42,835 research outputs found
Mathematical models of martensitic microstructure
Martensitic microstructures are studied using variational models based on nonlinear elasticity. Some relevant mathematical tools from nonlinear analysis are described, and applications given to austenite-martensite interfaces and related topics
Interplay of growth mode and thermally induced spin accumulation in epitaxial Al/CoTiSi/Al and Al/CoTiGe/Al contacts
The feasibility of thermally driven spin injectors built from half-metallic
Heusler alloys inserted between aluminum leads was investigated by means of
{\em ab initio} calculations of the thermodynamic equilibrium and electronic
transport. We have focused on two main issues and found that: (i) the interface
between Al and the closely lattice-matched Heusler alloys of type CoTi
( Si or Ge) is stable under various growth conditions; and (ii) the
conventional and spin-dependent Seebeck coefficients in such heterojunctions
exhibit a strong dependence on both the spacer and the atomic composition of
the Al/Heusler interface. The latter quantity gives a measure of the spin
accumulation and varies between ~V/K and ~V/K near ~K,
depending on whether a Ti-Ge or a Co-Co plane makes the contact between Al and
CoTiGe in the trilayer. Our results show that it is in principle possible
to tailor the spin-caloric effects by a targeted growth control of the samples.Comment: 16 pages, 13 figure
Review on Slip Transmission Criteria in Experiments and Crystal Plasticity Models
A comprehensive overview is given of the literature on slip transmission
criteria for grain boundaries in metals, with a focus on slip system and grain
boundary orientation. Much of this extensive literature has been informed by
experimental investigations. The use of geometric criteria in continuum crystal
plasticity models is discussed. The theoretical framework of Gurtin (2008, J.
Mech. Phys. Solids 56, p. 640) is reviewed for the single slip case. This
highlights the connections to slip transmission criteria from the literature
that are not discussed in the work itself. Different geometric criteria are
compared for the single slip case with regard to their prediction of slip
transmission. Perspectives on additional criteria, investigated in experiments
and used in computational simulations, are given.Comment: in Journal of Materials Science, 201
Nano-structures at martensite macrotwin interfaces in
The atomic configurations at macrotwin interfaces between microtwinned martensite plates in material are investigated using transmission electron microscopy. The observed structures are interpreted in view of possible formation mechanisms for these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular \{110\} austenite planes, enclose a final angle larger or smaller than . Two different configurations, a crossing and a step type are described. Depending on the actual case, tapering, bending and tip splitting of the smaller microtwin variants are observed. The most reproducible deformations occur in a region of approximately 5-10nm width around the interface while a variety of structural defects are observed further away from the interface. These structures and deformations are interpreted in terms of the coalescence of two separately nucleated microtwinned martensite plates and the need to accommodate remaining stresses
Growth and thermal stability of TiN/ZrAlN: Effect of internal interfaces
Wear resistant hard films comprised of cubic transition metal nitride (c-TMN) and metastable c-AlN with coherent interfaces have a confined operating envelope governed by the limited thermal stability of metastable phases. However, equilibrium phases (c-TMN and wurtzite(w)-AlN) forming semicoherent interfaces during film growth offer higher thermal stability. We demonstrate this concept for a model multilayer system with TiN and ZrAlN layers where the latter is a nanocomposite of ZrN- and AlN- rich domains. The interfaces between the domains are tuned by changing the AlN crystal structure by varying the multilayer architecture and growth temperature. The interface energy minimization at higher growth temperature leads to formation of semicoherent interfaces between w-AlN and c-TMN during growth of 15 nm thin layers. Ab initio calculations predict higher thermodynamic stability of semicoherent interfaces between c-TMN and w-AlN than isostructural coherent interfaces between c-TMN and c-AlN. The combination of a stable interface structure and confinement of w-AlN to nm-sized domains by its low solubility in c-TMN in a multilayer, results in films with a stable hardness of 34 GPa even after annealing at 1150 °C.Peer ReviewedPostprint (author's final draft
Level Set Jet Schemes for Stiff Advection Equations: The SemiJet Method
Many interfacial phenomena in physical and biological systems are dominated
by high order geometric quantities such as curvature.
Here a semi-implicit method is combined with a level set jet scheme to handle
stiff nonlinear advection problems.
The new method offers an improvement over the semi-implicit gradient
augmented level set method previously introduced by requiring only one
smoothing step when updating the level set jet function while still preserving
the underlying methods higher accuracy. Sample results demonstrate that
accuracy is not sacrificed while strict time step restrictions can be avoided
The asymptotic homogenization elasticity tensor properties for composites with material discontinuities
The classical asymptotic homogenization approach for linear elastic composites with discontinuous material properties is considered as a starting point. The sharp length scale separation between the fine periodic structure and the whole material formally leads to anisotropic elastic-type balance equations on the coarse scale, where the arising fourth rank operator is to be computed solving single periodic cell problems on the fine scale. After revisiting the derivation of the problem, which here explicitly points out how the discontinuity in the individual constituents’ elastic coefficients translates into stress jump interface conditions for the cell problems, we prove that the gradient of the cell problem solution is minor symmetric and that its cell average is zero. This property holds for perfect interfaces only (i.e., when the elastic displacement is continuous across the composite’s interface) and can be used to assess the accuracy of the computed numerical solutions. These facts are further exploited, together with the individual constituents’ elastic coefficients and the specific form of the cell problems, to prove a theorem that characterizes the fourth rank operator appearing in the coarse-scale elastic-type balance equations as a composite material effective elasticity tensor. We both recover known facts, such as minor and major symmetries and positive definiteness, and establish new facts concerning the Voigt and Reuss bounds. The latter are shown for the first time without assuming any equivalence between coarse and fine-scale energies (Hill’s condition), which, in contrast to the case of representative volume elements, does not identically hold in the context of asymptotic homogenization. We conclude with instructive three-dimensional numerical simulations of a soft elastic matrix with an embedded cubic stiffer inclusion to show the profile of the physically relevant elastic moduli (Young’s and shear moduli) and Poisson’s ratio at increasing (up to 100 %) inclusion’s volume fraction, thus providing a proxy for the design of artificial elastic composites
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