Influence of sub-glass transition heat-treatment on physical and structural properties of Cu46Zr44Al8Hf2 metallic glass

The metastability of metallic glasses (MGs) can be altered via post-heat treatment around its glass transition temperature Tg. Here, the influence of the short-term heat treatment slightly below the glass transition on the thermal, thermomechanical, and structural properties are investigated. The water-cooled copper mold casting is used to produce the Cu46Zr44Al8Hf2 MGs under argon gas. Heat treatment was performed by continuous heating at 20 °C/min to 400 °C, followed by immediate cooling. Samples were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) via continuously heating through its Tg and Tx (crystallization temperature) and by X-ray diffraction (XRD) in Bragg-Brentano (θ-2θ) geometry. Main findings observed are (1) an increase in the thermal stability, Tx-Tg, registered by DMA and DSC, (2) a remarkable drop of the relaxation enthalpy in the DSC trace calculated from the change in specific heat beforeTg, (3) appearance of a third broad peak after heat-treatment in XRD, and (4) steady profile of E^' below Tg correlated to the β-relaxation in DMA. The unexpected increase in ∆T is linked to the release of the residual stresses accumulated during fast-quenching of the molten ingot down to room temperature. This stress can be estimated as 240 MPa on the outer surface and 100 MPa in the center of the cast CuZr-MG plates, which can be eliminated by the introduced heat-treatment method. Overall, this study suggests a direct and practical way to enhance the thermal stability and high strength retention of CuZr-based metallic glasses at elevated temperatures.