In Situ Metal Matrix Nanocomposites: Towards Understanding Formation Mechanisms and Microstructural Control

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.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163192/1/reesecw_1.pd

Novel processes for large area gallium nitride single crystal and nanowire growth.

III-nitrides (InN, GaN, AlN) are some of the most promising materials for making blue light emitting diodes (LED), blue laser diodes (LD) and high power, high temperature field effect transistors (FET). Current techniques produce GaN films with defect densities on the order of 107/cm2 or higher. The performance and life-time of the devices critically depend upon the defect densities and high power, high frequency devices require the defect densities to be lower than 104/cm2. So, the need for new processes to produce large size GaN crystals with defect densities less than 107/cm2 is immediate. In addition to large area single crystals (or wafers), the nanowires also present as an alternating platform for making devices. So, the processes for controlled synthesis, modifying and integrating sub 100 nm nanowires into electronic devices are of great interest. This thesis presents a new concept of ‘self-oriented growth\u27 of GaN platelets shaped crystals on molten gallium to produce near single crystal quality GaN films over large areas (\u3e 1 cm²). The process involves direct nitridation of Ga films using nitrogen plasma at low pressures (few mTorr). GaN flakes with areas over 25 mm² have been successfully obtained. Raman spectra of the resulting GaN crystals show no stress and low native donor concentration on the order of 1017/cm³. XRD texture analysis showed an overall c-axis tilt of 2.2o between GaN domains within the flake. The cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small mis-orientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8o and 1o, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces resulting with both twisted and perfect attachments. This thesis also presents direct synthesis approach for GaN nanowires with control on growth directions using modified nitridation conditions. The nitridation in the presence of hydrogen or ammonia resulted in oxide sheath free GaN nanowires as thin as 20 nm and long as 100 Ìm in \u3c0001\u3e direction. The nitridation using low Ga flux in a vapor transport set-up resulted in sub 100 nm GaN nanowires with \u3c10-10\u3e growth direction. The difference in the nucleation and growth mechanism allowed control on the nanowire directions. Homo-epitaxial experiments onto pre-synthesized GaN nanowires with the above two growth directions using the vapor transport of Ga and dissociated ammonia exhibited different morphologies, e.g. micro hexagonal columns for \u3c0001\u3e nanowires and micro belts for nanowires with \u3c10-10\u3e growth direction. The results further illustrate a new phenomenon of enhanced surface diffusion on nanowires in general but more pronounced for wires with \u3c0001\u3e growth direction. The results from homo-epitaxy experiments suggest that the \u3c10-10\u3e direction wires could be used as seeds for growing large area GaN crystals in vapor phase homo-epitaxy schemes

Boron Arsenide and Boron Phosphide for High Temperature and Luminescent Devices

The crystal growth of boron arsenide and boron phosphide in the form of bulk crystals and epitaxial layers on suitable substrates is discussed. The physical, chemical, and electrical properties of the crystals and epitaxial layers are examined. Bulk crystals of boron arsenide were prepared by the chemical transport technique, and their carrier concentration and Hall mobility were measured. The growth of boron arsenide crystals from high temperature solutions was attempted without success. Bulk crystals of boron phosphide were also prepared by chemical transport and solution growth techniques. Techniques required for the fabrication of boron phosphide devices such as junction shaping, diffusion, and contact formation were investigated. Alloying techniques were developed for the formation of low-resistance ohmic contacts to boron phosphide. Four types of boron phosphide devices were fabricated: (1) metal-insulator-boron phosphide structures, (2) Schottky barriers; (3) boron phosphide-silicon carbide heterojunctions; and (4) p-n homojunctions. Easily visible red electroluminescence was observed from both epitaxial and solution grown p-n junctions

A study on the cleaning and modification of metal surfaces by direct current cathodic electrolytic plasma process

The processes of surface treatments or surface modifications, more formally known as surface engineering, tailor the surfaces of engineering materials. The treatments are usually intended to change physical properties such as thermal or electrical conductivity, modify the surface dimensions, i.e. roughness, etc. A novel surface treatment method, Electrolytic Plasma Process (EPP), was developed for coating purposes, in its early developmental stage. In this work, the major effort is to avoid using any environmental hazardous chemical (coatings) and to extend such its application to some new fields. The general response of the substrate material under electrolytic plasma process was summarized including the morphology and its thermodynamic background, microstructural change and its physical process. Selection of operation parameters in direct current EPP was addressed with experimental results. The major component of this work covers the corrosion improvement on widely used structural materials and its electrochemical analysis. The improvement of corrosion resistance over a long time testing condition was confirmed on steel material. These improvements are closely related to the unique surface micro-features with EPP treatment. But the improvement in aqueous corrosion is material dependent. The adverse impact in corrosion was found on EPP treated aluminum alloy

The effect of transition metal additions on double oxide film defects in Al alloy castings

This work investigated the effect of transition metal additions on the double oxide film defects in Al alloys. A bubble trapping experiment was initially conducted, which deliberately trapped an air bubble inside the aluminium melt for a period of time in three different Al alloys (Commercial purity aluminium, 2L99(Al-7Si-0.35Mg) and Al-5Mg alloy), as an analogy of the consumption of the entrapped atmosphere in double oxide film defects in castings. Several elements, namely, Mo, Ti, Zr, Hf, Sc were selected and added into the aluminium melt. The result suggested that the three different alloys behaved differently with regard to the consumption of the entrapped bubble and the different oxide/nitride layers formed. Only the addition of Sc and Mo altered the structure of the oxide surface and promoted the consumption of the air in the trapped bubble in the 2L99 aluminium alloy melt. Sand casting was subsequently conducted for 2L99 alloy with different element additions. Mo and W were found to improve the Weibull moduli of the UTS. Statistical analysis confirmed that such improvement was significant. For the castings with Mo addition, a nitride was found in some double oxide film defects, on the fracture surface of the tensile testbars. This was unusual, as bi-film defects in aluminium castings usually have a short solidification time and do not have enough time to consume the majority of their entrapped oxygen. The formation of the nitride on the surface of bi-film defects in +Mo castings, suggested that the majority of the oxygen was depleted and a reaction was going on between nitrogen and liquid aluminium. The formation of the permeable nitride surface layer on the bifilm defect might promote the consumption of the entrapped gas. This should lead to reductions in the bi-film size and an improvement in mechanical properties. For +W casting, W containing intermetallic compound might be nucleated on the sides of the bifilm and drag the bi-film to the bottom of the casting, resulting in a clean melt and improving mechanical properties. The effect of Si modifier addition on the bi-film defect (Na, Sr and Ba) in 2L99 sand castings was also investigated, which suggested the addition of modifiers resulted in a reduction in the mechanical properties of the 2L99 castings while the bi-film defect content in the casting was high but significantly improved the Weibull moduli of the UTS of 2L99 castings while the bi-film defect content was reduced. The results suggested that the modifier addition tended to aggravate the effect of bi-film defects on mechanical properties by increasing the defect size. Ti and Mn additions into 2L99 alloy were found to not significantly affect the mechanical properties of the casting. A porous oxide structure was found on only one of the fracture surfaces (out of ten) of Ti containing testbar examined, which hardly affect the mechanical properties of the casting. For the +Mn casting, the composition and structure of the oxide was not affected by the addition. The reduction of the mean value of the UTS for both element additions could be due to bi-film defects being introduced during master alloy preparation, master alloy addition and during degassing before casting

The behaviour of double oxide film defects in the processing of liquid Mg alloys

The global demand for Mg alloys continually grew in the last 20 years, motivating a wide interest in the improvement of the mechanical properties of Mg-alloy castings. In addition, the existence of double oxide film defects, which were widely recognised as a major factor in the quality and reproducibility of the properties of light-alloy castings, has been demonstrated in Mg-alloy castings. Thus it became important to understand behaviour of double oxide film defects formed in Mg-alloys. In the work reported here, three different Mg alloys (commercial pure Mg, AZ91 alloy, and Mg-Y alloys) and two cover gases (SF6/air and SF6/CO2), were used, in order to involve different doubled oxide films which may have different behaviours. Direct and cross-sectional observations of the double oxide film defects formed the Mg-alloy castings protected by different cover gases were obtained via a Scanning Electron Microscopy (SEM), and the focus ion beam milling (FIB) technique. In addition, oxide films growing on the corresponding Mg-alloy melt surfaces were also investigated. Based on the observed film structures in conjunction with a thermochemical calculation, evolution processes of the different double oxide film defects were suggested. The quality of Mg-alloy castings was evaluated by the Weibull modulus, which is popularly used to discriminate “good” and “bad” castings. A shortcoming of the traditional Weibull estimation method (i.e. linear least square method) was demonstrated, and a new estimation method was therefore come up with. The Weibull modulus result revealed that air can confer an improvement in the quality of AZ91 castings, compared with CO2

Argonne National Laboratory Report ANL-6600

This seventh Annual Report is a summary of some of the progress in scientific and engineering research and development carried on at Argonne National Laboratory during 1961. As is customary in this series, only those portions of the total program that have reached such a stage that they may be of general interest are recorded. Thus, a comparison with the Annual Reports for 1959 (ANL-6125) and for 1960 (ANL-6275) will reveal the description of a generally different set of scientific activities. A more detailed presentation of any work covered in this report or of the many ANL projects not mentioned may be obtained by perusing the various progress and topical reports issued by the Laboratory during 1961. A list of the publications in the scientific journals during 1961 by Argonne personnel has been given as an Appendix