Mysterious failure in load-free superalloys under repeated thermal shocks. (1st August 2020)

• Record Type: Journal Article
• Authors: Wen, Junxia; Cao, Rui; Gao, Yanfei
• Is Part Of: Acta materialia. Volume 194(2020)
• Page Start: 276
• Page End: 282
• Publication Date: 2020-08-01
• Subjects: Superalloy -- Thermal fatigue -- Concomitant oxidation-diffusion-creep processes -- Stokes-Herring-Suo model -- Preferential oxidation; Materials -- Periodicals; Materials science -- Periodicals; Materials -- Mechanical properties -- Periodicals; Metallurgy -- Periodicals; Chemistry, Inorganic -- Periodicals; 620.112
• DOI: 10.1016/j.actamat.2020.05.002
• Languages: English
• ISSNs: 1359-6454

Abstract

Abstract: The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or "fingers" is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions. Graphical abstract: – Superalloys under repeated thermal shocks but without the externally applied load are found to exhibit microcracks from surfaces and to have shortened lifetime. – Stresses may arise from concomitant oxidation-diffusion-creep processes, as modeled by the Stokes-Herring-Suo formalism. – Oxide fingers, if present under thermal shock condition, will elongate rather than thicken because of the low oxidation stress. Image, graphical abstract