An Ultrasonic Study on the Polyamorphic Transition in La/Ce-based Bulk Metallic Glass to 12 GPa

Abstract:

Bulk metallic glasses (BMGs), also known as amorphous alloys, are one of the most promising materials in the 21^st century. With their unique structures, BMGs are at the cutting edge of materials study for both commercial applications and fundamental studies. In the light of engineering applications, some metal-metal alloy BMGs are stronger than steels but able to be shaped and molded like plastics. As a new member of glass family, metallic glasses are also good models for studies of fundamental problems in condensed matter physics.

Pressure-induced polyamorphism in Ce-based bulk metallic glasses have attracted interest in recent years. Ultrasonic measurements of the compressional and shear wave velocities not only provide critical information about their interatomic forces, but also offer a unique approach for precise determination of mass densities under pressure that are quite difficult to access by other methods. In this study, the acoustic velocities of La₃₂Ce₃₂Al₁₆Ni₅Cu₁₅ bulk metallic glass were measured up to 12.3 GPa using ultrasonic interferometry in a multi-anvil apparatus at room temperature. Both compressional and shear waves exhibited softening behaviors at 0-4 GPa, followed by a continuous increase (stiffening) with pressures. Measurements under decompression exhibit a hysteretic behavior compared to that on compression. The mass density as a function of pressure revealed three different amorphous states and the transition pressures were accompanied by distinct changes in the pressure derivatives of elastic moduli. The interpretation of the residual densification and the softening/stiffening across the polyamorphic transition can be sought via the topological rearrangement of the solute-centered clusters in medium-range order and the possibility of 4f electron localization/delocalization in Ce.