I realize that you have only just turned three years old, but I thought it might be interesting for you at some time in the future to read about some of the things that concern your Grandpa Jake today. In recent years, episodes of a popular television program opened with this statement, “Space, the Final Frontier!” I have worked for many years on the development of structural metals, putting most of my attention recently on the metal beryllium. And I have been feeling lately as though we need something similarly dramatic to be said about this metal, such as “Beryllium, the Final Frontier,” or perhaps more appropriately, “Beryllium, Stairway to the Final Frontier!” I want to share these thoughts with you because of two important truths about structural metals. One is that Metallurgy is like Medicine: They only call you up when something goes wrong. And the other truth is that structural materials, when they work, are invisible. So we have to work harder in order to gain the attention that structural materials deserve. Beryllium is truly the last structural metal that has outstanding properties that have not been fully exploited by society in order to meet the needs for greater structural efficiency and the savings in energy and effort that result from meeting these societal needs. So, on with “Beryllium, the Final Frontier!”

Nanocrystalline materials have been attracting rapidly increasing interest in the last decade and have the potential of revolutionizing traditional materials design in many applications via atomic-level structural control to tailor the engineering properties. In addition to interesting physical properties in the areas of magnetics, catalysis, and optics, this class of materials exhibits a broad range of fascinating mechanical behavior. Superplastic deformation behavior has been observed at significantly lower temperatures in ceramic nanoscale powders. Ultrahigh hardnesses have been measured in nanoscale superlattices made of metallic and ceramic materials. Tensile and compressive strengths in nearly all material systems studied have shown anomalously high values at the nanometer-length scale.

The development of nanostructured materials is now raising the question of how the different properties change as the microstructural scale is reduced to nanometer dimensions. Among potential applications of nanostructured materials, the design to achieve optimum mechanical properties is a common concern. Traditionally the mechanical strength σ of crystalline materials is believed to be largely controlled by the grain size d, often in the manner described by the Hall-Petch relationship σ= kd−1/2 +σ0. As the structural scale reduces to the nanometer range, the limits to the conventional descriptions of yielding need to be established, and new mechanisms that may come into play at these very small dimensions need to be explored and studied. In addition the intrinsically high interface-to-volume ratio of the nanostructured materials may enhance interface-driven processes to extend the strain to failure and plasticity. These potential gains will have profound technological impact in a wide range of engineering applications, and need to be validated and exploited.

Nanometer-scale multilayer materials exhibit a wealth of interesting structural and mechanical property behaviors. Physical-vapor-deposition technology allows almost unlimited freedom to choose among elements, alloys, and Compounds as layering constituents and to design and produce materials with compositional and structural periodicities approaching the atomic Scale. These materials have tremendous interface area density, approaching 106 mm/mm3, so that a Square centimeter area of a one-micron-thick multilayer film with a bilayer period of 2 nm has an interface area of roughly 1,000 cm2. Hence interfacial effects can dominate multilayer structure and properties leading to unusually large strains and frequently stabilization of metastable structures. The atomic-scale layering of different materials also leads to very high hardnesses and good wear resistance. These materials are a test-bed for examination of the fundamental aspects of phase stability and for exploring mechanical strengthening mechanisms. They are also becoming increasingly interesting for applications such as hard coatings, x-ray optical elements, in microelectromechanical Systems (MEMS), and in magnetic recording media and heads.

In this article, we review some of the interesting structures and mechanical properties that have been observed in nanometer-scale artificial multilayer structures.

Superlattice thin films are readily deposited by vapor-phase techniques such as sputter deposition, evaporation, and chemical vapor deposition, as well as by electrochemical deposition. Superlattice deposition Systems are similar to conventional film deposition Systems, except for the provision to modulate the fluxes and thereby produce alternating super-lattice layers.

The appeal of nanolayered materials from a mechanical viewpoint is that, in principle, plastic deformation can be confined to small volumes of material by Controlling both the frequency and magnitude of obstacles to dislocation motion. As we shall see, the spacing of obstacles can be used to impart large plastic anisotropy and work hardening. However, how strong can such materials be made as layer thickness (and therefore obstacle spacing) is decreased to the nanoscale level? In perspective, large, micron-scale, polycrystalline materials generally display improved yield strength (and fracture toughness) as grain size is decreased. This behavior at the micron scale can be explained via modeis that are built on two assumptions: (1) the strength of obstacles to crystal slip is sufficiently large to require pileups of numerous dislocations in order to slip past them; and (2) the strength of such obstacles does not change, even if their spacing is decreased. The modeling presented here shows that these assumptions may break down at the nanometer scale. The result is that there is a critical layer thickness in the nanometer range, below which improvement in strength does not occur.

Our discussion to follow briefly outlines a more macroscopic, micron-scale approach to determine yield strength, and then contrasts that with a sequence of events leading up to yield in nanolayered materials. We also address whether nanoscale materials are expected to exhibit more uniform or coarse slip than micron-scale materials. Finally, a semi-quantitative model of yield strength is developed which requires, as input, the strength of an interface to crystal slip transmission across it. We discuss several contributions to the interfacial strength and apply the theory to demonstrate a peak in strength for a 50 vol% Cu-50 vol% Ni multilayered sample.

Almost all thin films deposited on a substrate are in a state of stress. Fifty years ago pioneering work concerning the measurement of thin-film stresses was conducted by Brenner and Senderoff. They electroplated a metal film onto a thin metal substrate strip fixed at one end and measured the deflection of the free end of the substrate with a micrometer. Using a beam-bending analysis, they were able to calculate a residual stress from the measured deflection of the bimetallic film-substrate system. A variety of other, more sensitive methods of measuring the curvature of the surface of a film-substrate system have since been developed using, for example, capacitance measurements and interferometry techniques.

When a monochromatic x-ray beam is incident onto a curved single crystal, the diffraction condition is satisfied only for regions of the crystal where the inclination angle with respect to the incident beam exactly matches the Bragg angle. When a parallel beam plane-wave source is used, the diffracted beam from a particular set of (hkl) planes gives rise to a single narrow-contour band. If the crystal is rocked by an angle ω, the contour band will move by a certain distance D. The radius of curvature R of the crystal lattice planes is given by

where θ is the Bragg angle. Equal rocking angles produce equivalent D values for uniform curvature, or varied D values for nonuniform curvature. Using this procedure, detailed contour maps of the angular displacement field of the crystal can be mapped in two dimensions.

It is generally recognized that the capillary forces associated with internal and external interfaces affect both the shapes of liquid-vapor surfaces and wetting of a solid by a liquid. It is less commonly understood that the same phenomenology often applies equally well to solid-solid or solid-vapor interfaces.

The fundamental quantity governing capillary phenomena is the excess free energy associated with a unit area of interface. The microscopic origin of this excess free energy is often intuitively simple to understand: the atoms at a free surface have “missing bonds”; a grain boundary contains “holes” and hence does not have the optimal electronic density; an incoherent interface contains dislocations that cost strain energy; and the ordering of a liquid near a solid-liquid interface causes a lowering of the entropy and hence an increase in the free energy. In what follows we shall show how this fundamental quantity determines the shape of increasingly complex bodies: spheres, wires, thin films, and multilayers composed of liquids or solids. Crystal anisotropy is not considered here; all interfaces and surfaces are assumed isotropic.

Consideration of the equilibrium of a spherical drop of radius R with surface free energy γ shows that pressure inside the droplet is higher than outside. The difference is given by the well-known Laplace equation:

This result can be obtained by equating work done against internal and external pressure during an infinitesimal change of radius with the work of creating a new surface.

The reduction of grain size to the nanometer range (˜2-100 nm) has led to many interesting materials properties, including those involving mechanical behavior. In the case of metals, the Hall-Petch equation, which relates the yield stress to the inverse square root of the grain size, predicts great increases in strength with grain refinement. On the other hand, theory indicates that the high volume fraction of interfacial regions leads to increased deformation by grain-boundary sliding in metals with grain size in the low end of the nanocrystalline range. Nanocrystalline ceramics also have desirable properties. Chief among these are lower sintering temperatures and enhanced strain to failure. These two properties acting in combination allow for some unique applications, such as low-temperature diffusion bonding (the direct joining of ceramics to each other using moderate temperatures and pressures). Mechanical properties sometimes are affected by the fact that ceramics in a fine-grained form are stable in a different (usually higher pressure) phase than that which is considered “normal” for the ceramic. To the extent that the mechanical properties of a ceramic are dependent on its crystal-lographic structure, these differences will become evident at the smaller size scales.

It is uncertain how deformation takes place in very fine-grained nanocrystalline materials. It has been recognized for some time that the Hall-Petch relationship, which usually is explained on the basis of dislocation pileups at grain boundaries, must break down at grain sizes such that a grain cannot support a pileup. Even some of the basic assumptions of dislocation theory may no longer be appropriate in this size regime. Recently considerable progress has been made in simulating the behavior of extremely fine-grained metals under stress using molecular-dynamics techniques. Molecular-dynamics (MD) simulations of deformation in nanophase Ni and Cu were carried out in the temperature range of 300–500 K, at constant applied uniaxial tensile stresses between 0.05 GPa and 1.5 GPa, on samples with average grain sizes ranging from 3.4 nm to 12 nm.

Ductility is defined as the ability of a material to change shape without fracture. It is of critical importance for engineering materials for both manufacturability and Performance. Measures of ductility include percent elongation (uniform plastic flow prior to mechanical instability—necking—or fracture) and percent reduction in area. Fracture toughness is also some measure of potential ductility. Engineering materials exhibit wide variations in ductility which can often limit their application.

Ductility is a property of nanocrystalline materials which might be predicted to be enhanced by extrapolation of its grain-size dependence in conventional polycrystalline materials. It has been predicted that extrapolation of the grain size, or the scale of the microstructure, to the nanoscale will lead to both strengthening and an increase in ductility. As far as failure and ductility are concerned, this idea is based on experience with conventional materials, where the yield and fracture stress show different dependencies on the grain size. The fracture stress typically increases faster than the yield stress with decreasing grain size such that ductile/brittle transitions can occur. For example, the ductile / brittle transition temperature in mild steel can be lowered about 40°C by reducing the grain size by a factor of five. In terms of how ductility may be affected by the extreme grainsize reduction to the nanoscale, we consider the following. Firstly, it may be recalled that obtaining ductility relies simply on plastic deformation occurring without the catastrophic onset of failure mechanisms, and therefore we can examine possibilities of changing ductility in terms of avoiding failure.