In Situ Metal Matrix Nanocomposites: Towards Understanding Formation Mechanisms and Microstructural Control
Reese, Caleb
2020
Abstract
Lightweight materials are critical to meet the ever-increasing demands for improved fuel economy in the automotive, aerospace and defense industries. Consequently, aluminum alloys have been employed extensively in these industries for structural applications owing to their high strength-to-weight ratio. However, Al alloys suffer from several shortcomings, such as poor thermal stability of mechanical properties, limiting their usage for components operating in elevated temperature environments. Recently, the incorporation of nano-scale particles in the Al matrix, termed metal matrix nanocomposites (MMNCs), has been identified as a promising approach to improved ambient and elevated temperature mechanical properties, while still retaining the lightweight benefits of Al. MMNCs manufactured through typical ex situ incorporation methods, wherein pre-made particles are mixed into the matrix, can suffer from precursor contamination and undesirable particle/matrix interfacial reactions, making incorporation and large-scale processing difficult. In situ processing alternatives, where particles are created directly in the melt via direct reaction, have been demonstrated to exhibit improved particle/matrix interface stability and easier incorporation within the matrix. However, the ability to reliably control critical mechanical property-dependent particle characteristics (i.e., particle size, volume fraction, and dispersion) remains a barrier to large-scale processing of in situ MMNCs. The research for this dissertation is aimed at elucidating the mechanisms governing formation of the particles and provide guidance to controlling the resulting microstructure of MMNCs processed via in situ methods, for the purposes of informing large-scale processing efforts. In this work, we investigate the processing-microstructure-mechanical property relationships for two in situ processing methods, namely: in situ gas-liquid reaction (ISGR) for Al-AlN MMNCs and thermite-assisted self-propagating high-temperature synthesis (SHS) for Al-TiC MMNCs. We find that the SHS process is more capable of readily producing nano-scale TiC particles in a wide variety of volume fractions and dispersions dependent on processing conditions. Additionally, we report on successful SHS processing, at our industry partner, of commercial pilot-scale quantities of in situ Al-TiC MMNCs exhibiting enhanced mechanical properties for relatively low amounts of particle addition. The preliminary results are a promising demonstration of the potential for commercial-scale processing of in situ MMNCs. Building upon our study of large-scale processing of MMNCs, we then perform a more detailed investigation into understanding the formation mechanisms and microstructural control of the thermite-assisted SHS process. By leveraging 2D and 3D microstructural quantification techniques with a thermodynamic-based analysis, we identify three potential direct- and indirect- reaction pathways governing TiC formation and the conditions under which they are active. We also demonstrate an approach for correlating processing-property relationships via multivariate statistical analysis (i.e., canonical correlation analysis (CCA)). Using CCA, we report on the dominant processing variables affecting final MMNC microstructure and particle characteristics and discuss the link between processing variables, reaction pathways, and resultant microstructural signatures. Our results and analysis are expected to inform a more rational approach to process control of in situ SHS MMNCs, as well as being applicable to other in situ processing methods.Subjects
Aluminum metal matrix nanocomposites In situ nanocomposite processing Self-propagating high-temperature synthesis In situ gas-liquid reaction Nanoparticle formation mechanisms
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