The modifications of aluminium-silicon alloys with strontium

Jenkinson, David Charles (1972). The modifications of aluminium-silicon alloys with strontium PhD Thesis, School of Engineering, The University of Queensland. doi:10.14264/uql.2015.562

       
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Author Jenkinson, David Charles
Thesis Title The modifications of aluminium-silicon alloys with strontium
School, Centre or Institute School of Engineering
Institution The University of Queensland
DOI 10.14264/uql.2015.562
Publication date 1972-01-01
Thesis type PhD Thesis
Supervisor Unknown
Total pages 2 v.
Language eng
Subjects 091207 Metals and Alloy Materials
Formatted abstract
High purity aluminium-silicon alloys in the range 9-17% silicon, and containing strontium additions of 0 - 0.1%, have been directionally frozen at rates from 11 to 7500 µm/sec. with imposed temperature gradients from 90 to 410°C/cm.

Three microstructural types have been defined - "unmodified" "modified" and "modified plus a dendrites". The conditions of growth rate and strontium level required to produce each type in the various silicon concentrations have been accurately defined. Temperature gradient was monitored but not intentionally varied.

A region of coupled growth, in which modified eutectic alone grows, has been defined and the effect of strontium level on the shape of this region determined. The transition from unmodified to modified growth was found to occur in the range 480 - 1000 µm/sec. at an estimated 6 - 9 C undercooling below eutectic temperature, depending on the silicon level.

The composition of particles produced during over modification with strontium has been determined and an interparticle spacing/growth rate relationship defined for strontium modified alloy.

Scanning electron microscopy has been used to study both unmodified and modified silicon forms and the transition from one to the other. Transmission electron microscopy of extraction replicas has shown the modified fibres to contain a high density of {111} twins and to grow in a preferred <100> direction.

Electro-anodising revealed the a matrix in unmodified alloys to be polycrystalline, and in modified alloys largely monocrystalline. Laue back reflection X-rays were used to confirm the results. X-ray diffractometry could detect no change in silicon lattice parameter on inoculation.

All results are consistent with the proposal that unmodified eutectic is 'abnormal' with silicon leading at the interface and the a phase repeatedly nucleating on its side, while modified eutectic is 'normal' with -an approximately isothermal interface and a monocrystalline a matrix. The silicon phase is continuous in both cases.
Keyword Silicon alloys
Strontium
Aluminum alloys
Additional Notes Other Title:Aluminium-silicon alloys with Strontium.

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
 
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