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MÖSSBAUER STUDIES ON THE STATE OF TIN ATOMS SEGREGATED AT THE GRAIN
BOUNDARY OF IRON AND IRON ALLOYS
T. Ozawa, Y. Ishida
To cite this version:
T. Ozawa, Y. Ishida. MÖSSBAUER STUDIES ON THE STATE OF TIN ATOMS SEGREGATED
AT THE GRAIN BOUNDARY OF IRON AND IRON ALLOYS. Journal de Physique Colloques,
1979, 40 (C2), pp.C2-551-C2-552. �10.1051/jphyscol:19792191�. �jpa-00218569�
MOSSBAUER STUDIES ON THE STATE OF TIN ATOMS SEGREGATED AT THE GRAIN BOUNDARY OF IRON AND IRON ALLOYS
T. Ozawa and Y. Ishida
Institute of Industrial Science, University of Tokyo, 7 Roppongi, Minato, Tokyo, Japan
Abstract.- The binding state of tin atoms segregated at the grain boundary of fine grained iron and iron alloys is investigated by Mossbauer source experiments. It is found that the electronic state differs considerably from that of solid solution tin even though the average binding force itself decreases only by a small fraction. The grain boundary embrittlement seems to be related to the chan- ge in the binding characteristics of iron next to the segregated element in the grain boundary.
1 . Introduction.-As the result of recent intensive investigations, a large amount of knowledge has been accumulated about the structure of grain boundaries.
Knowledge about the electronic or the lattice dyna- mical state, however, is scarce in spite of their
significance to understand various basic properties of grain boundaries.
The Mossbauer effect should be potentially a powerful technique to investigate the binding state of individual atoms in the boundary. Generally, howe- ver, the number of atoms associated with the bounda- ry was too small to render a measurable contribution to the Mossbauer spectrum. The problem which is ty- pical for the application of the Mossbauer technique to the study of lattice defect may be solved in the case of the grain boundary if the analysis is made on fine grained specimen and for the segregation of a Mossbauer source nucleus of very low matrix solu- bility.
The segregation of IVb to VIb elements in the grain boundary is a well known cause of grain boun- dary embrittlement in low alloyed steels. It is in- teresting to know whether the phenomenon is explai- ned by the binding state of the segregated tin atoms in the boundary.
2. Experimental procedure.- Electro-deposited pure iron is used as the standard material, while Fe- 3wt.% Mn, Fe-lwt.% Ti and Fe-lwt.% Ni alloys are examined to investigate the influence of the alloy elements. Specimens were cross-rolled by 96^98% to thickness 0.2mm and rapidly heated in iced brine by the application of electronic current. Small grain
diameter of the order of l'vlOym was produced by the heat-t reatment.
Since the Mossbauer source was supplied in the form o f 1 1 9 mS n C l2 in 4N HC1 aqueous solution, it was mounted after coating the specimen surface with NaOH aqueous solution in order to avoid attacks by acids.
The Mossbauer nuclei ''9mSn were doped from the sur- face in hydrogen atmosphere at 673K. The average concentration of tin atoms including carriers were about 5 at. % of a monolayer in the boundary. The measurements were carried out in the conventional transmission geometry by means of ELSCINT AME31. The moving absorber BaSnCh (5 g/m2 1 1 9Sn) was hold at room temperature.
3. Experimental results.- The Mossbauer peaks deri- ved from tin atoms in the grain boundary were iden- tified from the change in the spectrum upon the re- moval of the surface layer of the specimen /l/. In addition, autoradiography and micro-Auger spectros- copy /2/ were used to prove that tin atoms are real- ly segregated within several atomic layers of the grain boundary.
The spectrum of tin atoms segregated at the grain boundary showed a broad single peak at room temperature with the exception of an Fe-l%Nialloy. The broad widths suggest either the quadrupole splitting due to the non-cubic configuration in the boundary or the distribution of the binding state. The spec- trum of tin atoms in the boundary of pure iron did not show any magnetic splitting even at 4.2K in sharp contrast to that of solid solution tin. In the case of an Fe-l%Ni alloy, however, a small magnetic JOURNAL DE PHYSIQUE
Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-551
Résumé.- Les états liés d'atomes d'étain ségrégés aux joints de grains dans du fer ou des alliages finement cristallisés sont étudiés par spectrométrie Mossbauer d'émission. On trouve que les états électroniques diffèrent considérablement de ceux de la solution solide, même si la force de liaison moyenne ne décroît que faiblement. La fragilisation aux joints de grains semble être due aux change- ments des caractéristiques de liaison des atomes de fer situés au voisinage des éléments ségrégés aux joints de grains.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792191
JOURNAL DE PHYSIQUE
splitting was observed 131.
The isomer shift of tin atoms in the boundary is commonly larger than that of solid solution tin as is in the case of A1 alloys 141, 151 and confirms electronically the weaker binding of tin with the surroundings. The isomer shift was reduced with the addition of titanium. The Ti atoms are thought to behave as sinks for the excess electrons of tin atoms.
The changes in the recoilless fraction are explained by the harmonic lattice vibration at low temperatures. The measured average amplitude and kinetic energy of the zero-point vibration of tin atoms in the boundary of pure iron was 3 . 5 ~ 1 0 - ~ n m and 1.9x10-~~ J respectively. At above the room tem- perature, however, the anharmonic effect appears unlike the case of tin in solid solution. The har- monic approximation, however, is already not valid at 200K with the second order Doppler shift.
At above 630K, line broadenings due to fre- quent jump of tin atoms at the grain boundary were observed. The measured value of the activation ener- gy is 96 kJ/mole and the change in the entropy is very small.
4. Discussion.- The spectrum of tin atoms in the grain boundary did not change with the concentration possibly because the interaction between tin atoms in the boundary saturated even in the low concentration range of tin atoms.
The empirical relation between the isomer shift 6 and the Debye temperature OD(-2) is shown in figure 1. The figure is similar to that of Bry- khanov et al. 161.
The value {OD(-2)j2 is thought to be proportional to the average force constant of tin. It should be no- ticed that the Mb'ssbauer resonance due to tin atoms in the grain boundary are placed normally at the lower position along the curve than that of solid solution tin. It indicates that the binding state of tin atoms in the boundary is metallic in nature even though the binding force is weakened, and the sur- rounding host atoms are bound more weakly than in the grain. The state of d-level of iron atoms near tin, appears anomalous because magnetic splittings are absent with the spectrum of tin atoms in the grain boundary. It is possible that the behavior of the iron and tin atoms is really the cause of the grain boundary embrittlement. In the case of
low carbon Ni-Cr steels, the increase in the ducti- le-brittle transition temperature due to antimony segregation was reduced drastically by the addition of Ti 171. It was suggested /7/ that the element with empty d-level such as titanium abstracts the excess electrons of tin from going to iron and pre- vent the embrittlement. The observed reduction of s electron density at the nucleus site of tin by the addition of titanium to iron implies that the ele- ment is indeed a powerful sink to the excess elec-
,
trons.In the case of pure iron, the new peak near 0.0 mm/s appeared at above 650K and remained when the specimen was cooled down to room temperature.
The new peak is thought to reflect the formation of the stannic oxide in the grain boundary. A large recoilless fraction of the spectrum indicates that the compounds are compressed. The peak near 0.0 m / s , however, did not appear in Fe-3ZMn alloys even though the force constant of tin atoms in the grain boundary was weak. Therefore, the embrittlement due
to the ionic bond formation may be limited in the grain boundary of pure iron only. The mixture of the two binding states of tin atoms in the boundary was suggested as a cause of the embrittlement of
pure iron boundary /I/.
References
Isomer shift
/
m . s - 1Fig. 1 : The relation between isomer shift 6 and Debye temperature O (-2) of tin atoms in various matrix and the grain boundary. D
/I/ Ozawa, T. and Ishida,Y., Scr.Met.
11
(1977) 835./2/ Ishida,Y., Iida,F., Koyama,N.and Shimizu,H., Scr.Met.
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(1976) 1021./3/ Ozawa,T., Ogawa,S.and Ishida,Y., unpubTished.
/4/ Ishida,Y.and Ozawa,T., Scr.Met.9 (1975) 1103.
/5/ Ozawa,T., Yashiro,K., Ishida,Y.and Kato,M., Seisan Kenkyu
28
(1976) 331.161 Brykhanov,V.A., Delyagin,N.N.and Shpinell,V.S., Sov.Phys. JETP
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(1965) 55.171 McMahon,C.J., Mater.Sci.Eng.
