STP1263

    Crack Initiation in an Aluminide Coated Single Crystal During Thermomechanical Fatigue

    Published: Jan 1996


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    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.

    Keywords:

    thermomechanical fatigue, aero engines, superalloy, single crystal, coating, crack initiation, surface crack


    Author Information:

    Martínez-Esnaola, JM
    Centro de Estudios e Investigaciones Técnicas de GuipÚzcoa (CEIT), San Sebastián,

    Arana, M
    Centro de Estudios e Investigaciones Técnicas de GuipÚzcoa (CEIT), San Sebastián,

    Bressers, J
    Institute for Advanced Materials - Joint Research Centre, Petten,

    Timm, J
    Institute for Advanced Materials - Joint Research Centre, Petten,

    Martín-Meizoso, A
    Centro de Estudios e Investigaciones Técnicas de GuipÚzcoa (CEIT), San Sebastián,

    Bennett, A
    Rolls-Royce plc, Derby,

    Affeldt, EE
    Motoren- und Turbinen-Union MTU, München,


    Paper ID: STP16447S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP16447S


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