Grain refinement leads, in general, to a decreased tendency to hot tearing, a more dispersed and refined porosity distribution, and improved directional feeding characteristics during solidification. Reduced as-cast grain size can also lead to improved mechanical properties and wrought processing by reducing the recrystallized grain size and achieving a fully recrystallized microstructure. It is now well established that the two key factors controlling grain refinement are the nucleant particles including their potency, size distribution and particle number density, and the rate of development of growth restriction, Q, generated by the alloy chemistry which establishes the undercooling needed to trigger nucleation events and facilitates their survival. The theories underpinning our current understanding of nucleation and grain formation are presented. The application of the latest theories to the light alloys of Al, Mg and Ti is explored as well as their applicability to a range of casting and solidification environments. In addition, processing by the application of physical processes such as external fields and additive manufacturing is discussed.To conclude, the current challenges for the development of reliable grain refining technologies for difficult to refine alloy systems will be presented.

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The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller, Jackson, Rutter, and Chalmers (1953). On that basis, Winegard and Chalmers (1954) further considered 'supercooling and dendritic freezing of alloys' where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

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a b s t r a c tThe influence of alloying on the ignition and flammability was studied. One end of a cylindrical specimen was exposed to a free diffusion flame. Ignition required at least partial melting. Burning extinguished once the flame was withdrawn. Specimen tips of pure Mg, AZ61, and AZ91 ignited upon prolonged flame exposure. There was smouldering and delayed ignition for Mg-1Y. There was no ignition for Mg-5Y specimen tips, attributed to a protective surface oxide containing Y. The results indicate that (i) vigorous burning requires a continued supply of Mg vapour, and (ii) a critical alloy concentration is required to change ignition behaviour.

Five randomised controlled trials show that warfarin reduces the risk of ischaemic strokes and death in patients with atrial fibrillation.' We suspected that many such patients were not receiving warfarin and therefore reviewed the records of previous inpatients with atrial fibrillation to see how many were receiving warfarin or aspirin and whether those who were not had a specific contraindication. Patients, methods, and resultsWe performed a retrospective analysis of case notes of patients admitted from January 1991 to September 1992. The clinical coding department provided a list of 726 admissions (in random order) in which the patient was discharged with a diagnosis of atrial fibrillation; the first 100 notes were requested, of which 95 were available.Fifty seven patients were drawn from general medical firms and the rest from the departments of cardiology (17), geriatrics (14), and surgery (seven). Forty eight were men. The median age was 79 (range 32-100). Nineteen patients had paroxysmal atrial fibrillation. The median duration of atrial fibrillation was 20 (4-226) months and it most commonly had an underlying cardiovascular cause (table).Cause ofatnalfibriUation in 95patients

This work reports the results of real-time X-ray radiography of grain refined and unrefined Al-Si alloys solidified at the SPring-8 synchrotron. The nucleation events were observed and the grain density and growth rate following nucleation were measured. Nucleation of the grain refined alloy samples occurred from the coolest to the hottest parts of the field of view in a sequence that mimicked a forward moving wave. No additional nucleation events occurred between the first nucleated grains and there was no evidence of grains being generated by fragmentation. Measurements of both grain density and growth rate show the effect of Si content and grain refiner (Al3Ti1B master alloy) additions on grain size. While the total number of grains increases in the alloys with added Al3Ti1B master alloy, the growth rates tend to be slower. Furthermore, the growth rate for all alloy compositions fluctuates between slow and fast velocities during the initial stages of growth and then tends towards a low steady state value. This decreasing trend is explained in terms of thermal and solutal field interactions between adjacent growing grains and subsequent grain impingement. These measurements result in a better understanding of the role of nucleation and solute content in influencing further nucleation and the subsequent change in the solid-liquid growth rate.

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