Thermomechanical Response of Polycrystalline alpha-Uranium

The research conducted represents an entirely new application of the self-consistent modeling strategy originally developed by Kroner to describe the elastic behavior of polycrystalline aggregates. The modeling is employed along with experiments to investigate the unique thermo-mechanical response of polycrystalline α-uranium. The orthorhombic crystal structure leads to high levels of anisotropy in its thermal, elastic and plastic response, as compared to cubic, or even hexagonal metals. Most interesting, the anisotropic thermal expansion response has one direction with a negative coefficient of thermal expansion, which in a polycrystal induces thermal residual stresses in polycrystals during cooling from thermo-mechanical processing temperatures. Unlike most metals, a fully annealed sample of α-uranium has strong thermal residual stresses. Furthermore, these thermal residual stresses are strong enough to induce plastic flow, which makes it unique compared to other metals which exhibit thermal stresses such as Zr alloys. During thermal cycling, the plastic strains accumulate and lead to a unique response of permanent shape change, termed thermal ratcheting. Textured polycrystalline α-uranium samples have a permanent shape change as a result of cooling and heating back to an initial temperature. This response has been well documented and qualitatively explained, but a complete explanation of the ratcheting response that incorporates all of the key factors, especially preferred orientation has been missing, until now.
To approximate the development of the thermal residual stress state and understand its effects on mechanical deformation, a combination of neutron diffraction experiments and self- consistent based polycrystalline plasticity modeling are employed. Initially, the thermal stresses were approximated assuming the stresses were relaxed by the known slip modes, predominately the chimney mode. This approach lead to quantitatively accurate estimates of the internal stresses. Subsequently, theses internal thermal stresses are shown to affect the deformation modes and internal strain evolution, but have little effect on the macroscopic stress-strain response. The same model and experimental techniques showed that the presence of hydrogen embrittles the material without affecting the mechanisms of plastic deformation which control macroscopic flow. Additionally, the model is used to explore the effects of temperature on the mechanical deformation up to 150 C. It was seen that the elevated temperature shifted the thermal stress state, and provided an apparent increase in twin resistance.
Finally, as a capstone to the dissertation research, additional in−situ neutron experiments conducted during thermal cycling and literature review suggested that essentially isotropic, rate sensitive plastic deformation occurs during temperature cycling. A rate sensitive poly- crystalline plasticity model was developed and used to predict thermal ratcheting including the important effect of crystallographic texture. The thermal ratcheting response is explained through mechanisms of creep deformation. Diffusional flow mechanisms explain the ratcheting, and the rate dependence and temperature sensitivity of the plastic deformation are revealed to be critical to predict thermal ratcheting.