Development and characterization of Iron-Cobalt (FeCo) based bulk metallic glass (BMG) materials

Abstract

Bulk metallic glass (BMG) is an important class of materials possessing unique set of properties. Iron Cobalt (FeCo) based BMGs have properties especially suited for applications where high strength and wear resistance is required like surgical tools and mechanical precision elements. These can also be used in electromechanical conversion devices. The current thesis is aimed at the study of the mechanical properties of the FeCo based BMGs. Three FeCo-based BMGs with the compositions (Fe0.5Co0.5)69-xNb6B25+x (x = 0, 2, 4) were cast using electric arc melting and suction casting technique. For each composition three samples were prepared, tested, and analysed in as-cast, annealed and partially crystalline conditions. The as-cast alloys possessed high hardness that was found to increase with annealing and partial crystallization. Hardness values approaching 1400 HV were obtained for partially crystalline BMG alloy having the maximum boron content. In X-ray diffraction scans no peaks that indicate crystallinity in the BMG alloys were observed in the as-cast and annealed condition while small broad peaks were present in the diffraction patterns of partially crystalline alloy samples. Glass transition (Tg) and crystallization (Tx) temperatures of all the three BMG alloys in as-cast condition were above 800 K. Saturation magnetization (MS) has shown a decrease while magnetic coercivity (HC) increased with an increase in boron content of the alloy. The maximum value of MS (92 emu/g) was obtained for annealed BMG alloy with the maximum iron-cobalt content. Indentation of the BMG alloys resulted in the formation of the deformation zone underneath the indents that consisted of shear bands. Annealing of the cast FeCo based BMG alloys resulted in a decrease in the size of the deformation zone as compared to as-cast amorphous alloys mainly because of increase in strength of the alloys. Plastically deformed zone size calculations through available models show an overestimation of the zone size. Indentation of partially crystalline alloys (intrinsic composites) produced corner cracks while no cracks were observed in case of as-cast and annealed alloys. Palmqvist crack morphology was observed for corner cracks at all the indentation loads used. Indentation fracture toughness of intrinsic composites was obtained by calculations based on the measured corner crack lengths. Indentation fracture toughness values ranging in 1.85 MPa√m to 2.9 MPa√m, were obtained. Best mechanical properties and thermal stability was shown by the alloy with the highest boron content (29 at%) while better magnetic properties were exhibited by the other two BMG alloys.