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Spurious Grain Formation During Directional Solidification in Microgravity

Upadhyay, Supriya Ramashankar
http://rave.ohiolink.edu/etdc/view?acc_num=csu1527780881822168

Abstract Details

2018, Master of Science in Chemical Engineering, Cleveland State University, Washkewicz College of Engineering.
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.
Surendra Tewari (Committee Chair)
Orhan Talu (Committee Member)
Jorge Gatica (Committee Member)
Christopher Wirth (Committee Member)
92 p.
Aerospace Engineering; Chemical Engineering; Chemistry; Engineering; Materials Science
Directional Solidification; Marangoni Convection; Dendritic Growth; Phase Diagram

Recommended Citations

Citations

  • Upadhyay, S. R. (2018). Spurious Grain Formation During Directional Solidification in Microgravity [Master's thesis, Cleveland State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=csu1527780881822168

    APA Style (7th edition)

  • Upadhyay, Supriya. Spurious Grain Formation During Directional Solidification in Microgravity. 2018. Cleveland State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=csu1527780881822168.

    MLA Style (8th edition)

  • Upadhyay, Supriya. "Spurious Grain Formation During Directional Solidification in Microgravity." Master's thesis, Cleveland State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=csu1527780881822168

    Chicago Manual of Style (17th edition)

csu1527780881822168
652
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This open access ETD is published by Cleveland State University and OhioLINK.