Self-repair Of Shear Bands In Zr-based BMGs And Its Effect On Thermal Properties

Amorphous alloys have many physical and chemical properties better than metal crystal materials due to their special atomic space arrangement,so that they have potential applications in many areas,such as sporting goods,aerospace,biological application and so on.However,at room temperature,plastic deformation of metallic glasses is highly localized into shear bands,resulting in a limited plastic strain,which impedes their further application.The self-repairing of metallic glasses shear bands can delay the propagation of shear bands.Therefore,the study on the self-repair behaviors of shear bands in the alloys has important theoretical and practical significance.In this paper,the self-repairing of shear bands in the Zr₆₅Cu₁₅Ni_12.5Al_7.5 bulk metallic glasses was studied by heat treatment in its supercooled liquid region,and the effect of self-repairing on its thermal properties was further investigated by thermal analysis methods.The samples for experiments included as-casted specimen?without treatment?,deformed specimen?with 2% compression deformation?and self-repaired specimen?self-repaired after heat treatment?.The self-repairing of shear bands on surface of the specimens was studied after different heat treatment conditions.The results showed that the shear bands on the surface of the samples were repaired at 633 K,643 K and 653 K for 10 min,20 min and 30 min,respectively.At the same temperature,with the increase of repair time,the healing degree is more and more obvious.When the repair time is the same,with the increase of temperature,the degree of repair of shear bands is better.The samples after healing at 633 K and 643 K for 10 min respectively are still amorphous,but the other heat treatment samples have crystallized.The effect of self-repair on the thermal properties was studied by DSC.It is indicated that during the continuous heating process,the change of T_g is not obvious after repaired at 650 K for 10 min,but T_x,T_p and ?T_x reduce significantly.The activation energy was calculated by Kissinger method.The glass transition activation energies of the as-casted,deformed and self-repaired specimens are 252 kJ/mol,263 kJ/mol and 270 kJ/mol respectively;and the crystallization activation energies are 238 kJ/mol,255 kJ/mol and 296 kJ/mol respectively.In the processes of isothermal crystallization,the incubation time of self-repaired samples decreases significantly.The Avrami index was calculated by Johnson-Mehl-Avrami?JMA?method.The Avrami index ranges of the as-casted,deformed and self-repaired specimens are 1.698¹.991,2.273².44 and 2.15².30,respectively.It is found that the isothermal crystallization processes of all the studied samples are the primary crystal or eutectic crystal whose growth is controlled by diffusion,and the nucleation and growth rate decrease with time.The effective activation energies of the three samples are 448 kJ/mol,457 kJ/mol and 333 kJ/mol,respectively.