Processing and toughening of dual-metallic phase dual-carbide composites

• Autor(a) principal: Ravanan, Arivumani
• Data de Publicação: 2013
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/11784
• topic: Engenharia de materiais - Teses de mestrado; Compósitos metálicos - Dureza - Tenacidade; Ligas de alumínio; Carboneto de silício; Carboneto de boro; Diagramas de fases

This work on the processing and study of the indentation fracture composites of dual-metallic phase of Al-Si and dual carbide phase of B4C and SiC uses the reactive melt infiltration method to prepare nearly fully dense samples. Batches of single and twice smelted Al-Si alloys of hypereutectic composition were produced by mixing aluminium and silicon powders in the proportion of 25 wt% and 35 wt% Si and melting at 1000 ºC for 60 minutes in argon atmosphere. Boron carbide powder of 10 μm average particle size, containing free carbon, mostly graphite, was used to prepare nominally 50% dense cylindrical pellets of 7.5 mm of height by uniaxial pressing followed by cold isostatic pressing. B4C preforms become spontaneously infiltrated by the reactive melt of the Al-Si alloy at 1200 °C and 1300 °C in 5 minutes using a resistive heated graphite furnace with inert argon atmosphere. Defects related to infiltration are described in the study, the conditions and contamination in oxygen of the alloy and furnace atmosphere that could have caused partial or absence of infiltration of the B4C preforms are discussed. A set of fully infiltrated samples of the composites was obtained representing different Si proportions in the infiltrate and two temperatures where infiltration became feasible within the range of experimental conditions of this study. Polished longitudinal sections of the pellets were further investigated by using quantitative XRD methods with standards for the volume fractions of the main crystalline phases, Al, Si, B4C and SiC and scanning electron microscopy with standard-less energy dispersive analysis SEM/EDS for the element distribution in the phases. The reaction path accounting for changes in the volume fraction of the phases from top, the melt infiltration face, to the bottom areas were represented in isothermal section of the Al-Si-B4C phase diagram. The corresponding representations of the average elemental composition of the composites in the Al-Si-C and Al-Si-B-C phase diagrams expose a large content of Al, mostly Al metal in the composites which explains the comparatively low values of hardness. An indentation size effect of the Vickers hardness is observed for indentation loads of 9.8 N, 98 N and 294 N, the hardness of the composites determined at 98 N being correlated to the Si concentration of the Al-Si infiltrate. In spite of the scatter of indentation fracture toughness determined at 98 N, the high values toughness of the composites in this study are correlated to the comparatively low values of hardness in the same way as recently published in a similar study of reactive melt infiltration of B4C composites with Al-Si alloys of high Si content. The way the toughening mechanism may depend on the distribution of the brittle phases and ductile Al and interphase de-bonding is further discussed.