(Dr. David McDowell, advisor)

"Measuring Inhomogeneous Deformation Fields in Polycrystalline OFHC Copper"

Abstract

Polycrystal plasticity models based upon the work of Taylor (1938) are founded upon the assumptions that the fundamental unit of deformation is at the scale of the grain, and that the displacement field is homogeneous amongst all grains in the aggregate. This has shown to be a reasonable first-order assumption (Kalidindi et. al., 1991), but it is widely known that grains deform in a non-uniform fashion. Recent work has shown that grain subdivision processes associated with the formation of dislocation substructures promote gradients of deformation at scales well below that of the grain (Hughes and Hansen, 1991, 1992), and that in some situations intermediate-scale mesobanding or grain-boundary sliding may occur (Panin, 1999; Muto and Sakai, 2000). Furthermore, although trends are valid, comparison of numerical simulations with experiments have shown that Taylor s assumptions lead to more highly developed textures peaks than observed experimentally for complex loading histories (Butler et. al., 1998). To date only limited experiments have been performed to investigate displacement fields well below the grain scale over a wide field of view in realistic grain size polycrystals. This thesis explores high-resolution material stretch and rotation fields measured at the sub-grain scale for ensembles of 10-20 grains. Using photolithographic techniques, a series of micron-scale grids were deposited onto the surface of OFHC copper specimens. The specimens were then monotonically compressed to strains up to unity. The resulting grid patterns exhibit strong gradients of deformation both intergranularly and intragranularly, providing a wealth of information on which to judge the performance of polycrystal plasticity theories. Out-of-plane displacement measurements are also presented in statistical form to show the influence of out-of-plane deformations on in-plane projections.