Observations in 3D of Tensile Twinning and Slip in Zr

Low symmetry crystals and polycrystals have anisotropic mechanical properties which, given better understanding of their deformation modes, could lead to development of next generation materials. Understanding how grains in a bulk polycrystal interact will guide and improve material modeling. Here, we show that tensile twins, in hexagonal close-packed metals, form where the macroscopic stress does not generate appropriate shear stress and vice versa. In other way, Schmid factors are not a reliable guide for predicting the onset of twinning. We use nondestructive near-field High Energy X-ray Diffraction Microscopy to map local crystal orientations in three dimensions over a series of tensile strain states in a zirconium polycrystal. Twins and intragranular orientation variations are observed and it is found that deformation induced rotations in neighbor grains are spatially correlated with many twins. We conclude that deformation twinning involves complex multigrain interactions. Comparisons are made with self-consistent and full-field polycrystal plasticity models

Tools for linking modeling and experiments to enable materials design at the mesoscale

There have been substantial advances in modeling and simulation of microstructure in 3D. These have been accompanied by equally significant advances in characterization techniques, with serial sectioning, synthetic microstructure generation, and synchrotron radiation all contributing strongly. Image-based methods for solving elastic, viscoplastic and elasto-viscoplastic problems are now available to complement finite element methods. The image-based methods sidestep the difficulty of generating meshes that conform to 3D microstructures while preserving mesh quality. The resolution available permits many aspects of heterogeneity in deformation to be investigated. Materials can also be orientation mapped nondestructively in 3D thanks to penetrating radiation at synchrotrons, which permits microstructural evolution to be characterized. Synthetic microstructure generation now accounts for distributions of orientation, grain boundary character, and grain morphology, even fitting the tails of distributions. Software packages such as Dream3D substantially facilitate the exchange of 3D data between experimental systems and simulation programs. Examples of applications are drawn from a number of different projects including one on understanding the origins of void nucleation under dynamic loading

Fast Fourier transform-based modelling for the determination of micromechanical fields in polycrystals

International audienceEmerging characterization methods in Experimental Mechanics pose a challenge to modelers to devise efficient formulations that permit interpretation and exploitation of the massive amount of data generated by these novel methods. In this overview we report on a numerical formulation based on Fast Fourier Transforms, developed over the last 15 years, which can use the voxelized microstructural images of heterogeneous materials as input to predict their micromechanical and effective response. The focus of this presentation is on applications of the method to plastically-deforming polycrystalline materials

Formation of annealing twins during recrystallization and grain growth in 304L austenitic stainless steel

International audienceUnderstanding of the mechanisms of annealing twin formation is fundamental for grain boundary engineering. In this work, the formation of annealing twins in a 304L austenitic stainless steel is examined in relation to the thermo-mechanical history. The behavior of annealing twins of various morphologies is analyzed using an in-situ annealing device and EBSD. The results confirm that there is a synergistic effect of prior strain level on annealing twin density generated during recrystallization. The higher the prior strain level, the higher the velocity of grain boundary migration and the higher the annealing twin density in the recrystallized grains. This effect decreases as the recrystallization fraction increases. The existing mathematical models (Pande's model and Gleiter's model), which were established to predict annealing twin density in the grain growth regime, can not predict this phenomeno

Development of Recrystallization Texture and Microstructure in Cold Rolled Copper

Oxygen free electronic copper, 99.995% purity, of two initial grain sizes, 50 {mu}m and 100 {mu}m, has been cold rolled to six strains of 1.0, 1.5, 2.0, 2.65, 3.5 and 4.5 (von Mises equivalents). The rolled materials were partially and fully recrystallized to study the development of recrystallization textures as a function of grain size, strain and fraction recrystallized. The initial textures were relatively random and the deformation textures show the classic {beta} fiber development. As strain is increased both materials produce increasingly intense cube recrystallization textures, (100), as measured both by x-ray diffraction and the electron backscatter pattern (EBSP) techniques. The strong cube recrystallization textures are a product of a higher than random frequency of cube nucleation sites. An additional factor is that cube regions grow larger than non-cube regions. The explanation of the cube frequency advantage is based on the development of large stored energy differences between cube orientations and neighboring orientations due to recovery of cube sites. Of several possible explanations of the cube orientation size advantage, the most plausible one is solute entrapment. At the higher strains the boundaries of cube grains encounter the deformation texture S components, (123), changing the boundary character to one of 40{degrees}. These boundaries are more resistant to solute accumulation than random high angle boundaries, allowing the boundaries to migrate with less of a solute drag effect than a random high angle boundary

Reduction of Annealing Times for Energy Conservation in Aluminum

Carnegie Mellon University was teamed with the Alcoa Technical Center with support from the US Dept. of Energy (Office of Industrial Technology) and the Pennsylvania Technology Investment Authority (PTIA) to make processing of aluminum less costly and more energy efficient. Researchers in the Department of Materials Science and Engineering have investigated how annealing processes in the early stages of aluminum processing affect the structure and properties of the material. Annealing at high temperatures consumes significant amounts of time and energy. By making detailed measurements of the crystallography and morphology of internal structural changes they have generated new information that will provide a scientific basis for shortening processing times and consuming less energy during annealing

Ultrafast X-Ray Imaging of Laser-Metal Additive Manufacturing Processes

The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts