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The macroscopic properties of most materials depend directly on their microstructure and its local variability at the atomic level. Recent trends in high resolution electron microscopes (HREMs) have led to resolving powers on this scale, which in turn has made these instruments invaluable to many materials science investigations. The purposes of this short review are firstly to outline some of the fundamentals of high resolution image formation and interpretation and then to summarise some of the latest instrumental developments. Some recent applications are briefly described to provide some appreciation of the wide range of materials currently being investigated with the HREM. The impact of this work should be apparent from reference to other papers in this volume as well as several recent reviews [1–3] and special conference proceedings [4–5]. The likelihood of further developments in instrumentation and the necessity for complementary information from other techniques are also briefly considered.
Measurements of the diffuse X-ray (or neutron) scattering allow the detailed investigation of point defects in crystalline solids. The method can be applied for defect sizes ranging from isolated point defects up to large dislocation loops. The diffuse scattering intensity close to the Bragg reflections, Huang Diffuse Scattering and Asymptotic Diffuse Scattering, is of special interest as the intensities from lattice distorting defects are high and the scattering theory is most straightforward for this region of the reciprocal lattice. After a short introduction to the theoretical background and to the experimental techniques the capabilities and limitations of the method will be demonstrated with examples of experimental results. i) The structure of interstitial atoms has been investigated for low temperature irradiated crystals and for metals with interstitially dissolved solute atoms. ii) The mobility and growth of interstitial agglomerates during annealing stage II of irradiated metals is discussed. The influence of impurities on the cluster growth is demonstrated for the example of Nibase alloys. iii) Defect clusters and defect distributions within cascades as observed after different types of irradiations are discussed.
X-ray diffuse scattering has been used to study the thermal annealing of vacancy and interstitial loops in Ni-ion irradiated copper. The diffuse scattering formalism is reviewed and diffuse scattering measurements are reported on liquid-He temperature Ni-ion irradiated copper after annealing to 40, 275, and 300 °C. Size distributions are presented for vacancy and interstitial loops after each anneal and the thermal-induced changes are discussed in terms of loop dissolution and coalescence.
Hydrogen in metals is a textbook example for fast diffusing defects in solids. The jumping frequency is in the case of H in Nb (Stump et al, 1975) even at room temperature in the range of 1011 1/s – 1012 1/s.
The perturbed angular correlation (PAC) method was used to study vacancy trapping at 111In atoms in quenched Al single crystals containing up to 10 ppm In. During annealing from 200–230 K, two different electric field gradients appeared, characterized by the interaction frequencies ν 01 = 67 MHz with asymmetry parameter η = 0.41, and νQ2 = 133 MHz with η = 0. The assignment of ν01 to a single vacancy-In atom pair is discussed. The results indicate that the migration energy of single vacancies in Al is = 0.55 eV.
Previous neutron scattering studies on nominally pure Nb (Chang et al, 1977, Werner et al) showed unusual diffuse scattering peaks at Q = (4/3 4/3 4/3)2π/a with varying intensities from sample to sample.
The effect of temperature on radiation-induced defect production was investigated in Cu single crystal films doped with ∼ 100 ppm Be. Frenkel pair production was determined at 55K and 190K as a function of electron energy and incident beam direction using in situ electrical resistivity measurements in the high voltage electron microscope. The crystallographic anisotropy of defect production was found to be considerably greater for irradiations at 55K than for irradiations below 10K. This difference is related largely to an observed anisotropy in the stage I defect recovery. It was found in isochronal annealing experiments after irradiation nearthreshold that the resistivity remaining after stage I was nearly twice as large for recoils along <110> as compared to <100>.
Positrons localize in trapped states at a variety of defect sites in solids, from which they subsequently annihilate with unique observable characteristics. As such, the positron is a valuable probe for the study of these defects. Positron annihilation spectroscopy (PAS) has made significant contributions in recent years to the determination of atomic defect properties in metals and alloys, and in molecular solids as well. It has also been used extensively in the monitoring and characterization of vacancy-like microstructure development, as occurs during post-irradiation annealing. The characterization of defects using PAS is selectively reviewed and some possibilities for using the positron as a localized probe of the atomic and electronic structure of atomic defects and their aggregates are discussed.
Vacancy formation was studied in the refractory bcc metals, tungsten, chromium, and niobium, using the positron annihilation spectroscopy Doppler broadening technique, between room temperature and the respective melting temperatures, under ultra-high vacuum conditions. Temperatures were measured by optical and infrared pyrometry, a W(Rh) thermocouple, and the power delivered to the sample, with calibrations against known melting temperatures. For W, a trapping-model analysis of the data from the temperature range 300–3633 K yielded a vacancy formation enthalpy of 3.76 ± 0.39 eV. For Cr, a similar fit to the data from 296–2049 K yielded a vacancy formation enthalpy of 2.0 ± 0.2 eV. The results are discussed in relation to previous vacancy formation and self-diffusion studies. Measurements on Nb as a function of temperature and oxygen content are also presented.
Recent advances in the use of positron annihilation to study defect ensembles in and on the surfaces of metals, are pointing the way towards studies where particular positron-electron annihilation modes may be identified and studied in the presence of one another. Although a great deal is understood about the annihilation of positrons in ostensibly defect free metals, much less is understood when the positron annihilates in complex defect systems such as liquid metals, amorphous solids, or at or near the vacuum-solid interface. In this paper the results of three experiments, all of which demonstrate means by which we can resolve various positron annihilation channels from one another, will be discussed.
Neutrons possess a number of attributes which make neutron scattering a powerful technique for the investigation of problems in materials science. These properties are discussed with emphasis on their importance to materials characterization. A number of recent experiments using neutron scattering will be described. These studies include the areas of damage accumulation; residual stress; porosity in ceramics; polymer chain configurations; phase changes.
The high intensity and large beam size of a synchrotron radiation source have been exploited in order to obtain double crystal X-ray topographs of whole 2in. and 3in. slices of semi-insulating LEC GaAs single crystals. Exposure times, typically 30 minutes for high resolution topographs, are at least one order of magnitude down on those required when using a conventional source. Variations in relative lattice parameter and lattice tilt have been measured as a function of position on the slice. The defect structure has been imaged and dislocations are seen in cellular configurations, slip bands and linear arrays (lineage), the latter of which are shown to be associated with small lattice tilts, typically 30”. The defect structure revealed on the topographs has been correlated with 1μm infrared absorption micrographs which are believed to represent the concentration of the dominant deep level EL2.
Coarsening kinetics was studied in a A190. 6 Li9. 4 alloy aged at 100°C, 128°C, 165°C, 178 ° C, and 209 °C. The time-dependent behavior of the average precipitate radius followed t1/3 behavior. The precipitate/ matrix interfacial energy obtained from analysis of the kinetic constant was 0.015 J/M2 and was largely independent of temperature. The scattering curves exhibit interparticle interference which prevents us from calculating a particle size distribution.
The dynamic behavior of the melt growth process of Sn with low dislocation density has been investigated by means of synchrotron Laue topography using an X-ray sensing high resolution camera tube. No defect images are observed at the point where the melting begins. No dislocations are generated during the temperature rise to the melting point,although the dislocation configuration changes slightly. Most of dislocations which are contacting the melting interface do not propagate into a newly grown crystal part and the homogeneous solidification results in a nearly perfect crystal.Dislocations and slip bands are generated from a part where the solidification is completed. It was found that the growing interface appears atomically rough and extends through a considerably large number of layers.
High resolution section topographs are made with synchrotron radiation using exposure times that are 1/1000 or less of those necessary with conventional x-ray sources. The imaging technique is employed to study oxygenrelated defects and their spatial distribution in a large number of annealed p-type (100) Czochralski silicon wafers. A two-step annealing in nitrogen consisted of a nucleation cycle at 800 °C and at 825 °C for 20 h followed by a denuding cycle at 1100 °C for 6 h. Denuded zones measuring from zero to 100 μm are seen in the synchrotron section topographs made after oxidation of the wafers at 1100 °C for 2 h. The denuded zone formation is found to depend strongly on the part of the crystal from which the wafer is cut. The wafers have the same interstitial oxygen concentration but different carbon concentrations. Large spatial variations up to 100 μm of the denuded zone width are observed in some wafers. Synchrotron section topographs of the wafers in which CMOS devices were fabricated showed a denuded zone only if the wafers were thremally annealed before the IC process.
Recent results on the formation of voids and dislocation substructure in fatigued copper single crystals and on the early stages of decomposition in Ni base alloys are presented to illustrate some of the advances in the application of small-angle scattering techniques.
This Raman study indicates that the boroxol ring concentrations for glasses in the B2 O3-GeO2 and B2 O3-GeO2-SiO2 systems are dependent upon temperature and composition. Ring breakage and formation occur well below Tg. An Arrhenius plot of intensity data for borogermanate g glass indicates that the heat of ring rupture is ca. 7.7 Kcal/mole. The boroxol ring concentrations in various glasses were also investigated at room temperature after various heat treatments. The structural interpretation from Raman scattering data was compared with that from IR data.
Reflection electron microscopy (REM) utilizes the Bragg reflected high energy electrons to form the image of a crystal surface. Images of dislocations, atomic steps, reconstructions of surface layers of atoms and adatoms, stacking faults and twinning, superlattices, etc., have been successfully observed on a wide variety of specimens. Contrast is mainly due to diffraction and phase, which distiguished REM as a unique method for high spacial resolution and high sensitivity imaging of the surfaces of bulk specimens. REM can be effectively performed under UHV as well as under the moderate vacuum of an ordinary commercial electron microscope.
Rearrangements of atomic columns on extended gold surfaces have been imaged directly using a 500kV high resolution electron microscope. The (100), (110) and (111) surface profiles were all found to be highly mobile and microscopically rough, with (111) in particular developing a characteristic hill-and-valley morphology. The presence of surface steps had a marked influence on the direction of surface diffusion only for the (100) surface. The observations establish that high-resolution profile imaging can provide unique information about surface self-diffusion which is unobtainable by other techniques.
Thin films of α-alumina have been produced by annealing oxide films produced by the thermal oxidation of aluminum substrates. Grains of αalumina which formed in the basal orientation show well-defined surface steps. When these steps move they interact with particles which have nucleated on the surface. Further annealing allows direct observation of the movement of these surface steps.