In-situ radiographic observations of alloy solidification

Abstract

Theories evolved to explain solidification behaviour in alloys involve a consideration of the growth morphology of the solid and of the associated solute distribution. However, direct observations of growth morphology in metallic systems have been limited almost exclusively to the results of surface studies [1], the only available transmission evidence having been obtained from thin (3000 ix) film studies of bismuth [2] and the alkali metals [3]. Such restrictions in experimental conditions require that some caution be exercised in attributing these results to bulk metal behaviour, and perhaps more valid experimental evidence for solidification theory has been yielded by work on various transparent organic and inorganic analogues [4]. Radiography of bulk material has been carried out [5] but this does not reveal the morphology of the solid-liquid interface. We would like to report here the successful development of a new technique allowing the observation in transmission of both growth morphology and solute distribution during solidification of specimens more representative of bulk material, in a wide range of commercially important alloys. Contact microradiography has been long established as a method of examining secondary phases in a metallic matrix [6]. The nature of the secondary phase can vary widely from a discrete inclusion to a region of solute segregation in an otherwise homogeneous matrix, provided that differential absorption contrast is generated. The contrast and geometric resolution considerations of contact microradiography have been applied in making a high-resolution radiographic study of alloy solidification. The microradiographic specimen approximately 300/~m thick is contained within a graphite crucible. A large current is then passed through the crucible, producing sufficient resistive heating to melt the specimen. A subsequent regulated decrease of the power input imposes controlled solidification, during which period a series of radiographs is taken. In radiographing a solidifying structure an additional resolution parameter is introduced by the movement of the interface during exposure. Interracial movement blurring may be reduced by minimizing the exposure time. However, in doing so, it becomes necessary to arrive at a compromise between the high inherent resolution of a Lippmann-type emulsion and the speed of a coarser