Structural changes and physical properties of Fe–Ni-based metallic glasses rapidly heated by pulsed electrical currents

Fe–Ni–Si–B metallic glasses have been annealed and crystallized using short electrical current pulses. Two types of electrical heat treatment have been used. The first one is an isothermal annealing treatment using a very high initial heating rate while the second one is a thermal spike applied on an amorphous sample held at various initial temperatures. The microstructure of the alloys after heat treatment has been characterized by x-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy. The thermal and magnetic properties of the samples measured by DSC and hysteresis loop tracer have been studied after the various heat treatments and correlated with the microstructure of the alloys. The crystallization at high temperatures produces the gamma phase only, while at low temperatures, a mixture of the gamma and alpha phases (the alpha phase being predominant) is usually observed. The samples initially held at liquid nitrogen temperature and heat treated with a thermal spike remain amorphous and show improved magnetic properties (lower coercive field and higher induction at saturation) without loss of ductility.