Even superalloys get creeped out from stress

Researchers study how internal stress evolves in high-performance alloys used in jet engines and power turbines.

Image courtesy of Oak Ridge National Laboratory/Jill Hemman

The Science

Superalloys are composite materials with outstanding strength. Researchers design superalloys by embedding particles in a metal matrix. When subjected to stress, the particles and matrix can deform differently. This can cause components made with the alloys to fail. Researchers employed neutrons to probe the internal stresses experienced by two different superalloys at high temperatures and loads typical of real-world applications. The measurements provided critical insights on the deformation mechanisms at relevant conditions. The results also helped validate the mathematical models researchers use to predict the materials’ behavior.

The Impact

High-performance industrial alloys are widely used in gas turbines, aircraft engines, and other applications. Scientists design these alloys to operate for long times under harsh conditions such as extreme pressures and temperatures. Long-term operation under extreme conditions can adversely affect the reliability of these materials. For example, they can suffer from creep—permanent deformation in the alloy’s atomic structure that occurs over time. Researchers are developing an accurate understanding of the internal processes that lead to creep. They are especially interested in the initial stages (called primary creep). This will guide better alloy design and improve component life with higher reliability.

Summary

In nickel-based superalloys comprising a metal matrix with embedded particles for reinforcement, the evolution of internal stresses created by extreme operating conditions leads to effects such as primary creep and metal memory of prior deformations. In this study, researchers conducted in situ neutron scattering measurements using the VULCAN instrument at the Spallation Neutron Source, a Department of Energy Office of Science user facility. VULCAN helped the researchers better understand the nature of such internal stresses and their evolution under simultaneous high temperature and high pressure conditions. The neutron data allowed characterization of phase-specific elastic strain, calculated from various diffraction peak parameters with respect to the applied load direction. Researchers used these strains, in conjunction with the proposed mathematical model, to estimate the internal resistance to deformation (internal stress or back-stress) in the deformed alloys. They obtained good agreement between theory and experiment on superalloy samples with two different microstructures, providing experimental validation of the proposed model. The resulting novel methodologies that combine state-of-the-art in situ neutron measurements and validated mathematical models will be broadly useful for the characterization of mechanical properties of components under relevant conditions and for the design of new materials that offer greater reliability and longevity of industrial components.

Contact

Sanket Sarkar
GE Research
Sanket.Sarkar@ge.com

Ke An
Oak Ridge National Laboratory
kean@ornl.gov

Funding

This research was funded by the Advanced Technology Program of GE Research, GE Aviation, and GE Power. This work used the Spallation Neutron Source, a DOE Office of Science user facility.