W.G. Burgers

On the process of transition of the cubic-body-centered modification into the hexagonal-close-packed modification of zirconium

Summary The process of transition of the cubic-body-centred modification into the hexagonal-close-packed modification of zirconium can be described by means of a combination of shearing- and dilatation-processes parallel to definite crystallographic directions. The transition has, therefore, “homogeneous” or “oriented” character. See for particulars the detailed summary at the end of the paper.

“Oriented” oxidation of barium

Summary Layers of metallic barium were obtained by volatilizing the metal in the (evacuated) electron diffraction apparatus and depositing it on a flat polished copper disc. Depending on the conditions of evaporation (temperature), they show either random orientation of the crystallites, or a preferential orientation characterised by a [111]-direction perpendicular to the plane of the supporting disc. The oriented layers, on oxidation, yield likewise oriented oxide layers. In these latter, the normal to the support is nearly parallel to a [110]-direction of the oxide lattice, deviating from it by about 10°–15° towards [100]. It is thus found that a direction of closest packing of the metal atoms in the face-centred oxide lattice (i.e. a [110]-direction) is closely parallel to a corresponding direction in the body-centred metal lattice (i.e. a [111]-direction). The relationship is further discussed in connection with that found by Mehl, McCandless and Rhines to occur between FeO and Fe on oxidation of a single crystal of iron. It was found that the [11 Indirection in the oriented barium layers, formed under conditions of oblique incidence of the vapour beam, deviates slightly from the normal to the supporting disc towards the direction of the incident beam, a behaviour analogous to that observed previously with layers of calciumfloride, deposited in vacuum from the vapour.

On the crystalline state of thin calciumfluoride films

Summary Thin transparent films of calcium fluoride, evaporated in vacuum from a hot-glowing filament covered with this salt on to metal- or celluloid underlayers, have been investigated by means of the method of electron-diffraction. It is shown that such films are crystalline, with random orientation of the crystallites in the first-deposited layers; in films of 0.1 μ thickness however, a preference orientation in the outer layers has already set in viz. with (111)-planes parallel to the substratum in the case of practically perpendicular incidence of the salt-vapour beam, and slightly inclined towards the direction of this beam in the case of strongly oblique incidence. It has been concluded from this that the films are built up of crystallites bounded by octahedral planes, the grains being more or less equiaxed in the first-deposited layers, and tending to form flat plates perpendicular to the vapour-beam and parallel to the underlayer on continued deposition of salt. This is a confirmation of the conception of the structure of these films, arrived at from adsorption-measurements by De Boer. The fact that heating of the films in extreme vacuum to about 400° does not alter the diffraction-diagram, although (as the decrease of their adsorption-power indicates) a decrease of the “free-surface” takes place, indicates that the heating causes a kind of “sintering” of the composing “lamellae”, a process different from recrystallisation. The latter may, however, take place on heating the films under less rigorous vacuum conditions.

Electronoptical observation of metal surfaces: III. Crystal growth and allotropic transition in Zirconium

Summary the transition of the hexagonal α- into the cubic β-modification of zirconium was investigated with the aid of the electron microscope. It was observed that, after passing the transition temperature, the more or less equiaxed texture of β-crystallites was replaced by a lamellar α-texture, the direction of the “lamellae” being different in each region, originally occupied by a β-crystallite. Considered as a whole, the α-texture closely resembled the so-called “martensitic” or “Widmanstatten” type of texture, exhibited by martensitic steels and other alloys. On retransformation the original β-crystallites reappeared, which in their turn could again be transformed into practically the same lamellar pattern of α-crystallites. This behaviour may be considered to confirm the conclusion, reached in former work, that the transition is brought about by a homogenous transformation (by shearing and dilatation) of coherent lattice regions from one crystalline state into the other, parallel to definite crystallographic directions, the “sense” of the transformation being opposite in successive α → β and β → α transitions, the whole process thus resembling closely the way martensite needles are formed in steel. A temperatures above 1150°C, the β-crystallites showed phenomena of crystal growth, whereby the capacity for growth of the individual crystallites could be influenced by the insertion of a temporary transition of the metal into the α-modification.

Lattice distortion and coercive force in single crystals of nickel-iron-aluminium

Zusammenfassung Die Erscheinung der sogenannten Ausscheidungshartung wird rontgenographisch und magnetisch verfolgt an Nickeleisenaluminium-Einkristallen, welche durch einseitige Kuhlung aus der Schmelze erhalten wurden. Die Linien der auszuscheidenden γ-Phase sind nur durch Erhitzen auf Temperaturen oberhalb 1000° sichtbar zu machen. Unter den gewohnlichen Ausscheidungsbedingungen bleiben die γ-Linien unsichtbar. An den Linien der α-Phase aber sind im Laufe des Prozesses ganz karakteristische Verformungen zu beobachten, welche bis zum Zustand, wo die hochste Koerzitivkraft eintritt, ihr eigenes Karakter beibehalten. Es erscheint dabei das scharfe Kα Dublett auf einen nach Intensitat und Breite stark wechselnden kontinuierlichen Hintergrund. Erst nach Durchlaufen des magnetisch hartesten Zustandes tritt eine allgemeine Verbreiterung ein, wobei das Dublett verschwimmt. Auf Grund der beobachteten Erscheinungen wird ein Bild entworfen, nach dem sich im ursprunglichen Einkristallgitter durch Diffusion Gebiete (“Kerne”) mit abweichender Zusammenstellung ausbilden, welche durch ihren verschiedenen Gitterabstand die Umgebung ein wenig verzerren. Beim Anwachsen dieser Keime oder Vermehrung ihrer Zahl werden die Verzerrungen immer grosser, da die Konzentrationsunterschiede zunehmen und die Gradiente schroffer werden. Der Zuruckgang in der Koerzitivkraft, welcher schliesslich eintritt, kann auf zwei Weisen erklart werden, namlich erstens dadurch, dass das Material bei zunehmender Verzerrung plastisch nachgibt und sich dabei erholt (F. Preisach), oder dadurch, dass die γ-Umsetzung einsetzt, welche Umsetzung bekanntlich auch verbunden sein kannn mit einem vorubergehenden Zustand mechanischer Schwache (G. Wassermann).

Secondary electron emission of soot in valves with oxidecathode

Summary When a carbon surface is covered with baryum atoms, the capability for secondary emission rises in the first instance. However, soot directly precipitated from the atmosphere round a burning flame is composed of very small particles and was found to behave exceptionally. It is apparent from the change of the secondary emission with time that the baryum atoms, precipitated on this carbon modification, fade away by a moderately strong electron bombardment. Owing to local heatings the baryum migrates then to soot particles at greater depth. Soot, sprayed on a metal surface, shows this migration in less degree. In this modification the small particles are conglomerated to larger ones. This is confirmed by electron diffraction experiments.

Electronoptical observation of metal surfaces: I. Iron: formation of the “crystal pattern” on activation

Summary After an introductory part regarding the construction of the cathoderay tube, the phenomena are discussed, and illustrated by means of a number of photographs, taking place on activating iron with strontium (starting from strontiumcarbonate), so that an intensive emission pattern of its crystalline texture is obtained. It is suggested that the applied method of activation has some features in common with the activation of thoriated tungsten.

Electronoptical observation of metal surfaces: II. Phenomena observed on transition of α into γ iron

Summary The changes occurring in the emission pattern of iron on transition of the α- into the γ-modification and vice-versa are illustrated by means of a number of photographs. The observed phenomena are discussed as well from a metallographical point of view as in connection with questions of electron emission. The occurrence of a zone of temporarily abnormal emission in front of a growing crystallite has been considered to be an indication that the transition process is accompanied by a migration of activating atoms in the growing zone.

Electronoptical observation of metal surfaces: IV. Appearance of “lines” of high emissivity on nickeliron crystals

Summary A nickeliron cathode was activated with strontium and barium, so that a “normal” emission pattern, showing its crystal structure, was observed in the electron microscope. If now a fresh supply of strontium (together with strontiumoxide) was deposited on the cathode while the latter was heated at a temperature of about 900°C, the emission, apart from incresing in intensity over the whole surface of the cathode, became excessively intensive along systems of parallel lines (each line being composed of a number of separate spots), the directions of which were related to the orientation of the underlying crystal lattice: this may be deduced from the fact that (a) in most cases the lines run from crystal boundary to crystal boundary and (b) reappear with practically the same direction if, after the cathode has been freed from the surplus of activator by heating for a short time at a higher temperature, again activating atoms are desposited on its surface under the same conditions as before.

Texture of thinly rolled tungsten foil

Summary Electron diffraction photographs of very thinly rolled tungsten foil show the presence of a pronounced rolling texture, the crystallites lying with a cube plane parallel to the plane of the foil and a cube edge (with very little spreading) at 45° to the direction of rolling. On oxidation, an oxide layer is formed consisting of crystallites oriented at random.