JOURNAL OF MATERIALS SCIENCE LETTERS 17 (1998) 1169±1171 X-Ray diffraction and neutron diffraction study of Fe±(15 to 29)Cr± 11Ni±0.5N M. BAEVA Institute of Solid State Physics, Bulgarian Academy of Sciences, So®a 1784, Bulgaria E-mail: mbaeva@center.phys.acad.bg A. BESKROVNYI Joint Institute of Nuclear Research, Dubna, Russia S. DANILKIN Institute of Physics and Power Engineering, Obninsk, Russia E. JADROVSKI Joint Institute of Nuclear Research, Dubna, Russia Nitrogen stainless steels are extensively investigated in view of their high-promising mechanical proper- ties. The enlargement of the temperature region of stability of the ã-phase is reached by addition of nitrogen [1], which appears as an interstitial impurity in contrast to the Cr and Ni, which are substitu- tionals in the fcc crystal lattice (ã-phase). In the past 15 years there has been a tendency to use nitrogen in the steels because it appears as a stabilizer of the ã- phase and stabilizes the austenitic phase no worse than does nickel, which is more expensive. Both types of impurities change the lattice parameter (LP), the exact value of which is necessary in the calculation of all the microstructure characteristics of the material (Burger's vector, dislocation density, microstrains). Through the mass density and volume changes, the LP values concern the basic engineering modules (shear, bulk and longitudinal modules), as well [2]. Investigations have been published on the LP changes of four-component Fe±19Cr±11Ni±N aus- tenite steels [3] under variable nitrogen content (0.039 to 0.24 wt%). However, we are not aware of similar studies at ®xed nitrogen content and increas- ing substitutional Cr atoms. In this letter, we aim to ®ll the lack of such experimental data. The casting of the steels was made in a 10-kg induction autoclave and 50-kg induction furnace. The steels were cast in a nitrogen medium at a pressure of 2 MPa (20 atm) in order to reach 0.5 wt % N content. The solubility of nitrogen at 1873 K and atmospheric pressure is only 0.13 wt % for Fe±15Cr±15Ni alloy [4]. As nitrogen-bearing additives gas-nitri®ed Cr±N (with 10±12% N) foundry alloys were used. After homogenization at 1200 8C during 10 h the melts were quenched in 5% water solution of NaCl. For the X-ray diffraction (XRD) measurements we used powders obtained by ®ling at room temperature and seeded through 80 ìm mesh. To establish the exact concentration of all the elements, half the amount of each powder was submitted to: 1) ¯uorescent gas analysis to measure the nitrogen concentration and 2) chemical analysis to determine the concentration of other elements. In Table I the exact composition of the samples is listed. For removing the residual stresses, the powders (sealed in evacuated to 10 4 MPa quartz ampules) were annealed at 1250 8C for 60 min and quenched in water. The X-ray peak 222 is measured by Fe K á radiation, Mn ®lter for K â . The diffractometer was calibrated by Fe powder standard. The peak positions of the split 222 doublet pro®les, shown in Fig. 1, are used. The calculated values are averaged by means 0261-8028 # 1998 Kluwer Academic Publishers TA B L E I Chemical composition of the steels. All concentrations are given in wt %; the remainder to 100% is Fe. For all the samples: C , 0:08%, S  (0:0060:009)%, Si  (0:010:014)%. In the last two columns, the values of the LP(nm) are given Sample Cr Ni N C a (nm) X-ray a (nm) neutrons 1 15.23 11.67 0.497 0.004 0.3596 0.3593 2 20.3 11.23 0.504 0.006 0.3604 0.3601 3 20.58 10.33 0.532 0.08 0.3603 Ð 4 25.13 11.17 0.530 0.07 0.3614 0.3609 5 29.04 10.94 0.574 0.07 0.3616 0.3614 Intensity (arb. units) 30 25 20 15 10 5 0 134 136 138 140 2θ (degrees) 1 2 5 Figure 1 XRD 222 peaks of the samples with different Cr content. 1169