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Title: Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys

Abstract

Morphological evolution and selection of angular primary silicon is investigated in near-eutectic Al-Si alloys. Angular silicon arrays are grown directionally in a Bridgman furnace at velocities in the regime of 10 -3 m/sec and with a temperature gradient of 7.5 x 10 3 K/m. Under these conditions, the primary Si phase grows as an array of twinned bicrystalline dendrites, where the twinning gives rise to a characteristic 8-pointed star-shaped primary morphology. While this primary Si remains largely faceted at the growth front, a complex structure of coherent symmetric twin boundaries enables various adjustment mechanisms which operate to optimize the characteristic spacings within the primary array. In the work presented here, this primary silicon growth morphology is examined in detail. In particular, this thesis describes the investigation of: (1) morphological selection of the twinned bicrystalline primary starshape morphology; (2) primary array behavior, including the lateral propagation of the starshape grains and the associated evolution of a strong <100> texture; (3) the detailed structure of the 8-pointed star-shaped primary morphology, including the twin boundary configuration within the central core; (4) the mechanisms of lateral propagation and spacing adjustment during array evolution; and (5) the thermosolutal conditions (i.e. operating state) at the primarymore » growth front, including composition and phase fraction in the vicinity of the primary tip.« less

Authors:
 [1]
  1. Iowa State Univ., Ames, IA (United States)
Publication Date:
Research Org.:
Ames Lab., Ames, IA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
897368
Report Number(s):
IS-T 2475
TRN: US200705%%56
DOE Contract Number:
AC02-07CH11358
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; CONFIGURATION; DENDRITES; EUTECTICS; FURNACES; MORPHOLOGY; SILICON; TEMPERATURE GRADIENTS; TEXTURE; TWINNING

Citation Formats

Jung, Choonho. Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys. United States: N. p., 2006. Web. doi:10.2172/897368.
Jung, Choonho. Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys. United States. doi:10.2172/897368.
Jung, Choonho. Sun . "Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys". United States. doi:10.2172/897368. https://www.osti.gov/servlets/purl/897368.
@article{osti_897368,
title = {Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys},
author = {Jung, Choonho},
abstractNote = {Morphological evolution and selection of angular primary silicon is investigated in near-eutectic Al-Si alloys. Angular silicon arrays are grown directionally in a Bridgman furnace at velocities in the regime of 10-3 m/sec and with a temperature gradient of 7.5 x 103 K/m. Under these conditions, the primary Si phase grows as an array of twinned bicrystalline dendrites, where the twinning gives rise to a characteristic 8-pointed star-shaped primary morphology. While this primary Si remains largely faceted at the growth front, a complex structure of coherent symmetric twin boundaries enables various adjustment mechanisms which operate to optimize the characteristic spacings within the primary array. In the work presented here, this primary silicon growth morphology is examined in detail. In particular, this thesis describes the investigation of: (1) morphological selection of the twinned bicrystalline primary starshape morphology; (2) primary array behavior, including the lateral propagation of the starshape grains and the associated evolution of a strong <100> texture; (3) the detailed structure of the 8-pointed star-shaped primary morphology, including the twin boundary configuration within the central core; (4) the mechanisms of lateral propagation and spacing adjustment during array evolution; and (5) the thermosolutal conditions (i.e. operating state) at the primary growth front, including composition and phase fraction in the vicinity of the primary tip.},
doi = {10.2172/897368},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}

Thesis/Dissertation:
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  • Analytical solutions were obtained for the thermal stress and strain of both concentric spheres and concentric cylinders comprised of dissimilar materials. Previously derived formulations for elastic interactions between the materials were extended to include a plastically deforming shell for both the spherical and cylindrical models. Thermal expansion measurements were performed on Al-Si alloys, aluminum alloy silicon carbide composites, and aluminium alloy tungsten composites. These measurements include detailed observations on heating and cooling over repeated temperature cycles between room temperature and 500/sup 0/C. Changes in the expansivity were observed in various temperature intervals that cannot be related to volume fraction rulesmore » of mixtures or to composite theory based on purely elastic behavior of the materials. The models developed, in part, relate the differences in the observed expansivity between heating and cooling to plastic deformation of the shell material. The differences in expansivity between heating and cooling result in linear changes in dimensions after thermal cycling.« less
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  • The effect of temperature on the fatigue crack propagation rate was investigated in niobium-hydrogen alloys. The study was undertaken with the ultimate goal of determining the role of hydrogen, in conjunction with test temperatures, in the embrittlement and fracture processes of niobium as a representative of group VA metals. Compact tension specimens (CTS) of pure niobium, as well as niobium containing hydrogen in solid solution and in the form of hydride, were used. The fatigue tests were performed on these specimens using a tension-tension loading in a closed-loop electrohydraulic machine operating in a sinusoidal loading cycle. The tests were mademore » at a frequency of 4 Hz, load ratio of 0.05, and at test temperatures of 273 K and 400 K. Microhardness was also measured on the fatigued specimens in order to determine the plastic zone size. The fracture surfaces of specimens were further examined by means of a scanning electron microscope. Analysis of this behavior suggests that stress induced hydride formation cannot be responsible for the embrittlement of niobium with hydrogen, and that some more mobile form of hydrogen interacting with dislocations plays an important role in the embrittlement process.« less