Effect of the Prior Microstructure Degradation on the High Temperature/Low Stress Non‐Isothermal Creep Behavior of CMSX‐4® Ni‐Based Single Crystal Superalloy

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.

Investigating the Dislocation-Driven Micro-mechanical Response Under Non-isothermal Creep Conditions in Single-Crystal Superalloys

“…As a result, the 9:3 test has the lowest deformation per cycle but the highest strain rate overall, while the 27:3 condition accumulates the most strain per cycle. Similar relationships with varying time at base temperature condition have been published by Raffaitin et al [10] and Giraud et al [4] for thermal cycles to 1150°C.…”

“…The micro-mechanical response under non-isothermal conditions has been shown to reduce the lifetime of an alloy relative to its isothermal creep life. [2,3] By varying the non-isothermal exposure cycles in the temperature range of 1050°C to 1150°C, Giraud et al [4] concluded that one crucial factor for the lifetime reduction is accredited to the development of rafts. This observation is at odds with the well-established beneficial role of c¢-rafting under isothermal conditions.…”

Creep of a nickel-based single-crystal superalloy during very high-temperature jumps followed by synchrotron X-ray diffraction

“…Recent studies have shown that the creep rate under thermal cycling conditions is higher than under isothermal ones and that the lifetime at high temperature is shorter during non-isothermal creep [44,45]. The same last observation was also done by Cormier et al in the case of a single overheating performed after different creep durations under high-temperature/low-stress conditions [46].…”

Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy

“…Previous research has shown that pre-rafts parallel to the loading direction can enhance TMF properties [36] as well as the non-isothermal creep properties. [21] Another study has shown that rafts parallel to the loading direction may slow down the process of dislocation climb and therefore improve high temperature creep properties. [18] At 1223 K (950°C), the h001i direction is stronger in compression than tension, however the opposite relationship is observed at 1023 K (750°C), where no rafting is visible.…”

“…[20] Another study has also shown how N-type of rafts are deleterious to non-isothermal creep properties while rafting of P-type seems to decrease creep rates and therefore enhance the non-isothermal creep properties. [21] The creep performance is strongly influenced by the crystal orientation of the single-crystal superalloy. Literature often concludes that the h011i direction has worse creep properties compared to the h001i and h111i directions.…”