Localized Deformation And Damage Of FCC/HCP Metallic Materials Investigated By Synchrotron-based X-ray Diffraction

Localized deformation extensively occurs in deformed metal polycrystals.The behaviors vary with different types of metals,processing,and severing conditions that often involve complex physical mechanisms and evolution laws.Making clear the relation between microstructure and mechanical behaviors will contribute to improving the properties of structural metallic materials.However,due to some limitations especially on characterization scales of past experimental techniques,the localized deformation related failure mechanism is still not understood well,which prohibited the design,simulation,and evaluation of structural components.Studying the critical issues by employing multiscale research methods will benefit a further understanding on the evolution of deformation microstructures,thus magnifying the processing to improve the mechanical properties.Synchrotron-based diffraction techniques,as non-destructive characterization methods with high penetration,are extensively used in various scientific researches.The establishment and development of the third-generation synchrotron sources enable the techniques of the ability to do researches with much more excellent resolution in both spatial and time scales.It is especially useful in conducting in-situ experiments at synchrotron beamlines,which brings opportunities for various scientific fields in both academic research and industry application.The development of synchrotron techniques also paves new ways for multiscale researches on complex long-term questions,e.g.,localized straining.Here the synchrotron-based techniques combined with conventional characterization methods were employed to answer several challenging questions on behaviors and evolution of deformation structures.These studies provide models for metals deformation researches by using synchrotron-based diffraction techniques.(1)Shear banding is a ubiquitous phenomenon of severe plastic deformation,and damage accumulation in shear bands often results in the catastrophic failure of a material.Despite extensive studies,the microscopic mechanisms of strain localization and deformation damage in shear bands remain elusive due to their spatial-temporal complexities embedded in bulk materials.In chapter four,the mechanical fatigue behavior of AL6XN stainless steel as a typical type of planar slip alloy was investigated by in-situ neutron diffraction and synchrotron-based X-ray microdiffraction(?XRD)methods.Under cyclic loading at a high strain amplitude(±0.8%),the fatigue damage originated mainly from the accumulation of statistically stored dislocations,as clearly evidenced from a continuous increase in diffraction peak width with increasing the number of load cycles.However,under cyclic loading at a low strain amplitude(±0.3%),the density of statistically stored dislocations became saturated just after a hundred loading cycles,and the fatigue damage was mainly dominated by the accumulation of persistent Luders bands(PLBs)and the complex interactions among various PLBs as evidenced through X-ray microdiffraction measurements.It was further found that there exists obvious grain-orientation-dependent local damage in the low-strain-amplitude fatigued sample.In particular,fatigued grains orientated with[001]paralleling the loading direction are subjected to compressive stress and contain a large number of broad PLBs in boundaries arraying the edge dislocation pile-ups,which generate a large stress gradient leading to local plastic instability.The highly localized stress field at PLBs in the cyclically-deformed sample at a low strain amplitude may explain the obvious cyclic stress softening.We further conducted synchrotron-based X-ray microdiffraction experiments to map out the 3D lattice strain field with a submicron resolution around fatigue shear bands in stainless steel.Both in situ and postmortem?XRD results revealed large lattice strain gradients at intersections of the primary and secondary shear bands.Such strain gradients resulted in severe mechanical heterogeneities across the fatigue shear bands,leading to reduced fatigue limits in the high-cycle regime.The ability to spatially quantify the localized strain gradients with submicron resolution through ?XRD opens opportunities for understanding the microscopic mechanisms of damage and failure in bulk materials.(2)Mechanical twinning has been extensively observed in the deformed hexagonal closed-packed(HCP)polycrystalline metals,significantly governing the strength and other mechanical/physical properties.However,the local stress balance related to the formation and propagation of mechanical twins and its influence on crystallographic orientation evolution is still mysterious in materials science and engineering.Here,both spatial-resolved orientation and lattice strain of grains buried inside a titanium polycrystal were mapped in-situ under tensile loading by synchrotron-based X-ray microdiffraction technique,providing the direct experimental evidence on the significant stress heterogeneity caused by mechanical twinning and its interaction with slip bands.The dislocation pile-ups blocked by the mechanical nano-twins generate a very high-stress gradient,reaching 84 MPa/?m.Our in-situ experiments also reveal the significant role of multiscale geometrically necessary components,i.e.,a type of twin-and dislocation-configured complexity in sustaining the high local stress gradient for maintaining the continuum of plastic strain during deformation of HCP metals.Meanwhile,the microstructure and stress gradient of the laser shock peening treated titanium polycrystal was characterized by the synchrotron-based high energy X-ray diffraction(HE-XRD),in-situ revealing the dynamics of the hierarchical structures.The result indicated that the grain reorientation is suppressed in the deformed surface layer.(3)Metal additive manufacturing(AM)has become a new revolutionary technique of high-performance structural materials building for industrial manufacturing systems.Here the HE-XRD combined with in-situ EBSD was employed to study the deformation scenario of 316L austenitic stainless steels fabricated by coaxial powder feeding(CPF)and selected laser melting(SLM)methods upon uniaxial tensile loading along the AM building direction at room temperature,finding that the effect of slipping induced reorientation is reduced with significant heterogeneous deformation being generated.The deformation in AM components was confined by printed hierarchical structures,i.e.,irregular grain boundaries,inhomogeneous orientation-grains,and intragranular cell structures,improving the interface stiffness and enhancing the grain-to-grain accommodation.In CPF-316L,the localized strain was carried by heterogeneous slipping among different grains,inducing extensive grain-to-grain interactions to accommodate local mechanical mismatch.In SLM-316L,extensive deformation twinning offsets the plasticity that should be provided via grain rotation to sustain the deformability.The analysis of this study provides a new understanding of the deformation mechanisms of AM materials and helps to evaluating the service stability of similar components.