Date of Award

Fall 2012

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Supervisor

Sumanth Shankar

Language

English

Abstract

Al-Si cast alloys have been in the fore-front of commercial casting application for more than a century. Iron containing intermetallic phases that evolve during the solidification of these alloys play a major role in the resultant mechanical properties and performance of the cast products. Changes in alloy composition and casting parameters significantly alter the evolution of the Al-Si-Fe intermetallic phases. There was a lack of clear understanding of the complex relationships between the solidification parameters and nature intermetallic phases in these alloys. Current thermodynamic model predictions for the nature of these intermetallic phases in the Al corner of the Al-Si-Fe ternary system are strikingly different from the experimental results in this project. Trace levels of Sr (about 0.02wt%) are typically added to the Al-Si commercial alloys to effect a morphological modification of the eutectic phases to improve the properties and performance of the cast products.

The nature of the Fe containing intermetallic phases have been characterized as a function of alloy composition (Si, Fe and Sr) and cooling rates during solidification. There was an anomalous evolution of the t5-Al8Fe2Si phase which transformed into the t6-Al9Fe2Si2 phase during solidification at lower cooling rates and higher Fe concentration in the alloy, alike. Further, Sr addition to these alloys prevented the evolution of the t5 phase and promoted the evolution of an unidentified Al5Fe2Si3 phase which was noted as k in this dissertation; the k phase also transformed into the t6 phase at lower cooling rates and higher Fe concentration in the alloy, alike.

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