Thermodynamics of microsegregation and influence of elements on structure of unalloyed and alloyed cast iron

Abstract

Graphitizing elements in cast iron such as Si, AI, Cu, and Ni are subject to so-called ‘negative’ microsegregation during crystallization of primary austenite dendrites in hypoeutectic cast irons, while chill-forming elements such as Mn and Cr are subject in the same conditions to ‘positive’ microsegregation. This general rule is developed quantitatively and the theory of micro segregation of the components of cast iron is linked to the theory of graphitization. On this basis a new aspect of the thermodynamics of graphitic and non-graphitic iron alloys of the cast-iron type was worked out, using methods of geometrical thermodynamics developed from those proposed by Hillert in the early 1950s. A simple equation has been obtained which gives a relationship between the activity of components of ternary systems and the slope of the tie-line in the corresponding two-phase region of the phase diagram. The paper also includes the results of computations in the Fe-C-Si and Fe-C-Al systems. The results are analysed and compared with those obtained on the basis of relationships between the slope of eutectic lines and the angle at the summit of eutectic tie-line triangles.