In situ Characterization of Deformation Mechanisms in Harmonic Structure Nickel

There is an ever-increasing demand for structural metals with higher strength. Metals can be strengthened by reducing grain size in their microstructure, but ductility is also concomitantly reduced. High strength and ductility are a desirable combination of properties for most structural engineering materials. By arranging fine grains in a continuous network that surrounds islands of coarse grains, in a so-called harmonic structure, it is possible to increase the strength without reducing ductility. Such a synergetic effect has been attributed to an accelerated work-hardening rate in harmonic structures.

This thesis aims at deepening the understanding of the synergetic effects, which can help the optimization of harmonic structures in the future. The distributions of stress and strain were measured during tensile testing as they are deemed important for understanding the interplay between coarse and fine grain fractions. Stress distribution among the grain fractions were measured at unprecedented detail in harmonic structures materials through synchrotron X-ray powder diffraction and individual grains through high resolution reciprocal space mapping. To achieve individual investigation of grain fractions, a new algorithm was elaborated in this work for the separation and analysis of diffraction data in a single- phase material with two fractions of grain sizes. The distribution of strain was measured with digital image correlation in optical microscopy.

The main achievements in this work reveal that the constriction of coarse grains by fine grains in harmonic structures increases the yield strength of coarse grains compared to homogenous counterparts. At elastic-plastic transition, stress partitions between the grain fractions and back stresses develop in coarse grains along with forward stresses in fine grains. With further macroscopic strain, the local strains also clearly partition between the grain fractions. The stress-strain behaviour of the grain fractions is similar to the homogenous counterparts when the local and macroscopic strains are similar. The high work-hardening rate of the harmonic structure is the superposition of fine grains with inherited high work-hardening rate and coarse grains with low work-hardening rate. Beyond elastic-plastic transition, the acceleration of work-hardening rate in the coarse-grained fraction is found, which coincides with the strain partitioning. The evolution of strain distributions shortly before macroscopic fracture indicates that the microstructure might suppress strain localization. However, full understanding of fracture mechanisms requires further investigation.