MICROMECHANISMS OF DAMAGE AND FAILURE IN POLYCRYSTALLINE MATERIALS FROM X-RAY AND NEUTRON DIFFRACTION

The room-temperature plastic behavior of several FCC alloys was examined with in-situ neutron-diffraction and with polychromatic X-ray microbeam diffraction (PXM). The measurements characterize the local dislocation density distribution as a function of loading and combined with modeling, provide insights into damage and failure in polycrystalline materials. Both, monotonic-tension and low-cyclefatigue experiments were conducted as a function of stress. The plastic behavior during deformation is discussed in light of the relationship between the stress and dislocationdensity evolution. The observed dislocation density evolution finds that the monotonic tensile and low-cycle-fatigue samples have similar dislocation densities at small strain, but that latter have much lower dislocation densities than the former at high strain. INTRODUCTION: Plastic deformation in polycrystalline solids is the focus of intense research due to an increasing demand for high-performance structural materials. Of central importance is the need to understand the mechanisms that control damage and which ultimately lead to failure. The recent development of powerful X-ray and neutron sources allows nondestructive materials characterization at the length scales needed to advance our understanding of damage nucleation and evolution. For example, nondestructive three-dimensional (3D) measurements of submicron strain and unpaired dislocation-density distributions are now possible due to an emerging class of instrumentation: the 3D x-ray crystal microscopes. These instruments use ultra-intense synchrotron x-ray sources and advanced x-ray optics to probe polycrystalline materials with submicron x-ray beams. By employing polychromatic x-ray microbeams (PXM) and a virtual pinhole camera method, called differential aperture microscopy, 3D distributions of the local crystalline phase, orientation (texture) and elastic and plastic strain tensor distributions can be measured with submicron resolution in all directions [Larson et al. 2002; Ice and Barabash, 2007]. Neutrons also provide valuable information about damage processes and can be used to study in-situ bulk deformation within gauge volumes that are sufficiently large to provide good statistical information, but are sufficiently small to avoid surface or geometrical complications. Because neutrons are nondestructive, measurements can be made in-situ on a single sample, which avoids the difficulties of ex-situ or multiple specimen