Date of Award

2017

Degree Type

Thesis

Degree Name

Master of Science in Chemical Engineering

Department

Washkewicz College of Engineering

First Advisor

Tewari, Surendra

Subject Headings

Chemical Engineering

Abstract

The purpose of this research is to examine the dendrite array morphology, macrosegregation, and defect formation caused by the fluid flow at the abrupt cross-section changes during directional solidification of Pb-6% Sb alloy. Four 24-cm long cylindrical alloy samples were directionally solidified in graphite crucibles: two having a constant diameter (9-mm) grown at 10.4 and 63.1 μm s-1 , one having an abrupt cross-section decrease (from 12.7 to 6.35 mm) and one having an abrupt increase (from 6.35 to 12.7 mm) by pulling down the alloy containing cylindrical graphite crucibles from the upper hot-zone of a stationary vertical furnace into its cold-zone below. Microstructures were examined on transverse slices cut along the length of the directionally solidified samples. Dendrite spacing and distribution were characterized on these transverse sections. The Pb-6% Sb alloy was selected as a low melting point analog for commercially used multicomponent nickel-base superalloys, because its thermophysical properties are well characterized. Also, a density inversion occurs in the inter-dendritic melt in the “mushy-zone” during directional solidification of this alloy, because the density of the melt decreases as Sb content increases from the array tips at the top of the mushy zone to the eutectic at their bottom. In constant cross-section crucibles, the formation of dendrite-trees in the mushy zone will be subject only to this “plume type” convection as solidification proceeds from the bottom end of the crucible to its top. Whereas in crucibles with abrupt cross-section change, the solidifying mushy-zone will be subject to additional “cross-section change induced” solidification shrinkage flow, when the speed of the liquid flowing downwards to feed the solidification shrinkage occurring below, will either suddenly accelerate or decelerate, because of the abrupt area change. This sudden change in the incoming fluid speed may break slender side-branches of dendrite trees. These broken dendrite fragments may rotate, sink, and grow further to develop into misaligned “spurious” grains. The “plume type of flow” is different than the “steepling convection flow” which was recently examined during directional solidification of Al-19% Cu and Al-7% Si alloys by Dr. Masoud Ghods in our laboratory.

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