Visualization and Analysis of Nanoscale Microstructure Evolution of In situ Metal Matrix Composites

Abstract

Creating new, light-weight materials is a critical engineering problem required to meet the ever-increasing demands for improved fuel economy and electric vehicle range in the automotive, aerospace, and defense industries. Aluminum and its alloys have gained increased usage due to their high strength-to-weight ratio, low cost, and machinability. However, Al alloys suffer from poor thermal stability of mechanical properties, thus limiting their usage for components that operate in elevated temperatures. Therefore, there is much interest in methods for improving mechanical performance of Al-based materials at both ambient and elevated temperatures to expand the use of lightweight materials into new applications. One possible solution to this problem is the use of Al-based metal matrix nanocomposites (MMNCs), a mixture of aluminum and nanoscale reinforcement, as they have improved mechanical properties at both ambient and elevated temperatures over base Al alloys, without sacrificing the lightweight benefits of Al. MMNCs are typically made via ex situ processing, where pre-manufactured reinforcing particles are incorporated into the Al matrix. These routes of MMNC production have a few main issues including the cost of the reinforcing nanopowders, reinforcement contamination, and undesirable particle-matrix interface reactions, which make particle incorporation and large-scale processing difficult. In contrast, in situ MMNC processing methods generate particles directly in the melt via a reaction between precursors and have shown improved particle-matrix interface stability and easier particle incorporation with the matrix. However, there is much work to be done to reliably control key particle characteristics, such as particle size, dispersion, and volume fraction, when creating in situ MMNCs. The research for this dissertation is focused on studying of the formation mechanisms of particles and controlling the resulting microstructure of in situ MMNCs. In this work we explore the processing-microstructure-properties relationships for two in situ processing methods, metal-based polymer pyrolysis (MBPP) and salt-flux reaction synthesis (SFRS), used to generate Al-based MMNCs. We find that there are multiple commercially available precursor polymers that can be used for generating MMNCs via MBPP and study their thermal degradation behavior to inform the best processing parameters for MMNC production. We report on successful MBPP processing to generate Al powder-based MMNCs that show improved mechanical properties. The results of the MBPP experiments demonstrate it as a potential method for in situ MMNC production. We find that SFSR is a facile technique for generating Al/TiC MMNCs that has the possibility of scaling into industrial production. We use 2D and 3D microstructural analysis techniques to perform a detailed investigation of the MMNCs generated via SFRS and investigate the formation mechanisms of reinforcing TiC particles and intermetallic Al3Ti. We find that the TiC particles are formed first and directly by the SFRS reaction, not through an indirect reaction using Al3Ti, and that their presence in the melt affects the intermetallic morphology in multiple modes. We also investigate the effect of increasing Si content in the Al matrix on the microstructure and properties of the MMNCs. We find that Si affects the microstructure in multiple ways, changing both the intermetallics and the carbides which form, and discuss possible methods which its presence causes these changes. Our analysis and results will assist in forming a more rational approach to processing in situ SFRS MMNCs and is an important step towards scaling up in situ processing methods.