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Characterisation of the quaternary system Co-Al-W-Ta for development of y' - strengthened Co-base alloys

Subject Area Metallurgical, Thermal and Thermomechanical Treatment of Materials
Term from 2012 to 2016
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 220692189
 
Final Report Year 2016

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.

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