All-Russian Scientific-Research Institute of Aviation Materials (VIAM), Moscow, Russia:

E. A. Lukina, Leading Researcher, e-mail: lukinaea@viam.ru
S. V. Ovsepyan, Head of Laboratory
E. A. Davydova, Leading Engineer
M. V. Akhmedzyanov, Second Category Engineer

There were investigated the structural and phase features of VZh171 (ВЖ171) heat-resistant Ni – Co – Cr-based alloy after chemical heat treatment (nitriding), and after nitriding with following annealing in vacuum. Nitrides with different morphology are formed in chemical-heat treatment process. Branched form nitrides have FCC crystal lattice with lattice period close to the (Ti, Cr)N composition nitride. Titanium and chromium are redistributed in nitrides during annealing, which may be connected with reduction of total nitrogen content in alloy after vacuum annealing. Solid solution near large nitride particles is enriched with nitrogen and chromium. Disintegration of supersaturated solid solution occurs during annealing with release of fine chromium-containing nitride particles. Electron microscopic methods studied the processes of nitride particle interaction with dislocation structures. Accumulation of dislocations near nitride particles are revealed in samples exposed to short-term trials. The characteristic arrangement of dislocations confirms the assumption about the inhibition of dislocation motion under the stress fields near particles and characterizes the strain hardening process. Additional annealing before short-term tests at room temperature leads to formation of uniform dislocation structure in the tested samples. Microtwins are formed, corresponding to diffraction effects in the form of elongated rods on the electron diffraction. The twinning plane correlates with the slip planes in FCC metals. Formation of nanodispersed nitride precipitates after annealing and appearance of microtwins leads to higher elongation of the samples at high temperature for a long term tests by 25% and time to failure by 35% and also leads to additional hardening in the alloy VZh171 and provides the improved mechanical properties.
There researches were carried out with the financial support of Russian Federation for Basic Research — grant No. 11-08-12095.

1. Kablov E. N. Strategicheskie napravleniya razvitiya materialov i tekhnologiy ikh pererabotki na period do 2030 goda (Strategic ways of development of materials and their processing technologies for the period till 2030). Aviatsionnye materialy i tekhnologii = Aviation materials and technologies. 2012. No. S. pp. 7–17.
2. Ospennikova O. G. Strategiya razvitiya zharoprochnykh splavov i staley spetsialnogo naznacheniya, zashchitnykh i teplozashchitnykh pokrytiy (Strategy of development of heat-resistant alloys and special steels, protection and heatprotection coatings). Aviatsionnye materialy i tekhnologii = Aviation materials and technologies. 2012. No. S. pp. 19–36.
3. Kablov E. N. Aviatsionnoe materialovedenie: itogi i perspektivy (Aviation materials science: results and prospects). Vestnik Rossiyskoy akademii nauk = Herald of the Russian Academy of Sciences. 2002. Vol. 72, No. 1. pp. 3–12.
4. Letnikov M. N., Lomberg B. S., Ovsepyan S. V. Issledovanie kompozitsiy sistemy Ni – Al – Co pri razrabotke novogo zharoprochnogo deformiruemogo intermetallidnogo splava (Investigation of compositions of Ni – Al – Co system during the development of new heat-resistant deformed intermetallic alloy). Trudy Vserossiyskogo instituta aviatsionnykh materialov = Proceedings of All-Russian Scientific-Research Institute of Aviation Materials. 2013. No. 10. Available at : http://viam-works.ru/plugins/content/journal/uploads/articles/pdf/250.pdf

5. Chollet S. et al. Plasma assisted nitriding of Ni-based superalloys with various microstructures. Surface and Coatings Technology. 2013. Vol. 235. pp. 318–325.
6. Mindivan F., Mindivan H. Comparisons of wear performance of hardened Inconel 600 by different nitriding processes. Procedia Engineering. 2013. Vol. 68. pp. 730–735.
7. Krishna Srivastava S. Cobalt-chromium-iron-nickel alloys amenable to nitride strengthening. Patent US, No. 8075839. Haynes International, Inc. Published: March 20, 2008.
8. Sudha C. et al. Nitriding kinetics of Inconel 600. Surface and Coatings Technology. 2013. Vol. 226. pp. 92–99.
9. Ovsepyan S. V., Lukina E. A., Filonova E. V., Mazalov I. S. Formirovanie uprochnyayushchey fazy v protsesse vysokotemperaturnogo azotirovaniya svarivaemogo zharoprochnogo deformiruemogo splava na osnove sistemy Ni – Co – Cr (Formation of reinforcing phase in the process of high-temperature nitration of welded heat-resistant deforming alloy on the basis of Ni – Co – Cr system). Aviatsionnye materialy i tekhnologii = Aviation materials and technologies. 2013. No. 1. pp. 3–8.
10. Kablov E. N., Petrushin N. V., Svetlov I. L., Demonis I. M. Nikelevye liteynye zharoprochnye splavy novogo pokoleniya (Nickel casting heat-resistant alloys of new generation). Aviatsionnye materialy i tekhnologii = Aviation materials and technologies. 2012. No. S. pp. 36–52.
11. Petrova L. G., Chudina O. V. Primenenie metodologii upravleniya strukturoobrazovaniem dlya razrabotki uprochnyayushchikh tekhnologiy (Application of methodology of structure formation control for development of reinforcing technologies). Metallovedenie i termicheskaya obrabotka metallov = Metal Science and Heat Treatment. 2010. No. 5. pp. 31–41.
12. Fahrmann M., Srivastava S. K. Nitridation of Haynes® NS-163® alloy: thermodynamics and kinetics. JOM. 2012. Vol. 64, No. 2. pp. 280–287.
13. Kablov D. E., Kraposhin V. S., Gerasimov S. A. Kristallograficheskiy mekhanizm obrazovaniya dvoynikov pod vliyaniem azota pri vyrashchivanii monokristallov zharoprochnykh nikelevykh splavov (Crystalligraphic mechanism of twinning under the influence of nytrogen during the heat-resistant nickel alloy monocrystals growing). Zagotovitelnye proizvodstva v mashinostroenii = Blank Production in Mechanical Engineering. 2012. No. 7. pp. 37–41.
14. Kablov E. N., Bondarenko Yu. A., Kablov D. E. Osobennosti struktury i zharoprochnykh svoystv monokristallov <001> vysokorenievogo nikelevogo zharoprochnogo splava, poluchennogo v usloviyakh vysokogradientnoy napravlennoy kristallizatsii (Peculiarities of structure and heat-resistant properties of monocrystals <001> of high-rhenium nickel heat-resistant alloy, obtained in the conditions of high-gradient oriented crystallization). Aviatsionnye materialy i tekhnologii = Aviation materials and technologies. 2011. No. 4. pp. 24–31.
15. Chabina E. B., Filonova E. V., Lomberg B. S., Bakradze M. M. Struktura sovremennykh deformiruemykh nikelevykh splavov (Structure of modern deformed nickel alloys). Vse materialy. Entsiklopedicheskiy spravochnik = Polymer Science. Series D. 2012. No. 6. pp. 22–27.
16. Yakovleva I. L., Karkina L. E., Kabanova I. G., Schastlivtcev V. M., Zubkova T. A. Elektronnomikroskopicheskoe issledovanie mikrodvoynikov austenita i ikh vliyanie na kristallograficheskie osobennosti perlitnogo prevrashcheniya (Electrone microscopic investigation of austenite microtwins and their influence on crystallographic peculiarities of perlite transformation). Izvestiya Rossiyskoy akademii nauk. Seriya “Fizika” = Bulletin of the Russian Academy of Sciences. Physics. 2010. Vol. 74. pp. 1599–1605.
17. Chabina E. B., Alekseev A. A., Filonova E. V., Lukina E. A. Primenenie metodov analiticheskoy mikroskopii i rentgenostrukturnogo analiza dlya issledovaniya strukturno-fazovogo sostoyaniya materialov (Application of methods of analytical microscopy and X-ray structure analysis for investigation of structure-phase state of materials). Trudy Vserossiyskogo instituta aviatsionnykh materialov = Proceedings of All-Russian Scientific-Research Institute of Aviation Materials. 2013. No. 5. Available at: http://viam-works.ru/plugins/content/journal/uploads/articles/pdf/37.pdf
18. Hirsch P., Howie A., Nicholson R., Pashley D., Whelan M. Electron Microscopy Of Thin Crystals. London : Butterworths, 1965. 575 p.
19. Kodentsov A. A., Gulpen J. H., Cserhati C., Kivilahti J. K., Van Loo F. J. J. High-temperature nitridation of Ni – Cr alloys. Metallurgical and Materials Transactions A. 1996. Vol. 27, No. 1. pp. 59–69.