Stuart D. McDonald

Modification of Al-Si alloys with Ba, Ca, Y and Yb

Abstract Modification of Al–Si alloys is known to result in a depression of the eutectic arrest temperature. It has been suggested that a larger depression is related to increased modification. The effects of different concentrations of separate additions of Ba, Ca, Y and Yb on the eutectic arrest in an A356.0 (Al–7%Si–Mg) alloy have been studied by thermal analysis. All of these elements cause a depression of the eutectic arrest, however Ba and Ca result in fibrous eutectic Si while Y and Yb result in a refined plate-like eutectic silicon. Analysis of the effects of the elements on eutectic nucleation and growth temperatures and the recalescence shows two different trends. Addition of Ba and Yb both causes linear changes with increased concentration, while addition of Ca and Y result in an instantaneous effect with the first addition and no further significant changes with increased concentration.

Development of high-temperature solders: Review

The development of lead-free solder alloys for high-temperature applications is required to meet increasing demands for reliable replacements for lead-containing alloys. This paper provides a review of recent research on suitable replacement alloys, as well as traditional Pb–Sn alloys, collating relevant properties and identifying areas where further development is required. The main candidate alloys covered are derived from the Au–Sn, Au–Ge, Zn–Al, Zn–Sn, Bi–Ag and Sn–Sb alloy systems. Each of these systems is discussed with respect to the advantages and disadvantages associated with their use in soldering applications. It is concluded that further development of alloys suitable for high-temperature lead-free soldering applications is required.

Mechanisms of eutectic solidification in Al–Si alloys modified with Ba, Ca, Y and Yb

Abstract The eutectic solidification mechanisms in an A356.0 (Al–7%Si–Mg) alloy modified by barium, calcium, yttrium and ytterbium have been determined. The crystallographic orientations of aluminium in the eutectic and the surrounding aluminium dendrites were measured by electron backscattering diffraction mapping, and samples were also quenched at different stages during the eutectic arrest and examined by optical microscopy. The combination of these two techniques shows that each of the elements added promote heterogeneous nucleation of eutectic grains in the interdendritic liquid, while the aluminium in the unmodified alloy grows epitaxially from the dendrites. Furthermore, calcium and yttrium result in a strong dependency of eutectic solidification on the thermal gradient, i.e. the eutectic evolves from the walls towards the centre of the sample on a macro-scale. These differences in eutectic solidification mode show a correlation with some thermal characteristics of the eutectic arrest.

Grain-refinement mechanisms in titanium alloys

Despite the importance of the prior-beta grain structure in determining the properties of titanium-based alloys, there are few published studies on methods of controlling the size of these grains in commercial alloys. The existing research raises questions about the relative importance of solute elements in grain-refining mechanisms, particularly the common alloying elements of aluminum and vanadium. The effect of these elements was investigated by producing a series of castings in a nonconsumable arc-melting furnace, and the results were interpreted with the aid of available phase-diagram information and solute-based models of grain refinement. A small reduction in grain size was obtained with increasing solute additions; however, this was not expected from the theoretical analysis. Possible reasons for this discrepancy are discussed.

Porosity formation in aluminium alloy A356 modified with Ba, Ca, Y and Yb

Abstract An increased level of porosity is normally reported to accompany modification of Al–Si alloys. In this study, the effects of additions of barium, calcium, yttrium and ytterbium on porosity formation in an A356.0 (Al–7%Si–Mg) alloy are examined. The permanent mould casting consisted of a vertical plate connected directly to a runner at a hot-spot junction. Casting defects observed include surface shrinkage and internal porosity in the hot spot and also hot tearing of the casting and runner. All additions increased the porosity level compared to the unmodified alloy and it increased with increased addition level. The results show that additions of Ca and Y caused porosity to become increasingly concentrated in the hot spot. Additions of Ba and Yb resulted in small, round, dispersed porosity. When porosity formation is considered based on the feeding mechanisms, particularly interdendritic feeding, it is possible to rationalise the effects of the elements on porosity distribution based on their impact on the eutectic solidification mode. In Ca and Y containing alloys the eutectic evolves from the surface towards the centre of the hot spot, while heterogeneous nucleation of eutectic grains across the hot spot occurs with additions of Ba and Yb.

The influence of solder composition on the impact strength of lead-free solder ball grid array joints

This study aims to investigate the shear and tensile impact strength of solder ball attachments. Tests were conducted on Ni-doped and non-Ni-doped Sn–0.7wt.% Cu, Sn–37wt.% Pb and Sn–3.0wt.% Ag–0.7wt.% Cu solder ball grid arrays (BGAs) placed on Cu substrates, which were as-reflowed and aged, over a wide range of displacement rates from 10 to 4000 mm/s in shear and from 1 to 400 mm/s in tensile tests. Ni additions to the Sn–0.7wt.% Cu solders has slowed the growth of the interface intermetallic compounds (IMCs) and made the IMC layer morphology smooth. As-reflowed Ni-doped Sn–0.7wt.% Cu BGA joints show superior properties at high speed shear and tensile impacts compared to the non-Ni-doped Sn–0.7wt.% Cu and Sn–3.0wt.% Ag–0.7wt.% Cu BGAs. Sn–3.0wt.% Ag–0.7wt.% Cu BGAs exhibit the least resistance in both shear and tensile tests among the four compositions of solders, which may result from the cracks in the IMC layers introduced during the reflow processes.

Solidification mechanisms of unmodified and strontium-modified hypereutectic aluminium-silicon alloys

The effects of strontium on the solidification mode of hypereutectic aluminium–silicon alloys have been studied. Samples were prepared from an aluminium–17 wt% silicon-based alloy and strontium was added at several different concentrations. The development of the microstructure was investigated by cooling curve analysis, interrupted solidification experiments and optical and scanning electron microscopy. It was found that nucleation of primary silicon is suppressed by additions of strontium. The suppressed nucleation results in supersaturation of the liquid prior to nucleation, and an increased growth rate after nucleation. As a result, the silicon crystals become less faceted and more dendritic with increasing strontium additions. Increasing the strontium concentration slightly refined the eutectic spacing and introduced a small amount of fibrous silicon. Electron back-scatter diffraction measurements were performed to determine the crystallographic relation between the primary and eutectic silicon phases. The eutectic silicon in the unmodified alloy does not have any crystallographic relationship with the primary silicon crystals. In contrast, the eutectic silicon crystals in the strontium-modified alloys often share an identical or twin relationship with nearby primary silicon crystals. The incidence of twinning within primary silicon crystals was relatively low and did not appear to increase with strontium additions.

Non-metal reinforced lead-free composite solder fabrication methods and its reinforcing effects to the suppression of intermetallic formation: short review

To increase the solder joint robustness, researches and studies on composite solder carried out by many researchers in an effort to develop viable lead-free solders which can replace the conventional lead-based solders.This paper reviews the fabrication processes of the lead-free composite solder and its non-metal reinforcing effects to the suppression of intermetallic formation. Most researchers using different solder fabrication methods have found that byadditions of non-metal reinforcement from micron up to nanoparticle size had suppressed the intermetallic compound formations of lead-free composite solders.

XRD study of the kinetics of β ↔ α transformations in tin

The transformation kinetics of βSn (white tin) to and from αSn (grey tin) are studied by synchrotron X-ray diffraction of seeded powder samples of 99.99% Sn. An analysis of thermal expansion behaviour revealed that the volume change of transformation increases as the temperature decreases. The βSn → αSn transformation was well described by Johnson–Mehl–Avrami kinetics with an Avrami exponent of 3, which was confirmed to be due to three-dimesional growth from pre-existing nuclei by a microstructural study. The αSn → βSn transformation exhibited a decreasing Avrami exponent from ∼4 to ∼1 during the transformation. Time–Temperature–Transformation curves are plotted from the isothermal transformation data and are compared with past work.

Effects of boron on microstructure in cast zirconium alloys

Trace additions of boron to cast zirconium result in significant microstructural changes similar to those observed with additions of boron to titanium alloys. These changes include the promotion of dendritic growth and a refinement in both the prior beta and alpha grain size. The refinement of the prior beta grain size is explained using a model of grain refinement in association with values calculated from the binary Zr-B phase diagram. It is proposed that the refinement of the alpha phase occurs through a combination of increased nucleation and altered diffusion mechanisms during cooling through the beta transus.