Crystallization of highly undercooled metallic melts and metallic glasses around the glass transition temperature

Abstract

Crystallization in highly undercooled melts can be studied either after severe undercooling of the melt or after heating up metallic glasses above their glass transition temperature. Whereas the crystallization of silicate glasses proceeds only above the glass transition temperature, the crystallization of metallic glasses can occur in both temperature ranges. Below the glass transition temperature, nucleation and crystal growth are controlled by diffusivity with an Arrhenius-type temperature dependence; above the glass transition, crystallization kinetics can be better described by the Vogel-Fulcher-Tammann equation which is usually used to describe the temperature dependence of shear viscosity. The different behaviour in comparison with silicate glasses is assumed to be due to the metallic bonding which allows atomic exchange of the glass-forming elements by diffusion even at temperatures below the glass transition temperature. Usually, metallic glasses are found to crystallize very rapidly at temperatures close to the glass transition, thus hiding the glass transition itself. For example, metal-metalloid glasses ( e.g. Fe 75B 25) and zirconium based transition metal glasses ( e.g. Co 33Zr 67 or Co 50Zr 50) are known to crystallize within a few seconds in this temperature range. Zr 60Ni 25Al 15 glasses, however, can be held without crystallization for relatively long times in the highly undercooled state, i.e. in the temperature range above the glass transition temperature. During primary crystallization of metallic glasses, e.g. FINEMET (Fe 73.4Cu 1Nb 3.1Si 13.4B 9.1, with size-dependent growth rates, the microstructure can be controlled by the addition of slow diffusing elements such as Nb and/or elements such as Cu or Au which enhance the nucleation rate.