The Microstructure, Morphology and Mechanical Properties of Rapidly Solidified Al-10wt%Si Alloy

• Author / Creator

  Hearn, William

• Hypoeutectic Al-Si alloys are widely used in both the automotive and aerospace industries, due to their strong corrosion resistance, good castability and relatively high strength-to-weight ratio. Despite this wide use, these alloys have a major limitation related to the Si phase that forms within the eutectic structure. In the as-cast state this eutectic Si phase forms a flaky lamellar morphology that combined with the inherent brittle nature of Si significantly reduces the ductility and the mechanical property performance of these alloys. To improve and modify this eutectic Si phase, alloy additions and/or rapid solidification can be used. However, the underlying mechanisms behind this Si phase refinement are poorly understood, especially for modification via rapid solidification. As such, the research in this thesis aimed to shed light on how rapid solidification affects the microstructure of hypoeutectic Al-Si alloys. To conduct this analysis a rapidly solidified Al-10wt%Si alloy, that was produced under varying rapid solidification conditions, was examined. The microstructure of this Al-10Si alloy was found to consist of a primary α-Al phase and a secondary Al + Si eutectic structure. From an analysis of the primary α-Al phase two distinct growth directions were identified: a growth and a growth. In addition, the formation of a “seaweed” or “coral-like” α-Al structure was observed. The transition in growth from to , along with the propensity for seaweed growth, was found to become more prevalent as solidification became more rapid, suggesting that rapid solidification caused this shift in the α-Al morphology.The analysis of the secondary Al + Si eutectic structure was done via a characterization of the Si growth morphology, where the observed Si morphologies were mapped as a function of the local eutectic solidification conditions. From this analysis it was found that the Si morphology transitioned from flaky  fibrous  globular + fibrous  globular as solidification became more rapid; where the globular morphology was considered to be the finest morphology and the flaky morphology was considered the coarsest morphology. The beginning of the flaky  fibrous, fibrous  globular + fibrous and the globular + fibrous  globular transitions occurred at local eutectic cooling rates of ~60 K/s, ~350 K/s, and ~1200 K/s respectively.Further analysis related these microstructural changes to the hardness of the Al-10Si alloy, and in doing so it was found that the alloy hardness increased monotonically with the Si morphology. By shifting to a globular Si the Al-10Si alloy was able to achieve improvements in hardness of up to 24%. Indicating that control of this morphology is an important factor when considering the mechanical properties and characteristics of hypoeutectic Al-Si alloys. Given that this Si morphology is the key to improving mechanical property performance in hypoeutectic Al-Si alloys, this work recommends that other rapid solidification processes adjust their processing conditions to invoke this globular Si morphology.

• Subjects / Keywords

  • Microstructure
  • Impulse Atomization
  • Rapid Solidification
  • Aluminium Alloys
  • Hypoeutectic Al-Si alloys

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3X34N69P

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.