CRYSTALLIZATION OF HIGHLY UNDERCOOLED METALLIC MELTS AND METALLIC GLASSES AROUND THE GLASS-TRANSITION TEMPERATURE

Crystallization in highly undercooled melts can be studied either afte r 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 temperat ure, the crystallization of metallic glasses can occur in both tempera ture ranges. Below the glass transition temperature, nucleation and cr ystal growth are controlled by diffusivity with an Arrhenius-type temp erature dependence; above the glass transition, crystallization kineti cs can be better described by the Vogel-Fulcher-Tammann equation which is usually used to describe the temperature dependence of shear visco sity. The different behaviour in comparison with silicate glasses is a ssumed 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 transit ion, thus hiding the glass transition itself. For example, metal-metal loid glasses (e.g. Fe75B25) and zirconium based transition metal glass es (e.g. Co33Zr67 or Co50Zr50) are known to crystallize within a few s econds in this temperature range. Zr60Ni25Al15 glasses, however, can b e held without crystallization for relatively long times in the highly undercooled state, i.e. in the temperature range above the glass tran sition temperature. During primary crystallization of metallic glasses , e.g. FINEMET (Fe73.4Cu1Nb3.1Si13.4B9.1, with size-dependent growth r ates, the microstructure can be controlled by the addition of slow dif fusing elements such as Nb and/or elements such as Cu or Au which enha nce the nucleation rate.