B. Ralph

The strain and grain size dependence of the flow stress of copper

Abstract Tensile stress strain data for 99.999% copper at room and liquid nitrogen temperature as a function of grain size are presented together with some microstructural observations made by transmission electron microscopy. It is shown that the flow stress data, at constant strain may be expressed in terms of a Hall-Petch relationship. At low strains an inhomogeneous distribution of dislocations is seen whilst at higher strains (0.1–0.2) a more regular cell structure begins to develop. This tends to have a minimum size near to grain boundaries. These microstructural observations are correlated with the mechanical test data.

The recrystallization of nickel-base superalloys

The effects of recrystallization on the γ′ distribution in four nickel-base superalloys of varying γ′ volume fraction (Nimonics PE16, 80A and 115, and Udimet 720) have been studied by transmission electron microscopy. These effects are explained in terms of high solubility and diffusivity in the recrystallization interface, and it is suggested that high diffusivity assumes greater importance as the amount of solute dissolved in the boundary increases. Some attention is given to the nucleation of recrystallization. It is shown that in one of the alloys (Udimet 720), nucleation at grain boundaries involves subgrain coalescence. Subsequent growth of the nucleus occurs by strain-induced boundary migration.

The effect of grain size on deformation twinning in a textured zinc alloy

The incidence of deformation twinning in samples of a commercial rolled zinc-0.1% aluminium-0.05 wt % magnesium alloy has been measured at room temperature as a function of strain, grain size and direction of loading relative to the rolling direction. The volume fraction of twinned material has been measured at the surface and in the bulk. It is shown that this volume fraction is a linear function of both strain and grain size in most cases. Bands of heavily twinned grains are found to form inhomogeneously across the gauge length of tensile specimens, by an autocatalytic mechanism. Several examples are given of the interaction of twins and slip bands at grain boundaries which illustrate the formation and the accommodation of twins. The smaller volume fraction of twins found at the surface compared with the bulk reflects a relaxation of Von Mises criterion.

The application of convergent-beam electron diffraction to the detection of small symmetry changes accompanying phase transformations I. General and methods

Abstract This paper considers the origins of convergent-beam diffraction patterns (CBDPs) and the particular uses of high-order Laue zone (holz) lines which are a feature of these patterns. Small changes in the arrangement of the holz lines, corresponding to changes in the lattice parameter and symmetry of the crystal giving rise to them, are discussed, and a technique for their simulation described. The advantages and limitations of the convergent-beam diffraction technique are considered.

Contrast from stacking faults and partial dislocations in the field-ion microscope

Abstract The effects of partial dislocations and stacking faults intersecting the surface of a field-ion tip are predicted from the geometrical properties of the defects. The surface configuration is related to the contrast expected in field-ion micrographs. The displacements producing partial dislocations and stacking faults lead to ‘steps’ and ‘kinks’ on the tip surface which give contrast in field-ion micrographs. Parameters are defined which conveniently and generally describe the contrast to be expected from dissociated total dislocations and faulted loops. Examples of faulted loops in iridium are discussed in terms of this theory.

A field ion microscope study of some tungsten-rhenium alloys

Abstract Field ion microscope observations have been made of the effects of alloying tungsten with rhenium, and a series of alloys have been imaged and examined. The structure of two alloy grain boundaries has been determined, stacking-faults imaged and a [sgrave]-phase precipitate particle examined. Tentative explanations are put forward to account for the micrographs obtained and it is concluded that under suitable conditions atomic species could be distinguished.

On the ageing of an aluminium-lithium-zirconium alloy

Electron microscopy has been used to follow the ageing of an aluminium alloy containing 3.08 wt % lithium and 0.19 wt % zirconium over the temperature range 433 to 553 K. A dispersion of Al3Zr particles was present before these ageing treatments and is unmodified by them. Two dispersions of Al3Li (δ′) are produced by these ageing treatments, one is formed homogeneously in the matrix while the other nucleates and coarsens on the Al3Zr/matrix interface. From the data it appears that there is little interaction between the lithium and zirconium in solution and that the precipitation processes occur chemically independently. The coarsening characteristics of both dispersions of δ′ have been investigated as has the discontinuous precipitation of δ′.

The interpretation of field-ion micrographs: The image from an order/disorder alloy

Abstract The micrographs obtained from the alloy Pt-Co in several states of order are analysed in terms of the atomic structure of the lattice concerned, and the particular principles involved in the technique of field-ion microscopy. It is shown how such analysis yields information on the metallurgical features of the disorder to order transformation.

A model of the equilibrium structure of F.C.C./B.C.C. interfaces

Abstract An extension to the 0-lattice model due to Bollmann is given which allows the relative favourabilities of partially coherent boundaries to be calculated in terms of the dislocation arrays of which they are comprised. Some predictions of this model for f.c.c./b.c.c. interfaces in a number of orientation relationships are given and, where possible, compared with experimental observations. A calculation is made of the additional relative energy which arises due to those components of the structure of interfaces not directly related to the dislocation content.

The creation and accommodation of extrinsic dislocations at grain boundaries

Abstract This paper considers the observation of extrinsic grain boundary dislocations in a commercial austenitic steel. It is shown that the stability of these non-equilibrium defects is a sensitive function of the precise thermo-mechanical history of the specimen material and is often controlled by the presence of active pinning centres in the grain boundary (e.g. solute and/or precipitate)