Laura Carroll

Low Cycle Fatigue and Creep-Fatigue Behavior of Alloy 617 at High Temperature

Alloy 617 is the leading candidate material for an intermediate heat exchanger (IHX) application of the Very High Temperature Nuclear Reactor (VHTR), expected to have an outlet temperature as high as 950 degrees C. Acceptance of Alloy 617 in Section III of the ASME Code for nuclear construction requires a detailed understanding of the creep-fatigue behavior. Initial creep-fatigue work on Alloy 617 suggests a more dominant role of environment with increasing temperature and/or hold times evidenced through changes in creep-fatigue crack growth mechanism/s and failure life. Continuous cycle fatigue and creep-fatigue testing of Alloy 617 was conducted at 950 degrees C and 0.3% and 0.6% total strain in air to simulate damage modes expected in a VHTR application. Continuous cycle specimens exhibited transgranular cracking. Intergranular cracking was observed in the creep-fatigue specimens, although evidence of grain boundary cavitation was not observed. Despite the absence of grain boundary cavitation to accelerate crack propagation, the addition of a hold time at peak tensile strain was detrimental to cycle life. This suggests that creepfatigue interaction may occur by a different mechanism or that the environment may be partially responsible for accelerating failure.

Developing Dislocation Subgrain Structures and Cyclic Softening During High-Temperature Creep–Fatigue of a Nickel Alloy

The complex cyclic deformation response of Alloy 617 under creep–fatigue conditions is of practical interest both in terms of the observed detriment in failure life and the considerable cyclic softening that occurs. At the low strain ranges investigated, the inelastic strain is the sole predictor of the failure life without taking into consideration a potentially significant environmental influence. The tensile-hold creep–fatigue peak stress response can be directly correlated to the evolving dislocation substructure, which consists of a relatively homogenous distribution of subgrains. Progressive high-temperature cycling with a static hold allows for the rearrangement of loose tangles of dislocations into well-ordered hexagonal dislocation networks. The cyclic softening during tensile-hold creep–fatigue deformation is attributable to two factors: the rearrangement of dislocation substructures into lower-energy configurations, which includes a decreasing dislocation density in subgrain interiors through integration into the subgrain boundaries, and the formation of surface grain boundary cracks and cavity formation or separation at interior grain boundaries, which occurs perpendicular to the stress axis. Effects attributable to the tensile character of the hold cycle are further analyzed through variations in the creep–fatigue waveform and illuminate the effects of the hold-time character on the overall creep–fatigue behavior and evolution of the dislocation substructure.

The Role of Environment on High Temperature Creep-Fatigue Behavior of Alloy 617

Alloy 617 is the leading candidate material for an intermediate heat exchanger (IHX) application of the Very High Temperature Nuclear Reactor (VHTR), expected to have an outlet temperature as high as 950°C. Acceptance of Alloy 617 in Section III of the ASME Code for nuclear construction requires a detailed understanding of the creep-fatigue behavior. Initial creep-fatigue work on Alloy 617 suggests a more dominant role of environment with increasing temperature and/or hold times evidenced through changes in creep-fatigue crack growth mechanism/s and failure life. Furthermore, previous work on corrosion of nickel base alloys in impure helium has suggested that this environment is far from inert with respect to Alloy 617. Continuous cycle fatigue and creep-fatigue testing of Alloy 617 was conducted at 950°C and 0.3% and 0.6% total strain in air to simulate damage modes expected in a VHTR application. Continuous cycle and creep-fatigue specimens exhibited intergranular cracking, but did not show evidence of grain boundary cavitation. Despite the absence of grain boundary cavitation to accelerate crack propagation, the addition of a hold time at peak tensile strain was detrimental to cycle life. This suggests that creep-fatigue interaction may occur by a different mechanism or that the environment may be partially responsible for accelerating failure.Copyright

Strain Rate Sensitivity of Alloys 800H and 617

The flow stress of many materials is a function of the applied strain rate at elevated temperature. The magnitude of this effect is captured by the strain rate sensitivity parameter “m”. The strain rate sensitivity of two face–center cubic solid solution alloys that are proposed for use in high temperature heat exchanger or steam generator applications, Alloys 800H and 617, has been determined as a function of temperature over that range of temperatures relevant for these applications. In addition to determining the strain rate sensitivity, it is important for nuclear design within Section III of the ASME Boiler and Pressure Vessel Code to determine temperature below which the flow stress is not affected by the strain rate. This temperature has been determined for both Alloy 800H and Alloy 617. At high temperature the strain rate sensitivity of the two alloys is significant and they have similar m values. For Alloy 617 the temperature limit below which little or no strain rate sensitivity is observed is approximately 700°C. For Alloy 800H this temperature is approximately 650°C.Copyright