Correlações entre parâmetros microestruturais, parâmetros térmicos e resistência mecânica de ligas Sn- Bi e Sn-Bi-(Cu,Ag)
Abstract
The present research aims to develop a theoretical/experimental analysis of the
combined effects of solidification thermal parameters, Bi content and addition of
ternary elements (Cu, Ag) on the final microstructure aspects and on the final
mechanical resistances of directionally solidified Sn-Bi, Sn-Bi-Ag and Sn-Bi-Cu
solder alloys under transient heat flow conditions. Hence, experimental
interrelations between microstructure and thermal parameters and between
mechanical properties and microstructure could be established. The
microstructures regarding the Sn-34wt.%Bi and Sn-52wt.%Bi alloys show the
presence of β-Sn dendrites with Bi precipitates on their own, being enveloped
by a lamellar binary Sn-Bi eutectic. The Sn-58wt.%Bi eutectic alloy show a
variety of microstructures along the length of the directionally solidified casting,
which includes binary eutectic, Bi plates, Bi trifoils and fishbone eutectic. In the
case of the ternary Sn-Bi-Ag and Sn-Bi-Cu chemistries, microstructures are
constituted by β-Sn dendrites decorated with Bi particles, Bi-Sn eutectic and
Cu6Sn5 and Ag3Sn intermetallic particles for the Cu and the Ag bearing alloys,
respectively. Experimental growth laws have been derived for both dendritic (λ1,
λ2, λ3) and eutectic (λfine, λcoarse) arrangements considering the following alloys:
binary Sn-34wt.%Bi, Sn-52wt.%Bi e Sn-58wt.%Bi alloys and ternary Sn-
34wt.%Bi-0.1wt.%Cu, Sn-34wt.%Bi-0.7wt.%Cu e Sn-33wt.%Bi-2wt.%Ag.
Considering the binary Sn-Bi, it has been observed that increasing Bi content
(34wt.%-->52wt.%-->58wt.%Bi), may cause a decrease on both strength and
ductility, except for the sample at P=6mm of the Sn-52wt%Bi alloy. Hall-Petch
type functional correlations have been able to represent the evolution of the
tensile mechanical properties for the examined Sn-Bi and Sn-Bi-X alloys.