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Description
This thesis reports on the processing and properties of hybrid tin (Sn) composites. The effects of varying hybrid (half alumina (Al2O3), half graphite by volume percent) reinforcement content, up to 30 volume percent, on the microstructure, hardness and indentation creep behavior of Sn-Al2O3-Graphite composites were studied. Each hybrid metal matrix composite was processed using powder metallurgy sequence of mixing, uniaxial compaction and extrusion at a tin homologous temperature of ~0.6 and extrusion ratio of ~17.4:1. Microscopy (optical and scanning electron) were used in conjunction with image analysis, to study key microstructural features of the processed composites, including the average matrix grain size, reinforcement particle size and interparticle spacing. Effects of the reinforcements on mechanical properties was studied via Vickers hardness testing with varied loads, and indentation creep tests and analyses (also using varied loads). The hardness was found to increase at 5% reinforcement content then decline until 30 vol.% reinforcement content where the hardness was found to increase again. Inherently the hardness was found to depend primarily on the matrix grain size in agreement with the Hall-Petch relationship. Composites with 5 and 10 vol% reinforcement displayed superior creep resistance to unreinforced tin. Creep analyses confirmed that the dominant creep mechanism in operation was dislocation creep. It was found that alumina particles did not wet the tin matrix well and so introduced intra agglomerate porosity in addition to interfacial gaps between the alumina reinforcements and the Sn matrix. These ultimately resulted in the composites not achieving theoretically predicated mechanical behavior. However, the composite with 5% hybrid reinforcement had the best creep performance and the highest Vickers hardness value at 16.5.
