Grain size control and microstructural evolution in nanocrystalline Ni-W alloys

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.Includes bibliographical references (leaves 119-131).Nanocrystalline materials have been studied extensively over the past two decades because of their useful and interesting physical properties. In most cases, these properties derive from the fine characteristic length scale of nanocrystalline structures and are superior to those realized in traditional coarse-grained materials. A fundamental challenge, however, involves the synthesis of high-quality specimens, which represent a classical far-from-equilibrium state due to the large presence of high-energy interfaces. Alloying presents a possibility to reduce this energy penalty through solute segregation and thermodynamic stabilization of the grain boundaries. The present work exploits this concept in the nanocrystalline Ni-W system. Atomistic computer simulations are used to evaluate the potential for stabilization based on the equilibrium solute distribution and energetics of nanocrystalline structures. Following this, a synthesis technique based on electrodeposition is developed where precise control over the alloying addition correlates with precise control over grain size.(cont.) Investigations of the microstructure involving techniques such as transmission electron microscopy, x-ray diffraction, and atom probe tomography provide a detailed view of the structure and solute distribution in these materials, and the results are compared with atomistic simulations and thermodynamic models of nanostructure stabilization. The elevated temperature behavior of experimental specimens is also evaluated and compared to analytical models of microstructural evolution, showing that grain boundary relaxation is an important mechanism for the finest nanocrystalline grain sizes, having a significant influence on properties. With a new degree of control over the nanostructure, Ni-W alloys are produced over a broad range of grain sizes to investigate hardness trends and the breakdown of a classical scaling law in the nanocrystalline regime. An extension of the synthesis technique is also demonstrated for the production of functionally graded and nano-scale composite materials. Potential benefits of the methods developed in this work are highlighted for both fundamental scientific investigations and practical applications.by Andrew J. Detor.Ph.D

Residual stress measurement and microstructural characterization of thick beryllium films

Beryllium films are synthesized by a magnetron sputtering technique incorporating in-situ residual stress measurement. Monitoring the stress evolution in real time provides quantitative through-thickness information on the effects of various processing parameters, including sputtering gas pressure and substrate biasing. Specimens produced over a wide range of stress states are characterized via transmission and scanning electron microscopy, and atomic force microscopy, in order to correlate the stress data with microstructure. A columnar grain structure is observed for all specimens, and surface morphology is found to be strongly dependent on processing conditions. Analytical models of stress generation are reviewed and discussed in terms of the observed microstructure

Gallium incorporation into phosphate based glasses: bulk and thin film properties

The osteogenic ions Ca2+, P5+, Mg2+, and antimicrobial ion Ga3+ were homogenously dispersed into a 1.45 mum thick phosphate glass coating by plasma assisted sputtering onto CP grade titanium. The objective was to deliver therapeutic ions in orthopedic/dental implants such as hip prosthesis or dental screws. The hardness 4.7 GPa and elastic modulus 69.7 GPa, of the coating were comparable to plasma sprayed hydroxyapatite/dental enamel, whilst superseding femoral cortical bone. To investigate the manufacturing challenge of translation from a target to vapour condensed coating, structural/compositional properties of the target (P51MQ) were compared to the coating (P40PVD) and a melt-quenched equivalent (P40MQ). Following condensation from P51MQ to P40PVD, P2O5 content reduced from 48.9 to 40.5 mol%. This depolymerisation and reduction in the P-O-P bridging oxygen content as determined by 31P-NMR, FTIR and Raman spectroscopy techniques was attributed to a decrease in the P2O5 network former and increases in alkali/alkali-earth cations. P40PVD appeared denser (3.47 vs. 2.70 g cm-3) and more polymerised than it’s compositionally equivalent P40MQ, showing that structure/ mechanical properties were affected by manufacturing route