CHANGE OF THE STRUCTURE OF A HEAT-RESISTANT ALLOY DOPED BY RHENIUM AND LANTHANUM DEPENDING ON THERMAL PROCESSING

The properties of high-temperature nickel alloys for modern engineering were determined by thermal stability of structure, size, shape and amount of hardening γ′-phase, and the strength characteristics of γ′-solid solution. These alloys are strengthened by alloying with rhenium and lanthanum. The aim of this work was qualitative and quantitative study of the alloy structure and the phase composition of a nickel heat-resisting alloy additionally doped with rhenium (0.4  %  at.) and lanthanum (0.006  %  at.). The investigations were carried out by two methods: the method of transmission diffraction electron microscopy and the scanning electron microscopy method. Investigation of the alloy’s structure was carried out in three states: sample 1 – initial state (after directional crystallization (DC)); sample 2 – DC, annealing at 1150  °C for 1 hour, annealing at 1100 °C for 480 hours;sample 3  –  DC, annealing at 1150 °C for 1 hour, annealing at 1100 °C for 1430  hours. The studies showed that the phases observed in the superalloy can be classified into primary and secondary phases. The main phases are γ′ and γ. They form structure of the alloy and are present as quasi-cuboids of γ′-phase separated by γ′-phase interlayers. The remaining phases are secondary. It was found that doping with rhenium and lanthanum leads to the appearance of secondary phases, namely: β-NiAl, AlRe, NiAl₂ Re; σ; χ; Ni₃La₂ . The formation of secondary phases introduces a serious violation into the structure of quasi-cuboids (γ  +  γ′)-phases. Rhenium and lanthanum do not fill the entire volume of the alloy uniformly but are present only in local areas. Therefore, in three states of the alloy only a part of the volume of quasi-cuboids (γ  +  γ′)-phases was affected. The morphology of the secondary phases was studied. It was found that the particles of the σ-phase are thin needles, while the Ni₃La₂ particles have an internal structure with a characteristic contrast and have a finite thickness. An interesting feature is that the σ-phase and Ni₃La₂ phase are distinguished in the same places. It was established that the introduction of lanthanum and rhenium changes phase composition of the alloy, suppressing the formation of γ-phase. Particles of secondary phases are localized in individual sections of the alloy with a certain periodicity. The resulting secondary phases are refractory: the melting point of the β-phase is approximately 1600  °C, for the σ-phase it is 2600  °C, and for the χ-phase it is 2800  °C. The formation of secondary refractory phases and their periodic distribution in the structure contributes to hardening of the superalloy doped with rhenium and lanthanum.

1. Zharoprochnye stali i splavy [Heat-resistant steels and alloys]. Mos-cow: VIAM, 2012, 60 p. (In Russ.).

2. Ross E.W., Sims C.T. Nickel-Base Alloys. In: Superalloys II – High temperature materials for aerospace and industrial power.  NY:  John Wiley & Sons, Inc., 1987, pp. 97–133.

3. Superalloys II: High-Temperature Materials for Aerospace and Industrial Power. Chester T. Sims, Stoloff N.S., Wolliam C. Hagel eds.  New York: Wiley, 1987, 615 p. (Russ.ed.: Supersplavy II: Zharoprochnye materialy dlya aerokosmicheskikh i promyshlennykh energoustanovok. Sims Ch.T., Stoloff N.S., Hagel W.C. eds. Moscow:  Metallurgiya, 1995, 384 p.).

4. Gerasimov V.V., Petrushin N.V., Visik E.M. Improvement of composition and development of a technology for casting single-crystal  blades from a heat-resistant intermetallic alloy. Trudy VIAM. 2015,  no. 3, Electronic resource. Available at URL: http://viam-works.ru/ ru/articles?art_id=783 (Accessed 02.02.2017). (In Russ.).

5. Hirsch P.B., Howie A., Nicholson R., Pashley D.W., Whelan M.J.  The Electron Microscopy of Thin Crystals. Butterworths, 1965, 549 p. (Russ.ed.: Hirsh P., Hovi R., Nicholson R. Elektronnaya mikroskopiya tonkikh kristallov. Moscow: Mir, 1968, 574 p.).

6. Utevskii L.M. Difraktsionnaya elektronnaya mikroskopiya v metal-lo vedenii [Diffraction electron microscopy in metallurgy]. Moscow:  Metallurgiya, 1973, 584 p. (In Russ.).

7. Kozlov E.V., Nikonenko E.L., Popova N.A., Koneva N.A. Structure  and composition of higher-rhenium-content superalloy based on La-alloyed Ni-Al-Cr. Advanced Materials with Hierarchical Structure for New Technologies and Reliable Structures AIP Conf. Proc. 2015,  vol. 1683, pp. 020101-1–020101-4.

8. Diagrammy sostoyaniya dvoinykh metallicheskikh sistem: spravochnik v 3-kh t. T. 2.  [State  diagrams  of  double  metal  systems:  Refe rence  book  in  3  vols.  Vol.  2]  Lyakishev  N.P.  ed.  Moscow:  Mashinostroenie, 1996, 992 p. (In Russ.).

9. Kolobov  Yu.R.,  Kablov  E.N.,  Kozlov  E.V.,  Koneva  N.A.,  Povarova  K.B.,  Grabovetskaya  G.P.,  Buntushkin  V.P.,  Bazyleva  O.A.,  Muboya dzhyan S.A., Budinovskii S.A. Struktura i svoistva inter-metallidnykh materialov s nanofaznym uprochneniem  [Structure  and properties of intermetallic materials with nanophase hardening].  Moscow: ID MISiS, 2008, 328 p. (In Russ.).

10. Povarova  K.B.,  Kazanskaya  N.K.,  Drozdov A.A.,  Morozov A.E.  Physicochemical laws of the interaction of nickel aluminides with  alloying  elements:  I.  Formation  of  nickel  aluminide-based  solid  solutions.  Russian Metallurgy (Metally).  2006,  vol.  2006,  no.  5,  pp.  415–426.

11. Reed R.C. The superalloys – fundamentals and applications. Cam-bridge: Cambridge University Press, 2006, 372 p.

12. Segersäll  M.  Nickel-based single-crystal superalloys.  Linköping,  Sweden: 2013, 56 p.

13. Göken, M., Kempf M. Microstructural properties of superalloys investigated by nanoindentation in an atomic force microscope. Acta Mat. 1999, vol. 47, no. 3, pp. 1043–1052.

14. Rae  C.M.F.,  Reed  R.C.  The  precipitation  of  topologically  close-packed phases in rhenium-containing superalloys. Acta Materialia. 2001, vol. 49, no. 19, pp. 4113–4125.

15. Kozlov E.V., Nikonenko E.L., Popova N.A., Koneva N.A. Effect of  thermal treatment and Re-dopingon the volume fraction of the γ′ phase  in complex-doped Ni-Al-Cr-based superalloy. Bulletin of the Russian Academy of Sciences. Physics. 2014, vol. 78, no. 4, pp.  267–270.

16. Sugui T., Minggang W., Huichen Yu., Xingfu Yu., Tang Li., Benjiang Qian. Influence of element Re on lattice misfits and stress rupture properties of single crystal nickel-based superalloys. Materials Science and Engineering. 2010, vol. 527, no. 16-17, pp. 4458–4465.

17. Nikonenko Е.L., Popova N.A., Koneva N.A., Kozlov E.V. Structure  and phase composition of the superalloy on the basis of Ni-Al-Cr  alloyed by Re and La. IOP Conf. Series: Materials Science and Engineering. 2016, vol. 112, pp. 012036–012040.

18. Volek A., Pyczak F., Singer R.F., Mughrabi H. Partitioning of Re  between γand γ′phase in nickel-base superalloys. Scripta Mat. 2005,  no. 52, pp. 141–145. 

19. Kozlov E.V., Nikonenko E.L., Popova N.A., Koneva N.A. Change  in the phase composition and defect structure of a multicomponent  ordered Ni-based alloy upon high-temperature annealing. Bulletin of the Russian Academy of Sciences. Physics. 2013, vol. 77, no. 9,  pp. 1108–1111.

20. Sato A., Harada H., Yokokawa T., Murakumo T., Koizumi Y., Kobayashi T., Imai H. The effects of ruthenium on the phase stability  of fourth generation Ni-base single-crystal superalloy. Scripta Ma-terialia. 2006, vol. 54, pp. 1679–1684.