by Yeung Ching Chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 69-71). / Text in English; abstract also in Chinese. / by Yeung Ching Chung. / Table of contents --- p.i / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- General overview --- p.1 / Chapter 1.2 --- The present study --- p.3 / Chapter Chapter 2 --- Sample preparation and characterization / Chapter 2.1 --- Sample preparation / Chapter A. --- General description --- p.5 / Chapter B. --- The thermal pulse furnace --- p.7 / Chapter C. --- The substrates --- p.9 / Chapter D. --- Sample preparation --- p.10 / Chapter 2.2 --- Sample characterization / Chapter A. --- Micro Raman system --- p.11 / Chapter B. --- Rutherford backscattering spectrometry (RBS) --- p.12 / Chapter C. --- X-ray powder diffraction --- p.13 / Chapter D. --- AFM. SEM and Surface Profiler --- p.13 / Chapter Chapter 3 --- Results and discussion / Chapter 3.1 --- The surface morphology / Chapter A. --- General description --- p.15 / Chapter B. --- The as-deposited amorphous film --- p.15 / Chapter C. --- The crystalline Ge film --- p.16 / Chapter D. --- The alloy film --- p.17 / Chapter E. --- The role of a-Si layer --- p.22 / Chapter 3.2 --- The depth profile (RBS) / Chapter A. --- General description --- p.24 / Chapter B. --- Peak temperature dependence --- p.27 / Chapter C. --- Heating rate dependence --- p.30 / Chapter 3.3 --- The near surface composition measured by Raman scattering / Chapter A. --- General description --- p.33 / Chapter B. --- Peak temperature dependence --- p.43 / Chapter C. --- Heating rate dependence --- p.45 / Chapter 3.4 --- Preferred growth direction / Chapter A. --- General description --- p.47 / Chapter B. --- Peak temperature dependence --- p.48 / Chapter C. --- Heating rate dependence --- p.51 / Chapter 3.5 --- Discussion / Chapter A. --- The particle size --- p.55 / Chapter B. --- The participation of Si substrate --- p.58 / Chapter C. --- The alloy formation --- p.58 / Chapter D. --- The abnormally fast interdiffusion --- p.63 / Chapter Chapter 4 --- Conclusion --- p.65 / Appendix --- p.67 / References --- p.69

Aluminum-silicon-copper cast alloys of the 319-type have attained a commercially important status because of their widespread use. Artificial aging treatments are routinely applied to these alloys in order to obtain precipitation hardening and improve their mechanical properties. Standard treatments may not always yield the optimum achievable properties, thus Mg and Sr are commonly added to improve the response of the alloy to aging and to modify the eutectic Si morphology from acicular to fibrous, respectively. The present study was carried out to investigate aging behavior of four 319-type alloys in regard to such mechanical properties as their ultimate tensile strength, yield strength, microhardness, percent elongation and impact toughness. Non-conventional aging cycles were applied so as to evaluate the degree of the improvement in strength obtainable. These treatments, labeled in this study as T6- and T7-type multi-temperature and interrupted aging treatments, involve several heating stages at different temperatures, as opposed to the single stage at constant temperature specifications of the standard T6 or T7 heat treatment regimes. Scanning electron microscopy was used to examine the fracture surfaces of selected tensile-tested samples to compare the fracture behavior. Transmission electron microscopy was used to reveal and identify the tiny precipitates which appear in the microstructure as a result of the precipitation-hardening process due to artificial aging. It was found that the main strengthening phase is theta-Al2Cu in the form of needles; other phases were observed as minor constituents in this alloy, including the binary beta-Mg2Si, the ternary S-CuAlMg 2 and the quaternary Q-Al5Cu2Mg7Si 7. The results show that while Mg and Sr additions improve the properties of the alloy, the standard T6 treatment may not be the best available option to produce optimum properties. In fact, when the peak-aged (T6) condition is desired, the optimum treatment consists of a continuous artificial treatment at 170°C for 8 h; when the overaged (T7) condition is desired, a T7-type multi-temperature treatment consisting of underaging at 170°C for 1 h, then at 190°C for 1 h, and finally overaging at 240°C for 2 h is the best option.

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