Advances in engineering science are often driven by material destruction under actual production or service conditions. As the jewel in the crown of structural materials research, superalloys are often used in extremely complex and harsh working environments. Although this type of alloy has near-limiting creep resistance, oxidation resistance and corrosion resistance, people have some knowledge of the high temperature fatigue failure of such alloys under cyclic loading, but they still cannot predict the creep in practical applications more accurately. Failure caused by change-fatigue-oxidation. In the study of such problems, people will often encounter unprecedented and incredible failure modes. Existing theories have a better understanding of fatigue crack propagation, but like many fatigue problems, the mechanism of fatigue crack initiation is still a difficult problem. In view of the initiation of fatigue cracks, this study proposes a mechanism for the evolution of the oxide layer of the cobalt-based superalloy from a uniform growth to a local "finger" morphology under low-cycle thermal shock, which reasonably explains the failure of the cobalt-based superalloy without external load phenomenon.
Recently, the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals and the team of Professor Cao Rui of the School of Materials Science and Engineering of Lanzhou University of Technology and Professor Gao Yanfei of the University of Tennessee (co-corresponding author) reported a case of no external load Mechanical explanation of thermal shock fatigue failure mechanism of cobalt-based superalloys. Related achievements were published on Acta Materialia under the title "Mysterious Failure in Load-Free Superalloys under Repeated Thermal Shocks".
Paper link:
https://doi.org/10.1016/j.actamat.2020.05.002
This work is based on the comparative study of the oxidation behavior of Co-Cr-W superalloys under isothermal and thermal cycling conditions.
The author expands the oxidation-diffusion-creep constitutive relationship proposed by Academician Suo Zhigang of Harvard University (this article is named Stokes-Herring-Suo model), and the peculiarity of finger oxides under no applied load The growth pattern was explained reasonably. The growth of chromium oxide requires chromium to diffuse from the far-field matrix. The divergence of chromium diffusion flux is not zero, so an arbitrary volume element must shrink. However, due to the limitation of the lateral geometry, the lateral shrinkage of the micro-element can only be compensated by the lateral elongation caused by creep. The former is a volumetric process, while the latter is a deviatoric process, and the coordination of the two results in the generation of lateral stress and longitudinal compression deformation. The key of the model is that the time required for the establishment of the oxidation stress is controlled by the creep rate and the elastic modulus.