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Degradation of LCF properties of CMSX-4 single crystal superalloy, after creep exposure at 850 °C and 950 °C

pubblicazioni - Articolo

Degradation of LCF properties of CMSX-4 single crystal superalloy, after creep exposure at 850 °C and 950 °C

In the frame of the COST 538 programme one important aspect of the activity was the collection of quantitative information about the influence of service degradation on mechanical properties of Ni-based alloys, used in the hot parts of gas turbines.

To reach such goal material degradation was obtained by preliminary creep exposure to conditions similar to those experienced by blades during service. Preaged specimens were tested to determine the tensile and LCF behaviour of the degraded material at high temperature. The experiments were performed with CMSX-4 single crystal superalloy. Specimens with double heads for creep and LCF were machined from as delivered bars. In this paper the influence of creep ageing in the two following conditions is described: 2500h both at 850°C and at 950°C with applied stress of 320 and 150 MPa respectively, which let to obtain deformation levels in the range 0.3 to 0.5%. On the superalloy degraded in the two conditions LCF tests were performed at the same temperatures of the pre-ageing. In both cases a reduction in LCF resistance close to a 2 factor was observed on respect that of the virgin alloy. This can be explained by the results of fractographic and metallographic observations performed in the three material conditions (virgin, aged at 850°C and at 950°C) . At the lower temperature only an initial microstructural degradation level (mainly increase of the gamma channels width and some coarsening) was observed, as expected; rafting was observed on the material degraded at the higher temperature, in agreement with predictions of published models of microstructural degradation of CMSX-4 due to creep aging

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