A theory of branching limited growth of irregular eutectics

Abstract

It has long been recognised that irregular eutectics, such as AlSi or FeC are characterised by large average phase spacings, compared to those of regular eutectics growing at the same rate and by large interfacial undercoolings. More recently, experimental evidence has accumulated to show that both spacings and undercoolings are decreased upon increasing the imposed temperature gradient in steadystate unidirectional growth. None of the existing models of eutectic growth explain any of these facts. In the present paper, some new results are presented which clarify the mode of growth of irregular eutectics. On the basis of these, a first attempt is made to incorporate the various disparate facts, concerning their growth, into a coherent growth model. In particular, the usual, minimum undercooling criterion is abandoned in favour of one based on the morphological instability and branching of the phases. Despite numerous approximations, the correct order of magnitude of spacing and undercooling is predicted, as is the effect of the imposed temperature gradient.