Influence of crystallinity on thermo-process ability and mechanical properties in a Au-based bulk metallic glass

The present investigation addresses the impact of crystallinity on the mechanical properties: hardness (HV) and toughness (KIC) in an Au49Cu26.9Ag5.5Pd2.3Si16.3 BMG which appears especially attractive for applications in jewelry and watch making industries. Thermal stability is first determined using differential scanning calorimetry (DSC), X-ray diffraction and thermo-mechanical analysis (TMA). Then the conditions (time, temperature) in which crystallization is observed (during annealing above the glass transition temperature) and the kinetics are determined. Results show an increase of hardness proportional to the volume fraction of the crystalline phase (ϕ) (from about 350 HV up to about 480 HV), and a drastic reduction in fracture toughness from about 20 MPa√m down to 1.5 MPa√m for fully crystalline structure. Finally the conditions required achieving a good compromise between hardness and toughness are established.