Formation of {alpha}{double_prime} iron nitride in FeN martensite: Nitrogen vacancies, iron-atom displacements, and misfit-strain energy
- Delft Univ. of Technology (Netherlands). Lab. for Materials Science
The precipitation of {alpha}{double_prime} iron nitride in FeN martensite (containing about 5.9 N/100 Fe) is studied as a function of tempering time successively at 333 and 373 K by means of X-ray diffractometry (XRD). On the basis of peak (=lattice parameter) shifts and intensity changes of main and superstructure reflections, it is concluded that, during tempering, the structure of the formed {alpha}{double_prime} precipitates changes. At 333 K, initially stoichiometric Fe{sub 16}N{sub 2} is formed and, with further tempering, structural vacancies are introduced, i.e., Fe{sub 16}N{sub 2{minus}x} is formed. At 373 K, the vacancies are partially filled. The change of the {alpha}{double_prime} structure at 333 K is caused by the change of the matrix from martensite to ferrite, i.e., the misfit between the {alpha}{double_prime} and the matrix is initially relatively small and then increases. By allowing nitrogen-deficient {alpha}{double_prime}, the increase of the misfit can be diminished at the expense of some increase of the volume of {alpha}{double_prime}. Gibbs free energy calculations (at 333 K), in which differences in the misfit energy are decisive, indeed show that, in a martensite matrix, Fe{sub 16}N{sub 2} is favored, whereas, in a ferrite matrix, Fe{sub 16}N{sub 2{minus}x} is favored. Further, in this article, methods are presented to calculate structure factors for nitrogen-deficient {alpha}{double_prime} and to obtain, from measured (=strained) lattice parameters, strain-free {alpha}{double_prime} lattice parameters from which the composition can be deduced.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 438584
- Journal Information:
- Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science, Vol. 28, Issue 1; Other Information: PBD: Jan 1997
- Country of Publication:
- United States
- Language:
- English
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