Kazuhiro Nogita

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.

Formation and growth of intragranular fission gas bubbles in UO2 fuels with burnup of 6–83 GWd/t

Abstract The detailed characteristics of intragranular fission gas bubbles in UO2 fuel pellets (burnup: 6–83 GWd/t) before and after postirradiation annealing at 1600 and 1800°C have been examined by TEM and SEM fractography. In the base-irradiated fuels, a high density of small bubbles of about 2 nm in diameter precipitated uniformly in the matrix. When increasing burnup above 44 GWd/t, larger bubbles of 10–20 nm newly appeared in addition to the small bubbles. On heating at high temperatures, bubble growth was saturated within a few minutes. Enhanced coarsening of bubbles was found preferentially near the grain boundaries in the middle burnup fuels of 16–28 GWd/t and throughout the grains in the high burnup fuels of 44 and 83 GWd/t. The bubble growth during annealing was associated with a remarkable decrease of the bubble number density, and the relationship between bubble density Nb and mean diameter db was expressed as Nbαdb−2.6. The coarsening was attributed to coalescence via bubble migration for moderately large bubbles of up to 50–60 nm, and to Ostwald ripening accompanied by a sufficient vacancy supply from external vacancy sources such as free surfaces, grain boundaries or irradiation-induced sub-grain boundaries for huge bubbles above 100 nm.

Microstructural change and its influence on fission gas release in high burnup UO2 fuel

Abstract The microstructural change of UO2 fuel pellets (burnup: 6–83 GWd/t), base irradiated under LWR conditions, has been studied by detailed postirradiation examinations. The lattice parameter near the fuel rim in the irradiated UO2 increased with burnup and appeared to become constant beyond about 50 GWd/t. This lattice dilation was mainly due to the accumulation of radiation induced point defects. Moreover, the dislocation density in the UO2 matrix developed progressively with burnup, and eventually the tangled dislocations organized many sub-grain boundaries in the highest burnup fuel of 83 GWd/t. This sub-grain structure induced by accumulated radiation damage was compatible in appearance with SEM fractography results which revealed sub-divided grains of sub-micron size in as-fabricated grains. The influence of burnup on 85Kr release from the UO2 fuels has been examined by means of a postirradiation annealing technique. The higher fractional release of high burnup fuels was mainly due to the burnup dependence of the fractional burst release evolved on temperature ramp. The fractional burst release was represented in terms of the square root of burnup from 6 to 83 GWd/t.

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.

Crystallography of zirconium hydrides in recrystallized Zircaloy-2 fuel cladding by electron backscatter diffraction

Precipitation morphology and habit planes of the δ-phase Zr hydrides, which were precipitated within the a-phase matrix grains and along the grain boundaries of recrystallized Zircaloy-2 cladding tube, have been examined by electron backscatter diffraction (EBSD). Radially-oriented hydrides, induced by residual tensile stress, precipitated in the outside region of the cladding, and circumferentially-oriented hydrides in the stress-free middle region of the cladding. The most common crystallographic relationship for both types of the hydrides precipitated at the inter- and intra-granular sites was identical at (0001)α // {111}δ, with {10&1macr;7}α // {111}δ, being the occasional exception only for the inter-granular radial hydrides. When tensile stress was loaded, the intra-granular hydrides tended to preferentially precipitate in the grains with circumferential basal pole textures. The inter-granular hydrides tended to preferentially precipitate on the grain faces opposite to tensile axis. The change of prioritization in the precipitation sites for the hydrides due to tensile stress could be explained in terms of the relaxation effect of constrained elastic energy on the terminal solid solubility of hydrogen at hydride precipitation.

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

Columnar to equiaxed transition of eutectic in hypoeutectic aluminium–silicon alloys

Abstract 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 high-purity 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.

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.