Tuning of the magnetic properties of Hf2Co11B alloys through a combined high pressure torsion and annealing treatment

The evolution of crystalline structure induced by heat treatment and high pressure torsion (HPT) deformation and its influence on the magnetic properties of amorphous and partially crystalline Hf2Co11B ribbons were investigated. Slight improvement in thermal stability of the as-quenched sample after deformation was observed as indicated by the shift of the first crystallization peak from 592 to 598 °C. Plastic deformation of the initially annealed partially crystalline alloy led to its amorphization, as confirmed by X-ray diffraction (XRD). However, the presence of small volume fraction of needle-like nanocrystals was indicated by transmission electron microscopy (TEM). The annealed sample subjected to high pressure torsion was characterized by the reduced coercive field, from 0.7 to 0.2 kOe, while the subsequent reannealing of the deformed sample enhanced the coercivity up to 1.3 kOe. Transmission electron microscopy analysis revealed the existence of nanocrystals with large lattice constant of 8 Å characteristic of Hf2Co11 phase. Magnetic measurements also confirmed that some nanocrystals of the hard magnetic phase were present in the sample after deformation, indicating their importance for maximization of coercivity. The isolation of this quite elusive phase is clearly linked to magnetic performance and the method combining severe plastic deformation (SPD) and heat treatment was found to allow tuning of the structure and improving the hard magnetic properties. •Amorphization of Hf2Co11B by high pressure torsion (HPT).•Thermal stability of Hf2Co11B is slightly improved by HPT deformation.•Reversible changes in magnetic properties by HPT/annealing treatment.