590 research outputs found
Rise of correlations of transformation strains in random polycrystals
We investigate the statistics of the transformation strains that arise in random martensitic polycrystals as boundary conditions cause its component crystallites to undergo martensitic phase transitions. In our laminated polycrystal model the orientation of the n grains (crystallites)
is given by an uncorrelated random array of the orientation angles Ξ_i, i = 1, . . . ,n. Under imposed boundary conditions the polycrystal grains may undergo a martensitic transformation. The associated transformation strains Δ_i, i = 1, . . . ,n depend on the array of orientation angles, and they can be obtained as a solution to a nonlinear optimization problem. While the random variables Ξ_i,
i = 1, . . . ,n are uncorrelated, the random variables Δ_i, i = 1, . . . ,n may be correlated. This issue is
central in our considerations. We investigate it in following three different scaling limits: (i) Infinitely
long grains (laminated polycrystal of height L = â); (ii) Grains of finite but large height (L » 1); and (iii) Chain of short grains (L = l_0/(2n), l_0 « 1). With references to de Finettiâs theorem, Rieszâ
rearrangement inequality, and near neighbor approximations, our analyses establish that under the
scaling limits (i), (ii), and (iii) the arrays of transformation strains arising from given boundary
conditions exhibit no correlations, long-range correlations, and exponentially decaying short-range
correlations, respectivel
The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals
Additively manufactured (AM) metals exhibit highly complex microstructures,
particularly with respect to grain morphology which typically features
heterogeneous grain size distribution, anomalous and anisotropic grain shapes,
and the so-called columnar grains. In general, the conventional morphological
descriptors are not suitable to represent complex and anisotropic grain
morphology of AM microstructures. The principal aspect of microstructural grain
morphology is the state of grain boundary spacing or grain size whose effect on
the mechanical response is known to be crucial. In this paper, we formally
introduce the notions of axial grain size and grain size anisotropy as robust
morphological descriptors which can concisely represent highly complex grain
morphologies. We instantiated a discrete sample of polycrystalline aggregate as
a representative volume element (RVE) which has random crystallographic
orientation and misorientation distributions. However, the instantiated RVE
incorporates the typical morphological features of AM microstructures including
distinctive grain size heterogeneity and anisotropic grain size owing to its
pronounced columnar grain morphology. We ensured that any anisotropy arising in
the macroscopic mechanical response of the instantiated sample is mainly
associated with its underlying anisotropic grain size. The RVE was then used
for meso-scale full-field crystal plasticity simulations corresponding to
uniaxial tensile deformation along different axes via a spectral solver and a
physics-based crystal plasticity constitutive model. Through the numerical
analyses, we were able to isolate the contribution of anisotropic grain size to
the anisotropy in the mechanical response of polycrystalline aggregates,
particularly those with the characteristic complex grain morphology of AM
metals. Such a contribution can be described by an inverse square relation
Revealing per-grain and neighbourhood stress interactions of a deforming ferritic steel via three-dimensional X-ray diffraction
The structural performance of polycrystalline alloys is strongly controlled by the characteristics of individual grains and their interactions, motivating this study to understand the dynamic micromechanical response within the microstructure. Here, a high ductility single-phase ferritic steel during uniaxial deformation is explored using three-dimensional X-ray diffraction. Grains well aligned for dislocation slip are shown to possess a wide intergranular stress range, controlled by per-grain dependent hardening activity. Contrariwise, grains orientated poorly for slip have a narrow stress range. A grain neighbourhood effect is observed of statistical significance: the Schmid factor of serial adjoining grains influences the stress state of a grain of interest, whereas parallel neighbours are less influential. This phenomenon is strongest at low plastic strains, with the effect diminishing as grains rotate during plasticity to eliminate any orientation dependent load shedding. The ability of the ferrite to eliminate such neighbourhood interactions is considered key to the high ductility possessed by these materials
Revealing per-grain and neighbourhood stress interactions of a deforming ferritic steel via three-dimensional X-ray diffraction
The structural performance of polycrystalline alloys is strongly controlled by the characteristics of individual grains and their interactions, motivating this study to understand the dynamic micromechanical response within the microstructure. Here, a high ductility single-phase ferritic steel during uniaxial deformation is explored using three-dimensional X-ray diffraction. Grains well aligned for dislocation slip are shown to possess a wide intergranular stress range, controlled by per-grain dependent hardening activity. Contrariwise, grains orientated poorly for slip have a narrow stress range. A grain neighbourhood effect is observed of statistical significance: the Schmid factor of serial adjoining grains influences the stress state of a grain of interest, whereas parallel neighbours are less influential. This phenomenon is strongest at low plastic strains, with the effect diminishing as grains rotate during plasticity to eliminate any orientation dependent load shedding. The ability of the ferrite to eliminate such neighbourhood interactions is considered key to the high ductility possessed by these materials
Modelling Polycrystalline Materials: An Overview of Three-Dimensional Grain-Scale Mechanical Models
International audienc
Review of the Synergies Between Computational Modeling and Experimental Characterization of Materials Across Length Scales
With the increasing interplay between experimental and computational
approaches at multiple length scales, new research directions are emerging in
materials science and computational mechanics. Such cooperative interactions
find many applications in the development, characterization and design of
complex material systems. This manuscript provides a broad and comprehensive
overview of recent trends where predictive modeling capabilities are developed
in conjunction with experiments and advanced characterization to gain a greater
insight into structure-properties relationships and study various physical
phenomena and mechanisms. The focus of this review is on the intersections of
multiscale materials experiments and modeling relevant to the materials
mechanics community. After a general discussion on the perspective from various
communities, the article focuses on the latest experimental and theoretical
opportunities. Emphasis is given to the role of experiments in multiscale
models, including insights into how computations can be used as discovery tools
for materials engineering, rather than to "simply" support experimental work.
This is illustrated by examples from several application areas on structural
materials. This manuscript ends with a discussion on some problems and open
scientific questions that are being explored in order to advance this
relatively new field of research.Comment: 25 pages, 11 figures, review article accepted for publication in J.
Mater. Sc
The behavior of grain boundaries in the Fe-based superconductors
The Fe-based superconductors (FBS) are an important new class of
superconducting materials. As with any new superconductor with a high
transition temperature and upper critical field, there is a need to establish
what their applications potential might be. Applications require high critical
current densities, so the usefulness of any new superconductor is determined
both by the capability to develop strong vortex pinning and by the absence or
ability to overcome any strong current-limiting mechanisms of which grain
boundaries in the cuprates are a cautionary example. In this review we first
consider the positive role that grain boundary properties play in the metallic,
low temperature superconductors and then review the theoretical background and
current experimental data relating to the properties of grain boundaries in FBS
polycrystals, bi-crystal thin films, and wires. Based on this evidence, we
conclude that grain boundaries in FBS are weak linked in a qualitatively
similar way to grain boundaries in the cuprate superconductors, but also that
the effects are a little less marked. Initial experiments with the textured
substrates used for cuprate coated conductors show similar benefit for the
critical current density of FBS thin films too. We also note that the
particular richness of the pairing symmetry and the multiband parent state in
FBS may provide opportunities for grain boundary modification as a better
understanding of their pairing state and grain boundary properties are
developed.Comment: To appear in Reports on Progress in Physic
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