. However, nanocrystalline materials are produced by means of powder metallurgy and they have a residual porosity of about 5 to 7~. This i~ not the case in SMG samples produced by the strain-heating method /2-6/. Hence, they can be looked at as objects convenient for studing intercrystalline boundaries using conventional methods of solid state physics. Quite recently, in /i0/, a study using M6ssbauer spectroscopy indicated that in SMG iron there are atoms in states, distinguished by the parameters of their hyperfine electric a n d magnetic structures. One of the states corresponds to the grain phase (GP) and the other one is associated with the grain boundary phase (GBP).The fact that the hyperfine structure (HFS) parameters of the grain boundary phase atoms are strictly enough fixed enables us to speak about the grain boundary phase characteristic thickness which is an order of magnitude larger than the intercrystalline boundary thickness deduced from crystallographic observations The difference of GP atoms from GBP ones in manifested first of all in their dynamic behaviour. Thus, at present, there are a number of facts that provide convincing evidence for the existence of GBP in SMG metals. At the same time, many features of the GBP nature are still vague. The present paper is concerned with performing Mossbauer investigations on SMG iron samples with different mean grain sizes and at different temperatures in order to clarify the nature of the grain boundary phase. Experimental Procedure This paper as well as the earlier one /10/ deals with a study of SMG iron samples (99.97~ pure) using the method of Mossbauer spectroscopy. To produce a SMG structure, the samples (as in [2,3]) were subjected to intensive plastic torsional straining under pressure until a true logarithmic deformation of e=6 was achieved (the time of straining was less than 5 min ), followed by annealing at different temperatures. As a result, SMG samples with different mean grain sizes were obtained. For comparison, coarse grained samples with a mean grain size of 15 ~m were studied. Structures were studied in an electron microscope JEM-2000 EX.
2717 0036-9748/91 $3.00 + .00 Copyright (c) 1991 Pergamon Press plc
2718
SUBMICROMETER GRAINED Fe
The Mossbauer analysis of the samples at room temperature mathematical processing of experimental spectra were performed as The sample with d=0.23 ~m was studied at 77 K.
Vol. 25, No. 12
and the in [i0].
Results and Discussion Figure I shows the SMG iron structure produced by annealing after intensive plastic deformation. One can see that anneallng at 400°C for 1 hour has led to the formation of a granular structure with random high-angle boundaries. A mean grain size as defined by means of the linear intercept method was 0.12 ~m. The density of dislocations was about 108 cm -2. When the temperature of annealing rose to 400°C , the grains size became 0.22 ~m and the density of dislocations in the grains decreased. Annealing at 500 and 600 ° C for i hour yielded samples with mean grain sizes of 0.26 and 0.65 ~m respectively. The Mossbauer spectroscopy of the SMG iron samples with different mean grain sizes performed at room temperature has indicated that each experimental spectrum is a superposition of two subspectra (i and 2)(Fig. 2), referring to the grain and the grain boundary phase, respectively. Mathematically processed results presented in Table i indicate that for each mean grain size value, subspectra i and 2 have very similar widths for the respective peaks at i/2 of the peak height, but they are considerably different in their HFS parameters (efficient magnetic field Her r on the iron atom nuclei and isomeric shift ~) as well as in the values of the intensities I i and I2 The presence of the second subspectrum cannot be explained by the contamination of the sample. Firstly, sufficiently pure iron was studied. Secondly, a dependence of the second spectrum intensity I2on grain size has been observed in the experiment: with an increase of the grain size the value of 12decreases and practically disappears at d>2 ~m. Thirdly, the presence of interstitial impurities in Fe usually results in a positive isomeric shift [ii], while a negative shift is obtained in the experiment. All this confirms data reported in /i0/ on the existence of two different states of iron atoms in the SMG material under study which refer to the grain and grain boundary phase, respectively. The values of effective field, isomeric shift and the width of all the six peaks for the more intensive subspectrum number i corresponding to t h e grain phase, ~n fact coincide with the parameters of the calibrating (coarse grained) ~-Fe. The above listed parameters for subspectrum 2, referring to the GBP, are practically constant in spite of the changes of the grain size of the SMG iron. This fact indicates that the grain boundary phase h a s a physical nature which does not vary despite the difference in the graln sizes. The relative intensity of subspectrum 2 decreases with an increase of the grain size. To evaluate the thickness of the GBP (i.e. the physical width of IB) Ad and to find out how this value is related to the grain size, the relationship of the experimental values of the GBP atom fractlon c in the structure to the inverse quantity of the corresponding mean grain size, was analyzed (Fig. 3).The experimental fractional size was established by the relative integral intensity of subspectra c=IJ(I1+I2). It turned out that the data satisfactorily match the straight line, which indicates that the Ad value in studied range of grain size is approximately constant, because c ~.Ad/d, when Ad { d. Calculations indicated that the thickness of GBP (physical width of IB) was nd=(8.4±0.8)nm. This result agrees with values obtained through the measurement of specific heat and saturation magnetization and by the earlier
Vol. 25, No. 12
SUBMICROMETER GRAINED Fe
2719
M6ssbauer analyses of SMG materials [4, 5, i0]. It is important to note that very similar values of the peak widths of subspectra 1 and 2 indicate [Ii] that the GBP and GP crystalline structures are very much alike. This agrees with the HREM observatlons of atomic planes near the intercrystalline boundary and within the grains of nanocrystalline palladium [12]. The reduction of a hyperfine field Her f observed in the GBP can be assgciated with a number of peculiarities of the GBP electron structure /ii, 13/-: I) a decrease in the magnetic moment of the iron, atoms as compared with the grain size; 2) a reduction of the concentration of conduction electrons; 3) weakening of the direct and/or indirect (through conduction electrons) exchange of iron atoms in GBP and a respective decrease in the z-projection of the iron atom magnetic moments. The contribution of each of these effects must be evaluated additionally. A negative shift of the GBP subspectrum as computed to the GP subspectrum cannot be assigned to a trivial effect of the increasing lattice parameters in the vicinity of the crystallographic intercrystalllne boundary, which could result in the reduction of the density p of GBP, because according to the relationship 8:-l.4Ap/p mm/c /ii/, this would lead to a positive shift. Proceeding from the existing theoretical postulates, two possible explanations of the nature of the negative shift of the GBP subspectrum can be offered. One of them is associated with a possibility of increasing the density of s- electrons on the STFe nuclei due to the transition of %ome of the 4s conduction electrons into states localized on the iron atoms , i.e. by reducing "metallic" features in the interatomic bonds in GBP. The other explanation is a possible softening of the phonon spectrum (a decrease in the Debye temperature eD). To evaluate the Debye temperature of GBP, M6ssbauer spectra of samples with a mean grain size 0.23 um obtained at room temperature and at 77 K were used. The experiments confirmed that GBP also exists at 77 K. Computer data for the experimental spectra are given in Table 2. As the intensity of the Mossbauer spectrum lines is proportional to probability f of the phononless absorption of N-quanta, the diminishing ratio of the subspectrum 2 intensity relative to that of subspectrum 1 accompanying the rise of temperature indicates that the decrease of probability f with a rise in temperature is faster in the GBP than in the GP. This fact can be explained by a lower Debye temperature of the GBP as compared to the GP. The f value for the GBP was calculated using the experimgntal data obtained and the Debye approximation for the probability f [ii]:
3ER f
:
exp ~
T •
+
~
2 •
0
d
(i)
where E R is the free atom recoil energy,and k is the Boltzmann constant. In the calculation we made the natural assumption that the GP Debye temperature is equal to the eDvalue for monocrystalline or coarse grained Iron at 467K /14/.It was also assumed that the fraction of the GBP atoms in the structure was the same both at 77 K and room temperature (otherwise, if according to /5/ we assume that the GBP fraction grows with a rise of the temperature, the difference obtained below in the e D values for the GBP and the GP will increase).Calculations made with these assumptions indicated that the Debye " individually or in combination "" The transition of 3d-electrons shielding s-electrons from the iron atoms is hardly possible.
2720
SUBMICROMETER GRAINED Fe
Vol. 25, No. 12
temperature of the GBP is (240±50)K, i.e. lower by about 200K than that of the GP. The reduction of the efficient Debye temperature in nanocrystalline iron was also observed in [9]. Returning to the analysis of the shift of the GBP subspectrum against the GP subspectrum, we can see that a rather high difference in the Debye temperatures of GBP and GP is unable to provide an unequivocal explanation of the shift of the subspectra ~=-(0.3+0.4) mm/s observed in the experiment. The value of this shift calculated using the experimentally obtained difference in the temperatures e and the following formula /11/:
a= - 2 . 7 1 5 . 1 0 -4
D + 8T - ~
0
d
(2)
is -0.02 mm/s a t room temperature and - . 0 5 mm/s a t 77 K, which in a b s o l u t e f i g u r e s i s an o r d e r of magnitude s m a l l e r than t h a t o b t a i n e d e x p e r i m e n t a l l y . Hence, to provide a d e f i n i t i v e e x p l a n a t i o n of the s u b s p e c t r a s h i f t we should assume t h a t the l o c a l i z a t i o n of conduction e l e c t r o n s takes place in the GBP too. Results presented in this paper are in good agreement with results obtained during the study of other magnetic and thermal properties of SMG metals performed earlier /4, 5/. The observed increase in the specific heat of SMG metal is consistent with the specific heat of the GBP induced by a drop in the GBP Debye temperature. A decrease in the saturation magnetization of a ferromagnetic SMG nickel agrees with a possible decrease in the GBP atom magnetic moment. A decrease in the magnetic Curie Temperature T c of a submicrometer grained nickel as compared to T c of a coarse grained metal can be associated with the presence of GBP in the SMG metal, this GBP having a lower value of efficient magnetic field Her r and a higher (see Table 2) rate of the H e f f decrease with temperature as compared to the GP. Conclusions i. Mossbauer analysis of iron with mean grain sizes ranging from 0.12 to 0.65 ~m have conflrmed the existence of a specific state of atoms in a SMG metal identified with the existence of a specific.grain boundary phase. The GBP thickness (physical width of intercrystalline boundaries) in the studied range of grain sizes is approximatally constant and amounts to (8.4±0.8) rim. 2. No difference in the crystalline structures of the grain boundary and grain phases have been detected. The observed difference in the hyperfine structure parameter of GBP and GP testifies to the difference in their electron structures and phonon spectra, that apparently determines the difference in dynamic properties of their atoms. 3. A growth of the ratio of the GBP and GP intensity subspectra with the temperature decreasing from 300 down to 77 K has been observed. The difference in the ratios indicated that the Debye temperature of GBP was by 200 K lower than that of GP. Acknowledgements The authors would like to express their sincere thanks to A. Yu. Arkhipenko and I. M. Safarov for their help in carrying out the experiments.
i.
2.
References Morokhov I.D., Trusov L.I., and Lapovok V.I Physical Phenomena in Ultra d i s p e r s e Media.(Nauka p u b l . , Moscow, 1984) i i n R u s s i a n ) . Smirnova N.A., L e v i t V . I . , P i l y u g i n V.P., Kuznetsov R . I . , Degtyarov M.V.
Vol. 25, No. 12
SUBMICROMETER GRAINED Fe
2721
Metal.Phys.Metallogr. 62, 566(1986); Valiev R.Z., Kaibyshev O.A., KuzneCsov R.I. et a l . Proc.Acad. Sci.USSR. 301 t 864 ( 1 9 8 8 ) . 4. V a l l e y R . Z . , Mulyukov R.R., Mulyukov Kh.Ya., Novikov V.I. and Trusov L.I J . T e c h . P h y s . L e t t . 15,78 (1989). 5. V a l i e v R . Z . , Vishny-~oy Ya.D., Mulyukov R.R. and F a i n s h e i n G.S. Phys. S t a t . S o l . ( a ) 117,549 (1990). 6. I s l a m ~ a l i e v R.K7-., Akhmadeev N.A. Mulyukov R.R. and V a l i e v R.Z. Phys. S t a t . ~ o l . ( a ) 118, K27 (1990). ' 7. B i r r i n g e r R. a---n-~ G l e i t e r H., in Encyclopedia of M a t e r i a l s Science and Engineering, S u p p l . 1 , ed. by Cahn (Pergamon T r e s s , Oxford, 1988) p.339. 8. Rupp J. and B i r r i n g e r R. Phys.Rev. 36 , 788 (1987). 9. Herr U . , . J i n ~ J . , B i r r i n g e r R., Gon-~-v U. and G l e i t e r H. Appl.Phys. L e t t . 50, 472 (1987). 10. V a l i e v R . Z . , Mulyukov R.R. Ovchinnikov V.V. Phyl.Mag. L e t t . 6 2 , 253 (1990 11. L i t v i n o v V., Karakishev 0 D. and Ovchinnikov V.V. "Nuclear-G~-mma-Resonance Spectroscopy of Alloys". (Metallurgia, Moscow. 1982) (in Russian). 12. Thomas G.F., Siegel R.W. and Eastman F.A. Scr.Met.24,201 (1990): 13 Ovchinnikov V.V. and Geld P.V. Metal.Phys.Metallggr.6-S]-23 (1988) 14 American Institute of Physics Handbook. 3rd ed (Ame-ffYcan Institute of P h y s i c s , New York, 19720, p.4.
3.
Hyperfine IThe sample mean grain size, ~m
parameters Subspectrum
0.12 0.23 0.23* 0.26 0.65
of subspectra
Efficient magnetic field, Heff, kOe
TABLE 1 1 and 2 of different size
sample
mean grain
Isomeric shift against the model ~-Fe, mm/s
Width of the external peaks of spectrum at i/2 of the height mm/s
0.00 ± 0.06 -0.36 _ 0.06
0 . 3 2 __. 0 . 0 5 0.32 ± 0.05
0.00 i 0.06 -0.35 + 0.06
0 . 3 3 __. 0 . 0 5 0 . 5 4 _+ 0 . 0 5
0.00 ± 0.06 -0.25 5 0.06
0 . 3 3 -,- 0 . 0 5 0.35 5 0.05
0.00 5 0.06 -0.38 ± 0.06
0.34 + 0.05 0.38 5 0.05
1
5 5 5 5 5 5 5 5 330 5 5
2
302 5 5
0.00 ± 0.06 -0.30
0 . 3 0 ± 0.05 0.40 5 0 . 0 5
1 2 1 2 1 2 1 2
331 301 331 301 329 229 331 299
± ± ± 5 5 5 ± 5
_+ 0 . 0 6
* The data are taken from an earlier paper [i0]. TABLE 2 Hyperfine parameters of subspectra 1 and 2 of sample different temperature
with
Sub- Efficient mag- Isomeric shift Width of the exspec- netic field, against the mo- ternal peaks of del ~-Fe, mm/s spectrum at 1/2 of trum Heff(kOe) the heigth, mm/s
77K
300K
77K
300K
77K
300K
340±5
330±5
0.0
0.0
0.32±0.05 0.28±0.05
32055
30055
-0.34
-0.38
0.40±0.05 0.23±0.05
d=0.23
~m
for
Ratio of integral intensities (areas) of subspectra, I2/I l
77K
300K
0.14
0.09
2722
SUBMICROMETER GRAINED Fe
Vol. 25, No. 12
2 4
1.00
=o 0.96
0.g2
I 0
-5
i 5
Velocity (mm/s) Fig.1 The SMG from structure (d=0.12 ~m)" light field TEM image and microdiffraction pattern of an area of 0.75 um in diameter.
25
Fig.2 The SMG iron Mossbauer spectrum (d = 0.23 #m) at room temperature (isotope 57Co in Cr). The continuous lines indicate the division of the experimental spectrum into two subspectra.
!Y
20 .~.15 o10 5
I/d (1/jU... rn)
13
Fig.3 The dependence of the GBP atom fraction in the sample on the mean grain size. The line coresponds to the GBP thickness physical width of IB) which is 8.4 nm.
[