The milling of tin bronze with a Cu-24.6wt%Sn composition

• Main Author: Williams, Garth
• Other Authors: Lang, Candy
• Format: Doctoral Thesis
• Language: English
• Published: University of Cape Town 2014
• Subjects: Materials Engineering
• Online Access: http://hdl.handle.net/11427/8280

Includes bibliographies. === The effects of high energy milling on tin bronze with the composition Cu-24.6wtSn have been examined using hardness testing, optical microscopy, scanning electron microscopy, transmission electron microscopy and x-ray diffraction. High energy milling has caused mechanical alloying of an elemental copper and tin powder blend, and mechanical milling of a cast powder and a melt quenched powder. Nanocrystalline grains with a size between 5 nm and 50 nm have been directly observed in the final milled powder. The powder consist of the a phase and 8 phase and is partially amorphous. An extension of the solid solution solubility has also been detected due to milling. The formation of the metastable tin-rich 11 phase has been observed in the intermediate stage of mechanical alloying of the elemental powder blend due to the higher diffusivity of tin in copper over copper in tin. The formation of the 11 phase during mechanical alloying of tin bronze with the composition Cu-24.6wtSn has not been reported before. The morphological development of the three initial powders has proceeded by different mechanisms during milling due to the different hardness and toughness of the starting powders. Milling of the elemental powder blend and the cast powder proceeds via classic mechanisms for milling of ductile powders and brittle powders respectively, while milling of the tougher melt quenched powder proceeds via a combination of the two mechanisms. An attempt to process the milled powder into a bulk state using various thermomechanical techniques while still retaining a nanocrystalline grain size has not succeeded. The high diffusivity of the material at elevated temperatures has led to grain growth into the micrometer range even at relatively low thermo-mechanical processing temperatures. The milled powders have poor compaction properties due to the highly deformed structure and therefore the processed material has poor properties compared to a cast material.