The role of Co/Al ratio in glass-forming GdCoAl magnetocaloric metallic glasses

Abstract

In this work, a prominent improvement of glass-forming ability is achieved through the adjustment of Co/Al ratio in the ternary GdCoAl system. The critical diameter of Gd55Co17.5Al27.5 bulk metallic glass (BMG) is up to 8 mm, which is larger than that of the Gd55Co22.5Al22.5 BMG with a critical diameter of 3 mm and most ternary Gd-based BMGs. Thermodynamic investigations in terms of specific heat capacity and Gibbs free energy difference indicate that the Gd55Co17.5Al27.5 BMG possesses a lower driving force for crystallization. Kinetic analyses based on viscosity and dynamic relaxation spectrum reveal a stronger liquid behavior and more sluggish kinetics of the Gd55Co17.5Al27.5 BMG compared with the Gd55Co22.5Al22.5 BMG. Through the combination of the linear thermal expansion behavior and relaxation, the distinct difference of the contraction in supercooled liquid region of Gd55Co17.5Al27.5 and Gd55Co22.5Al22.5 BMGs is well explained. In addition, the magnetic entropy change is further enhanced by appropriate annealing treatment through the formation of more complex structures comprising short-range order, medium-range order and nano-crystallized structure.