Characterisation of the quaternary system Co-Al-W-Ta for development of y' - strengthened Co-base alloys
Final Report Abstract
The project was aimed on the investigation of the quaternary system Co-Al-W-Ta as a base for new Co-base alloys strengthened by the intermetallic γ’-phase Co3(Al,W). The main task of the project was to support the groups developing these alloys by relevant metallurgical information about this system. The performed investigations provided a large amount of qiuantitative information which can be used for alloy development, e.g.: • The two phase area γ/γ’ in the Co-Al-W-Ta phase diagram at 900°C. • The compositional dependences of the critical temperatures (γ’-solvus, solidus, liquidus). • The element partitioning between γ- and γ’-phases. • The compositional dependence of the γ/γ’-misfit. • The diffusion mobility in the Co-Al-W-Ta system. Two selected quaternary Co-Al-W-Ta alloys were solidified as [001] single-crystals and mechanically tested at temperatures between 20°C and 1200°C. It was shown, that at temperatures below 800-900°C the strength of quaternary Co-base alloys can be at the same level or even higher than of Ni-base superalloys. However at higer temperatures the Co-alloys are much weaker due to the lower γ’-solvus temperature Tsolv . It was shown, that Ta additions, up to about 3 at%, can rise Tsolv slightly above 1100°C but that a further increase of Ta results in an extended precipitation of other undesirable phases. It was found that the concentration ratio Ta/W has to be optimized to get the stable γ/γ’-microstructure. Additionally to the original project plan, two multicomponent γ’-strengthened Co-base alloys with additions of Ti, Cr, Mo and Re were cast, solidified as [001] single-crystals and tested under tension at temperatures between 20°C and 1000°C as well tested for microstructural degradation at 700°C and 800°C. This investigation resolved two problems in development of γ’-strengthened Co-base alloys. The first one regards γ-solution strengthening. The problem is, that the γ-strengthening elements, either preferably (W, Mo) or significantly (Re), partition into the γ’-phase, which reduces their concentration in the γ-matrix. The second problem is the instability of the γ/γ’-microstructure during long thermal exposure due to the γ’→β transformation. Summing up the project results one can draw the following conclusions. The Co-base alloys can not substitute the Ni-base superalloys in high temperature applications (T≥1050°C) such as blades of hot section of aircraft jet engines. But they could substitute some alloys used at lower temperatures (T≤900°C), e.g. conventional Co-base alloys. This field of application could be interesting for γ/γ’ Co-alloys. It was somewhat surprising that the application of the method of gradient DS-castings was not so effective for the investigated system Co-Al-W-Ta compared with that for the Ni-base alloys. The reason is the weaker segregation of the alloying elements in the Co-Al-W-Ta system. On the other hand the application of different types of gradient specimens (diffusional couples, specimens with surface gradients) was found to be very useful.
Publications
- Advanced EBSD pattern interpretation through iterative post-processing, Microscopy and Microanalysis, Vol. 19, Supplement S2, August (2013) pp. 728-729
G. Nolze, E. Payton, A. Winkelmann
- The backscatter electron signal as an additional tool for phase segmentation in electron backscatter diffraction, Microscopy and Microanalysis, Vol. 19, Issue 4, August (2013) pp. 929-41
E. J. Payton, G. Nolze
- Diffusion processes in multicomponent nickel-base superalloy-nickel system, The Physics of Metals and Metallography, Vol. 115 (2014) pp. 21-29
A. I. Epishin, T. Link, G. Nolze, I. L. Svetlov, B. S. Bokstein, A.O. Rodin, R. Salivan Neumann, G. Oder
(See online at https://doi.org/10.1134/S0031918X14010050) - Exploring structural similarities between crystal phases using EBSD pattern comparison, Cryst. Res. Technol. Vol. 49, No. 7 (2014) pp. 490–501
G. Nolze, A. Winkelmann
(See online at https://doi.org/10.1002/crat.201400091) - Segregation of alloying elements during flat front solidification of γ'-strengthened Co-Al-W-Ta superalloy, Herald of the Russian Foundation of Basic Research, no. 2 (2015) pp. 11-17
N. V. Petrushin, U. S. Elyutin, U. V. Filonova, R. M. Nazarkin
- Single-crystal Co-base superalloy strengthened by γ'-precipitates: structure and mechanical properties, Advanced Engineering Materials, Vol. 17 (2015) pp. 755–760
N. Petrushin, K. Hvatzkiy, V. Gerasimov, T. Link, A. Epishin, G. Nolze, G. Gerstein, A.
(See online at https://doi.org/10.1002/adem.201500088) - Structural stability of a Co-base superalloy strengthened by intermetallic precipitates of γ'-phase, Deformation and Fracture of Materials, no. 3 (2015) pp. 17-22
A. I. Epishin, N. V. Petrushin, T. Link, G. Nolze, Yu. V. Loshchinin, G. Gerstein
- Modelling interdiffusion in CMSX-10/Ni diffusion couple, Journal of Phase Equilibria and Diffusion, Vol. 37 (2016) pp. 201-211
A. Chyrkin, A. Epshin, R. Pillai, T. Link, G. Nolze, W. J. Quadakkers
(See online at https://doi.org/10.1007/s11669-015-0444-9)
