Refractory products for the production units of amorphous metals and alloys

Abstract

In recent years, extensive studies have been carried out on the synthesis of amorphous metals and alloys having improved physicochemical properties. Centrifugal-casting units are used for maintaining the melt in its amorphous state during the course of its solidifcation. Their operation is based on rotating a bowl at a speed of approximately i0,000 rpm and continuously feeding a thin jet of the molten material having a temperature of 1500-1600~ onto its bottom portion.The melt comes into contact with the bottom of the bowl and forms a film having a thickness up to 200 ~m under the action of the centrifugal force. This film is driven upwards to the edge of the bowl and is separated from it to form granules having a size of approximately 200 zm. The bowl is cooled using liquid nitrogen (T = 173 K) so as to ensure retention of the amorphous state of the granules during their fall and to avoid ~heir crystallization.Until recently, graphite bowls or water-cooled copper bowls have been used for carryin g out atomization of the melts. However, the extent of their corrosion under the action of the melt is so significant that they can not be reused after the very first melting. Furthermore, the molten metal is contaminated with the material of the bowl. Bowls made from fused quartz are also not being used widely because of cristobalitization and cracking during the course of cooling.This paper deals with a study of the possibility of replacing such bowls and other products used under the conditions involving the action of aggressive chemical media and temperature gradients with ceramic components. When developing the components, it was borne in mind that they must possess the following set of properties simultaneously: a high thermal shock resistance (heating internally up to 1550-1600~ and cooling from the outside up to 100~ nonwettability with the melts; ability to retain mechanical strength under the conditions involving prolonged action of temperature gradients along (across) the wall of the bowl; and sufficient mechanical strength for withstanding the stresses develped in the edge of the bowl under the action of significant centrifugal loads and the shear stresses developed during the operation.The accumulated experimental and the theoretical data show that some of the properties (for example, refractoriness and the limits of temperature compatibility of the products) are controlled by the physicochemical nature of the compact ceramic materials. However, their mechanical strength and corrosion resistance can be improved using different technological routes and new heterophase materials having a specific structure [i].In view of the fact that the structure consists of an aggregate of pores and a solid region containing crystals, a glass, and an amorphous phase, one can develop materials having the required service characteristics by changing the quantitative and the qualitative ratios of the structural elements and establishing the structure--property relationships.It is known that the nature of the pore structure of ...