Abstract
Creep-fatigue tests on a single crystal Ni-Base superalloy, CMSX-2, were carried out under trapezoidal creep-fatigue loading with a hold time in tension at a temperature of 1223 K in vacuum and in air. During the study special attentions have been paid to understand the following articles: (i) microstructural aspect of crack initiation, (ii) early growth of naturally-occurring creep-fatigue small cracks of the order of microns, compared with those of physically long cracks of the order of millimetres; (iii) effect of loading frequency on the creep-fatigue small crack propagation; and (iv) effect of environment on the creep-fatigue small crack propagation. The experimental results under different loading frequency in air and in vacuum indicated that the small crack propagation under the trapezoidal creep-fatigue loading was essentially dominated by a time-dependent creep mechanism. The results also revealed a significant difference in crack growth rates between the small and long cracks under the creep-fatigue condition, as well as under the fatigue dominant condition: the small cracks exhibited growth rates remarkably higher than long cracks at a given stress intensity factor range.
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