Synthesis and characterization of bulk metallic glass matrix composities

摘要

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.ududComposites with a bulk metallic glass matrix are synthesized and characterized. This was made possible by the recent development of bulk metallic glasses that exhibit high resistance to crystallization in the undercooled liquid state. In this thesis, experimental methods for processing metallic glass composites are introduced. Three different bulk metallic glass (BMG) forming alloys were used as the matrix materials. Ceramics such as SiC, WC, or TiC, and metals W or Ta were introduced as reinforcement into the metallic glass.ududStructure, microstructure and thermal stability of the composites are studied by X-ray diffraction, optical microscopy and differential scanning calorimetry. The metallic glass matrix remained amorphous after adding up to 30 percent volume fraction of particles or short wires. X-ray diffraction patterns of the composites show only peaks from the second phase particles superimposed on the broad diffuse maxima from the amorphous phase. Optical micrographs reveal uniformly distributed particles in the matrix. The thermal stability of the matrix did not deteriorate after adding the particles. In the case of SiC, the matrix becomes even more robust with respect to crystallization.ududThe reactions at the interfaces between the matrix and the different reinforcing materials are investigated with scanning electron microscopy, transmission electron microscopy, and electron microprobe. At the interfaces between the matrix and the WC or SiC particles, ZrC layers foiled. W and Si diffused into the matrix, repectively. At the interface between W and the matrix, a thin layer of nanocrystals is observed after cooling the liquid/particulate mixture.ududThe mechanical properties of the composites are studied in compression and tension. Compressive strain to failure increased by over 300% compared to the unreinforced [...] and the energy to fracture of the tensile samples increased by over 50% adding 15 vol. % W.ududThe effect of silicon on the glass forming ability of [...] bulk metallic glass during composite processing is studied. Composites of the [...] bulk metallic glass, reinforced with up to 30 vol. % SiC particles are synthesized. During processing of the composites, a TiC layer forms around the SiC particles and Si diffuses into the [...] matrix stabilizing the supercooled liquid against crystallization. The small Si addition between 0.5 and 1 at. % increases the attainable maximum thickness of glassy ingots from 4mm for Cu-Ti-Zr-Ni alloy to 7mm for Cu-Ti-Zr-Ni-Si alloys. DSC analyses show that neither the thermodynamics nor the kinetics of the alloy is significantly affected by the Si addition. This suggests that Si enhances the glass forming ability by chemically passivating impurities such as oxygen and carbon that cause heterogeneous nucleation in the melt.