UCLA

Solidification processing such as casting is of paramount significance for the mass production of complex materials and components throughout human history. Phase control during solidification processing is vital to achieve desired structures and properties for numerous applications. However, it is a long-standing challenge to achieve effective phase control during solidification processing of materials. Conventional phase control approaches have gradually encountered certain technical and/or fundamental limits. The emerging nanotechnology could provide a new pathway to control the phase evolution during solidification to break these limits. The objective of this study is to significantly advance the fundamental understanding of nanoparticles enabled phase control during solidification processing of materials and to overcome the limits of the current methods by the incorporation of nanoparticles. More specifically, fundamental studies were conducted to understand how nanoparticles interact with liquid-liquid phase in immiscible alloys, solid-liquid phase during solidification of pure metals for unprecedented grain refinement and solid-solid phase for the grain structure stability at high temperatures.

To investigate the nanoparticle enabled phase control in solidification processing, key issues of nanoparticles incorporation and dispersion should be tackled first. In this study, a novel salt-assisted incorporation method was developed to fabricate metals containing uniformly distributed and dispersed nanoparticles. Molten salt such as KAlF4 can effectively remove the oxide layer at the top of the metal melt and readily assist the incorporation of nanoparticles into molten metal. This new salt-assisted incorporation paves a new pathway for mass production of metal matrix nanocomposites. In this thesis, metal matrix nanocomposites of Al-TiC, Al-TiB2 and Cu-WC were successfully fabricated by this method through solidification process.

In addition, to enhance the Orowan strengthening and study the phase control effect from nanoparticles of smaller diameter, a novel and facile molten salt reaction method was developed to synthesis small TiC nanoparticles with a diameter about 10 nm, which is not commercially available. The size of the TiC nanoparticles can be well controlled by the reaction template made of diamond nanoparticles in this study. To further improve and simplify the processing of Al nanocomposites reinforced by TiC (10 nm) nanoparticles, a novel in-situ reaction and incorporation method was explored, which enable one-step synthesis and incorporation. Al-TiC nanocomposites with 10 nm TiC nanoparticles show significantly improved mechanical properties.

Cu-WC nanocomposites with various volume fractions of nanoparticles were also successfully fabricated by this molten salt method after different salts were tested for optimized processing. KAlF4, borax and NaCl were all effective for the incorporation of WC nanoparticles into the molten Cu. WC nanoparticles can be uniformly distributed and dispersed in Cu matrix by this method and provide much enhanced mechanical properties (strength, hardness and Young’s modulus) without significant deterioration of the electrical conductivity, which suggests that the new Cu-WC nanocomposites could serve as high strength and high electrical conductivity materials for a widespread range of applications. .

The nanoparticles enabled phase control for liquid-liquid phase were studied in immiscible alloys both experimentally and theoretically to advance the fundamental understanding and to solve the long-standing challenges in solidification processing of immiscible alloys. It is shown that nanoparticles can move to the interface between the primary phase and secondary phase forming a coating and slowing down the diffusional growth of the secondary phase, which result in a significantly refined microstructure. TiC and TiC0.7N0.3 nanoparticles were successfully utilized in Al-Bi immiscible alloy for scalable manufacturing of high performance self-lubrication bearing materials. It was also found that the addition of Cu element can significantly enhance the mechanical properties without microstructure deterioration. Tungsten (W) nanoparticle is also demonstrated to be effective for the processing Zn-Bi immiscible alloys. In addition, our study show that nanoparticles not only worked for processing of metallic immiscible alloys, but also function well in organic immiscible systems such as B4C nanoparticles in the organic SCN-CTB immiscible system.

The solid-liquid phase control by nanoparticles were investigated by studying the grain refinement effect during solidification. The grain refinement by nanoparticles for pure metals is studied in different model material systems such as Cu-WC, Al-TiB2 and Zn-WC. In this study, a new discovery that nanoparticles can refine metal grains down to ultrafine or even nanoscale by instilling a continuous nucleation and grain growth restriction mechanism during slow cooling (< 100 K/s) is reported for the first time. When casting pure Cu with WC nanoparticles, the grain sizes of Cu are refined substantially ultrafine and even nanoscale, which is approximate to the inter-particle spacing. Differential scanning calorimetry (DSC) studies showed that Cu-WC samples need significantly longer time (by 83%) to complete the solidification than pure Cu, indicating nanoparticles can substantially slow down the solidification process. It is proposed that nanoparticles can restrict the grain growth by the Gibbs-Thompson effect, thus allowing a continuous nucleation throughout the solidification process. Furthermore, this newly revealed grain control mechanism is successfully applied in other materials systems such as Al-TiB2 and Zn-WC for ultrafine grains via slow cooling. This revolutionary method paves a pathway for the mass production of bulk stable ultrafine-grained (UFG)/nanocrystalline materials. This method may be readily extended to any other processes that involve cooling, nucleation and phase growth for widespread applications.

The solid-solid phase control by nanoparticles was investigated by studying the thermal stability of UFG/nanocrystalline Cu-WC nanocomposites. It is shown that the as-solidified bulk ultrafine/nanocrystalline Cu reveals an unprecedented thermal stability up to 1023 K (0.75 melting point of Cu) by the Zener pinning effect from WC nanoparticles.

In summary, this dissertation presents experimental methods and a theoretical framework to understand and utilize the newly discovered nanoparticles enabled phase control mechanisms during solidification processing. Cases studies of liquid-liquid phase (e.g. immiscible alloys), solid-liquid phase (e.g. grain refinement) and solid-solid phase (grain structure thermal stability) of pure metals show that nanoparticle is extremely effective to control phase evolution. This approach breaks the fundamental and/or technical limits for solidification processing of numerous metals with unusual properties for broad applications.