ResumoO estudo de ligas livres de Pb envolvendo o emprego de junções metálicas sob altas temperaturas tem recebido atenção especial, sendo desenvolvidas novas ligas com qualidade e desempenho adequados. Ligas Bi-Ag consistem em alternativas promissoras para a substituição das ligas de solda contendo Pb para tais aplicações, por isso devem ser melhor caracterizadas sob o aspecto de estruturas brutas de fusão. Assim, o presente trabalho objetiva investigar a liga hipoeutética Bi-1,5%Ag (%em peso) solidificada unidirecionalmente sob regime transitório de fluxo de calor. Será analisada a influência dos parâmetros térmicos de solidificação como velocidade de solidificação (VL), taxa de resfriamento (Ṫ) gradiente de temperatura (G) na microestrutura e na evolução de dureza da liga citada. Para determinação do perfil de dureza, foram realizados ensaios de dureza Vickers. Para obtenção e caracterização das microestruturas presentes no fundido Bi-1,5%Ag, fez-se o uso da microscopia ótica e da fluorescência de raios-x. Uma estrutura de dendritas facetadas de Bi circundadas pelo eutético Bi-Ag prevaleceu ao longo do lingote Bi-1,5%Ag. Verificou-se um aumento gradativo dos espaçamentos dendríticos primário (1) e secundário (2) à medida que a distância da base refrigerada foi aumentada. Em geral, os valores de dureza diminuíram para maiores valores de 1 e 2. Palavras-chave: Ligas Bi-Ag; Microestrutura; Solidificação; Dureza. AbstractThe study of Pb-free alloys involving the use of metallic connections under high temperatures has received special attention in the last few years, being developed new alloys with adequate quality and performance. Bi-Ag alloys appear as potential candidates to replace solder alloys containing Pb. Hence, such alloys are needed to be better characterized considering their as-cast or assoldered microstructures. The aim of this study is to investigate a lead-free Bi-1.5wt%Ag solder alloy solidified unsteady-state upward unidirectional conditions. The influence of thermal parameters as tip growth rate (VL), cooling rate (Ṫ) and thermal gradient (G) on the final microstructure and hardness properties will be examined. A hardness Vickers tester was used in order to determine the hardness profile of the Bi-1.5wt%Ag alloy. The directionally solidified microstructure was characterized by both light microscopy and fluorescence spectrometer. It was found that the as-cast structure was arranged by faceted Bi-rich dendrites sorrounded by a eutectic mixture (Bi+Ag). In general, the hardness values decrease for higher of 1 and 2 values.

Abstract. In the present study a hypoeutectic Bi-1.5wt%Ag alloy was directionally solidified under transient heat flow conditions and the microstructure was analysed. Bi-Ag alloys are considered as potential alternatives to replace Pb-based alloys as solder materials for metallic connections under high temperatures. However, a lack of understanding regarding the effects of solidification thermal parameters (growth rate -V L , the cooling rate -Ṫ) on microstructural aspects is reported in literature. Another challenge is to improve properties and reliability. The results of the present study include the determination of the tip growth rate and the cooling rate by cooling curves recorded by thermocouples positioned along the casting length, metallography, X-ray fluorescence (XRF) and Vickers hardness. The entire directionally solidified Bi-1.5Ag microstructure was arranged by faceted Bi-rich dendrites surrounded by a eutectic mixture (Bi+Ag). The primary and secondary dendrite arm spacing ( 1 and  2 ), the interphase spacing (λ) and the diameter of Ag-rich particles were also measured along the casting length; and experimental growth laws. Relating these microstructural features to the experimental thermal parameters are proposed. IntroductionDuring the last decades lead-based eutectic or near-eutectic alloys have been massively used as solders in electronic packaging and assembly. Due to the inherent negative impacts to human health and environment, nowadays lead (Pb) is intended to be banned from such applications. As a consequence, alternative solder alloys have to be selected and tested in order to replace the traditional lead-based material. In the case of high-temperature applications the usage of the Pb-5wt.%Sn solder alloy is still considered since lead-free alternative alloys do not achieve enough performance considering characteristics such as wettability, reliability and cost. As a consequence, further research is necessary since the use of high lead content alloys is growing considerably [1]. In this context, near eutectic Bi-Ag alloys become promising solder alternatives once Bi is the least toxic of the heavy metals, allowing its application as a substitute for Pb. This would become a step toward an environmentally friendly soldering technique. Further, the temperature criterion is achieved since the melting temperature of the eutectic Bi-2.5wt%Ag is quite close to that of the Pb-5wt.%Sn alloy. This criterion is based on the fact that high-temperature solders usually have melting temperature in the range between 260°C and 400°C. It is obviously realized that both cited characteristics are not enough to guarantee soundness for a candidate alloy in order to substitute high Pb-alloy solders.The Bi-2.5Ag alloy is considered an interesting option due to compatible melting point and hardness as compared with Pb-based traditional solder alloys, with an actual possibility of having