Microstructure Evolution and Mechanical Properties of Haynes 282

Doktorsavhandling, 2018

Precipitation-hardened nickel-based superalloys find wide applications in aero engines and land-based gas turbines due to a combination of properties such as high temperature strength, resistance to oxidation and corrosion, fabricability, and creep strength. Structural engine components are traditionally cast to achieve higher degree of geometrical design freedom. However, the latest fabrication strategy to achieve low cost and light weight structural components is by joining materials based on temperature needs. The challenge in this strategy is to tailor the heat treatment to suit the multi-material structures and still be able to meet the desired property requirements. This requires a profound understanding of the process-structure-property relationships in these complex alloys. The newly introduced Ni-base superalloy Haynes 282 has been attracting interest due to its high-temperature properties and excellent weldability. These properties are achieved due to the precipitation of strengthening phase (γʹ, Ni3 (Al,Ti)) and grain boundary carbides (mainly M23C6 and M6C) during heat treatment.

As Haynes 282 has showed sensitivity to heat treatment temperatures within the typical tolerance limits around the conventional heat treatment, the main objective of this research was to understand the microstructural evolution and mechanical properties with changes in heat treatment conditions. The effect of heat treatment variations on microstructure and mechanical properties has been systematically studied. Its influence on microstructure and tensile properties between room temperature and 730 °C are presented.

The results show that γ׳ does not precipitate during rapid cooling but it precipitates as fine spherical particles during air cooling from the carbide stabilization temperature, and it changes to bimodal distribution with square and spherical morphology during slow cooling. During ageing, γ׳ is seen to precipitate intergranularly, as well as along the grain boundaries. The solvus temperature for this phase was above 1010 °C (higher than previously suggested), and depending on the combination of temperatures and times of the heat treatments, the γ׳ morphology changes from spherical to bi-modal to cuboidal. The grain boundary carbide morphology depends strongly on heat treatment temperature and is seen to change from continuous film to brick wall structure and finally to discrete particles. These microstructural changes strongly affect both strength and ductility of the material.

Furthermore, Haynes 282 forgings show ductility variations in short transverse direction. The lower limit of ductility in this direction is close to the design tolerance and thus creates a need to understand the underlying cause. In this part, the study is focused to understand ductility variation by microscopic investigations. Carbide segregation and banding is seen to influence the ductility when oriented perpendicular to the tensile axis. This influence is also qualitatively captured through micromechanical modelling.

 
Haynes 282, gamma prime, carbides, isothermal transformation, anisotropy ductility, heat treatment, microstructure, solution treatment, carbide stabilization treatment