Formation of crystal structures in pure aluminum

Abstract

This is the first progressive report on the crystallization of metals and alloys from liquid and describes the formation of macrostructures in pure aluminum. The controlling variables on forming macrostructures have been determined on the basis of the distance-time curves of the waves of freezing start and end taken from various sets of cooling curves.High purity, commercial purity and grain refined aluminum have particular types of macrostructures. Movement of the wave of freezing start is rather important than that of freezing end. The starting wave progresses parabolically at the early stage of directional freezing and linearly at the final stage. The transformation of structure from columnar to equiaxial occurs near this transitional point. The substantial location of the structural transformation is dependent on the purity of aluminum or existing stable nuclei. Although most of the thermal variables during freezing are not so important in forming the structure, only the thermal gradient in the liquid just ahead of growing solid is critical for each purity of aluminum. The ratio(linear freezing rate)1/2/(temperature gradient in liquid) first proposed by Tiller and Rutter probably depends on the temperature gradient to a great degree. The critical temperature gradient for structural transformation also depends on the stability and numbers of existing heterogeneous nuclei. Unfortunately, the substance, numbers, size and distribution of the heterogeneous nuclei have not yet determined particularly on the quantitative basis.