Impurity Atmosphere Research Articles

The ductile—brittle transition in body-centred cubic transition metals

Abstract The significance of the parameters [sgrave]i and k y in Petch's empirical relation [sgrave]y=[sgrave]i+k y d −1/2 between yield stress and grain size is discussed. It is concluded that [sgrave]i is made up of two terms [sgrave]i (L) and [sgrave]i (I). The first, [sgrave]i (L), represents the stress required to propagate a Luders band through a single crystal having no lattice friction stress. It is a function of the stress, [sgrave]d, required to unpin a dislocation from its impurity atmosphere and of the work-hardening properties of the metal. The second, [sgrave]i (I), is a friction stress probably resulting from the presence of impurity atoms. The grain size parameter, k y, measures the extra work-hardening which occurs as a result of forced slip during the propagation of yielding through a polycrystal. It is a function of the work-hardening rate of the metal and of the unpinning stress. A criterion for brittle fracture is derived which is based on the observation that, at and above the ductil...

Yield Phenomena in Copper-Arsenic Alloys

The occurrence of strain ageing in single crystals of arsenical copper (0.36% As) has been established by simultaneous determinations of stress-strain curves and changes in electrical resistivity. During yielding the electrical resistance increased more rapidly than normally. The change in resistance during yielding is explained in terms of Cottrell's model of an impurity atmosphere around dislocations.

LIX. The Electrical Properties of Dislocations in Germanium

ABSTRACT The electron energy levels at an edge dislocation in germanium are discussed. It is shown that these levels will be considerably modified by the presence of a Cottrell impurity atmosphere. This leads to an interpretation of otherwise conflicting experimental results. Further experiments are suggested. † Communicated by the Author