Date of Award

2018

Degree Type

Thesis

Degree Name

Master of Science in Chemical Engineering

Department

Chemical and Biomedical Engineering

First Advisor

Tewari, Surendra

Subject Headings

Aerospace Engineering, Chemical Engineering, Chemistry, Engineering, Materials Science

Abstract

This research is aimed at carrying out a systematic investigation of the nucleation, and growth of spurious “misoriented” grains during directional solidification in the low gravity environment of space. Three Al–7 wt. % Si alloy cylindrical samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified on the Space Station at growth speeds varying from 5 to 50 µms-1 under thermal gradients varying from 14 to 33 K cm-1 in alumina crucibles, under a joint NASA-ESA (European Space Agency) project called, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions). The primary purpose of directionally solidifying these three Al-7Si samples in the low gravity environment of space was to eliminate gravity-induced convection in the melt, and grow dendrite array morphology under purely diffusive transport conditions. However, when these directionally solidified samples were extracted from their alumina crucibles, they all showed evidence of surface pores along their length. We believe that these pores formed because in microgravity, there is no imposed force to pull the liquid column down on to the solidifying portion below to continue to feed the volume shrinkage due to liquid to solid phase transformation. There was no additional built-in mechanism, such as a piston and spring, in the MICAST ampoules to keep the melt column pressed onto the solid below. We also believe that even in the absence of gravity, a liquid coulmn which gets detached from the crucible internal waals (forming surface pores), under an imposed positive thermal gradient, would lead to the liquid-solid surface energy driven Marangoni convection. This convection may fragment fragile secondary or tertiary arms of the primary dendrite trees growing in the mushy zone. These broken dendrite solid fragments may lead to the nucleation and growth of spurious grains in the MICAST samples, where the orientation of primary dendrites would be very different from those in their unmelted seed portions. Our purpose is to examine the longitudinal and transverse microstructures of these MICAST samples to study the formation of spurious grains, and investigate if there is any correlation between the location of the observed surface pores and the formation of spurious grains.

Download
COinS