In-situ X-ray Video Microscopy Studies of Al-Si Eutectic Solidification

In-situ studies of Al-Si eutectic growth has been carried out for the first time by X-ray video microscopy during directional solidification of Al-Si-Cu alloys with and without Sr-addtions. The unmodified eutectics showed distinctive non-isothermal growth dynamics, where Si-crystals attained needle-like tip morphologies and progressed under significantly higher undercooling than Al, leading to formation of an irregular eutectic with Si as the leading phase and subsequent nucleation of Al on the Si-surfaces. In the Sr-modified alloys, the eutectic reaction was found to be strongly suppressed, occurring with low nucleation frequencies at undercoolings in the range 10-18 K. In the Cu-enriched melt, the eutectic front was found to attain meso-scale interface perturbations evolving into equiaxed cellular rosettes in order to accommodate to the long-range redistribution of Cu from the composite eutectic interface. The eutectic front also attained short-range microscale interface perturbations consistent with characteristics of a fibrous Si growth, however further improvements in spatial resolution is required in order to study microscale structure formation in greater detail. Evidence was found in support of Si-nucleation occurring on potent particles suspended in the melt. Yet, both with Sr- modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form on the Si-particles. To what extent nucleation mechanisms in the Cu-enriched systems are transferable to binary or commercial Al-Si alloys remains uncertain

Nucleation and Growth Dynamics of the α-Al / β-Al5FeSi Eutectic in a Complex Al-Si-Cu-Fe Alloy

Secondary-sourced recycled aluminium alloys can exhibit high levels of different impurities. It is well known that the presence of iron, the most common impurity, can lead to the formation of hard and brittle intermetallic phases which are detrimental to the machining properties and the mechanical behaviour of the material in service. The purpose of this work is to study the nucleation and growth of the β-Al5FeSi intermetallic phase in the framework of the eutectic reaction: Liquid → α-Al + β-Al5FeSi. In situ X-ray microtomography has been used to investigate the formation of the irregular eutectic β-phase plates during the solidification at low cooling rate of an Al-8Si-4Cu-0.8Fe alloy. The results show that only a few plates form, nucleating early near the sample surface. Next, growth occurs very rapidly in the principal growth direction and slowly in the thickness direction. The plates are highly branched and appear to form as a divorced eutectic, i.e. not coupled with the α-Al. These features are inconsistent with the common irregular eutectic solidification theory based on the Jackson and Hunt model

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

Treatment outcome of new culture positive pulmonary tuberculosis in Norway

BACKGROUND: The key elements in tuberculosis (TB) control are to cure the individual patient, interrupt transmission of TB to others and prevent the tubercle bacilli from becoming drug resistant. Incomplete treatment may result in excretion of bacteria that may also acquire drug resistance and cause increased morbidity and mortality. Treatment outcome results serves as a tool to control the quality of TB treatment provided by the health care system. The aims of this study were to evaluate the treatment outcome for new cases of culture positive pulmonary TB registered in Norway during the period 1996–2002 and to identify factors associated with non-successful treatment. METHODS: This was a register-based cohort study. Treatment outcome was assessed according to sex, birthplace, age group, isoniazid (INH) susceptibility, mode of detection and treatment periods (1996–1997, 1998–1999 and 2000–2002). Logistic regression was also used to estimate the odds ratio for treatment success vs. non-success with 95% confidence interval (CI), taking the above variables into account. RESULTS: Among the 655 patients included, the total treatment success rate was 83% (95% CI 80%–86%). The success rates for those born in Norway and abroad were 79% (95% CI 74%–84%) and 86% (95% CI 83%–89%) respectively. There was no difference in success rates by sex and treatment periods. Twenty-two patients (3%) defaulted treatment, 58 (9%) died and 26 (4%) transferred out. The default rate was higher among foreign-born and male patients, whereas almost all who died were born in Norway. The majority of the transferred out group left the country, but seven were expelled from the country. In the multivariate analysis, only high age and initial INH resistance remained as significant risk factors for non-successful treatment. CONCLUSION: Although the TB treatment success rate in Norway has increased compared to previous studies and although it has reached a reasonable target for treatment outcome in low-incidence countries, the total success rate for 1996–2002 was still slightly below the WHO target of success rate of 85%. Early diagnosis of TB in elderly patients to reduce the death rate, abstaining from expulsion of patients on treatment and further measures to prevent default could improve the success rate further

Nucleation and grain refinement

The nucleation dynamics and grain refinement during solidification are reviewed. Control of the nucleation of phases is an essential step in the manufacture of optimized materials. Development of inoculants is still based on trial-and-error. Using aluminium as an example, it is shown that there is much room for significant improvements. Areas for future research are identified and important materials with an urgent need for grain refiners are shown

Effects of Sr and B interactions in hypoeutectic Al-Si foundry alloys

Strontium is the most widely used and a very effective element for modifying the morphology of eutectic silicon, while boron is commonly present in commercial grain refiners used for Al-Si alloys. Recent work on the combined additions of Sr and Al-B master alloys and of Sr and Al5Ti1B master alloy has suggested that negative interactions occur between Sr and B added through the grain refiners. However, the effects of and mechanisms for such negative interactions are not fully understood. This paper documents the experimental work and results aimed at determining the effects of Sr and B interactions on the solidification of hypoeutectic Al-Si foundry alloys. The mechanisms responsible for such negative interactions are further discussed

Settling behaviour of different grain refiners in aluminium

Grain refiner is added to the melt in order to introduce particles which are effective nucleant substrates for the primary aluminium phase. The most widely used grain refiners are based on the Al-Ti-B system. Depending on the composition of the master alloy, TiB, (Al,Ti)B and AlTi particles can be introduced. Ideally, the nucleant particles should be homogeneously distributed throughout the melt to ensure a large number of nucleation events and a uniform microstructure. However, a difference in density exists between the grain refining particles (TiB density ∼4.5 g/cm) and the melt (pure aluminium density ∼2.3 g/cm) and therefore the particles sink, making it difficult to maintain a homogeneous distribution of grain refining particles. Particle characteristics (size and density) strongly influence the settling behaviour when the master alloy is diluted in the melt. This paper reports on the behaviour of three grain refiner master alloys containing particles of different size distribution and composition. In all three cases, it was found that the particles settled quite quickly and agglomeration of the refining particles also played an important role in the rate of settling

Refinement of eutectic silicon in hypoeutectic Al-Si alloys with calcium addition

The effect of Ca as a refiner for eutectic silicon in Al-Si alloys was investigated. Addition of Ca in excess of 100ppm transforms eutectic silicon from plate-like to a fine fibrous morphology. Increasing Ca contents causes a depression of the eutectic arrest temperature and a significant decrease in the nucleation frequency of Al-Si eutectic cells. In alloys well-modified by Ca addition, few eutectic cells nucleate independently of the dendrites in the inter-dendritic liquid and they grow with a strong dependency on the thermal gradient. This reduction in eutectic nucleation frequency has been suggested to be one of the mechanisms responsible for modification of the Al-Si eutectic. In this work it was found that Ca forms polygonal Al Si Ca intermetallics where phosphorus is also found to be localized. This suggests that Ca plays a key role in poisoning AlP nuclei for eutectic silicon and thereby reducing the number of Al-Si eutectic cells that nucleate. The interaction between Ca and aluminium grain refiner, ie. Al-Ti-B master alloy, was also investigated