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Title: The effect of solidification variables on the microstructure of hypereutectic Al-Si alloys
Author: Faraji, Masoumeh
ISNI:       0000 0001 3457 2259
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2007
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In this work, the effect of phosphorus, as a primary silicon refiner, and of strontium, as a eutectic silicon modifier, individually and simultaneously, on the microstructure ofhypereutectic AI-Si alloys was studied. AI-18.6Si-0.35Fe-0.02Cu-0.0014P (wt%) alloy was used for casting and phosphorus was added in a range of 0.015-0.08 wt% mainly by means of AI-6.75Fe4.91P (wt %), and in a few cases by AI-17.lCu-0.89P (wt%), master alloys. Eutectic silicon was modified using AI-5.93 wt% Sr over a range of 0.04-0.3 wt% Sr. Two different casting methods were used: bottom casting into sand moulds and chamber casting into steel moulds. The melt temperature was 800aC and holding time for each addition was 15 min. Adding 0.02 wt% P led to an increase in the number of primary silicon particles per unit volume (NV) by 1.5 times. NV was trebled by adding 0.08 wt% phosphorus during chamber casting into steel moulds. NV was decreased by about 20 times by adding 0.2 wt% Sr and 0.02 wt% Sr+0.02 wt% P to untreated alloy during bottom casting into a sand mould. Adding strontium increased primary silicon undercooling from 7.l±1.0 to 46.6±6.5 K, though phosphorus addition of 0.02, 0.04 and 0.08 wt% (for ingots chamber cast into steel moulds) gave a primary silicon undercooling of 18.7±10A, 8.5±1.2 and 9.l±0.9 K respectively (compared to 9.0±5.0 K for untreated ingots). The nucleation models (one surface-dependant and the other volumedependant) of Perepezko were applied to the observed nucleation behaviour of primary silicon. It was found that the nucleation temperature was the most crucial variable in both models. Applying the models to our results showed that contact angle from the surface-dependant model varied over the range of 24.3 to 30 degrees, compared with 17.7 to 23.5 degrees from the volume-dependant model. Additionally, Electron Back Scatter Diffraction (EBSD) was used to determine possible crystallographic relationships between neighbouring primary silicons in untreated and P-inoculated ingots. Between two connected primary silicon particles, the outermost layers of each silicon particle were often found to be twin-related by a 60° rotation around axis <111>.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available