L. M. Kaputkina, V. V. Sumin, and K. O. Bazaleeva Moscow Institute of Steel and Alloys
共Submitted April 13, 1999兲
Pis’ma Zh. Tekh. Fiz. 25, 50–54共December 26, 1999兲
The influence of nitrogen on the absolute coefficient of thermoelectromotive force in steel and the tendency of the material to form packing defects is studied. The absolute coefficient
of the thermo-emf increases monotonically with increasing nitrogen concentration. The tendency to form packing defects increases monotonically with increasing nitrogen concentration in the range where the austenite solid solution is stable. Near the phase transition
共austenite–martensite兲 this tendency increases sharply. The experiments described indicate that
nitrogen influences the electron subsystem of the metal. © 1999 American Institute of Physics. 关S1063-7850共99兲02412-X兴
The introduction of nitrogen is an extremely promising method of modifying various properties of steel. The influ- ence of nitrogen on the properties of iron alloys is in many respects comparable to the widely studied influence of car- bon. However, many authors have noted substantial differ- ences in the behavior of nitrogen and carbon in solid solu- tions of bcc and fcc iron and, specifically, different influences on the packing defect energy1 and the nature of the interatomic bonds.2 At the same time, specific data to estimate the structure and properties of iron–共substitutional dopant兲–nitrogen solid solutions are clearly inadequate and frequently often contradictory.
In the present study we report an experimental assess- ment of the influence of the nitrogen concentration on prop- erties closely related to the electron subsystem, i.e., the tem- perature dependence of the thermo-emf and the concentration of packing defects.
We investigated Fe–18%Cr–N alloys with nitrogen con- tent varying between 0.42 and 1.68 wt.%.
After being quenched from 1100 °C the alloys were in various phase-structural states. Since nitrogen fairly strongly stabilizes austenite, the steel having the lowest nitrogen con- tent, 0.42%, was in a two-phase state共martensite plus auste- nite兲 at room temperature. According to x-ray diffraction data, the sample containing 0.68% nitrogen possibly had a small quantity (⬍2%兲 of martensitic phase and then, as the nitrogen content increased共0.96–1.68% N兲, the steel became completely single-phase and the austenite was more stable.
In order to determine the position of the nitrogen atoms in the alloy we measured the lattice period; the results are plotted in Fig. 1. It can be seen that the lattice period of the austenite increases monotonically with concentration and the slope of the curve agrees fairly accurately with the experi- mental data.3,4
This behavior is evidence that almost all the nitrogen is present in solid solution after quenching the steel.
Results of measuring the temperature dependence of the thermo-emf E(T) of the alloys are plotted in Fig. 2. The
measurements were made in the range 50–200 °C by an in- tegral method5 where the hot junction acted as the contact between the tungsten tip contained within a microfurnace and the surface of the sample. It can be seen that the slope of E(T) increases monotonically with increasing nitrogen concentration. The absolute coefficient of the thermo-emf (S⫽⫺dE/dT) calculated using these data is plotted in Fig. 2b. The coefficient S also increases with increasing nitrogen content and for pure austenitic alloys 共0.94–1.68%兲 the de- pendence S⫽ f (N) is less steep and close to linear. We know that this coefficient is a function of the density of electron states at the Fermi surface (S⫽2k2T/(3兩e兩Nd())
⫻(Nd()/d)⫽, where Nd() is the density of states as a
function of energy andis the Fermi energy5兲 so that we are confident that this result qualitatively indicates that nitrogen influences the state of the metal electron subsystem.
The influence of nitrogen on the packing defect energy was estimated indirectly from the packing defect concentra- tion␣in these alloys under intensive deformation. The pack- ing defect concentration was determined in austenite by mea- suring the shift of the x-ray peaks.6 Results of numerous experiments show that, although this method does not give the correct absolute value of the packing defect energy in
FIG. 1. Lattice period of austenitic solid solution as a function of nitrogen concentration.
TECHNICAL PHYSICS LETTERS VOLUME 25, NUMBER 12 DECEMBER 1999
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certain cases, it provides a qualitatively true reflection of the tendency to form packing defects. We assume that the pack- ing defect energy and the inversely proportional parameter␣ in the solid solution are closely related to the density of electron states7 so that we could predict a correlation be- tween the values of S and␣. However, the tendency to pack- ing defect formation is also influenced by another factor, i.e., the packing defect energy decreases (␣ increases兲 near the
phase transition. Thus, in this case for two-phase alloys where the austenite solid solution is unstable 共0.42 and 0.677% N兲 the values of␣are exaggerated and they decrease on approaching the stable solid solution 共0.964% N兲. The second ascending branch of the curve in Fig. 3 clearly re- flects the influence of nitrogen on the electronic structure of the fcc solid solution and the increase in␣here is caused by a reduction in the packing defect energy accompanying the nitrogen doping.
1R. M. Banov and G. Z. Zlateva, Izv. AN SSSR. Metall. No. 2, 176共1977兲. 2H. Ino, K. Umezu, S. Kajiwara, and S. Uchawa, in Proceedings of the International Conference on Martensite Transformations, Nara, Japan, 1986, pp. 313–318.
3
M. Hansen and K. Anderko, Constitution of Binary Alloys, 2nd ed.
共McGraw-Hill, New York, 1958; Metallurgizdat, Moscow, 1962, 163 pp.兲.
4W. P. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys共Pergamon Press, Oxford, 1958兲, 258 pp.
5
F. J. Blatt et al., Thermoelectric Power of Metals共Plenum Press, New York, 1976; Metallurgiya, Moscow 1980, 248 pp.兲.
6Ya. D. Umanski, Yu. A. Skakov, A. N. Ivanov, and L. N. Rastorguev, Crystallography, X-Ray Diffraction Analysis, and Electron Microscopy关in
Russian兴, Metallurgiya, Moscow 共1982兲, 632 pp.
7
Ya. D. Vishnyakov, Packing Defects in Crystal Structure关in Russian兴, Metallurgiya, Moscow共1970兲, 215 pp.
Translated by R. M. Durham
FIG. 2. Temperature dependence of the thermo-emf for alloys containing 0.677% N共1兲, 0.964% N 共2兲, and 1.68% N 共3兲 共a兲; absolute coefficient of thermo-emf共50–200 °C兲 for these alloys 共b兲.
FIG. 3. Packing defect concentration for alloys having different nitrogen contents.
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