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 Burgers vectors of (a/2)⟨100⟩. The evidence for such a Burgers vector is strong and a structural justification is provided for its existence at these interphase boundaries. Observations and interpretation of faceting at the edges of these precipitates, and how the faceting changes on the breakdown of broad face coherency, are also given.