Materials Engineering

and Lu) on eutectic modification in Al-10 mass%Si has been studied by thermal analysis and optical microscopy. According to the twin-plane reentrant 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.

Directional solidification of unmodified and strontium modified binary, high-purity, aluminium-7 wt% silicon and commercial A356 alloys has been carried out to investigate the mechanism of eutectic solidification. The microstructure of the eutectic growth interface was investigated with optical microscopy and Electron Backscattering Diffraction (EBSD). In the commercial alloys, the eutectic solidification interface extends in the growth direction and creates a eutectic mushy zone. A planar eutectic growth front is observed in the high-purity alloys. The eutectic aluminium has mainly the same crystallographic orientation as the dendrites in the unmodified alloys and the strontium modified highpurity alloy. A more complex eutectic grain structure is found in the strontium modified commercial alloy. A mechanism involving constitutional undercooling and a columnar to equiaxed transition explains the differences between pure and commercial alloys. It is probably caused by the segregation of iron and magnesium and the activation of nucleants in the commercial alloy.

Cu 6 Sn 5 is a critical intermetallic compound in soldering and three-dimensional integrated circuit packaging technology and exists in at least five different crystal structures in the solid state, with a polymorphic phase transformation from hexagonal to monoclinic structures occurring on cooling. The kinetics of polymorphic transformations in Sn-rich Cu 6 Sn 5 and Cu-rich Cu 6 Sn 5 is systematically investigated in this study. This includes the generation of continuous cooling transformation diagrams as well as time-temperature transformation diagrams. Techniques used include variable temperature synchrotron powder X-ray diffraction and differential scanning calorimetry. The findings have important implications for the manufacture of solder joints and their in-service performance.

We investigate how 5 at. % Ni influences the relative stability of and Ј Cu 6 Sn 5. Synchrotron x-ray diffraction shows that, while Cu 6 Sn 5 exists as Ј at 25 and 150°C and transforms to on heating to 200°C, Cu 5.5 Ni 0.5 Sn 5 is best fit to throughout 25-200°C. Our first principles calculations predict that Ј is stable at T = 0 K in both Cu 6 Sn 5 and Cu 5.5 Ni 0.5 Sn 5 , but that the energy difference is substantially reduced from 1.21 to 0.90 eV per 22 atom cell by the Ni addition. This effect is attributed to Ni developing distinct bonding to both Cu and Sn in the phase.

Recently it has been shown that modification with strontium causes an increase in the size of eutectic grains. The eutectic grain size increases because there are fewer nucleation events, possibly due to the poisoning of phosphorus-based nuclei that are active in the unmodified alloy. The current paper investigates the effect of strontium concentration on the eutectic grain size. In the aluminium-10 wt.% silicon alloy used in this research, for fixed casting conditions, the eutectic grain size increases as the strontium concentration increases up to approximately 150 ppm, beyond which the grain size is relatively stable. This critical strontium concentration is likely to differ depending on the composition of the base alloy, including the concentration of minor elements and impurities. It is concluded that processing and in-service properties of strontium modified aluminium-silicon castings are likely to be more stable if a minimum critical strontium concentration is exceeded. If operating below this critical strontium concentration exceptional control over composition and casting conditions is required.

Aluminum Alloys Overview Understanding and controlling the eutectic solidifi cation 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 solidifi cation and porosity formation. New concepts in engineering eutectic solidifi cation are also explored.

In order to elucidate fuel wash-out behavior after the degraded secondary failure of LWR fuel rods, corrosion experiments in high temperature and high pressure water of 340°C and 15 MPa were conducted for unirradiated oxide fuels, two each of undoped UO 2 with two different grain sizes, 5 wt% Gd203-and 10 wt% Gd203-doped UO2. In degassed water of dissolved oxygen concentration (DO) equal to 10 ppb, no corrosion or oxidation was found, but the corrosion became noticeable above DO = 3 ppm. Pellet oxidation proceeded with U40 9 formation along the grain boundaries and subsequent U409 growth into enveloped UO 2 grains. Consequently, the grain boundary penetration rate in the oxygenated water followed a diffusion-controlled parabolic law. Mismatch in the O/U ratio between the grain boundaries and interiors led to weakened boundaries, which is the cause of fuel wash-out in heavily degraded fuel rods. Both increases of grain size and Gd20 3 content were found to provide improved corrosion resistance.

Plate-type specimens were cut from three different types of tungsten sheets fabricated under distinct rolling and heat treatment conditions in the directions parallel or perpendicular to the rolling planes. Amorphous carbon films were prepared by vacuum deposition on the specimen surfaces. Then, the specimens were heated at 1073 K in vacuum, and reaction products were analyzed by means of X-ray diffraction. The carbide formed was W 2 C and no peak of WC appeared. The growth rate of W 2 C was independent of the cutting directions of the specimens, although the grain boundary densities at the specimen surfaces were quite different. On the other hand, the rate of W 2 C growth was dependent on both reduction and heat treatment conditions, and a specimen with higher hardness showed a higher growth rate. By taking account of the observations by a transmission electron microscope, it was concluded that dislocations play important roles in W 2 C growth.

Nucleation and growth of the eutectic in hypoeutectic Al-Si foundry alloys has been investigated by the electron backscatter diffraction (EBSD) mapping technique using a scanning electron microscope (SEM). Sample preparation procedures for optimizing mapping have been developed. To obtain a sufficiently smooth surface from a cast Al-Si eutectic microstructure for EBSD mapping, an appropriate preparation technique by ion milling was developed and applied instead of conventional electropolishing. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites, the growth mechanism of the eutectic can be determined. Two different results were found, in isolation or sometimes together, but distinct for different strontium contents: (1) crystallographic orientations of aluminum in eutectic and surrounding primary dendrites are identical, and (2) wide variation in orientations of the aluminum in the eutectic.

Recent increasing applications for cast Al-Si alloys are particularly driven by the need for lightweighting components in the automotive sector. To improve mechanical properties, elements such as strontium, sodium and antimony can be added to modify the eutectic silicon from coarse and plate-like to fine and fibrous morphology. It is only recently being noticed that the morphological transformation resulting from eutectic modification is also accompanied by other, equally significant, but often unexpected changes. These changes can include a 10-fold increase in the eutectic grain size, redistribution of low-melting point phases and porosity as well as surface finish, consequently leading to variations in casting quality. This paper shows the state-of-the-art in understanding the mechanism of eutectic nucleation and growth in Al-Si alloys, inspecting samples, both quenched and uninterrupted, on the macro, micro and nano-scale. It shows that significant variations in eutectic nucleation and growth dynamics occur in Al-Si alloys as a function of the type and amount of modifier elements added. The key role of AlP particles in nucleating silicon is demonstrated.

Soldering alloys based on 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 from 0 to 1000 ppm. The alloys were investigated by thermal analysis during solidification, as well as optical/SEM microanalyses of fully solidified samples and samples quenched during solidification. It is concluded that Ni additions dramatically alter the nucleation patterns and solidification behaviour of the Sn-Cu 6 Sn 5 eutectic and that these changes are related to the superior soldering characteristics of the Ni-modified Sn-0.7 mass%Cu alloys.

This paper investigates the relationship between mechanical properties and microstructure in high pressure die cast binary Mg-Al alloys. As-cast test bars produced using high pressure die casting have been tested in tension in order to determine the properties for castings produced using this technique. It has been shown that increasing aluminium levels results in increases in yield strength and a decrease in ductility for these alloys. Higher aluminium levels also result in a decrease in creep rate at 150 C. It has also been shown that an increase in aluminium levels results in an increase in the volume fraction of eutectic Mg 17 Al 12 in the microstructure.

Phosphorus is often added to wave-solder baths as an anti-oxidation agent. Despite this practice, there is little information on how phosphorus influences the solidification and flow properties of new lead-free solders such as Sn-0.7Cu-0.05Ni. This paper investigates the effects of phosphorus content on microstructure and maximum fluidity length in Sn-0.7Cu-0.05Ni-xP alloys containing 0-0.08 mass% phosphorous. Ppm levels of phosphorous are found to cause Sn-xP, Ni-xP-(Sn) and Cu-xP-(Sn) intermetallic compounds to form in the liquid solder. The IMCs are less dense than liquid Sn and float towards the surface of the melt driven by buoyancy. It is shown that P-free Sn-0.7Cu-0.05Ni solidifies with a near-eutectic microstructure whereas, when P is added to this alloy, a significant volume fraction of primary Sn dendrites form once the P content exceeds $0:01 mass% P. It is further shown that P additions decrease the ability of Sn-0.7Cu-0.05Ni to flow as it solidifies.

The minimum Ni concentration required to prevent the g to g 4+1 polymorphic transformation of stoichiometric Cu 6 Sn 5 was found to be as low as 1 at.%. This is crucial in lead-free solder joints, especially in the ball grid array packages, in which only limited Ni can be supplied by micrometer-size solder balls. The Ni stabilization effect is present over a large temperature range with no phase transformation occurring between À120 and 250°C for the timeframes used in this study.

Microtome sections of proton exchange membrane cells produce a wide range of information ranging from macroscopic distribution of components through specimens in which the detailed distribution of catalyst particles can be observed. Using modern data management practices it is possible to combine information at different scales and correlate processing and performance data. Analytical electron microscopy reveals the compositional variations across used cells at the electrolyte/electrode interface. In particular analytical techniques indicate that sulphur concentrations are likely to diminish at the interface Nafion/anode interface.

It is widely accepted that the intermetallic layer that forms at the interface between a solder and a copper substrate has an effect on the properties of the solder joint, particularly under high strain rates such as occur in drop impact. The nature and extent of ...