Sub-grain deformation mechanisms of structural materials exposed through X-ray microscopy
Creators: name: Gustafson, Sven E
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Aerospace materials

description: Structural polycrystalline alloys are prevalent in industrial components, and it is of critical importance to prevent their failure while in service. To prevent this failure, engineers must understand the microstructural deformation mechanisms which occur within the material while under load. Such understanding requires measurements of the heterogenous deformation which occurs on the grain and sub-grain scale. Here, multiple 3D high energy X-ray microscopy techniques are employed to non-destructively measure the micromechanical state at these length scales before, during, and after deformation. Four studies are presented in this dissertation which use grain and sub-grain scale measurements of this heterogenous deformation to investigate residual stress, dynamic recovery, coherent twin boundaries, and triple junctions. First, residual stress in a component can be beneficial or detrimental to its performance. To explore the link between residual stress and strain rate, the residual stress state of individual grains before and after high strain rate and quasi-static loading are compared in a stainless steel alloy. A larger spread in residual stress was measured after high strain rate loading, with a distribution nearly twice that of the quasi-static test. Additionally, less grain scale lattice reorientation was found in the high strain rate test which led to the postulation that lattice reorientation during loading resulted in reduced residual stresses after deformation. Second, dynamic recovery is a microstructural phenomenon during thermo-mechanical loading. At elevated temperatures, dynamic recovery has been seen to affect the material properties of nickel-based superalloys, which are critical to their fatigue performance. Here, a nickel-based superalloy was thermo-mechanically loaded and heterogenous amounts of recovery were measured within individual grains during the hot loading state. Additionally, grain scale stresses were found to be insufficient to predict recovery, likely due to the mechanisms of recovery occurring on the sub-grain scale. Third, in nickel-based superalloys, coherent twin boundaries provide a combination of strength and ductility, but are known sites for fatigue crack initiation. As such, these critical regions require interrogation of their micromechanical state due to fatigue loading to capture the mechanisms driving fatigue failure. In this work, intragranular X-ray microscopy measured large gradients of elastic strain (stress) and misorientation near a coherent twin boundary. Coupling this finding with a crystal plasticity model revealed that the gradients imposed upon the microstructure were due to the existence of the twin boundary itself. This result connects sharp spatial gradients in stress and a metric of plasticity to a known site of crack initiation and highlights the importance of measured such gradients. Fourth, triple junctions are intersections of three or more grains and are known stress and strain concentrators due the compatibility requirements of multiple intersecting grains. To capture the heterogenous deformation caused by triple junctions, this study measures the intragranular misorientation and elastic strain of entire grains embedded within a polycrystal. Triple junctions were found to exhibit statistically higher values of the intragranular misorientation metrics than standard grain boundaries and the highest values of these metrics were measured to be spatially closer to triple junctions than grain boundaries. Intragranular misorientation, elastic strain, and dislocation density were all seen to vary greatly along a single triple junction, with two points exhibiting dislocation densities which differed by more than 5x the average value. Finally, the spatial gradients of lattice curvature and elastic strain were found to be comparable in magnitude, with the dislocation density at hotspots inaccurately calculated by to 37% of the maximum value when elastic strains were neglected. By capturing the heterogenous micromechanical fields at the grain and sub-grain scale, this dissertation works to provide insight into the mechanisms occurring during the deformation of polycrystalline structural alloys.