COMPRESSIVE BEHAVIOR OF BULK METALLIC GLASS UNDER DIFFERENT CONDITIONS - COUPLED EFFECT OF TEMPERATURE AND STRAIN RATE
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University of North Carolina at Charlotte
Metallic glass was rst reported in 1960 by rapid quenching of Au-Si alloys. But, due to the size limitation, this material did not attract remarkable interest until the development of bulk metallic glasses (BMGs) with specimen sizes in excess of 1 mm. BMGs are considered to be promising engineering materials because of their ultrahigh strength, high elastic limit and wear resistance. However, they usually suer from a strong tendency for localized plastic deformation with catastrophic failure. Many basic questions, such as the origin of shear softening and the strain rate eect remain unclear. In this thesis, the mechanical behavior of the Zr55Al10Ni5Cu30 bulk metallic glass and a metallic glass composite is investigated. The stress-strain relationship for Zr55Al10Ni5Cu30 over a wide range of strain rate (5x10-5 to 2x103 s-1) was investigated in uniaxial compression loading using both MTS servo-hydraulic system (quasi-static) and compression Kolsky bar system (dynamic). The eect of the strain rate on the fracture stress at room temperature was discussed. Based on the experimental results, the strain rate sensitivity of the bulk metallic glass changes from a positive value to a negative value at high strain rate, which is a consequence of the signicant adiabatic temperature rise during the dynamic testing. In order to characterize the temperature eect on the mechanical behavior of the metallic glass, a synchronically assembled heating unit was designed to be attached onto the Kolsky bar system to perform high temperature and high strain rate mechanical testing. A transition from inhomogeneous deformation to homogeneous deformation has been observed during the quasi-static compressive experiments at testing temperatures close to the glass transition temperature. However, no transition has been observed at high strain rates at all the testing temperatures. A free volume based model is applied to analyze the stress-strain behavior of the homogeneous deformation. To further examine the inelastic deformation of the Zr-based bulk metallic glasses, instrumented nanoindentation experiments were performed. A transition from discrete plastic deformation to continuous plastic deformation was found when strain rate is increased but still within the quasi-static strain rate region. Motivated by the metal matrix composite material, a tungsten reinforced BMG composite was investigated at quasi-static and dynamic strain rates. The mechanical behavior of the metallic glass matrix was improved signicantly by the addition of W particles.
BULK METALLIC GLASSESHIGH TEMPERATUREKOLSKY BARNANOINDENTATION
Kecskes, LaszloZhou, AixiZhang, HaitaoZhou, Jing
Thesis (Ph.D.)--University of North Carolina at Charlotte, 2013.
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