Transmission Electron Microscopy Of Thin Foils Research Articles

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ e or faulted γ→ ά. The orientation relationships between the three phases γ, e and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.

Preparation of direct-replica of steel specimens

Surface replication was the first widely used method which enabled surface structure of electron-opaque specimens to be studied in the electron microscope. Even today, with the advent of thin-foil transmission electron microscopy, replicas are still extensively employed. Replicas are prepared by either depositing a replicating material (carbon or silicon monoxide) directly on to the specimen followed by a separation process, or indirectly by using a two-stage technique. This involves the preliminary impression of the specimen surface in a suitable plastic, called the primary replica, followed by a secondary replica (using carbon or silicon monoxide) of this primary replica. It is now well recognized that the former technique, called replicas, are superior in resolution and contrast to the later technique, called the indirect replica [1]. The preparation of replicas, of steel specimens is based on the direct replica technique, and the stripping of the film is done either by electro-etching or chemical etching. The etchant generally used for steel is 10% HNO 3. However, this etchant strips the film with the evolution of gases which is unsatisfactory as it breaks up the film. 5~o bromine in methanol is also unsatisfactory as it requires a long etching time and is opaque, making it difficult to retrieve the carbon replica. It also results in the contamination of the carbon film from reaction products. This work describes the development of a fast and reliable direct replica preparation technique from a steel fatigue specimen (En 351) using a new etchant.

Dynamic recovery in aluminum

Dynamic recovery was studied in pure aluminum specimens deformed in tension at temperatures between 200° and 573° K, and strain rates between 0.004 and 2 min −1. The dislocation interactions and substructures were observed by means of thin-foil transmission electron microscopy. Activation energies were obtained for dynamic recovery and for tensile deformation. The values for deformation are ΔH d = 22.5 ± 3.5 kcal/ mol (at low temperatures) and ΔH d = 37.5 ± 2.5 kcal/ mol (at high temperatures). These two activation energies are related to the rate-controlling controlling processes of cross-slip and dislocation processes of cross-slip and dislocation climb, respectively. Thus, the activation energy for dynamic recovery, ΔH r = 22 ± 2.8 kcal/ mol, corresponds to the cross-slip of screw dislocations, which leads to the formation of stable dislocation networks and dislocation-free subgrains. At higher deformation temperatures, dislocation climb predominates, and subboundary disintegration and coalescence of subgrains are observed.

Precipitation in Ni-12 at. % Ti Alloy

Precipitation behaviour in Ni-12 at. % Ti alloy was studied by means of thin foil transmission electron microscopy and electron diffraction technique. Hardness measurements showed two stage age-hardening at 500°C and 600°C. Thin foil observations revealed that the initial rapid increase in hardness was associated with the presence of superlattice. Slower second hardening was caused by the formation of modulated structure or periodic ordered structure. The increase in hardness by these periodic structures was found to be due to the internal strain hardening as originally suggested by Mott and Nabarro.

The microstructure of superalloys

Some recent thin-foil transmission electron microscopy studies that contribute to an increased understanding of the microstructure of nickel- and cobalt-base superalloys are discussed. Emphasis has been placed on the interaction between imperfections and precipitates that cannot be studied by x-ray and replica methods. The effects of matrix stacking fault energy on the dislocation substructure and the role of different dislocation substructures in the nucleation of precipitates are summarized. Attention has been given to the intragranular precipitation of MC carbides in association with stacking faults in nickel-base alloys. Recent data on the behavior of γ′-phase in simpler nickel-base alloy systems are discussed with respect to the possible application of these results in understanding the behavior of commercial γ′-bearing superalloys. Finally, brief consideration is given to recent studies of order-disorder reactions in nickel-base alloys.

Electron Optical Study of Basal Dislocations in Graphite

Dislocations having a Burgers vector in the basal plane and lying in this plane, have been studied in graphite by the use of thin foil transmission electron microscopy. The dislocations are dissociated into ribbons consisting of two partials separated by a stacking fault. Specific models for these dislocations are presented. Hexagonal networks contain extended and contracted nodes. The presence of two types of stacking faults gives rise to singularities which are analyzed in terms of experimentally determined Burgers vectors. The dislocations and even the networks are very mobile along the basal plane. The stacking fault energy is determined as 3 to 5×10−2 ergs/cm2.