Samples extracted along the length of directionally solidified (DS) castings of three Sn-xBi alloys (x = 34 wt.%Bi, 52 wt.%Bi and 58 wt.%Bi) were first evaluated metallographically and then subjected to scanning electron microscopy and energy-dispersive x-ray spectroscopy analyses. The characteristic length scale of both eutectic and dendritic phases forming the microstructure were determined and correlated with solidification thermal parameters (growth rate V, and cooling rate T ˙). Tensile and Vickers hardness tests were performed to allow strength and ductility to be discussed as a function of both microstructure features and alloy solute content. The tertiary dendrite arm spacings along the length of the DS Sn-52 wt.%Bi alloy casting are shown to be lower than those obtained for the Sn-34 wt.%Bi alloy casting. The results of mechanical tests show that, with the decrease in the alloy Bi content, both tensile strength and hardness are improved. This is shown to be mainly attributed to the higher density of Bi precipitates decorating the Sn-rich dendrites, which are finer than the equivalent phase developed for the Sn-52 wt.% Bi alloy. However, the ductility is shown to be significantly improved for specimens associated with regions of more refined microstructure of the Sn-52 wt.%Bi alloy DS casting. A microstructure combining much branched dendrites, fine Bi particles within the β-Sn dendritic matrix and an important proportion of very fine eutectic formed by alternate Bi-rich and Sn-rich phase, seems to be conducive to this higher ductility. In this case, the fracture surface is shown to be more finely broken with presence of dimples for this particular condition, i.e., characteristic of a ductile fracture mode.

ResumoEsta proposta objetivou desenvolver uma análise teórico/experimental da influência de 2% Ag (em peso) em uma liga de solda Sn-40%Bi solidificada unidirecionalmente em regime transitório de fluxo de calor. Correlações experimentais entre os parâmetros térmicos de solidificação como velocidade de deslocamento da isoterma liquidus -V L , taxa de resfriamento -Ṫ L e parâmetros microestruturais como espaçamentos dendríticos secundários -λ 2 foram determinados. A caracterização microestrutural foi realizada por meio de microscopia ótica e eletrônica, difração de raios-x, além de fluorescência de raios-x para obtenção dos perfis de macrossegregação dos solutos (Bi, Ag). Para determinação do perfil de dureza foram realizados ensaios de dureza Vickers. As microestruturas brutas de fusão da liga Sn-40%Bi-2%Ag são compostas de dendritas ricas em Sn decoradas com partículas de Bi circundadas por uma mistura eutética (Sn+Bi) e partículas primárias de Ag 3 Sn. Os parâmetros térmicos decresceram com aumento da camada solidificada, permitindo uma ampla variação: Ṫ L → 0,1 a 10,0°C/s, V L → 0,2 a 0,9mm/s e G L → 0,4 a 11,0°C/mm. O teor de bismuto não variou significativamente, ao passo que a prata variou de forma efetiva, concentrando-se no início do lingote (até próximo de 35mm). O surgimento de braços dendríticos terciários em 15mm conteve a diminuição da microdureza Vickers, esperada para as posições mais afastadas da interface metal/molde. Palavras-chave:Ligas de soldagem Sn-Bi; Solidificação; Dureza. MICROSTRUCTURE, MICROHARDNESS AND THERMAL PARAMETERS OF Sn-40%Bi-2%Ag SOLDER ALLOY AbstractThis study aimed to develop a theoretical/experimental analysis of the influence of silver addition (2%Ag) in the Sn-40wt%Bi solder solidified directionally under unsteady-state conditions. Experimental correlations between thermal parameters as tip growth rate -V L , cooling rate -Ṫ L and microstructural parameters as secondary dendritic arm were determined. The microstructural characterization was carried out by light and electronic microscopy, x-ray diffraction, further of flurescence spectrometer to obtain the macrossegregation profiles of solutes (Bi, Ag). A hardness Vickers tester was used in order to determine the hardness profile of the Sn40wt%-2wt%Ag solder. The as-cast microstructures of Sn-Bi-Ag alloy were arranged by Sn-rich dendrites decorated with Bi precipitates in their own core surrounded with a eutectic mixture (Bi-rich and Sn-rich phases) and primary Ag 3 Sn intermetallic particles. The thermal parameters decreased with increase of the solidified layer, allowing a wide range: Ṫ L → 0.1-10.0°C/s, V L → 0.2mm-0.9mm/s and G L → 0.4-11.0°C/mm. The Bi content did not changed significantly, whereas Ag content has changed effectively, mainly at the beginning of the ingot (close to 35mm). The growth tertiary from 15mm contained the decrease of the hardness values, expected for the furthest positions of the metal/mold interface.

In this study, an extensive data set was based on existing literature records in order to enable the suitability of several predictive models, from Multiple Linear Regression (MLR) to Neural Networks (NN). The main objective was to, through regression analyses, generate model computations to correlate tensile properties (UTS- Ultimate Tensile Strength, YTS – Yield Tensile Strength and EF – Elongation-to-Fracture) to a given alloy composition and microstructural spacing. This investigation led to positive results, as the highest accuracies of the trained modules (in 80% of the database) were found to be above ~82% (UTS and EF) and a maximum of ~98% (YTS), when analyzing the results to a test data set. Later, these models were used to define trends for possible next solder alloy commercial compositions. Overall, using the standard model’s setup, the Random Forest and Decision Tree models showed the highest accuracy results, with 0.958 for YTS as opposed to 0.907 for MLR. Moreover, Multilayer Perceptron (MLP)-optimized models yielded the best results for each variable, with the highest increases in accuracy associated with the YTS and EF. The present contribution might imply an important milestone towards alloy design research based on data science guidelines to unlock the full potential of former experiments and their extensive set of results.

In the present study, the main purpose is to examine the quality of the interface formed between either binary Sn-Bi or ternary Sn-Bi-2wt.%Ga Thermal Interface Material-TIM alloys and a copper (Cu) substrate. The characteristics of the formed intermetallic compound (IMC) films were analyzed in the as-soldered joints, as well as during isothermal heat treatments at 100°C for long periods of time. Thermo-Calc computations were also used in return for a better comprehension of the mechanisms involved, including, for instance, those related to the presence of Ga within the interdendritic regions. This can be explained by the increase in the Ga-amount in the liquid-phase with the progress of solidification. In an effort to address the wettability issue, the contact angles between the molten alloy and the Cu substrate were also determined. Finally, tensile tests were performed in order to evaluate the effects of the minor Ga additions in the alloy's overall mechanical properties. Despite unsound alloy mechanical properties, gallium (Ga) additions effectively suppress the formation of the CuSn intermetallic by replacing it with a Cu 9 Ga 4 IMC layer in the as-soldered material, which grew with less deleterious morphology and finer thicknesses.