Grain Size and Solid Solution Strengthening in Metals

Sammanfattning: The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries.For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults.Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli.The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals.Keywords:strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD