Microstructure control in Sn-0.7mass%Cu alloys

Soldering alloys based oft the Sn-Cu alloy system are amongst the most favourable lead-free alternatives due to a range of attractive properties. Trace additions of Ni have been found to significantly improve the soldering characteristics of these alloys (reduced bridging etc.). This paper examines the mechanisms underlying the improvement in soldering properties of Sn-0.7 mass%Cu eutectic alloys modified with concentrations of Ni ranging front 0 to 1000 ppm. The alloys were investigated by thermal analysis during solidification, as well as optical/SEM microanalyses of fully solidified samples anti samples quenched during solidification. It is concluded that Ni additions dramatically alter the nucleation patterns and solidification behaviour of the Sn-Cu6Sn5 eutectic anti that these changes are related to the superior soldering characteristics of the Ni-modified Sn-0.7 mass%Cu alloys.

A rheological assessment of the effect of trace level Ni additions on the solidification of Sn-0.7Cu

The influence of trace level Ni additions on the eutectic solidification mode of Sn-0.7Cu has been studied using continuous torque experiments during solidification. The solid fraction at which resistance to paddle rotation at the thermal centre of the sample occurs is related to the spatial distribution of solid during solidification. The results indicate that a transition in solidification mode occurs in the range 0-300 ppm Ni. Growth occurs antiparallel to heat flow from near the mould walls in the Ni-free alloy, while equiaxed growth from distributed centres dominates in alloys containing at least 300 ppm Ni. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Evaluation of silicon twinning in hypo-eutectic Al-Si alloys

It is generally accepted that growth of eutectic silicon in aluminium-silicon alloys occurs by a twin plane re-entrant edge (TPRE) mechanism. It has been proposed that modification of eutectic silicon by trace additions occurs due to a massive increase in the twin density caused by atomic effects at the growth interface. In this study, eutectic microstructures and silicon twin densities in samples modified by elemental additions of barium (Ba), calcium (Ca), yttrium (Y) and ytterbium (Yb) (elements chosen due to a near-ideal atomic radii for twinning) in an A356.0 alloy have been determined by optical microscopy, thermal analysis, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Addition of barium or calcium caused the silicon structure to transform to a fine fibrous morphology, while the addition of yttrium or ytterbium resulted in a refined plate-like eutectic structure. Twin densities in all modified samples are higher than in unmodified alloys, and there are no significant differences between fine fibrous modification (by Ba and Ca) and refined plate-like modification (by Y and Yb). The twin density in all modified samples is less than expected based on the predictions by the impurity induced twining model. Based on these results it is difficult to explain the modification with Ba, Ca, Y and Yb by altered twin densities alone.

Effects of boron-strontium interactions on eutectic modification in Al-10 mass%Si alloys

The effects of boron and strontium interactions on the eutectic silicon in hypoeutectic Al-Si alloys have been studied. Samples were prepared from an AI-I 0 mass%Si base alloy with different Al-B additions, alone and in combination with strontium. In alloys containing no strontium, boron additions do not cause modification of the eutectic silicon, while in strontium containing alloys, boron additions reduce the level of modification of the eutectic silicon. Thermal analysis parameters and eutectic silicon microstructures were investigated with respect to the Sr to B ratio. In order to modify the eutectic silicon, a Sr/B ratio exceeding 0.4 is required.

Effects of boron on eutectic modification of hypoeutectic Al-Si alloys

The effects of boron on the eutectic modification and solidification mode of hypoeutectic Al-Si alloys have been studied adding different boride phases. The results show that boron does not cause modification of the eutectic silicon. Boron-containing samples display eutectic nucleation and growth characteristics similar to that of unmodified alloys. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Damage by eutectic particle cracking in aluminum casting alloys A356/357

The strain dependence of particle cracking in aluminum alloys A356/357 in the T6 temper has been studied in a range of microstructures produced by varying solidification rate and Mg content, and by chemical (Sr) modification of the eutectic silicon. The damage accumulates linearly with the applied strain for all microstructures, but the rate depends on the secondary dendrite arm spacing and modification state. Large and elongated eutectic silicon particles in the unmodified alloys and large pi-phase (Al9FeMg3Si5) particles in alloy A357 show the greatest tendency to cracking. In alloy A356, cracking of eutectic silicon particles dominates the accumulation of damage while cracking of Fe-rich particles is relatively unimportant. However, in alloy A357, especially with Sr modification, cracking of the large pi-phase intermetallics accounts for the majority of damage at low and intermediate strains but becomes comparable with silicon particle cracking at large strains. Fracture occurs when the volume fraction of cracked particles (eutectic silicon and Fe-rich intermetallics combined) approximates 45 pct of the total particle volume fraction or when the number fraction of cracked particles is about 20 pct. The results are discussed in terms of Weibull statistics and existing models for dispersion hardening.

Eutectic modification of Al-Si alloys with rare earth metals

The effects of different concentrations of individual additions of rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) on eutectic modification in Al-10mass%Si has been studied by thermal analysis and optical microscopy. According to the twin-plane re-entrant edge (TPRE) and impurity induced twinning mechanism, rare earth metals with atomic radii of about 1.65 times larger than that of silicon, are possible candidates for eutectic modification. All of the rare earth elements caused a depression of the eutectic growth temperature, but only Eu modified the eutectic silicon to a fibrous morphology. At best, the remaining elements resulted in only a small degree of refinement of the plate-like silicon. The samples were also quenched during the eutectic arrest to examine the eutectic solidification modes. Many of the rare-earth additions significantly altered the eutectic solidification mode from that of the unmodified alloy. It is concluded that the impurity induced twinning model of modification, based on atomic radius alone, is inadequate and other mechanisms are essential for the modification process. Furthermore, modification and the eutectic nucleation and growth modes are controlled independently of each other.

Eutectic solidification and its role in casting porosity formation

Understanding and controlling the eutectic solidification process in Al-Si alloys permits prediction of the formation of casting porosity, eventually leading to methods for its control and elimination. In addition, it enables control of eutectic structure, silicon morphology, and eutectic grain size to further improve the alloy properties. This paper presents the current understanding of eutectic solidification in hypoeutectic Al-Si foundry alloys and the relationship between eutectic solidification and porosity formation. New concepts in engineering eutectic solidification are also explored.

Eutectic nucleation in Al-Si alloys

In addition to a change in silicon morphology, modification of aluminium-silicon alloys with strontium or sodium increases the size of the eutectic grains. To determine the mechanism responsible, eutectic solidification in commercial purity and ultra-high purity aluminium-si I icon alloys, with and without strontium additions, was examined by a quenching technique. In the commercial unmodified alloy, nucleation was prolific while in the high-purity unmodified alloy few eutectic grains nucleated. The addition of strontium to the commercial alloy reduced the number of eutectic grains that nucleated. Addition of strontium to the high-purity alloy did not significantly alter nucleation. It is concluded that commercial purity alloys contain a large number of potent nuclei that are susceptible to poisoning by impurity modification. The flake-to-fibre transition that occurs with impurity modification is shown to be independent of any change in eutectic nucleation mode and frequency. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Aluminium phosphide as a eutectic grain nucleus in hypoeutectic Al-Si alloys

Aluminium phosphide (AlP) particles arc often suggested to be the nucleation site for eutectic silicon in Al-Si alloys, since both the crystal structure and lattice parameter of AlP (crystal structure: cubic K(4) over bar m; lattice parameter: 5.421 Angstrom) are close to that of silicon (cubic Fd3m, 5.431 Angstrom), and the melting point is higher than the Al-Si eutectic temperature. However, the crystallographic relationships between AlP particles and the surrounding eutectic silicon are seldom reported due to the difficulty in analysing the AlP particles, which react with water during sample preparation for polishing. in this study, the orientation relationships between AlP and Si are analysed by transmission electron microscopy using focused ion-beam milling for sample preparation to investigate the nucleation mechanism of eutectic silicon on AlP. The results show a clear and direct lattice relationship between centrally located AlP particles and the surrounding silicon in the hypoeutectic Al-Si alloy.

Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys

Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.