Growth temperatures and the limits of coupled growth in unidirectional solidification of Fe-C eutectic alloys
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1. I N T R O D U C T I O N A L T H O U G H the processes of solidification in cast irons have been studied now for several decades, the growth conditions governing formation of the various resulting morphologies are still neither fully defined nor properly understood. Steady state unidirectional growth studies 1-~4begun in the early sixties have shown that systematic changes in constitution, morphology and spacing occur with change of growth velocity and composition. Major impediments to the development of a quantitative model to account consistently for the main observations are i) the absence of direct determinations of growth temperatures of competing growth modes as a function growth conditions, ii) limited data (e.g. Ref. 5) on the effect of temperature gradient G, as opposed to growth velocity V, now known ~5,t6to significantly affect both growth temperature and spacing for irregular eutectics. Most of the available data 6,17-19on growth temperatures as a function of V has been derived from specimens quenched during isothermal growth, the operative V being estimated, subject to certain assumptions, from the amount of growth completed prior to the quench. The resulting plots of undercooling AT as a function of V show very substantial scatter (by as much as an order of magnitude in AT) and the comparatively limited data from the only
H. JONES is Senior Lecturer in Metallurgy, University of Sheffield, Sheffield, England. W. KURZ is Professor of Physical Metallurgy, Department of Materials Engineering, Swiss Federal Institute of Technology, Lausanne, Switzerland. Manuscript submitted August 14, 1979.
isovelocity growth studies available 6,17are in consistent disagreement by a factor of 5 in AT. The present aim is to report and interpret systematic measurements of growth temperature as a function of V for isovelocity unidirectional eutectic and competing austenite dendrite growth in an Fe-C alloy of near eutectic composition. The results and interpretation allow the conditions limiting coupled eutectic growth in Fe-C to be derived on a sound basis for the first time. A companion paper 2~will report corresponding measurements of eutectic spacing allowing the optimizing condition governing eutectic growth itself to be defined for the same system. 2. E X P E R I M E N T A L Alloy samples were made from 99.98 pct pure carbonyl iron in the form of 38 mm diam sintered and forged rod and spectrographic grade graphite (99.9997 pct pure) as rods 6mm in diam. These were vacuum induction melted together in a calcium oxide crucible and cast into clusters of alumina tubes to make cast rods 6 mm in diam and 150 mm long. Chemical analysis indicated 4.28 + 0.02 wt pct C, 0.035 _+ 0.005 wt pct S and 100 ___ 10 ppm Ca. These alloy rods were machined to ~5,7 mm diam to fit recrystallized alumina sheaths of 6 mm bore and 9 mm external diam for isovelocity unidirectional solidification. This was carried out vertically in a sillite rod tube furnace of 30 mm bore by lowering the sheath, containing the sample in a static argon atmosphe
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