No abstract

The effect of thermal cycling on the creep behavior and life of the second generation Ni-based single crystal superalloy CMSX-4® has been investigated in the very high temperature/low stress domain. Repeated overheatings at 1100°C and/or at 1150°C were introduced in the course of the creep life of the alloy at 1050°C/120 MPa. The detrimental effect of the thermal cycling compared to the isothermal creep behavior of the alloy is shown: thermal cycling leads to an increased average creep strain rate and a reduced creep life. This is always the case even when compared to the isothermal creep behavior at the highest temperature of the cycle (i.e. 1150°C/120 MPa). Moreover, the detrimental effect of the thermal cycling frequency is observed: the greater frequency the larger the creep properties decay. Compared to the first generation MC2 alloy under the same thermal cycling conditions, CMSX-4® exhibits superior non-isothermal creep properties in the very high temperature domain (T > 1100°C) due to its rhenium and ' contents.The effect of the microstructure degradation (i.e. ' rafting) was also investigated by pre-straining samples at 1050°C under tension (respectively under compression) prior to creep under thermal cycling conditions. These pre-strains allowed to generate a N-type rafting (respectively P-type). N-type ' rafting are shown to be detrimental to the non-isothermal creep properties. In contrast, P-type rafting leads to improved creep properties (i.e. reduced creep strain rate) compared to either N-type rafted or cuboidal ' morphologies. Despite a different behavior, the creep lives are identical for both P-type rafting and the cubical ' precipitates. From the present investigation, it is believed that: 1) P-type rafting improves non-isothermal creep properties due to a reduced dislocation climb activity 2) The transition from stationary creep to tertiary creep is controlled by the kinetics of Ntype rafting.