The federal budget process is often regarded as an arcane affair, the province of a select group of Washington insiders. This includes the mysteries behind the determination of budgets for R&D on materials. The purpose of this discussion is to demysrify the subject a little. It will focus largely on the role of Congress.

A common misconception is that the White House sets the budget for the federal government. What the White House does do is send detailed recommendations to Congress each year in January on what it believes the budget should be. This covers spending, tax policy, and economie predictions (the latter reflected in the anticipated tax revenues). In January 1990, for example, the administration submitted budget recommendations for fiscal year 1991, which began on October 1 and runs until September 30, 1991. These recommendations (with subsequent negociations with the Hill) can be quite influential, but serve primarily as a point of reference, which Congress can gladly accept, or completely ignore, as circumstances and preferences dictate.

At the 1989 MRS Fall Meeting the present guest editors and J.D. Jorgensen chaired a symposium on “Neutron Scattering for Materials Science.” Approximately 80 papers were presented, covering a great variety of topics which joined members of the materials science and neutron scattering communities. Because of that symposium's success, it was decided to bring the topic of Neutron Scattering to the wider attention of the materials community through this special issue of the MRS BULLETIN.

Our purpose is twofold. First, neutrons have increasingly come to play a crucial role, both here and especially in Europe, in our understanding of the structure and properties of materials. Through the manipulation of materials (radiation-induced effects, transmutation doping of semiconductors), nondestructive materials testing (residual stress measurements on industrial-sized objects, depth profiling of ion-implanted semiconductors) and structural and dynamical studies, academic, government, and industrial scientists and engineers are coming to recognize the broad utility of neutron methods. We would like to highlight some of the advances in this field for MRS BULLETIN readers.

Second, neutron scattering presents an excellent example of the contribution of our large research facilities to the solution of both basic and applied problems in materials science. Without the major neutron scattering centers we would be severely limited in the scope of our materials activities (no knowledge of magnetic structures and only primitive insight into polymer structures, for example).

This issue of the MRS BULLETIN is devoted to the technique of neutron scattering and its role in materials characterization. Compared with electrons and x-rays, the other major scattering probes, neutrons seem exotic and expensive. Why then are they so important? This article will address this question generically, with a few specific examples. Subsequent articles will address in greater detail a few of the more prominent techniques that are important for materials scientists.

We will begin by reviewing why scattering methods are used to study structure, when for many materials it is possible to employ electron microscopy (where, of course, the images are reconstructed from the interference among the scattered waves). One reason is that direct imaging often tells very little about the relationship between an observed structure and the material properties that derive from it. Figure 1, for example, shows a stereo image of a colloidal latex which is clearly crystalline. The stereo view of the same material in the melt seems random to the eye. If many thousands of such images are taken, however, and the number of pairs of particles separated by a given distance are plotted against the separation (this is the so-called pair distribution function), it is found that certain separations are highly probable, while others are equally improbable. In the systems of Figures 1 and 2, the pair distribution functions are not very different (particularly for separations up to several interatomic distances), and it is this function which governs the thermodynamic properties of the material.

For many classes of materials, neutron diffraction is the best way to obtain detailed atomic-level structural information. Diffraction experiments on single crystals provide the most precise data, but sufficiently large specimens (>0.1–0.5 mm3) are often not available. Steady development of instrumentation and data analysis techniques, however, has now made it possible to obtain comparably precise structural information from neutron diffraction experiments on powder samples. Such studies have played a prominent role in solid state physics, chemistry, and materials science in recent years. The special capabilities that have contributed to the success of this technique include atomic cross sections that are often favorable for a particular structural problem, high neutron penetrating power, the excellent resolution achieved with state-of-the-art diffractometers, and steadily advancing analysis techniques that facilitate obtaining structural information from a diverse range of polycrystalline materials.

As Axe, Pynn, and Hayter note in their introductory article in this issue of the MRS BULLETIN, atomic scattering cross sections for neutrons are not simply a function of atomic number, as is the case for x-rays. The scattering is predominantly from the nuclei (thus avoiding the form factor diminution observed for x-ray scattering), and coherent neutron scattering cross sections can, generally, be as large for light atoms as for heavy atoms. Light atoms, such as hydrogen (deuterium), oxygen, nitrogen, carbon, or lithium, can therefore be located in the presence of heavier atoms. This advantage has led to the widespread use of neutron powder diffraction for studing metal hydrides and, more recently, oxide superconductors.

Residual stresses are common in engineering materials. They are elastic stresses that exist in the absence of external forces and are produced through the differential action of plastic flow, thermal contraction, and/or changes in volume created by phase transformations. Differential plastic flow can occur during the forming of a part. For example, the grinding of a surface will plastically extend a thin surface layer relative to the underlying material through frictional forces. This tends to throw the near-surface region into compression, which is balanced by a tensile stress through the bulk of the part. In such a situation, the requirements of force equilibrium lead to a rather high level of compressive stress in the near-surface region and a low level of tensile stress through the bulk. Differential thermal contraction often occurs during the nonuniform cooling of a large part, in the vicinity of welds, and between the matrix and reinforcement phases of a composite. Differential volume changes occur during the precipitation of second phases, i.e., the atomic volume of the precipitating phase generally differs from that of the host matrix. If it is larger, the second phase is placed in compression, with the matrix in tension, and vice versa.

The residual stress states discussed in the previous paragraph are of two basic types: macrostress and microstress. Residual macrostresses are long-range relative to the scale of the microstructure––that is, they extend continuously across a part. Residual stresses arising from forming and joining are of this type. They generally vary with position and are extensive in nature. A plate with compressive residual stresses on the flat surfaces will deflect if the compressive region on one surface is removed. Destructive stress measurement techniques such as hole drilling or strain gauging and sectioning can be used to determine residual macrostresses.

During the past decade, scientific and technological interest in the properties of surfaces and interfaces has grown at an astounding rate. On first thought, one might not consider a neutron or even an x-ray photon to be a particularly sensitive surface probe given their relatively weak interactions with matter compared to that of a low-energy electron or atom. Indeed, low-energy electron diffraction and atomic beam scattering techniques have contributed significantly to our understanding of surface phenomena. Nonetheless, the very fact that electrons and atoms are so strongly interacting makes quantitative analysis of their scattering data difficult. The interaction of neutrons or x-rays with matter, on the other hand, is weak enough that the potential can be characterized by a relatively simple scattering amplitude. Presently attainable neutron intensities, though not yet comparable to those of x-ray synchrotron sources, are still of sufficient strength to permit a variety of surface or near surface reflectivity and grazing angle diffraction experiments. Because neutrons can distinguish between different isotopes of the same element, most notably hydrogen and deuterium, as well as couple to atomic magnetic moments via a dipolar interaction, they can be indispensable and complementary probes.

More conventional neutron diffraction techniques can also be applied to the study of interfacial phenomena and the effects of reduced dimensionality and compositional modulation in super-lattice structures grown by a variety of thin film deposition methods. In this article we will differentiate between reflectivity and diffraction measurements as follows: if the scattering occurs at a wavevector transfer low enough that the scattering medium appears as a continuum, so that amorphous and crystalline states are indistinguishable, then it will be considered to be in the reflectivity regime whereas diffraction will be taken to correspond to higher wavevector transfer where the precise arrangement of atoms is discernible.

Before the application of small-angle neutron scattering (SANS) to the study of polymer structure, chain conformation studies were limited to light scattering and small-angle x-ray scattering (SAXS) techniques. These experiments were usually conducted in dilute solution, and the methodology to measure radii of gyration, virial coefficients, molecular weights, etc., was well established in the classical works of Guinier, Zimm, Debye and Kratky, who pioneered these techniques during the 1940s and 1950s. This methodology could not be applied to concentrated solutions or bulk polymers because of the difficulty of separating the intra- and inter-molecular components of the scattering function. One attempt to circumvent this difficulty was the experiment by Krigbaum and Godwin, who end-labeled polystyrene molecules with Ag atoms. When dispersed in unlabeled polystyrene, the excess x-ray scattering could in principle be analyzed to provide the end-to-end distance, though in practice the signal-to-noise ratio of the experiment was insufficient for accurately determining this parameter. To our knowledge the first suggestion to use the difference in coherent scattering lengths of deuterium (bD = 0.66 × 10−12cm) and hydrogen (bH = −0.37 × 10−12cm) to create scattering contrast between deuterated and normal (hydrogenous) molecules and provide a direct determination of molecular dimensions was made independently by at least two groups in the late 1960s. By deuterating the whole molecule, as opposed to end-labeling, this proposal increased the signal-to-noise ratio of the experiment by several orders of magnitude and made possible for the first time the practical analysis of molecular conformations in bulk polymers. Even so, such experiments could not be undertaken until the completion in Europe of the first instruments employing long wavelength neutrons and large distances between the entrance slit, sample and detector, which allowed deuterium labeling methods to be successfully applied to polymers in the early 1970s.