Crack Initiation in an Aluminide Coated Single Crystal During Thermomechanical Fatigue

Abstract

Two different modes of coating crack initiation are observed in an aluminide coated single crystal nickel base alloy upon thermomechanical fatigue loading with a cycle where the lag of the strain cycle behind the temperature cycle was 135‡. Minimum and maximum cycle temperatures were 300‡C and 1050‡C, respectively. At applied mechanical strain ranges in excess of 0.8% the coating fails in a brittle manner, initiating a network of nearly equidistant parallel cracks. As opposed to this line initiation mode, a point initiation mode of multiple thumbnail crack origins is found for low strain ranges. Uncoated test specimens fail by point initiation at all strain ranges. The line initiation behaviour can be rationalized using a fracture mechanics approach based on energy release rate concepts to estimate the strain (or stress) to first coating crack and to predict the coating crack density during the test. This model highlights the effects of various parameters, such as coating thickness and temperature, on the failure mode. The influence of the ductile-brittle transition temperature and the stress-temperature coupling on the failure mode of the coating is discussed and the model predictions are compared with the experimental observations.

Influence of an Aluminide Coating on the TMF Life of a Single Crystal Nickel-Base Superalloy

“…As far as the mechanistic understanding is concerned it is clear that all conditions causing faster oxidation lead to localisation which is true for the interdendritic region and for carbides. Because the fracture resistance of the oxides is very small [Martinez 1996] it is realistic to assume that it will be broken in every cycle. Carbides and pores can additionally act as stress concentrator.…”

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

“…Then, tensile stress on the external surface is created [1]. Furthermore, the microstructure of the superalloys unavoidably becomes degraded due to operating at elevated temperatures and high rotation speeds [2,3]. Therefore, flexibility also becomes more important [4].…”

Damage mode analysis of MCrAlY overlay coatings subjected to isothermal stepwise tensile testing by usingin situvideo imaging and acoustic emission monitoring

“…[1][2][3][4] The damage processes and failure modes, which must be taken into account for the life assessment of such coatings, include oxidation, corrosion, plasticity, precipitate cracking, erosion and microstructural degradation caused by the interdiffusion between the substrate and the coating. [5][6][7][8][9] One of the most critical points in designing such protective coatings is the determination of the mechanical behaviour of the coating-substrate within the operational temperature spectrum, as it reveals the way the coating is degraded and how it contributes to the performance of the coating-substrate material system. A significant material parameter of these protective coatings is the ductileto-brittle transition temperature (DBTT), which delimits the temperature range where the coating behaves like a brittle material with negative consequences for its mechanical performance.…”