Mechanically induced solid-state reaction for synthesizing glassy Co₇₅Ti₂₅ soft magnet alloy powders with a wide supercooled liquid region

Abstract

A single phase of glassy Co₇₅Ti₂₅ alloy powders was synthesized by high-energy ball milling the elemental powders at room temperature, using the mechanical alloying method. The final product of the glassy alloy, which is obtained after ball milling for 86 ks, exhibits soft magnetic properties with polarization and coercivity values of 0.67 T and 2.98 kA/m, respectively. This binary glassy alloy, in which its glass transition temperature (T_g) lies at a rather high temperature (833 K), transforms into face-centered-cubic Co₃Ti (ordered phase) at 889 K through a single sharp exothermic reaction with an enthalpy change of crystallization (ΔH_x) of −2.35 kJ/mol. The supercooled liquid region before crystallization ΔT_x of the synthesized glassy powders shows an extraordinary high value (56 K) for a metallic binary system. The reduced glass transition temperature [ratio between T_g and liquidus temperatures, T_l (T_g/T_l)] was 0.56. We also demonstrated postannealing experiments of the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction). Its heat formation was measured directly and found to be −0.56 kJ/mol. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glassy phase.