Near-Surface Damage of Alloy 617 With and Without Barrier Layers During Static and Cyclic Creep Deformation at 800 C

Abstract

Alloy 617 – a Cr2O3-former – is the leading candidate material for heat exchangers of the Next Generation Nuclear Plants with high outlet temperatures (750-850°C). The first objective of this work was to understand the effect of oxidizing (He-≤0.1ppm O2) and carburizing/oxidizing (He-CO/CO2=1320) environments on creep deformation under static and cyclic stresses (σmax=50MPa and 75MPa), and specifically on damage in surface and near-surface regions of Alloy 617 at 800°C. Secondly, the work investigated the mitigation of environmental attack on by using combinations of Al2O3-forming coatings, NiAl and FeCrAlY. Interrupted creep studies in the two environments were conducted on uncoated 617, aluminized 617 (NiAl/617), clad-aluminized 617 (FeCrAlY/NiAl/617), aluminized 617 with FIB micro-notches, and clad-aluminized 617 having undergone extra heat treatment (>24h). In these test conditions, the environmental attack was limited to the surface of the uncoated alloy without affecting the bulk creep behavior, with cyclic stress accelerating the surface damage. Uncoated 617 suffered surface and internal oxidation in both environments, with thin/less porous Cr2O3 scales, and porous/extensive grain boundary Cr2O3 intrusions characterizing surface damage in He-≤0.1ppm O2 and He-CO/CO2=1320, respectively. The existence in unstressed specimens of different Cr2O3 growth rates due to Cr interstitials or Cr vacancies primary mobile species as a result of low or high oxygen partial pressures, respectively, was confirmed during the application of stress in this work. Also, the carburizing nature of the environment caused embrittlement of the near-surface regions of the alloy, hence exposing fresher alloy surfaces for inward oxidation to take place. In contrast, slow growing alumina scale on NiAl and FeCrAlY coating layers protected underlying 617 from environmental attack. Surface undulation, characteristic of NiAl layers under cyclic stresses, was absent on NiAl and hence no surface cracking was observed, nor were cracks initiated or propagated from FIB-notches planted to simulate defects in the layer. The NiAl layer was, however, vulnerable to cracking when a FeCrAlY cladding overlay was employed. In general, these coatings prevented environmentally-induced damage on the substrate. However, further investigations are needed to study the long-term implications of creep damage accumulation in both the underlying substrate and coating layers in non-accelerated tests.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116771/1/aokello_1.pd