Side branch morphology and coarsening in directionally solidified Pb-8.4 at. pct Au
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Side Branch Morphology and Coarsening in Directionally Solidified Pb-8.4 At. Pct Au S.N. TEWARI, V.V. NESARIKAR, and DAVID LEE
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A stable planar liquid-solid interface can be obtained during directional solidification of binary alloys in the presence of a sufficiently large positive thermal gradient. When the instability due to the solutal gradient in an alloy melt exceeds the stability provided by the combined effects of the positive thermal gradient and the liquid-solid surface energy, the planar liquid-solid interface breaks down. t~J This results in the formation of cellular microstructures. Formation of "unstable cells" occasionally has been observed for growth with very low gradients of constitutional supercooling ahead of the planar liquidsolid interface, t2i These unstable cells do not have the stable cellular morphology (i.e., they do not have the fixed shape, spacing, and cell length for a given growth condition) as observed for cellular microstructures. Instead, they appear to have a time-dependent and spatially varying morphology. At still higher growth speeds or lower thermal gradients, an increasing amount of constitutional supercooling develops in the intercellular regions, and perturbations (side branches) evolve on the sides of these cellular features near their tips, resulting
(c) Fig. 2--Typical substructures observed after cyclic deformation. All tests are fully reversed low-cycle fatigue (LCF) tests at a strain rate of 50 pct/min. In all cases g = (200), so that all stacking faults with (111) displacement vectors are imaged. (a) 20 ~ 6 cycles, Ayp = 0.12 pet. (b) 200 ~ 20 cycles, Ayp = 0.06 pct. (c) 400 ~ 52 cycles, Aye = 0.05 pet.
METALLURGICAL TRANSACTIONS A
S.N. TEWARI, Associate Professor, and V.V. NESARIKAR, Graduate Student, are with the Chemical Engineenng Department, Cleveland State University, Cleveland, OH
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