Platinum Group Metals Base Refractory Superalloys
as Ultra-High Temperature Materials

We have proposed "refractory superalloys" with fcc and L12 two-phase coherent structures similar to those in Ni-base superalloys and yet with higher melting temperature using platinum group metals. Ir and Rh were selected as base materials because the melting temperature of Ir and Rh are 2447 and 1960(C, respectively. We have investigated microstructure and deformation mechanism for binary alloys and challenged several methods to improve ductility and fracture mode as shown in Fig. 1.

1. Microstructure and deformation mechanism

We found that the fcc and L12 two-phase coherent structure formed in the Ir and Rh base binary alloys and precipitate shape depends on the lattice misfit between the fcc matrix and L12 precipitates,. Strengths of the Ir-base alloys are between 500 and 1200MPa at 1200(C. Strength mechanism of Ir-base alloys is solid solution hardening and precipitation hardening. The Ir-Zr alloy with plate-like precipitates shows largest precipitation hardening effect and deforms by shearing mechanism. Precipitate shape is effective for precipitation hardening in Ir-base alloys [1-9].

Based on above results, deformation behavior of Ir-Nb with cuboidal precipitates was investigated in this project. We found that deformation of Ir-Nb occurs by dislocation bypass in the narrow fcc channel. To understand which precipitate shape is suitable for high temperature materials, compression creep test has been tried for Ir-Nb and Ir-Zr alloys. The strain of Ir-Nb alloys after creep test at 1500(C under 98, 137, and 200MPa were 0.2, 1.2, and 1.8 %. Creep test for Ir-Zr alloys is undergoing now [10-13].

2. Improvement of ductility and fracture mode: Addition of third element

Ir-base binary alloys are brittle and exhibit intergranular fracture mode. To improve ductility and fracture mode, Ni was added to Ir-15at%Nb and Rh-15Nb alloys. The fcc and L12 coherent two-phase structure forms, the strength is improved, and the fracture mode is changed when Ni content is below the optimum amount [14-20].

Based on this result, the effect of other elements, B, C, Pt, Mo, Ta, and W have been investigated in this project. Thus, addition of B (<200 wppm), C (<4000wppm) and Pt (<30at%) are effective to change fracture mode with high strength and the fcc and L12 two-phase structure still remains. Mo, Ta, W are not effective to improve ductility and they destroy coherent two-phase structure [21-22].

3. Improvement of ductility and fracture mode: Development of quaternary alloys

This is another way to improve ductility and fracture mode. We considered if the Ir-base binary alloy and the Ni-Al alloy with the fcc and L12 two-phase structure are mixed, the quaternary alloys will also have two-phase structure and have advantages from both alloys, that is, high strength of Ir-base alloy and high ductility and low density of Ni-Al alloy. We found three-phase region, the fcc, Ir3Nb type L12 and Ni3Al type L12. In addition the three-phase region, the fcc and L12 two-phase region in both of Ir-base and Ni-base alloys side. Strengths of the alloy in three-phase region are between Ni-Al alloy and Ir-binary alloys [23-27].

In this project, the alloys with the fcc and L12 two-phase region in Ir-side and Ni-side were investigated. We found the coherent structure in these alloys. The strength of Ni-side alloys was slightly higher than that of Ni-Al alloy. The strength of the Ir-side alloys was higher than that of Ir binary alloys and Ir-Nb-Ni [28].

4. Processing by Spark Plasma Sintering (SPS)

In addition above three themes, development of processing technique has been started because it is difficult to process refractory superalloys that have high strength and are brittle. Sintering by SPS was tried for Ir3Nb with L12 structure using powder element or alloy powder. We have not succeeded sintering yet.

Collaboration work

1. MINTEK (South Africa): Pt-Al-X alloys
   Information exchange
   Compression test of Platinum alloys at high temperature (NRIM).
   Microstructure observation of Ir-Al supported by MINTEK (NRIM).
2. Hokkaido University (Japan): Rh-Al-X
   Information exchange
   Strength behavior of Ir baser L12 intermetallics (NRIM) and Rh-base L12 intermetallics (Hokkaido University)

Acknowledgement

We thank to Dr. Ro and Mr. Nakazawa in our group for helping mechanical test up to 1800(C. We are grateful to Mr. S. Nishikawa and T. Maruko of Furuya Metal Co. Ltd., for supporting materials and giving us a lot of technical supports.

Reference

1. Y. Yamabe, Y. Koizumi, H. Murakami, Y. Ro, T. Maruko, and H. Harada, Scripta Materialia., 35, 2, 1996, 211-215.

2. Y. Yamabe-Mitarai, Y. Koizumi, H. Murakami, Y. Ro, T. Maruko, and H. Harada, Mat. Res. Soc. Symp. Proc, 460, 1997, 701-706.

3. Y. Yamabe-Mitarai, Y. Koizumi, H. Murakami, Y. Ro, T. Maruko, and H. Harada, Scripta Materialia, 36, 4, 1997, 393-398.

4. Y. Yamabe-Mitarai, Y. Ro, T. Yokokawa, T. Maruko, and H. Harada, Proc. of the Asian Science Seminar, 1997, 277-281.

5. Y. Yamabe-Mitarai, Y. Ro, T. Maruko, T. Yokokawa, and H. Harada, " Structural Intermetallics 1997", eds. M. V. Nathal, R. Darolia, C. T. Liu, P. L. Martin, D. B. miracle, R. Wagner, and M. Yamaguchi, Seven Springs, PA, TMS, 1997, 805-814.

6. Y. Yamabe-Mitarai, Y. Ro, T. Maruko, and H. Harada, Met. Trans., 29A, 1998, 537-549.

7. Y. Yamabe-Mitarai, Y. Ro, T. Yokokawa, Yuefeng Gu, T. Maruko, H. Harada, "Materials for Advanced Power Engineering", Proc. of the 6th Liege conference, eds. J. Lecomte-Beckers, F. Schubert, and P. J. Ennis, Liege, EC, 5, part II, 1998, 1147-1154.

8. Y. Yamabe-Mitarai, Y. Ro, T. Maruko, and H. Harada, Intermetallics, 7, 1999, 49-58.

9. Y. Yamabe-Mitarai, Y. Ro, T. Maruko, and H. Harada, Scripta Materialia., 40, 1, 1999, 109-115.

10. Y. Yamabe-Mitarai, Yuefeng Gu, Y. Ro, S. Nakazawa, T. Maruko, and H. Harada, Scripta Materialia., 41, 3, 1999, 305-311.

11. Y. Yamabe-Mitarai, M-H. Hong, Y. Ro, and H. Harada, Phil. Mag. Lett., 79, 9, 1999, 673-682.

12. Y. Yamabe-Mitarai, Xihong Yu, Yuefeng Gu, Y. Ro, S. Nakazawa, T. Maruko, and H. Harada, International Conference on Advanced Technology in Experimental Mechanics '99 ATEM'99, 1999, 543-548.

13. Y. Yamabe-Mitarai, Xihong Yu, Yuefeng Gu, Y. Ro, S. Nakazawa, T. Maruko, and H. Harada, Key Engineering Materials,171-174, 2000, 625-632.

14. Yuefeng Gu, Y. Yamabe-Mitarai, Y. Ro, T. Yokokawa, and H. Harada, Scripta Materialia, 39, 6, 1998, 723-728.

15. Yuefeng Gu, Y. Yamabe-Mitarai, Xihong Yu, Y. Ro, T. Yokokawa,and H. Harada, "Aerospace materials", The first Oxford kobe Materials Seminar, 1998, Poster 1-7.

16. Yuefeng Gu, Y. Yamabe-Mitarai, Y. Ro, and H. Harada, Scripta Materialia., 40, 11, 1999, 1313-1319.

17. Yuefeng Gu, Y. Yamabe-Mitarai, Y. Ro, T. Yokokawa, and H. Harada, Met. Trans., 30A, 1998, 2696-2639.

18. Yuefeng Gu, Y. Yamabe-Mitarai, Y. Ro, Y. Yokokawa, and H. Harada, Mat. Res. Soc. Symp. Proc, 552, 1999, KK7.10.1-6.

19. Yuefeng Gu, Y. Yamabe-Mitarai, Xihong Yu, and H. Harada, Materials Letters,41, 1999, 45-51.

20. Yuefeng Gu, Y. Yamabe-Mitarai, Y. Ro, T. Yokokawa, T. Maruko and H. Harada, International Conference on Advanced Technology in Experimental Mechanics '99 ATEM'99, 1999, 549-554.

21. Yuefeng Gu, Y. Yamabe-Mitarai, and H. Harada, Scripta Materialia, 41, 10, 1999, 1079-1083.

22. Yuefeng Gu, Y. Yamabe-Mitarai, Y. Ro, and H. Harada, Key Engineering Materials, 171-174, 2000, 669-676.

23. Xihong Yu, Y.Yamabe-Mitarai, Yuefeng Gu, and H. Harada, "Aerospace materials", The first Oxford kobe Materials Seminar, 1998, Poster 17-21.

24. Xihong Yu, Y. Yamabe-Mitarai, Y. Ro, Yuefeng Gu and H. Harada, Scripta Materialia., 41, 6, 1999, 651-657.

25. Xihong Yu, Y. Yamabe-Mitarai, and H. Harada, Scripta Materialia., 41, 11, 1999, 1153-1159.

26. Xihong Yu, Y. Yamabe-Mitarai, Y. Ro, and H. Harada, Met Trans A, 31A, 2000, 173-178.

27. Xihong Yu, Y. Yamabe-Mitarai, Y. Ro, and H. Harada, New Developed quaternary refractory superalloys, IUMRS-ICAM'99, accepted.

28. Xihong Yu, Y. Yamabe-Mitarai, Y. Ro, and H. Harada, Key Engineering Materials, 71-174, 2000, 677-684.