On polymorphic crystal growth in a Ti-Ni-Cu-Fe system metallic glass at the glass-transition temperature

A polymorphic crystallization reaction forming spherical cP2/B2 crystals in a Ti50Ni22Cu22Fe6 metallic glass close to the glass-transition temperature was studied. The phase transformation was investigated by differential scanning calorimetry. X-ray diffractometry indicated the formation of the cP2/B2 crystalline phase after the initial exothermic crystallization reaction. In situ transmission electron microscopy observation allowed direct measurement of a crystal growth rate at a constant temperature close to the glass-transition temperature while the usage of an analytical microscope confirmed the polymorphic nature of the phase transformation. The results are interpreted in line with the diffusion data suggested from the viscosity obtained in creep experiments with a thermo-mechanical analyzer. Thermodynamic considerations are also taken into account.