We wrote it to be read by, and taught to, senior undergraduates and starting graduate students, rather than studied in a research laboratory. We wrote it using the same style and sentence construction that we have used in countless classroom lectures, rather than how we have written our countless (and much-less read) formal scientificpapers. In this respect particularly, wehave been deliberate in notreferencing the sources of every experimental fact or theoretical concept (although we do include some hints and clues in the chapters). However, at the end of each chapter we have included groups of references that should lead you to the best sources in the literature and help you go into more depth as you become more confident about what you are looking for. We are great believers in the value of history as the basis for under- standing the present and so the history of the techniques and key historical references are threaded throughout the book. Just because a reference is dated in the previous century (or even the antepenultimate century) doesn’t mean it isn’t useful! Likewise, with the numerous figures drawn from across the fields of materials science and engineering and nanotechnology, we do not reference the source in each caption. But at the very end of the book each of our many generous colleagues whose work we have used is clearly acknowledged.

Transmission Electron Microscopy

A typical commercial transmission electron microscope (TEM) costs about $5 for each electron volt (eV) of energy in the beam and, if you add on all available options, it can easily cost up to $10 per eV. As you’ll see, we use beam energies in the range from 100,000 to 400,000 eV, so a TEMis an extremely expensive piece of equipment. Consequently, there have to be very sound scientific reasons for investing such a large amount of money in onemicroscope. In this chapter (which is just a brief overview of many of the concepts that we’ll talk about in detail throughout the book) we start by introducing you to some of the historical development of the TEM because the history is intertwined with some of the reasons why you need to use a TEM to characterize materials. Other reasons for using a TEM have appeared as the instrument continues to develop, to the point where it can seriously be claimed that no other scientific instrument exists which can offer such a broad range of characterization techniques with such high spatial and analytical resolution, coupled with a completely quantitative understanding of the various techniques.

The Transmission Electron Microscope

Electron optical experiments on Al foils have revealed individual dislocations in the interior of the metal. The arrangement and movement of individual dislocations have been observed. Most of the dislocations occur in the boundaries of a substructure, the diameter of the subrains being of the order of 1 μ, or more. Tilt-boundaries, networks and dislocation nodes have been resolved. The results apply to aluminium recovered at 350°c after heavy deformation by beating; the dislocation density is 1010/cm2. The dislocations can be seen to move along traces of (111) slip planes; the motion of the dislocations can be either rapid or slow and jerky. Cross-slip by the screw dislocation mechanism has been observed frequently.

Direct Observations of the Arrangement and Motion of Dislocations in Aluminium

Electron transmission micrographs of polycrystalline aluminium quenched from ∼ 600°c into iced brine show that the specimens contain many dislocation loops with diameters of the order of several hundred A. Some of the loops are hoxagonal in shape, and occur on (111) planes, and have a Burgers vector ½ [110] not in the plane of the loop. The loops are thought to be producod by the collapse of discs of vacancies, and the corresponding vacancy concentration is 10−4 Some of the loops disappear, while under observation in the microscope, by a mechanism of prismatic slip. In the neighbourhood of grain boundaries there is a zone depleted of loops; similar zones are observed near dislocations, whose complex configurations suggest that climb may have taken place. The observations are discussed in relation to the problems of ‘annealing out’ of vacancies, of quench-hardening, and of the origin of the dislocation network.

Dislocation loops in quenched aluminium

The theory of the previous paper is applied to the case of stacking faults in close packed lattices. Electron optical experiments on thin foils of stainless steel are described, the results of which are in good agreement with theory.

Electron diffraction from crystals containing stacking faults: II

Bethe's dynamical theory of electron diffraction in crystals is developed using the approximation of nearly free electrons and Brillouin zones.The use of Brillouin zones in describing electron diffraction phenomena proves to be illuminating since the energy discontinuity at a zone boundary is a fundamental quantity determining the existence of a Bragg reflection. The perturbation of the energy levels at a corner of a Brillouin zone is briefly discussed and the manner in which forbidden reflections may arise at a corner pointed out. It is concluded that the kinematic theory is inadequate for interpreting electron images of crystalline films.An electrolytic method for preparing thin metal sections for electron microscopy and diffraction is introduced and its application to the structure of cold‐worked aluminum and an aluminum‐copper alloy demonstrated. It is concluded that cold‐worked aluminum initially consists of small, inhomogeneously strained and disoriented blocks about 200A in size. These blocks are n...

R. D. Heidenreich

Papers

Electron Microscope and Diffraction Study of Metal Crystal Textures by Means of Thin Sections