4,325 research outputs found
Crystal plasticity modelling and experiments for deriving microstructure-property relationships in γ-TiAl based alloys
Effect of aspect ratio on transverse diffusive broadening: A lattice Boltzmann study
We study scaling laws characterizing the inter-diffusive zone between two
miscible fluids flowing side by side in a Y-shape laminar micromixer using the
lattice Boltzmann method. The lattice Boltzmann method solves the coupled 3D
hydrodynamics and mass transfer equations and incorporates intrinsic features
of 3D flows related to this problem. We observe the different power law regimes
occurring at the center of the channel and close to the top/bottom wall. The
extent of the inter-diffusive zone scales as square root of the axial distance
at the center of the channel. At the top/bottom wall, we find an exponent 1/3
at early stages of mixing as observed in the experiments of Ismagilov and
coworkers [Appl. Phys. Lett. 76, 2376 (2000)]. At a larger distance from the
entrance, the scaling exponent close to the walls changes to 1/2 [J.-B. Salmon
et al J. Appl. Phys. 101, 074902 (2007)]. Here, we focus on the effect of
finite aspect ratio on diffusive broadening. Interestingly, we find the same
scaling laws regardless of the channel's aspect ratio. However,the point at
which the exponent 1/3 characterizing the broadening at the top/bottom wall
reverts to the normal diffusive behavior downstream strongly depends on the
aspect ratio. We propose an interpretation of this observation in terms of
shear rate at the side walls. A criterion for the range of aspect ratios with
non-negligible effect on diffusive broadening is also provided.Comment: 19 pages, 7 figure
Ab-initio simulation and experimental validation of beta-titanium alloys
In this progress report we present a new approach to the ab-initio guided
bottom up design of beta-Ti alloys for biomedical applications using a quantum
mechanical simulation method in conjunction with experiments. Parameter-free
density functional theory calculations are used to provide theoretical guidance
in selecting and optimizing Ti-based alloys with respect to three constraints:
(i) the use of non-toxic alloy elements; (ii) the stabilization of the body
centered cubic beta phase at room temperature; (iii) the reduction of the
elastic stiffness compared to existing Ti-based alloys. Following the
theoretical predictions, the alloys of interest are cast and characterized with
respect to their crystallographic structure, microstructure, texture, and
elastic stiffness. Due to the complexity of the ab initio calculations, the
simulations have been focused on a set of binary systems of Ti with two
different high melting bcc metals, namely, Nb and Mo. Various levels of model
approximations to describe mechanical and thermodynamic properties are tested
and critically evaluated. The experiments are conducted both, on some of the
binary alloys and on two more complex engineering alloy variants, namely,
Ti-35wt.%Nb-7wt.%Zr-5wt.%Ta and a Ti-20wt.%Mo-7wt.%Zr-5wt.%Ta.Comment: 23 pages, progress report on ab initio alloy desig
Polykristallmechanik Grundlagen
Mikrostruktur: Gesamtheit aller Gitterfehler, die sich nicht im thermodynamischen Gleichgewicht befinden (Haasen). Pfad: Prozeßgeschichte; meßbar nicht durch Prozeßparameter, sondern durch mikrostrukturelle Zustandsparameter (i.d.R. keine Analogie zu thermodynamischen Zustandsgrößen, Vorsicht bei Differentiation). Textur: Volumengewichtete Gesamtheit aller Kristallorientierungen in einer Probe. Materialeigenschaften: Makroskopische Reaktionen einer Probe auf makroskopische Anregungen. Konstitutive Gesetze: Quantitativ gefaßte Relationen zwischen makroskopischen Anregungen und makroskopischen Reaktionen (Feldgrößen, z.B. Hooke, evtl. auf Basis der Mikrostruktur). Konstitution: Thermodynamik der Phasen Isotropie: Richtungsunabhängigkeit (Tropos (gr.): Richtung) Anisotropie: Richtungsabhängigkeit (Morphologie, Toplogie, Textur, etc.) Fließort: Gesamtheit aller Spannungszustände, bei denen plastisches (zusätzlich zu elastischem) Fließen auftrit
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Mechanisms for the maintenance and eventual degradation of neurofilament proteins in the distal segments of severed goldfish Mauthner axons
Cellular mechanisms that might affect the degradation of neurofilament proteins (NFPs) were examined in the distal segments
of severed goldfish Mauthner axons (M-axons), which do
not degenerate for more than 2 months after severance. Calpain
levels, as determined by reactivity to a polyclonal antibody,
remained constant for 80 d postseverance in distal segments
of M-axons and then declined from 80 to 85 d
postseverance. Calpain activity in rat brain, as determined by a
spectrophotometric assay, was much higher than calpain activity
in control and severed goldfish brain, spinal cord, muscle,
or M-axons. Calpain activity was extremely low in M-axons
compared with that in all other tissues and remained low for up
to 80 d postseverance in distal segments of M-axons. Phosphorylated
NFPs, as determined by Stains-All treatment of SDS
gels, were maintained for up to 72 d postseverance and then decreased noticeably at 75 d postseverance when NFP breakdown
products appeared on silver-stained gels. By 85 d postseverance,
phosphorylated NFPs no longer were detected, and
NFP breakdown products were the most prominent bands on
silver-stained gels. These results suggest that the distal segments
of M-axons survive for months after severance, because
NFPs are maintained in a phosphorylated state that stabilizes
and protects NFPs from degradation by low levels of calpain
activity in the M-axon; the distal segments of severed M-axons
degenerate eventually when NFPs no longer are maintained in
a phosphorylated state and become susceptible to degradation,
possibly by low levels of calpain activity in the M-axon.This work was supported by an Advanced Technology Project Grant to G.D.B.Neuroscienc
The Role of Texture and Elastic-Plastic Anisotropy in Metal Forming Simulations
Metals mostly occur in polycrystalline form where each grain has a different crystallographic orientation, shape, and volume fraction. The distribution of the grain orientations is referred to as crystallographic texture. The discrete nature of crystallographic slip along certain lattice directions on preferred crystallographic planes entails an anisotropic plastic response of such samples under mechanical loads. While the elastic-plastic deformation of single crystals and bicrystals can nowadays be well predicted, plasticity of polycrystalline matter is less well understood. This is essentially due to the intricate interaction of the grains during co-deformation. This interaction leads to strong in-grain and grainto- grain heterogeneity in terms of strain, stress, and crystal orientation. Modern metal forming and crash simulations are usually based on the finite element method. Aims of such simulations are typically the prediction of the material shape, failure, and mechanical properties during deformation. Further goals lie in the computer assisted lay-out of manufacturing tools used for intricate processing steps. Any such simulation requires that the material under investigation is specified in terms of its respective constitutive behavior. Modern finite element simulations typically use three sets of material input data, covering hardening, forming limits, and anisotropy. The current research report issued by the Max-Planck-Institut für Eisenforschung is about the latter aspect placing particular attention on the physical nature of anisotropy. The report reviews different empirical and physically based concepts for the integration of the elastic-plastic anisotropy into metal forming finite Raabe, Texture and Anisotropy in Metal Forming Simulations Raabe, edoc Server, Max-Planck-Society - 3 - MPI Düsseldorf element simulations. Particular pronunciation is placed on the discussion of the crystallographic anisotropy of polycrystalline material rather than on aspects associated with topological or morphological microstructure anisotropy. The reviewed anisotropy concepts are empirical yield surface approximations, yield surface formulations based on crystallographic homogenization theory, combinations of finite element and homogenization approaches, the crystal plasticity finite element method, and the recently introduced texture component crystal plasticity finite element method. The report presents the basic physical approaches behind the different methods and discusses engineering aspects such as scalability, flexibility, and texture update in the course of a forming simulation. Published overviews on this topic can be found in Raabe, Klose, Engl, Imlau, F. Friedel, Roters: Advanced Engineering Materials 4 (2002) 169-180, Raabe, Zhao, Mao: Acta Materialia 50 (2002) 4379–4394, and Raabe, Roters: International Journal of Plasticity 20 (2004) p. 339-36
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