Microscopic Strain Localization and Damage in Multi-phase Alloys

Abstract

Identifying critical factors for microscopic strain localization and damage in multiphase alloys is challenging. In addition to distinct phase-specific properties, extra complexities arise from a broad spectrum of morphology and spatial distribution of phases. Multiple deformation mechanisms can also occur simultaneously, which renders it difficult to spatially and temporally resolve their contributions to strain heterogeneity. In this thesis, a comprehensive correlative approach is developed to address these challenges, which utilizes in situ scanning electron microscopy, in situ synchrotron X-ray diffraction, and various mapping techniques to analyze microstructure and micro-strain evolution. We reveal the governing microstructural mechanisms that control strain localization and damage in two most widely highlighted multi-phase alloys, an (α+β) titanium alloy and (α+α’+γ) transformation-induced plasticity (TRIP)-assisted steel. This thesis focuses especially on the effects of local texture, mechanical twinning, and mechanically-induced phase transformation. First, we study the influence of local crystallographic orientation in the two-phase titanium alloy. Quantitative analyses of local strain distribution demonstrate that the boundaries surrounded by soft and hard α grains are the most prone to strain localization. Second, we explore the role of mechanical twinning in modulating strain localization mechanisms during deformation and propose its potential use to retard damage development. The last focus of the thesis is the effects of mechanically induced martensitic transformation in a multi-phase quenching and partitioning (QP) steel. Our in situ tracking of metastable retained austenite unravels strong influences of neighborhood microstructure on its mechanical stability and also post-transformation behaviors. Based on our findings, micromechanically-guided microstructure design strategies to better optimize properties of these alloys are discussed.