Creep properties and deformation mechanisms of single-crystalline γ′-strengthened superalloys in dependence of the Co/Ni ratio

ABSTRACT

Co-base superalloys are considered as promising high temperature materials besides the well-established Ni-base superalloys. However, Ni appears to be an indispensable alloying element also in Co-base superalloys. To address the influence of the base elements on the deformation behaviour, high-temperature compressive creep experiments were performed on a single crystal alloy series that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr content. Creep tests were performed at 900 °C and 250 MPa and the resulting microstructures and defect configurations were characterised via electron microscopy. The minimum creep rates differ by more than one order of magnitude with changing Co/Ni ratio. An intermediate CoNi-base alloy exhibits the overall highest creep strength. Several strengthening contributions like solid solution strengthening of the γ phase, effective diffusion coefficients or stacking fault energies were quantified. Precipitate shearing mechanisms differ significantly when the base element content is varied. While the Ni-rich superalloys exhibit SISF and SESF shearing, the Co-rich alloys develop extended APBs when the γ′ phase is cut. This is mainly attributed to a difference in planar fault energies, caused by a changing segregation behaviour. As a result, it is assumed that the shearing resistivity and the occurring deformation mechanisms in the γ′ phase are crucial for the creep properties of the investigated alloy series.

KEYWORDS:

• Creep
• electron microscopy
• superalloy
• deformation mechanisms
• solid solution strengthening
• stacking fault energy

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Funding

The authors acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through projects A7 and B3 of the collaborative research center SFB/TR 103 ‘From Atoms to Turbine Blades – a Scientific Approach for Developing the Next Generation of Single Crystal Superalloys’.