Microsegregation in the Lead‐Antimony Alloys

Abstract

The freezing process occurring in lead and hypoeutectic lead‐antimony alloys was studied in single crystals and polycrystalline aggregates by examination of the metallographic structure with microscopic, microradiographic, and chemical replica techniques. The formation of dendritic lead crystals was also observed in other systems. The microsegregation occurring in the lead‐antimony alloys was found to be a result of the dendritic mode of growth. Production of a homogenous alloy was found to be impossible even when the amount of antimony was reduced to insignificant amounts. This was true whether the alloy was chill‐cast or very slowly cooled. Segregation is shown to be caused both by the small concentration gradient and the large amounts of lead crystallizing in the first intervals of freezing. Alloys of higher concentration are shown to have a more uniform solid solution of antimony in lead but actual segregation of antimony or an antimony‐rich phase is inevitable. The primary lead dendrites appear to grow by alternate dendritic extension and regular crystal growth, accounting for the structure observed in metallographic sections. The structure in these alloys, commonly referred to as eutectic, is not produced under eutectic conditions and is not representative of true eutectic structure. When chill cast, an alloy with 13 per cent antimony produces a normal eutectic crystallization. Normal eutectic crystallization could not be initiated in any slowly cooled alloy investigated. In general, the segregation that occurs in chillcast alloys is much more uniformly distributed than that in slowly cooled alloys.