| Historically, aluminum alloys have been in the forefront in satisfying the requirements of transportation and structural applications, mainly due to their high strength to weight ratio and excellent corrosion resistance. It has been identified that strong and lightweight materials are necessary ingredients for energy efficient transportation. Improving the competitiveness of aluminum alloys against other high strength, lightweight materials have been the topic of recent research and development efforts. Consequently, the focus has been on developing new aluminum alloys with higher strengths.;The major challenge in the development of high strength aluminum alloys is, to increase strength without reducing ductility very much. Aluminum by its nature, as the strength increases ductility reduces. When engineering applications are concerned, ductility is a key property that needs to be kept within adequate limits.;The main objective of this thesis was to establish a strong practical foundation for future research on high strength aluminum alloys by developing a new aluminum alloy with emphasis on grain refining elements. Compared to the methods that are used to strengthen aluminum alloys, grain refinement achieved by the addition of grain refining elements is identified as the most effective and economical method to improve strength while retaining a sufficient ductility. According to recent studies, Scandium (Sc) has shown better refining ability of the grain structure of aluminum than any other single element. However the extremely high price of Sc has restricted its use as a grain refining element in commercial alloys. Therefore instead of using Sc by itself, researchers have used Sc combined with titanium (Ti) and zirconium (Zr) which has also given better grain refining action.;In this research work hafnium (Hf) was identified as a candidate element that could combine with Sc, Ti and Zr. Hf has not previously been identified as a potential grain-refining element for aluminum.;The newly designed alloy composition was able to give 98% smaller grain size when compared to the grain size of pure aluminum and 97% smaller grain size when compared to Al-0.7Sc alloy. Further, this composition was able to give more than 92% smaller grain size when compared to two commercial alloys, A356 and AA2024-T3. This is a remarkable reduction in grain size by approximately two orders of magnitude, resulting in a major important step forward, applicable to all aluminum alloys.;The main conclusion of this research work is hafnium can be used as a successful grain refining element with combinations of Sc, Ti and Zr. |
