Supercooled liquid properties versus glass formation:application to Al‐rich alloys

Abstract

Glass formation when quenching from the liquid state is discussed in terms of the thermodynamics and kinetics of the stable/metastable competing phases. Recent developments have shown exceptionally good glass‐forming ability in a number of multicomponent systems which exhibit a wide supercooled liquid region before crystallisation, low critical cooling rate, (between 1 and 500 K · s−1) and, consequently, large critical sample thickness (up to 40 mm). Thermodynamics is required to relate the glass transition temperature, Tg, to the energetics of the supercooled liquid. Kinetic destabilisation of equilibrium solidification and, consequently, glass‐forming ability are favoured by the high‐viscosity values achieved under continuous cooling. The relative thermal stability of the supercooled liquid depends on the thermodynamic driving force and interfacial energy between each competing nucleating phase and the molten alloy. Following a procedure already established for binary systems, the thermodynamic optimisation of the Al‐rich part of the Al‐Ni‐Nd‐Cu system by the Calphad method is applied to the kinetics of the nanocrystallisation of the FCC‐(Al) for the Al87Ni7Nd3Cu3 alloy. The Al‐rich corner of the quaternary system is evaluated using the CALPHAD method. The thermodynamical data set obtained is used to calculate the thermodynamic functions needed to model nucleation and crystal growth from the undercooled liquid alloy. The glass formation is discussed in terms of the kinetics of the processes.