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Bulk metallic glasses and their composites : composition optimization, thermal stability, and microstructural tunability

  • Author(s): Khalifa, Hesham Ezzat
  • et al.
Abstract

A design protocol utilizing common elements for bulk metallic glass formation has been employed to develop novel, low cost Fe-, and Ti- based bulk metallic glasses. A critical obstacle that was successfully overcome in this work is the omission of beryllium in these alloys. Beryllium is of vital importance in many bulk metallic glass forming systems, but it is expensive and poses considerable health risks. Bulk metallic glasses in these novel Fe-, and Ti-based systems exhibit extremely high mechanical strength and excellent thermal stability. Devitrification and cooling rate experiments were used to identify crystalline phase formation and assess activation energy for crystallization, as well as to explore and develop ductile BMG composites. To better control microstructure in these BMG composites, a novel processing technique, called semi-solid forging was developed, wherein the alloy melt is heated to above the melt temperature of the glass, but below the melt temperature of the ductile crystalline phase. Such an approach permits the maintenance of a glassy, or nanocrystalline matrix phase, while simultaneously coarsening and homogenizing the ductile, secondary phase. This processing approach leads to enhanced ductility in the alloys, which, to this point, has not been observed using conventional casting methods. The combination of novel, low-cost, alloy compositions with semi-solid forging has been successfully utilized to develop new high strength structural materials with enhanced ductility and toughness. Microstrutural and mechanical properties of these novel, toughened, BMG composites are presented. A comprehensive analysis of the relationship between deformation mechanisms and microstructure reveals that enhanced ductility is predicated on matching fundamental mechanical and microstructural length scales in a Ti-Ni-Si-Mo BMG composite. Under optimized microstructural conditions, a maximum compressive strength exceeding 2400 MPa with ̃30% total strain to failure was achieved. Furthermore, a 400% increase in fracture toughness over the monolithic glass, and tensile ductility of 3.5% were observed. The design of conventional, crystalline, high-performance structural materials is governed by optimizing combinations of mechanical properties such as strength, ductility, toughness, and elasticity. In general though, high strength comes at the expense of ductility. The development of ductile BMG composites represents the dawn of a new era in structural materials that couples ultra high strength with considerable ductility and enhanced fracture toughness

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