Complex eutectic growth and Bi precipitation In ternary Sn-bi-cu and Sn-bi-ag Alloys

Sn-34 wt%Bi, Sn-34 wt%Bi-2wt%Ag and Sn-34 wt%Bi-0.7 wt%Cu alloys have been directionally solidified (DS) under a broad range of solidification cooling rates. Microstructures have been characterized with emphasis on both eutectic growth and precipitation of Bi within the beta-Sn dendritic matrix. The eutectic growth, for all alloys examined, is shown to be associated with the coexistence of coarse and fine lamellar structures with different length-scale of lamellar spacing (lambda). Experimental growth relations of l vs. the cooling rate have been proposed. The length-scale of the lamellae in the fine eutectic ranges from 0.8 to 2.5 mm while in the coarse eutectic from 1.8 to 4.0 mm. Taking as reference the Sn-Bi alloy, both the spacing between Bi precipitates (lambda(p)) and the fine eutectic spacing (lambda(fine)) increase with Cu and Ag additions, whereas lambda(coarse) remains roughly unaltered. Both ternary Sn-Bi-Ag and Sn-Bi-Cu alloys are shown to have worse distributions of both lamellae in the fine eutectic and of precipitates within Sn-rich dendrites, which resulted in decrease in hardness. (C) 2016 Elsevier B.V. All rights reserved.691600605Fundação de Amparo à Pesquisa do Estado de São Paulo -(FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -(CAPES)857/152013/08259-3; 2015/11863-5