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5. RECENT NEUTRON-SCATTERING RESEARCH IN TEE UNITED STATES; COMPARISONS WITH EUROPE In this chapter we present selected summaries of important research areas and accomplishments during the past 6 years. These summaries have been coordinated by experts on the panel in the appropriate scientific disciplines involved and reviewed by the panel as a whole. The focus is generally on U.S. neutron-scattering work, except in cases where activity has been dominated by special capabilities abroad (e.g., high-resolution spectroscopy, broad areas of polymer and chemical research). Where the work involves new areas of scientific applications of neutrons that have evolved recently (e.g., polymer and materials science, biology, chemical spectroscopy), we have tried to provide more background to explain the special role of neutrons in these cases. An attempt has also been made to point out both new directions in science and examples of broad areas where instrumentation at U.S. neutron sources does not allow critical scientific opportunities to be pursued. CONDENSED-MATTER PHYSICS Physicists were the first scientists to exploit the unique properties of the neutron in studying condensed matter, and after 30 years of intense activity the level of interest and vitality of the field are undiminished. In the following 34

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35 paragraphs we survey the contributions that neutron scattering has made to the various subfields. It is worth reflecting that most of the results discussed represent not only significant advances in our understanding of the physics of the materials but that neutron scattering provided unique information not available from other known techniques. Two examples, discussed more fully in subsequent sections, illustrate this particularly well. The first is the development of magnetic order in superconductors, where a series of key discoveries beginning in the late 1970s and involving a close interplay between imaginative materials synthesis and neutron-scattering studies have caused the simple dictum that "superconductivity and magnetism don't mix" to be greatly revised. In this case although thermodynamic and magnetic measurements suggested the occurrence of unusual phenomena in those new materials, neutron scattering was essential to establish its nature. A second example is provided by ongoing recent studies of the effect of disorder on the collective behavior of condensed-matter systems. Major conceptual difficulties have arisen in recent years as to the behavior of a simple magnetic model system when placed in a magnetic field that varies randomly in direction from site to site. Although the model has attracted much interest as a prototype of real physical disorder, the production of such a random field on an atomic scale is experimentally not possible. However, as it turns out, the application of a uniform magnetic field to a dilute random antiferromagnet produces a random antiferromagnetic internal field and a physical situation that simulates the ideal theoretical model. Neutron-scattering measurements on such systems are now providing the critical

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36 experimental tests of our understanding of simple random systems. Magnetic Systems The determination of magnetic structures was one of the first and most important applications of neutron diffraction. While many of the interesting magnetic structures of simple materials have now been determined, the ability to make these determinations on new materials will be essential for the indefinite future as long as new materials are developed. Most magnetic structures have been determined at mediums to low-flex sources using a two-axis diffractometer, but for very complex structures (Nd, PrAg, TmS), or for fine structural details (MnP), the polarization analysis technique can be extremely valuable. Use of this technique has been limited because there are only a few spectrometers in the world equipped for these measurements. Amorphous ferromagnets have enormous potential value in commercial applications, and neutron scattering has been used in a variety of ways to understand these materials on a microscopic basis. Diffuse scattering measurements have been used to obtain the radial distribution functions that describe the basic structure, inelastic neutron-scattering studies have given details of the spin dynamics, and small- angle scattering has been used to study the microstructure. Neutron experiments on these materials have been carried out worldwide. The advantage of using polarized neutrons has not yet been fully exploited in these studies. Another class of materials that has attracted great experimental and theoretical interest in recent years is that group of

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37 rare-earth materials in which the rare-earth ion apparently has a fractional valence--the intermediate- or mixed-valence materials. These materials do not show magnetic order, and the neutron-scattering spectra show a broad quasi-elastic line with an energy width in the range from 10 to 100 med. The shape of this line is consistent with a fast (10 13 see) relaxation process of the rare-earth spins. Actinides frequently show many characteristics of the unstable 4f moment systems. In the rock salt uranium compounds, for example, experiments have shown that, even though the systems order magnetically, spin waves need not exist. A combination of x-ray photoemission spectroscopy (XPS) and neutron spectroscopy suggests that the unusual damping is caused by strong Sf-6d electron interactions. Both critical and inelastic magnetic scattering has contributed greatly to our understanding of the solid-state physics of the last row of the periodic table. In another kind of experiment, polarized neutrons have been used to measure the induced magnetic form factor in order to gain a better understanding of the behavior of the f electrons in these materials. In general, these form factors are not dramatically different from free-ion form factors for integral valence ions. Interesting exceptions are CeSn and CePd, where evidence of some Sd polarization is found at low temperatures. As an example of the subtlety of magnetic systems and of the continuing long-term need for state-of-the-art neutron- scattering facilities, it should be noted that new and exciting facts about the most studied of all magnetic materials, Cr. Fe,and Ni, are still being discovered through neutron- scatteringexperiments. Recentpolarization analysisexper~ments of the diffuse scattering from Fe and Ni in the paramagnetic

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38 phase have revealed short-range magnetic order extending well above Tc. In another type of polarized-beam experiment, the appearance of n forbidden" magnons in Ni well below To indicates a deviation in the local and bulk magnetization directions. Commensurate diffuse excitations have been observed in the incommensurate spin-density wave state of Cr metal. These excitations were completely unexpected and are not yet understood. All of these experiments give valuable insights into the fundamental nature of the magnetic behavior of these important metals. Practically all areas of magnetic research would benefit from more and improved neutron polarization analysis spectrometers. Higher source fluxes and improved polarizers are both important for the growth of this technique. Fortunately, recent advances in this country and Europe toward better polarizers have been made. Pulsed-neutron sources show promise for enabling inelastic magnetic scattering at high-energy transfers. For example, recent measurements have already extended the spin-wave spectrum in Fe to higher values than previously possible, and such sources promise new opportunities to locate the single-particle (Stoner) continuum. The lack of very-high-resolution instruments, such as a neutron-spin-echo spectrometer, has limited the 0.S. research in important areas, such as the relaxation effects in spin glasses. Phase Transition From its inception, neutron scattering has made important contributions to the field of phase transitions and critical phenomena. Not surprisingly, this has continued to be the

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39 case for the past 6 years. Phase transitions occur in a wide variety of systems so that the work discussed in this section will overlap with results-presented in most of the other sections. There has been an interesting evolution in the class of materials and the types of problems being pursued. Initial work, especially in the period 1965 to 1975, concentrated on prototypical phase transitions in model systems. Examples include soft modes in ferroelectrics and in other systems exhibiting structural transitions such as SrTiO3, ordering in binary alloys such as CuZn, and magnetic transitions in simple magnets such as RbM~F3, E2NiF4, Fe, and Ni. These experiments were invaluable in elucidating the basic principles governing critical phenomena. As a result of these early studies and of important advances in theoretical understanding, it has proven possible to address much more complicated issues. Recent neutron work has concentrated on the phase-transition behavior of more exotic systems. Indeed a number of the more interesting experiments involve issues that were barely conceived of at the time of the previous NAS report. Examples include random field effects, spin-Peierls transitions, devil's staircase phenomena in incommensurate systems, re-entrant superconductivity, re-entrant spin-glass behavior, and other effects originating from competing interactions. In each case neutron-scattering experiments have provided a key to understanding the basic physics underlying these phase transitions. It is also interesting to note that with a few important exceptions, the crucial experiments have involved conventional elastic and inelastic neutron-scattering (including SANS) techniques. The most serious limitation typically has been sample size or, equivalently, neutron flux. In

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40 many cases major improvements in momentum or energy resolution, or both, currently available or under development in Europe, will be of increasing importance. In this brief report, it is impossible to survey all the beautiful experiments that have been performed in the last 6 years. Ve therefore limit ourselves to a few representative samples. The effects of randomness, especially in systems with competing interactions, are now being extensively explored. Particularly dramatic effects are observed in spin systems with random magnetic fields, that is, magnetic fields with zero average value but nonzero variance. Nentron experiments in both the United States and Europe have verified that a uniform field applied to a random antiferromagnet generates a random staggered magnetic field; this makes possible a systematic study of the phenomena. High-resolution experiments on both two- and three-dimensional systems have revealed that long-range magnetic order is not attained in the presence of a weak random field and that instead one sees with decreasing temperature a continuous evolution from the paramagnetic state to a frozen microdomain state. Such random field effects are undoubtedly important in a range of other physical systems, including randomranisotropy rare-earth amorphous alloys. Spin glasses have been the subject of intense experimental and theoretical study in recent years, and neutron techniques have contributed greatly to oorunderstandingof these materials. In Europe, a combination of the neutron-spin-echo and polarization-analysis techniques have been used to measure the time-dependent spin-correlation function of Cu-Mn alloys over a time range from 10 12 to 10 9 sec. In the United States, elastic polarization analysis measurements on single

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41 crystals of Cu-Mn have demonstrated strong short-range magnetic correlations associated with short-range nuclear order. Exotic effects are also observed in alloys with competing ferromagnetic and antiferromagnetic interactions. With increasing concentration of antiferromagnetic bonds one observes an evolution from ferromagnetism to spin-glass behavior. For ferromagnets with concentrations near the crossover point one sees "re-entrant behavior," that is, with decreasing temperature the system evolves from a paramagnet to a ferromagnet to a spin glass. The spin-wave frequency appears to soften at both the paramagnetic-ferromagnetic and ferromagnetic-spin glass transitions. In addition, unusual diffuse scattering is observed in the re-entrant phase, presumably originating from residual microdomains of the ferromagnetic network. These effects have been seen in such systems as Fe~Crl_~, Fe~Nil_~, PbAl, and FeMnPC. This is still an active and rather controversial area of research. It should be noted that the most precise experiments have been performed at Grenoble utilizing the small-angle scattering and high-energy resolution spectrometers. Systems with competing anisotropies rather than competing interactions have also been studied. Here again one observes new magnetic states whose basic structures and excitations could only be elucidated with neutrons. The study of phase transformations and other highly cooperative phenomena is perhaps the most challenging and subtle that condensed-matter physics has to offer. At the same time, fresh ways of thinking about such systems have greatly expanded our ability to understand such systems and suggested subjects for new studies. Such systems are very often magnetic because of their relative freedom from

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42 complicating extraneous interactions. As in the past, neutrons will continue to provide an indispensible tool for such studies. New Materials and Phenomena The unique capabilities of neutron studies are readily apparent in the study of the coexistence of magnetic and superconducting long-range order, which was discovered (nengineeredn is a better term) in the late 1970s. The phenomenon of superconductivity is quite tolerant of large concentrations of impurity atoms, so long as they are not magnetic in character, but is quickly destroyed by small concentrations of magnetic impurities. This peculiar fact was readily explained by the Bardeen, Cooper, and Schrieffer (BCS) theory, which showed how superconductivity can arise from a binding of pairs of electrons that travel in time-reversed orbits. Magnetic impurities that do not respect time-reversal symmetry destroy the pairing, whereas chemical impurities simply scatter the electrons into new time-reversed orbits leaving the superconductivity intact. The early studies of the magnetic suppression of superconductivity were carried out on simple binary alloys in which magnetic impurity atoms were substituted randomly into thelatticeof the superconducting metal. Reasoning that a more stringent test of coexistence would result if the electrons responsible for the superconductivity could be effectively separated from those electrons responsible for the magnetism, new studies were undertaken on more complex ternary systems, in which the magnetic atoms are separated from the atoms responsible

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43 for the superconductivity by a barrier of inert atoms, preventing strong interaction between the two. In the case of materials such as DyMO6S~, heat-capacity measurements showed the presence of a new kind of ordering occurring below the temperature of the onset of supercon- ductivity. Neutron-diffraction studies were necessary to establish the nature of the ordering, which was found to be simple antiferromagnetism in which planes of atoms with magnetic moments up and down alternate. The superconductivity is not destroyed, proving that superconductivity and antiferromagnetism can simultaneously coexist at the same temperature. In materials such as ErRh4B4, neutron-scattering studies have shown that ferromagnetic arrangement of magnetic moments arises at temperatures below that at which the sample becomes superconducting and that in the process the sample regains its normal conductivity. Thus, superconductivity and ferromagnetism can exist in the same material but apparently cannot coexist at the same temperature. Further insight into the nature of the competition between magnetism and superconductivity has recently come from studies of small- angle neutron scattering, which reveal- that even when the ferromagnetic state is marginally unstable, an entirely new type of order, taking the form of a long-wavelength oscillating magnetic disturbance can coexist with the superconducting state. These findings may have far-reaching implications for the directions of future research in superconductivity. Graphite is a prime example of a layered structure composed, in this case, of covalently bound sheets of carbon atoms with adjacent sheets held together more loosely by van der Waals forces. New compounds with unusual properties

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44 can be prepared by inserting, for example, alkali metal atoms between the graphite sheets. Among the fascinating and incompletely understood aspects of these resulting graphite intercalation compounds (GIC) is the structure of the inserted atoms. Neutron-scattering studies have elucidated the nature of the stacking of adjacent metal layers and have shown that at high temperatures the ordering is liquidlike. Unsaturated materials, prepared with low-metal-vapor pressure, have metal atoms inserted between every nth graphite layer, where n is a simple integer. This phenomenon is known as staging. Recent nentron-scattering experiments under hydrostatic pressure have revealed a new fractional staging sequence, related to the n = 3 stage by particle-hole symmetry, whose existence helps to decide between competing theories of the staging phenomenon. A recent first study of the dynamic structure factor and diffusion processes in higher-stage metal-graphite compounds has only been possible by joint U.S.-ILL experiments using high-intensity and high-resolution spectrometers at the ILL. Models of one-dimensional (1-D) magnetic systems show in addition to linear spin-wave excitations, localized large- amplitude excitations that preserve their integrity as they move along a chain. It is expected that these excitations, called solitons, exist in real materials, and considerable effort has been made to observe them by neutron scattering. Both planar 1-D ferromagnets and antiferromagnets in external magnetic fields are expected to have soliton excitations, although the effects are more difficult to interpret in the ferromagnetic case, where neutron measurements in CsNiF are still controversial. Soliton effects have been successfully observed in the nearly classical S = 5/2 antiferromagnet,

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79 ratios are considerable in systems of such low concentration, and the time for a single experimental run becomes unreasonably long. The dimensions of particles in colloids are comparable with those in polymers, contrast is established by substitution of deuterium for hydrogen, and therefore the SAN S requirements are of the same kind. One difference is that colloidal particles are compact, while polymer molecules are normally somewhat extended. As a result, the scattered signal from colloids is more intense, and experimental limitations tend to be less stringent. Quasi-elastic neutron scattering is used for examination of local motions such as methyl group rotation, polymer diffusion, and global dynamics of polymer molecules. The study of global dynamics is most demanding, and this aspect of polymer motion cannot be probed easily by other methods. Generally, the interest is in low-frequency motions in which long sections of the polymer molecule move in concert. The current instruments provide data at low frequency (down to 106 Ez) but are signal limited 0 . to q's greater than 0.02 A I, and this is not yet low enough to characterize chain dynamics fully. The limitation on q should be addressed in the development of new higher-q resolution spin-echo spectrometers. Some Results from Neutron Scattering The overall chain conformation of a polymer molecule in the bulk had been assumed alternatively to be (a) a random coil with no measurable excluded volume repulsive effects or (b) an ordered structure with neighboring chains strongly influencing mutual confirmational arrangements. In the

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80 absence of SANS, it was not possible to distinguish between these alternatives. SAN S measurements on a number of systems strongly support assumption (a), a conclusion that is now accepted universally. While the bulk state is important, it is only one line on the temperature-concentration (T- C) map of a polymer system. Most polymers are synthesized or processed under varying T-C conditions. SANS has made it possible to study the molecular behavior at these conditions. Although most measurements were made in Europe, scientists from the United States have also made major contributions in this area. Moreover, some of the modern theoretical predictions (renormalization group calculations and scaling theory, for example) have been critically compared with conventional theories (mean-field theory and perturbation theory, for example). Advancement in this area so far has mainly been due to comparison with the SANS results. Dynamical studies in this area would be greatly advanced if high-flux, small-angle spin-echo instrumentation and backreflection spectrometers became available to B.S. scientists. The verification of the kinetic theory of rubberelasticity, a theory based on analysis of chain statistics, has been dependent on measurements of network swelling and stress- strain behavior. SAN S measurements make it possible to study the deformation of the molecular chains directly. Indeed, it has already been shown from SAN S that chain deformation is substantially less than predicted. While this result has been criticized as arising from improperly prepared materials, it has provoked new efforts to improve the theory. Certainly, further measurements of this kind can be expected to stimulate new interest in the molecular theory of elastomers.

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81 Block polymer molecules contain within a single polymer chain at least two different chemical subchains. In some interesting cases, these subchains separate into two microphases owing to mutual incompatibility of different parts of many molecules. These microphases are compact, each unit containing parts of many molecules. This is a colloidal-type structure o of characteristic dimension 100 A or so. Much attention has been given to these block copolymers owing to both their theoretical interest and to their significant practical applicability. They can be and have been studied conveniently by small-angle neutron scattering in which one of the blocks is labeled by deuterium. In a few cases where this has been done, it has been possible to decide which of several theoretical analyses is preferable. Molecular conformation in binary systems and kinetics during phase decomposition is an important area in terms of future polymeric materials. Although it is still in its infant stage, it is already clear that neutron scattering will be one of the most important tools in this area. A high-flux, low-q SANS instrument with time-resolved measurement capability will be especially valuable. This is because the kinetics of microphage decomposition will be the major factor that controls the morphology and performance of the material. Thus, it will be of great scientific and technological interest to study the molecular details of phase decomposition kinetics.

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82 Instrumentation Needs Small-angle neutron scattering of high polymers and of colloids has become an essential research tool. More than 100 experimental studies have been published, the majority using neutrons from the Dll SAN S facility on a cold-neutron guide at the ILL. The largest concentration of polymer and colloid work in the United States has been done during the past three years at the NSF facility at Oak Ridge, but important contributions have been made by researchers at the National Bureau of Standards, the University of Missouri, and BrooLhaven National Laboratory. In Europe, SANS experiments have also been performed at J6lich, Saclay, and Harwell. Most modern instruments have two-dimensional detection, in which case scattering-intensity problems are less with isotropic materials than with oriented samples such as fibers, stretched rubber, or stretched plastics. This is a consequence of the fact that intensities for anisotropic materials are measured over a narrow azimuthal range, and much longer experiments are needed. It is significant that almost all results on anisotropic scattering have been reported from ILL, where fluxes are an order of magnitude greater than elsewhere. Similarly, studies on polyelectrolytes are most interesting at low concentration (one percent by weight or less), and reliable results from such weak scatterers require higher fluxes than are currently available. The needs for the future are clear. Higher neutron fluxes and an ability to attain a lower range in q are needed. With existing U.S. reactors, this could be obtained by use of a cold source and a monochromator of the velocity selector type. A guide hall adjoining the reactor building would

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83 be needed for optimal resolution and flexibility. Recent work in Japan has demonstrated that useful research on some problems can be done at modest-flux pulsed sources equipped with a cold source. The possibilities for using the pulse structure of such sources in time-dependent studies will provide new opportunities when much higher-flux sources are developed. Future needs for quasi-elastic scattering of polymers are focused on the problem of chain dynamics. The most promising spectrometer for this work is the "spin-echo" instrument, which permits a much better energy discrimination in the low-energy range than is otherwise obtainable. Thus far only a small number of experiments on polymers have been performed on the only spectrometer of this kind in use. However, the uniqueness and importance of the dynamical information that can be obtained from such measurements makes the construction of such facilities in the United States critical for research in polymer and biological systems. State-of-the-art cold-neutron time-of-flight and backreflection spectrometers would also make major contributions to future studies of polymer dynamics. MATERIALS SCIENCE AND ENGINEERING Neutron scattering offers a unique method of studying the response of materials to external variables such as stress, strain, temperature, and processing variables. The penetrating power of a neutron beam permits the sampling of a large volume of specimens, so that the investigation of bulk properties is possible. The energy of the neutrons is sufficiently

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84 low that the testing procedure itself does not introduce any additional damage, as may occur with electron microscopy. The sensitivity of SAN S to heterogeneities in the size range from a few nanometers to about a micrometer has made it possible to follow, often in detail, microstructural changes in metals and ceramics resulting from deformation, irradiation, or processing. The appearance or dissolution of carbides, precipitates, dispersoids, voids, and microcracks, for example, can be detected, and frequently with sufficient sensitivity and precision that the kinetics of the process can be ascertained and compared with theoretical models. In many cases, the information yielded by neutron scattering on material behavior can be obtained in such detail and with such accuracy by no other method currently available. Studies of Microstructural Changes Produced by Temperature and Deformation It was demonstrated several years ago in Europe that SANS has the potential to monitor thermal processing of complex alloys. Little work has been done to date in this area in the United States. Recently, a SANS study has been carried out to investigate the effect of austenitizing and aging conditions on the size and density of precipitates in a precipitation-hardening high-strength low-alloy steel. The results were correlated with mechanical behavior. A study such as this can be used not only to optimize processing variables but also to determine the allowable leeway in these variables before serious degradation of mechanical properties results. SANS has been used to followmicrostructural changes produced by prolonged exposure to high temperatures

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85 in a ferritic stainless steel developed for use in power- generation applications. Extended service causes microstructural alterations, which affect the mechanical properties of the steel such that it may no longer meet design requirements. Scattering experiments have given detailed information on the size and number of carbides ultimately produced at various temperatures. Deformation greatly hastens the microstructural changes associated with aging. Such changes were clearly picked up by SANS in steel samples fatigued for only a few hours. It may be concluded that service-induced microstructural changes can be detected and even analyzed in detail by SANS, even in complex alloys. In Europe a number of investigations have been carried out in deformation-induced changes in microstructure. Advantage has been taken of the high-flux densities available in order to obtain small-angle scattering patterns over sufficiently short time intervals that the changes can be followed in situ. It should be noted that the scattering features of interest in metallurgical and ceramic studies frequently are rather large (some tens of nanometers). Their density is likely to be low. These are conditions that necessitate high-flux densities and measurement at very low values of the scattering vector. Such requirements cannot always be satisfied at U.S. facilities. Use of SAN S in the Detection and Analysis of Damage Several recent experiments have pointed up the value of neutron scattering in the study of damage that appears in the form of cracks or voids. The ability of SAN S to provide statistical information on the number, size, and shape of

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86 microcracks has been demonstrated in an elegant investigation of the ceramic YCrO3, which undergoes extensive microcracking when it passes through a phase transformation at about 1100C. It was found that the small-angle scattering cross sections from the cracked YCrO3 could be fitted well by the form of the scattering expected from an ensemble of randomly oriented thin disks. By combining the SANS results with data on the elastic constants of the YCrO3, it was possible to determine the number density, average size, and shape of the cracks. Grain-boundary cavitation is a phenomenon found in many metals and ceramics subjected to deformation at elevated temperatures. It was virtually impossible to examine the details of this process until the advent of SANS made available statistical information on the kinetics of void nucleation and growth. It has been found that most of the voids produced by fully reversed cycling are surprisingly small (about 35 nary). At this size they should not be stable against surface energy forces. Void volume fractions of less than 10 6 can be measured and cavitation picked up by SAN S after only 15 see of fatiguing. No incubation time appears necessary for void nucleation. Since fatigue produces large numbers of small voids it is just possible to carry out SAN S studies in facilities currently available in the United States (although higher-flux and lower-q limits would improve the results). However, the void nucleation rate in creep is low, whereas the growth rate is high. Therefore, successful creep cavitation studies by SAN S demand the high-flux and low-q capabilities of the Dell instrument at ILL and are not now feasible in the United States. Recent results on crept Cu from ILL

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87 measurements show that, in contrast to the case of fatigue, no voids can be detected with sizes below the predicted value of the smallest stable void. As in the case of cyclic loading, no incubation time appears to be required for void nucleation. The measured void size distributions that evolve as creep proceeds have been modeled with remarkable accuracy on the basis of one of the well-known theories of void growth. The theory generally believed to be applicable for these experimental conditions was found to give poor agreement with the SAN S results. This is the first instance in which such a precise comparison between theory and measurement of grain-boundary cavitation has been possible. Measurement of Texture and Residual Stresses by Neutron Diffraction 1-ray diffraction techniques are well developed for the measurement of both texture and residual stresses. However, the limited penetrating power of ~ rays has restricted the determination ofthese quantities to surface layers. Measurement of bulk properties has required destruction of the sample. The z-ray methodology can be applied to neutron diffraction to permit the nondestructive evaluation of texture and residual stresses as a function of depth from the surface. This use of neutron diffraction has great potential. Several investigators have demonstrated that neutron diffraction can indeed be used to determine both texture and residual stresses throughout a component. Test pieces with known stress fields have been examined and satisfactory agreement found between measured and calculated values.

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88 The method now has been applied to the determination of stresses in a number of situations, e.g., in depleted uranium and in composites in cemented carbides. In the latter case, evidence was found of large hydrostatic components of stress between carbide and binder, which arise because of significant differences in their coefficients of thermal expansion. The stress is relaxed by creep or fatigue. Recent measurements have shown that pulsed-neutron time-of-flight neutron diffraction also can be used to investigate residual stresses. Grain interaction stresses in a deformed polycrystalline alloy were determined and found to be in accord with model calculations. (~-ray techniques are not suitable for measuring nonuniform stresses that have a wavelength on the order of the grain size.) Investigations of Phase Decomposition SAN S has proved to be a valuable tool in the study of precipitation phenomena, particularly spinodal decomposition. Information on the kinetics of decomposition has been obtained by following changes as aging proceeds in the maximum scattering cross section and the corresponding value of the scattering vector. Several investigators are studying spinodal decomposi- tion in the FeCr system. A coexistence curve determined from scattering patterns was found to be consistent with a curve deduced from reversion experiments but considerably different from calculated values. The high flux at ILL has permitted in situ observations to be made of the growth (or dissolution) of metastable precipitates. From the SAN S data it was possible to determine whether zone formation at a given aging temperature took

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89 place by nucleation and growth or by spinodal decomposition. In another recently reported in situ experiment at ILL, solute partitioning occurring during unmixing of a ternary alloy was studied. Three isotopes of one of the constituents were used in order to obtain independent scattering contrasts.