Vacancy-type Dislocation Loops Research Articles

Interfacial structure of platelike precipitates

Abstract By conventional and hot-stage electron microscopy, an attempt has been made to understand the mechanisms of coherency breakdown of platelike precipitates. The tH plate in Al-4% Cu alloy and the θ plate in Al-0·2% Au have been studied because they offer a suitably wide range of mismatch values. Hot-stage work on η has been particularly rewarding. Three mechanisms for the loss of coherency have been observed directly: (1) nucleation of dislocations at the periphery of the plates on the broad faces, (2) nucleation of vacancy type dislocation loops at the edges of the plates and their expansion through the precipitates, and (3) prismatic punching. The first two mechanisms operate frequently in both θ' and η, but the last mechanism was observed only in η, and rarely. The dislocations nucleated have Burgers vectors of the types (a/2)〈110〉 and a〈100〈. Detailed hot-stage observations have been made on how these dislocations multiply and subsequently interact to form equilibrium dislocation arrays with Bu...

The Production of Vacancies by Oxidation in Zinc and Magnesium

The growth of vacancy-type dislocation loops in zinc and magnesium during annealing has been studied as a function of small alloying additions and of the air pressure of the annealing environment. It is concluded that the excess vacancies necessary for loop growth are produced as metal atoms enter a thin oxide film on the surface and participate in the oxidation process. The results are discussed in terms of the current theories of oxidation.