Thermodynamics, Kinetics and Mechanical Behavior of Model Metallic Glasses

Abstract

The thermophysical properties and deformation behavior of a systematic series of model metallic glasses was investigated. For Zr-based metallic glasses with all metallic constituents, the activation energy of glass transition was determined to be in the range of 74-173 kJ/mol while the activation energy of crystallization was in the range of 155-170 kJ/mol. The reduced glass transition temperature was roughly the same for all the alloys (~ 0.6) while the supercooled liquid region was in the range of 100-150 K, indicating varying degree of thermal stability. In contrast, the metal-metalloid systems (such as Ni-Pd-P-B) showed relatively higher activation energy of crystallization from short range ordering in the form of triagonal prism clusters with strongly bonded metal-metalloid atomic pairs. Deformation mechanisms of all the alloys were investigated by uniaxial compression tests, strain rate sensitivity (SRS) measurements, and detailed characterization of the fracture surface morphology. For the metal-metal systems, plasticity was found to be directly correlated with shear transformation zone (STZ) size, with systems of larger STZ size showing better plasticity. In metal-metalloid amorphous alloys, plasticity was limited by the distribution of STZ units, with lower activation energy leading to more STZ units and better plasticity. The alloys with relatively higher plasticity showed multiple shear bands while the brittle alloys showed a single dominant shear band and vein-pattern on the fracture surface indicating sudden catastrophic failure. The effect of chemistry change on thermodynamics, kinetics, and deformation behavior was investigated for the model binary NixP100-x and CoxP100-x metallic glasses. Alloys with higher phosphorous content showed greater activation energy of crystallization, indicating better thermal stability. In addition, metallic glasses with higher % P showed greater hardness, modulus, and serrated flow behavior during indentation that is characteristic of inhomogeneous deformation.