Grain Boundary Engineered Superalloys for Irradiative and Corrosive Environments

E.Y. Chen, C.C. Chen
Transition45 Technologies, Inc., US

Keywords: grain boundary engineering, forging, corrosion, irradiation, superalloys

Summary:

Grain boundary engineering (GBE), or controlling the distribution and relative fractions of twin and other high coincident site lattices (CSL), has been demonstrated as an effective means to enhance durability, namely intragranular stress corrosion cracking (SCC), creep and fatigue crack growth resistance in nickel-base superalloys exposed to very high temperatures. These benefits have been successfully demonstrated on commercial cast/wrought and powder processed Ni-base superalloys for gas turbines, and should be transferable to irradiative and corrosive environments associated with nuclear reactors. Existing practices for GBE utilize multiple iterations of cold rolling followed by annealing. Since each iteration of deformation and annealing imparts a modest increase in the fraction of twin and special grain boundaries, multiple iterations are required to achieve a sufficiently high fraction of special grain boundaries that result in the improved properties. This approach therefore is not suitable for the GBE of large, complex structures and leads to added manufacturing lead time and cost. For this very reason, a revolutionary process for cost effective GBE of bulk Ni-base superalloy structures and components has been developed. This technology applies forge processing to impart the critical strain energy that induces the formation of twins and high CSL grain boundaries upon annealing.