Increasing the kinetic stability of bulk metallic glasses

Metallic glasses are non-equilibrium materials and the glass transition temperature upon heating Tg,h can be used to characterize the kinetic stability of the glass. Annealing below the glass transition is well-known to induce relaxation processes that reduce the glass enthalpy. We demonstrate that a liquid-cooled Au-based metallic glass can achieve very high kinetic stability by an optimal annealing treatment to yield a large increase in Tg,h of 28 K; this is 3–5 times larger than the increase usually reported. The measured enthalpy decrease of 1100 J/mol is about 50% of the difference between the as-cooled glass and the equilibrium crystalline state and reaches the extrapolated enthalpy of the supercooled liquid. The optimal annealing conditions can be determined by an enthalpy-temperature-time (ETT) diagram which is proposed for the first time based on the comprehensive examination of relaxation processes. At equilibrium, a direct relation is established between the increase in kinetic stability as measured by the increase in Tgh and the enthalpy decrease.

The 3-dimensional plot (a) of enthalpy decrease (ΔH) versus isothermal annealing time (logarithmic) and annealing temperatures. (b) The 2-dimensional contours of the ΔH. Each contour (from red to blue) represents a 50 J/mol decrease. From lower-left to upper-right, the diagram can be divided into two zones before crystallization: the nonequilibrium glass state and the metastable equilibrium supercooled liquid state. The dashed curve shows the optimal annealing temperatures to achieve the lowest enthalpy for a given annealing time.Figure optionsDownload high-quality image (512 K)Download as PowerPoint slide