Haydn N. G. Wadley

The topological design of multifunctional cellular metals

Abstract The multifunctional performance of stochastic (foamed) cellular metals is now well documented. This article compares such materials with the projected capabilities of materials with periodic cells, configured as cores of panels, tubes and shells. The implementation opportunities are as ultra-light structures, for compact cooling, in energy absorption and vibration control. The periodic topologies comprise either micro-truss lattices or prismatic materials. Performance benefits that can be expected upon implementing these periodic materials are presented and compared with competing concepts. Methods for manufacturing these materials are discussed and some cost/performance trade-offs are addressed.

Fabrication and structural performance of periodic cellular metal sandwich structures

Metallic sandwich panels with periodic, open-cell cores are important new structures, enabled by novel fabrication and topology design tools. Fabrication protocols based on the sheet forming of trusses and shell elements (egg-boxes) as well as textile assembly have allowed the manufacture of robust structures by inexpensive routes. Topology optimization enables control of failure mechanisms at the truss length scale, leading to superior structural performance. Analysis, testing and optimization have demonstrated that sandwich panels constructed with these cores sustain loads at much lower relative densities than stochastic foams. Moreover, the peak strengths of truss and textile cores are superior to honeycombs at low relative densities, because of their superior buckling resistance. Additional benefits of the truss/textile cores over honeycombs reside in their potentially lower manufacturing cost as well as in their multifunctionality.

Multifunctional periodic cellular metals

Periodic cellular metals with honeycomb and corrugated topologies are widely used for the cores of light weight sandwich panel structures. Honeycombs have closed cell pores and are well suited for thermal protection while also providing efficient load support. Corrugated core structures provide less efficient and highly anisotropic load support, but enable cross flow heat exchange opportunities because their pores are continuous in one direction. Recent advances in topology design and fabrication have led to the emergence of lattice truss structures with open cell structures. These three classes of periodic cellular metals can now be fabricated from a wide variety of structural alloys. Many topologies are found to provide adequate stiffness and strength for structural load support when configured as the cores of sandwich panels. Sandwich panels with core relative densities of 2–10% and cell sizes in the millimetre range are being assessed for use as multifunctional structures. The open, three-dimensional interconnected pore networks of lattice truss topologies provide opportunities for simultaneously supporting high stresses while also enabling cross flow heat exchange. These highly compressible structures also provide opportunities for the mitigation of high intensity dynamic loads created by impacts and shock waves in air or water. By filling the voids with polymers and hard ceramics, these structures have also been found to offer significant resistance to penetration by projectiles.

Atomic scale structure of sputtered metal multilayers

A combined theoretical and experimental approach has been used to study nanoscale CoFe/Cu/CoFe multilayer films grown by sputter deposition. Such films have applications in sensors that utilize the giant magnetoresistance effect, for example, read heads in high-density information storage devices. Atomistic simulations based on a molecular dynamics approach and an alloy form of the embedded atom method have been developed to accurately model the sputter deposition of the CoFe/Cu/CoFe multilayers. The simulations show that relatively flat interfaces are formed because of the energetic deposition conditions. However, significant intermixing at the CoFe-on-Cu interface, but not at the Cu-on-CoFe interface, was observed. An abnormal Fe depletion zone is also revealed at the CoFe-on-Cu interface. A three-dimensional atom probe method has been used for a nanoscale chemical analysis of the films. It provided direct verification of the simulations. The simulations have then been used to understand the mechanism responsible for the formation of the intermixing defects observed in the multilayers. A novel deposition technique is proposed which reduces both interfacial mixing and Fe depletion by controlling the incident adatom energies.

Compressive deformation and yielding mechanisms in cellular Al alloys determined using X-ray tomography and surface strain mapping

The mechanisms of compressive deformation that occur in both closed and open cell Al alloys have been established. This has been achieved by using X-ray computed tomography (CT) and surface strain mapping to determine the deformation modes and the cell morphologies that control the onset of yielding. The deformation is found to localize in narrow bands having widths of order of a cell diameter. Outside the bands, the material remains elastic. The cells within the bands that experience large permanent strains are primarily elliptical. A group of cells work collectively to allow large localized deformation. Size does not appear to be the initiator of the deformation bands. Equiaxed cells remain elastic. The implications for manufacturing materials with superior mechanical properties are discussed.

The structural performance of near-optimized truss core panels

Theoretical studies have indicated that truss core panels with a tetragonal topology support bending and compression loads at lower weight than competing concepts. The goal of this study is to validate this prediction by implementing an experimental protocol that probes the key mechanical characteristics while addressing node eccentricity and structural robustness. For this purpose, panels have been fabricated from a beryllium–copper alloy using a rapid prototyping approach and investment casting. Measurements were performed on these panels in flexure, shear and compression. Numerical simulations were conducted for these same configurations. The measurements reveal complete consistency with the stiffness and limit load predictions, as well as providing a vivid illustration of asymmetric structural responses that arises because the bending behavior of optimized panels is dependent on truss orientation. 2002 Elsevier Science Ltd. All rights reserved.

Cellular Metals Manufacturing

Open cell, stochastic nickel foams are widely used for the electrodes and current collectors of metal – metal hydride batteries. Closed cell, periodic aluminum honeycomb is extensively used for the cores of light, stiff sandwich panel structures. Interest is now growing in other cell topologies and potential applications are expanding. For example cellular metals are being evaluated for impact energy absorption, for noise and vibration damping and for novel approaches to thermal management. Numerous methods for manufacturing cellular metals are being developed. As a basic understanding of the relationships between cell topology and the performance of cellular metals in each application area begins to emerge, interest is growing in processes that enable an optimized topology to be reproducibly created. For some applications, such as acoustic attenuation, stochastic metal foams are likely to be preferred over their periodically structured counterparts. Nonetheless, the average cell s ize, the cell size standard deviation, the relative density and the microstructure of the ligaments are all important to control. The invention of more stable processes and improved methods for on-line control of the cellular structure via in-situ sensing and more sophisticated control algorithms are likely to lead to significant improvements in foam topology. For load supporting applications, sandwich panels containing honeycomb cores are much superior to those utilizing stochastic foams, but they are more costly than stochastic foam core materials. Recently, processes have begun to emerge for making open cell periodic cell materials with triangular or pyramidal truss topologies. These have been shown to match the stiffness and strength of honeycomb in sandwich panels. New cellular metals manufacturing processes that use metal textiles and deformed sheet metal are being explored as potentially low cost manufacturing processes for these applications. These topologically optimized systems are opening up new multifunctional applications for cellular metals.

Mechanisms, models and methods of vapor deposition

Abstract The condensation and assembly of atomic fluxes incident upon the surface of a thin film during its growth by vapor deposition is complex. Mediating the growth process by varying the flux, adjusting the film temperature, irradiating the growth surface with energetic (assisting) particles or making selective use of surfactants is essential to achieve the level of atomic scale perfection needed for high performance films. A multiscale modeling method for analyzing the growth of vapor deposited thin films and nanoparticles has begun to emerge and is reviewed. Ab-initio methods such as density functional theory are used to provide key insights about the basic mechanisms of atomic assembly. Recent work has explored the transition paths and kinetics of atomic hopping on defective surfaces and is investigating the role of surfactants to control surface atom mobility. New forms of interatomic potentials based upon the embedded atom method, Tersoff and bond order potentials can now be combined with molecular dynamics to investigate many aspects of vapor phase synthesis processes. For example, the energy distribution of atoms emitted from sputtering targets, the effects of hot atom impacts upon the mechanisms of surface diffusion, and the role of assisting ions in controlling surface roughness can all be investigated by this approach. They also enable the many activation barriers present during atomic assembly to be efficiently evaluated and used as inputs in multipath kinetic Monte Carlo models or continuum models of film growth. This hierarchy of modeling techniques now allows many of the atomic assembly mechanisms to be incorporated in film growth simulations of increasing fidelity. We identify new opportunities, to extend this modeling approach to the growth of increasingly complicated material systems. Using the growth of metal multilayers that exhibit giant magnetoresistance as a case study, we show that the approach can also lead to the identification of novel growth processes that utilize adatom energy control, very low energy ion assistance, or highly mobile, low solubility chemical species (surfactants) to control surface diffusion controlled film growth. Such approaches appear capable of enabling the creation of multilayered materials with exceptionally smooth, unmixed interfaces, with significantly superior magnetoresistance.

On the performance of truss panels with Kagomé cores

The performance characteristics of a truss core sandwich panel design based on the 3D Kagome has been measured and compared with earlier simulations. Panels have been fabricated by investment casting and tested in compression, shear and bending. The isotropic nature of this core design has been confirmed. The superior performance relative to truss designs based on the tetrahedron has been demonstrated and attributed to the greater resistance to plastic buckling at the equivalent core density.

Multifunctional microtruss laminates: Textile synthesis and properties

Open cell periodic metal truss structures can exhibit significantly higher stiffnesses and strengths than stochastic cellular metal structures of the same relative density while still providing high mechanical energy absorption and efficient heat exchange opportunities. Here, a potentially inexpensive textile-based approach to the synthesis of periodic metal microtruss laminates is reported. The process consists of selecting a wire weave, laying up the mesh and joining using a transient liquid phase. Example structures constructed from nichrome (Ni–24Fe–16Cr) wire cloth were made and tested. These exhibited a linear dependence of stiffness and strength upon relative density, absorbed large amounts of mechanical energy, and showed good potential for efficient heat exchange.