Microstructural Evolution And Mechanical Behavior In Nickel Based Alloys For Very High Temperature Reactor

Under the Gen IV advanced reactor development program, the Very High Temperature Reactor (VHTR) is the lead concept. Design and development steps are currently underway to construct a high-temperature reactor as the next generation nuclear plant (NGNP) at Idaho National Laboratory (INL). A major limitation of this system is the development and qualification of high-temperature materials for structural applications. Such materials must possess good high-temperature resistance. Among candidate materials, Alloy 617 and Alloy 230 are considered the most promising structural materials for the VHTR. In order to gain a better understanding of material performance during a high-temperature and long-term service life, various material tests and experiments were conducted to investigate the fundamental deformation mechanisms of the alloys and their long-term degradation process during thermal aging. First, mechanical properties of both alloys were studied by performing tensile tests at three different strain rates and at temperatures up to 1000o.C. This range covers time-dependent (plasticity) to time-independent (creep) deformations. At temperatures from 300 to 700o.C, the yield strength was found to be temperature independent as a result of additional strain hardening provided by dynamic strain aging. However, higher temperatures (>800o.C) activated additional deformation mechanisms, including dislocation creep and dynamic recrystallization, leading to a significant decrease in material strength. Consequently, the fracture mechanisms changed from inclusion particle cracks at temperatures up to 700o.C to triple junction cracks from 800 to 1000o.C. Through a strain-rate sensitivity analysis, the results of tensile tests were extended to approximate the alloys0́9 long-term flow stresses. According to the comparison with these estimated flow stresses, the allowable design stresses for either alloy in American Society of Mechanical Engineers (ASME) B&PV Code did not provide adequate degradation estimation for the long-term service life. However, rupture stresses for Alloy 617, developed in ASME code case N-47-28, can generally satisfy the safety margin estimated in the study following the strain-rate sensitivity analysis. Nevertheless, additional studies on material development are necessary in order for current VHTR conceptual designs to eventually meet design parameters defined by rupture stresses. Additionally, the effect of orientation on Alloy 617 was studied to provide proper guidance for engineering design and alloy development. Mechanical fibering, consisting of an alignment of inclusion particles and matrix crystals, was found to contribute to the mechanical anisotropy of Alloy 617 with varying performances across the studied temperature range. Second, long-term thermal aging experiments (up to 3,000 hours) were performed to investigate the dynamic process of microstructure evolution and, consequently, mechanical property degradation at 900 and 1000°C for both Alloy 617 and Alloy 230. This microstructural evolution was found to be characterized by diffusion-controlled precipitation and coarsening of carbide particles (mainly M23C6 and M6C). The kinetics of particle coarsening was studied though the measurement of volume increase of intergranular particles. The results of the mechanical tests were in good agreement with microstructure observations. Both hardness measurements and tensile tests showed a typical aging process characterized by short-term strengthening and long-term softening. Generally, both alloys aged at 900o.C attained higher yield and tensile strengths with a longer hardening time compared to samples aged at 1000o.C. Alloy 230 exhibited a longer age-hardening duration compared to Alloy 617, due to a lower diffusibility of Tungsten atoms (primary solute element in Alloy 230). Beyond the mechanical tests at room temperature, the long-term aging degradation for high-temperatures tensile properties was found to be comparable to the degradation for low-temperatures properties. Lastly, an advanced measurement technique, high-energy synchrotron radiation, was applied to Alloy 230 to investigate the deformation process during in-situ loading. The small volume fractions of carbides (i.e. ~6% of M6C in Alloy 230), which are difficult to detect using lab-based X-ray machines or neutron scattering facilities, were successfully examined using high-energy X-ray diffraction. The loading processes of the austenitic matrix and the carbide were separately studied by analyzing their different lattice strain evolutions, and thus, the response of each phase to the applied tensile load was clarified. Elastic anisotropy for various polycrystal planes (hkl) was also measured through various reflections for the austenitic matrix. The measured lattice strain can be converted to flow stress by a factor of Young0́9s modulus calculated by Kröner0́9s self-consistent method. The lattice strain measured from the (311) reflection is extensively studied, since it responds almost linearly to the applied stress in both the elastic and plastic regimes. The lattice strain evolution for carbides is different than that for the matrix. During the transition from the elastic regime to the plastic regime, carbide particles experience a dramatic loading process, and the internal stress reaches a critical value. The internal stress for the particles then begins to slowly decrease while the linear stress increases for the matrix. This indicates a continued particle fracture process during plastic deformations of the matrix. Finally, a high-energy diffraction technique was developed that combines synchrotron X-ray radiation and pressurized creep tubes and allows macroscopic creep strain and lattice strain to be simultaneously measured by a single X-ray exposure. A typical creep curve with an evidently identified secondary and tertiary creep was obtained by analyzing the X-ray diffraction patterns. In-situ observations of the development of dislocation densities and lattice strain make it possible to track the onset of accelerated creep void nucleation, growth and coalescence.