Influence of Thermal Treatment on Phase Formation Processes in Amorphous Alloys

Abstract

The use of amorphous materials in current nanotechnology allows us to obtain characteristics that could not be produced using materials with crystalline structure. The metastable amorphous state obtained by ultrafast quenching does not always have the best characteristics, however, so devising methods of controlled nanostructuring is now an important area of metallic glass research. The great attention devoted to the processing (heat and pressure treatment) of amorphous alloys is due to their ability to acquire new properties in the nanocrystalline state. Amorphous alloys are heterogeneous systems with amorphous matrices and frozen-in crystallization centers that exist in the metastable state; their properties thus substantially depend on the influence of external conditions (e.g., temperature, pressure, isothermal exposure time), particularly upon phase transitions. Our statement of the problem results from the theory of amorphous alloy thermodynamic stability, according to which external effects (isothermal annealing or long exposure at room temperature) can substantially shift the phase equilibrium in a heterogeneous system with an amorphous matrix and frozen-in crystallization centers. The process of purification of the amorphous matrix from the frozen-in crystallization centers is theoretically substantiated and experimentally implemented, on the basis of the analysis of the positions of the thermodynamic theory of high-temperature stability of amorphous alloys. Аccording to which theory there is an interval of temperatures in which the difference of chemical potentials between phases in a heterogeneous system of the amorphous matrix – frozen-in crystallization centers are negative, that is, the condition for the dissolution of frozen-in crystallization centers is fulfilled. It is shown that the proposed heat treatment modes allow to expand the thermal stability of amorphous alloys on the basis of iron by 20–40 K by purifying the amorphous matrix from the frozen-in crystallization centers by means of upward diffusion. On the basis of the analysis of the theory of high-temperature thermodynamic stability of amorphous alloys, a method for obtaining an amorphous–nanostructural state from the initial amorphous is proposed.