Browsing by Author "S.N. Tewari"

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Cellular/dendritic array tip morphology during directional solidification of Pb-5.8 Wt Pct Sb alloy
(Springer Boston, 1999) L. Yu; G.L. Ding; J. Reye; S.N. Ojha; S.N. Tewari
Cellular/dendritic array tip morphology has been examined in directionally solidified and quenched Pb-5.8 wt pct Sb alloy by a serial sectioning and three-dimensional image reconstruction technique. There is a large scatter in the tip radius, the nearest neighbor spacing, and the mushy zone length, even among the immediately neighboring cells and dendrites. This scatter may be caused by the natural convection (in the mushy zone and in the bulk melt at the array tip), which also produces macrosegregation along the length of the directionally solidified samples. Even in the presence of convection, however, the tip radii are observed to be approximately proportional to the square of the primary spacings, and the radii are in a good quantitative agreement with the predictions from the model due to Hunt-Lu.
Effect of growth rate on macrosegregation during directional solidification of a Pb-Sb alloy
(Maney Publishing, 2000) S.N. Ojha; S.N. Tewari
Directional solidification experiments on Pb-5.8 wt% Sb alloy have been carried out over growth rates varying from 0.8 to 20 μm s-1 in a positive temperature gradient of 140 Kcm-1. The microstructural examination of this alloy composition indicated cellular to dendritic transition at a growth rate of 1.5 μm s-1. The primary arm spacing was observed to decrease with growth rate in dendritic solidification regime. The maximum primary arm spacing occurred at the growth rate corresponding to the cellular to dendritic transition. Chemical analysis data revealed large amounts of macrosegregation in the longitudinal section of the alloy. A decrease in growth rate resulted in an increased degree of macrosegregation. These effects are discussed in light of thermo-solutal convection in the melt ahead of the solid-liquid interface during directional solidification of this alloy. A parameter ke is used to represent the extent of convection in the melt and the consequent degree of macrosegregation of the alloy with varying growth rates for solidification with a cellular-dendritic interface in the same way as it has been earlier used for the planar-front solidification condition.
Macrosegregation caused by thermosolutal convection during directional solidification of Pb-Sb alloys
(Minerals, Metals & Materials Soc (TMS), 1999) S.N. Ojha; G. Ding; Y. Lu; J. Reye; S.N. Tewari
Pb-2.2 and 5.8 wt pct Sb alloys were directionally solidified with a positive thermal gradient of 140 K cm-1 at growth speeds ranging from 0.8 to 30 μm s-1, and then quenched to retain the mushy-zone morphology. Chemical analysis along the length of the directionally solidified portion and in the quenched melt ahead of the dendritic array showed extensive longitudinal macrosegregation. Cellular morphologies growing at smaller growth speeds are associated with larger amounts of macrosegregation as compared with the dendrites growing at higher growth speeds. Convection is caused, mainly, by the density inversion in the overlying melt ahead of the cellular/dendritic array because of the antimony enrichment at the array tip. Mixing of the interdendritic and bulk melt during directional solidification is responsible for the observed longitudinal macrosegregation.
Macrosegregation during directional solidification of Pb-Sb alloys
(Indian Institute of Metals, 2009) S.N. Ojha; S.N. Tewari
Macrosegregation of Sb was investigated during directional solidification of binary Pb-Sb alloys containing 2.2 and 5.8 wt% Sb over growth rates varying from 0.8 to 30 μm s-1. The cellular to dendritic transition was observed at a growth rate of 3.0 μm s-1 in Pb-2.2 Sb alloy in contrast to a growth rate of 1.5 μm s-1 in Pb-5.8 Sb alloy. The chemical analysis data revealed considerable macrosegregation of Sb along the longitudinal section of alloys. The degree of macrosegregation increased with a decrease in the growth rate. This behavior is discussed in light of thermo-solutal convection in the mushy zone as well as that in the melt ahead of the solid-liquid interface. © 2009 TIIM, India.
Mushy zone characteristics and macrosegregation during directional solidification
(2004) S.N. Ojha; S.N. Tewari
Directional solidification of an alloy containing Pb-5.8 wt% Sb has been carried out over growth rates varying from 1 to 10 μm s-1 in a positive temperature gradient of 140 Kcm-1. The microstructural examination indicated cellular to dendritic transition at a growth rate of 1.5 μm s-1. An analysis of composition revealed a large macrosegregation of Sb along the longitudinal section of the directionally solidified alloys. The degree of macrosegregation was observed to increase with a decrease in the growth rate during solidification of the alloy. The microstructural analysis indicated a progressive increase in volume fraction of the solid phase, variation in shape factor of cells/dendtrites and decrease in the hydraulic radius as a function of distance from the solid-liquid interface in the mushy zone. These factors control the permeability of the mushy zone and resultant macrosegregation of the alloy. The macrosegregation behaviour of the alloy is discussed in light of thermosolutal convection in the mushy zone as well as in the melt ahead of the solid-liquid interface during directional solidification.
Mushy zone morphology during directional solidification of Pb-5.8 wt pet Sb alloy
(Springer Boston, 2000) L. Yu; G.L. Ding; J. Reye; S.N. Ojha; S.N. Tewari
The Pb-5.8 wt pct Sb alloy was directionally solidified with a positive thermal gradient of 140 K cm-1 at a growth speed ranging from 0.8 to 30 μm s-1, and then it was quenched to retain the mushy zone morphology. The morphology of the mushy zone along its entire length has been characterized by using a serial sectioning and three-dimensional image reconstruction technique. Variation in the cellular/dendritic shape factor, hydraulic radius of the interdendritic region, and fraction solid along the mushy zone length has been studied. A comparison with predictions from theoretical models indicates that convection remarkably reduces the primary dendrite spacing while its influence on the dendrite tip radius is not as significant.