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.

Alloy 625 is widely used for petrochemical, marine and aerospace applications owing to its outstanding corrosion and mechanical properties at high temperatures. However, this alloy is prone to complex microstructure evolutions above 500°C that may impact its mechanical properties. In this study, the impact of its micro structure evolutions occurring upon long-term thermal aging on its static and cyclic mechanical properties has been studied. Thermal exposures of up to ~ 2000 hours in the 550°C-900°C temperature range have been investigated. TTT diagrams of the 5 and y" phases were established based on high resolution scanning electron microscopy observations. The evolutions of secondary carbides distributions were also followed. It has been observed a steep increase of the room temperature microhardness after overagings performed at 650°C and 700°C due to the precipitation of the y" phase. Moreover, it is also clearly evidenced a strengthening effect of the 5 phase after long-term thermal exposures at temperatures in excess of 700°C. Finally, the impact of a thermal aging in the y" precipitation domain on the high temperature tensile and low cycle fatigue properties was evaluated. It is shown a beneficial effect of the y' ' precipitation to the fatigue life and to the tensile properties up to 750°C. It is also shown that the loss of high temperature ductility is not correlated to the precipitation of grain boundary secondary carbides. y'' precipitation and the y'' ^5 transformation are also shown to be faster under cyclic loading compared to pure thermal aging.