Early growth of creep-fatigue small cracks in an advanced cast Ni-based superalloys

Naturally initiated creep-fatigue small crack growth behavior in a cast single crystal Ni-base superalloy, CMSX-2, was investigated at a temperature of 1223 K in vacuum and in air, by using the solid cylindrical smooth specimen of the CMSX-2 within 5 degree from 〈001〉 crystallographic orientation. This paper covers the following topics: (i) the naturally initiated creep-fatigue small crack propagation behavior relevant to the microstructure, compared with those of the physically long cracks; (ii) effect of loading frequency on the creep-fatigue small crack propagation; (iii) comparison of crack propagation between the single crystal alloy and a conventional cast polycrystalline one; and (iv) effect of environment on the creep-fatigue small crack propagation. The experimental results revealed the lack of similarity law in crack growth rates between the small and long cracks was significant: the former exhibited the growth rates remarkably higher than the latter at a given stress intensity factor range under the same test condition. It was also shown by the comparison of the crack propagation between in air and in vacuum that the role of creep, as well as that of environment, were essential in the creep-fatigue small crack propagation process.