Correlation between subgrains and coherently scattering domains

Crystallite size determined by X-ray line profile analysis is often smaller than the grain or subgrain size obtained by transmission electron microscopy, especially when the material has been produced by plastic deformation. It is shown that besides differences in orientation between grains or subgrains, dipolar dislocation walls without differences in orientation also break down coherency of X-rays scattering. This means that the coherently scattering domain size provided by X-ray line profile analysis provides subgrain or cell size bounded by dislocation boundaries or dipolar walls

Nanostructures in Ti processed by severe plastic deformation

Metals and alloys processed by severe plastic deformation (SPD) can demonstrate superior mechanical properties, which are rendered by their unique defect structures. In this investigation, transmission electron microscopy and x-ray analysis were used to systematically study the defect structures, including grain and subgrain structures, dislocation cells, dislocation distributions, grain boundaries, and the hierarchy of these structural features, in nanostructured Ti produced by a two-step SPD procedure-warm equal channel angular pressing followed by cold rolling. The effects of these defect structures on the mechanical behaviors of nanostructured Ti are discussed

Origin of the Bauschinger effect in a polycrystalline material

There is a long and lively debate in the literature about the origin of the Bauschinger effect in polycrystalline materials, the most widely accepted explanation being the easier movement of dislocations during reverse loading causing the reduction of the yield stress. Other explanations include incompatible deformation at the grain scale and change of dislocation cell structures during forward and reverse loading, but recent publications show these phenomenological explanations of the Bauschinger effect are not holistic. In the experimental work presented here, we have investigated the role of micro residual lattice strain on the origin of the Bauschinger effect in type 316H austenitic stainless steel using in-situ neutron diffraction. Standard cylindrical specimens were tension-compression load cycled at room temperature with the loading interrupted at incrementally larger compressive and tensile strains followed by reloading to the tensile loop peak strain. Mirror symmetric compression-tension cyclic tests were also performed with tensile and compressive load interruptions followed by compressive reloading to the compressive loop peak strain. A strong correlation is demonstrated between the evolution of residual lattice strain in the grain families and the change in magnitude in macroscopic yield stress, peak stress and the shape of the yielding part of the stress-strain curve for both the cyclic tension yield and compression yield tests. This implies that the residual lattice strain generated by grain scale elastic and plastic deformation anisotropy is the primary source of the Bauschinger kinematic hardening effect observed in type 316H austenitic stainless steel

Origin of Creep-Fatigue Back Stress and its Effect on Deformation and Damage

Creep deformation of metals operating at a high temperature in electricity generation plant can limit the lifetime of components and pressurized systems. Assessment of a structure’s creep life under power plant operation conditions is a complex problem due to materials being exposed to cyclic load variations. The creep life of high-temperature steels can be significantly affected by the generation of internal back stress during monotonic and cyclic plastic loadings, originating from inhomogeneous deformation at grain and sub-grain length scales. This thesis examines origins of back stress developed in austenitic stainless steel and their influence on subsequent material deformation behaviour. In-situ neutron diffraction and transmission electron microscopy techniques were employed to study the contributions of intergranular and intragranular incompatible strains to the back stress that is introduced in type 316H austenitic stainless steel under monotonic and cyclic loading at room and elevated temperatures. The scope of testing included load controlled and displacement controlled creep dwells introduced at peak and intermediate positions of the cyclic loading curves. The origin of kinematic hardening in the same material was also examined by systematic loading interruptions during tension-compression cyclic loading, from which the observed variations in macroscopic yield stress were correlated with corresponding changes in intergranular strains. In addition, development of creep cavitation damage was characterized using small angle neutron scattering (SANS) and high-speed atomic force microscope (HS-AFM) techniques. Intergranular strains were found to significantly affect the minimum creep deformation rate of type 316H austenitic stainless steel, whereas no evidence of that for intragranular strains was observed, at the early stage of creep deformation studied here. It was found that, during tension-compression cyclic loading, the magnitude of intergranular strains not only depends on the stress and strain in the material but also on its loading path history. Intergranular strains were found to increase during the primary stage of load controlled creep, remain unchanged during the secondary stage and reduce during displacement controlled creep relaxation. A strong correlation between the evolution of intergranular strains and the kinematic hardening of this material were observed during interrupted cyclic loading test at room and elevated temperature, suggesting, that the observed Bauschinger effect in this material originates from the intergranular strains. SANS and HS-AFM were found to be powerful quantitative techniques for studying the nucleation and growth of creep cavities in stainless steel. The HS-AFM work also revealed that the cavities were faceted which highlights the oversimplification of current creep cavitation models that are based on an assumed spherical morphology. The experimental results have highlighted the significance of the effect of plasticity generated back stress on the creep and cyclic deformation of type 316H austenitic stainless steel. This demonstrates the importance of allowing for the evolution of back stress in high-temperature life assessment procedures

Mesoscale theory of grains and cells: crystal plasticity and coarsening

Solids with spatial variations in the crystalline axes naturally evolve into cells or grains separated by sharp walls. Such variations are mathematically described using the Nye dislocation density tensor. At high temperatures, polycrystalline grains form from the melt and coarsen with time: the dislocations can both climb and glide. At low temperatures under shear the dislocations (which allow only glide) form into cell structures. While both the microscopic laws of dislocation motion and the macroscopic laws of coarsening and plastic deformation are well studied, we hitherto have had no simple, continuum explanation for the evolution of dislocations into sharp walls. We present here a mesoscale theory of dislocation motion. It provides a quantitative description of deformation and rotation, grounded in a microscopic order parameter field exhibiting the topologically conserved quantities. The topological current of the Nye dislocation density tensor is derived from a microscopic theory of glide driven by Peach-Koehler forces between dislocations using a simple closure approximation. The resulting theory is shown to form sharp dislocation walls in finite time, both with and without dislocation climb.Comment: 5 pages, 3 figure

Deformation of Crystals: Connections with Statistical Physics

We give a bird's-eye view of the plastic deformation of crystals aimed at the statistical physics community, as well as a broad introduction to the statistical theories of forced rigid systems aimed at the plasticity community. Memory effects in magnets, spin glasses, charge density waves, and dilute colloidal suspensions are discussed in relation to the onset of plastic yielding in crystals. Dislocation avalanches and complex dislocation tangles are discussed via a brief introduction to the renormalization group and scaling. Analogies to emergent scale invariance in fracture, jamming, coarsening, and a variety of depinning transitions are explored. Dislocation dynamics in crystals challenge nonequilibrium statistical physics. Statistical physics provides both cautionary tales of subtle memory effects in nonequilibrium systems and systematic tools designed to address complex scale-invariant behavior on multiple length scales and timescales

Experimental investigations of internal and effective stresses during fatigue loading of high-strength steel

International audienceLow cycle fatigue tests are performed on a high strength tempered martensitic steel at different plastic strain amplitudes at room temperature. Internal and effective components of the flow stress are analyzed using Handfield and Dickson's method. The internal stress is affected by the plastic strain amplitude. Conversely, the evolution of the athermal component of the effective stress with the number of cycles is independent of the plastic strain amplitude. The thermal part of the effective stress increases with the plastic strain amplitude, but remains constant with plastic strain accumulation. Microstructural changes in the cyclically deformed material are investigated by means of transmission electronic mycroscopy and X-Ray characterizations. Internal and effective stress evolutions are discussed based on these observation