Modeling of Thermomechanical Fatigue Damage in Coated Alloys

Abstract

A life prediction model that determines the contribution of fatigue, creep and environmental damage to failure was developed for an aluminide coated nickel-based superalloy, Mar-M247. In the first phase of the study, isothermal (IF) and thermomechanical fatigue (TMF) experiments were conducted to investigate the experimental damage mechanisms. In the second phase, an analytical technique was advanced to compute the stress fields due to a surface inclusion in a half-space where the inclusion simulates the oxide spike. The technique is based on Eshelby's equivalent inclusion method and elucidates the mismatch in elastic moduli and thermal expansion coefficients of the matrix and the oxide spike on local strain fields. The fatigue life results of several experiments, along with the local stress-strain field in the vicinity of an oxide spike, were employed to define the model constants. Life prediction bounds are established corresponding to short-time coating protection, where the coating provides inconsiderable protection to substrate, and long-time coating protection, where the coating provides appreciable protection to the substrate. For in-phase loading, since the failure is governed by creep damage, the nature of coating protection did not influence the fatigue lives. The out-of-phase predictions corresponding to short-time coating protection and the experimental data coincided as the maximum temperature increased in the experiments, confirming that the coating provides unsubstantial protection at higher temperatures.

Failure behavior of coated nickel-based superalloy under thermomechanical fatigue

“…When the coating cracks, most of the imposed forces are transferred to the substrate. When the cracks arrive at the interface between the coating and substrate, it is thought that there is little load in the coating, and most of the applied forces are imposed on the substrate [ 21 ]. Therefore, the high stress acts on the substrate, and this causes more cracks to originate in the substrate.…”

Manipulate A2/B2 Structures in AlCrFexNi Alloys for Improved Mechanical Properties and Wear Resistance

“…From the EDS results, the oxygen distribution of representative alloys is displayed in the middle column (Figure 13), and corresponding atomic percentages of oxygen are measured as 6.8%, 15.8%, 15.7%, and 21.5%, respectively. The oxide scales involving the formation, damage, and peel/spall off during wear testing, are influenced by the mismatches in thermal expansion strain and mechanical strain between the oxide scale and the substrate [42,43]. For harder but less ductile alloys such as the AlCrFe 0 Ni alloy, the oxide scale formed on this alloy substrate may have a larger interfacial mismatch strain, making the oxide less adherent to the substrate.…”