The fact that metallic glasses combine both metallic properties and characteristics of amorphous materials lead to intensive research worldwide, exploring this interesting class of materials. Co-based metallic glasses, and Co43Fe20Ta5.5B31.5 in particular, offer the attractive combination of both high fracture strength and excellent soft magnetic properties. Within the framework of this thesis, the generation, validation and implementation of a first principles model for Co43Fe20Ta5.5X31.5 (X=B,Si,P,S) metallic glasses are described. This model allows for the identification of valence-electronconcentration and size-induced changes in structure, elastic and magnetic properties and hence contributes towards a knowledge based design of metallic glasses in the future.
Within the generated ab-initio model stoichiometric body centered cubic supercells are assumed as initial configurations. After annealing at 4000 K, all cells were quenched down to 0 K at different cooling rates and subsequently relaxed in terms of atomic positions and volumes. To validate this first principles model, density and elastic modulus as well as the pair distribution function and volume magnetization of sputtered Co-Fe-Ta-(B/Si) thin film metallic glasses were measured. The computationally obtained density and stiffness values as well as the theoretical pair distribution function of Co-Fe-Ta-B and the volume magnetization of the Si-containing alloy are consistent with experimental data obtained for thin films and with literature data. The infinite cooling rate to quench the molten alloy was determined to be sufficient.
The implementation of the for Co-Fe-Ta-(B/Si) successfully validated first principles model was realized by analyzing short range order, charge transfer and the bonding nature by means of density of states, Bader decomposition and pair distribution function analysis. For Co43Fe20Ta5.5X31.5 (X=B,Si,P,S) a clear trend of a decrease in density and bulk modulus as well as a weaker cohesion was observed as the valence electron concentration for the X element is increased by replacing B with Si and further with P and S. This observed trend upon X variation may be understood based on increased interatomic distances, variations in coordination numbers and the electronic structure changes: As the valence electron concentration of X is increased, the X bonding becomes more ionic. This factor disrupts the overall metallic interactions and leads to weaker cohesion and stiffness. Density of states as well as pair distribution functions are used to identify (Co,Fe)-X atomic pairs as the shortest and strongest constituents. These strong bonds may give rise to a comparatively large stiffness. The highest magnetic moments – and the largest population of unpaired transition metal d-states – are identified for X=S, despite the fact that the presence of X generally reduces the magnetic moment of Co. The interplay between transition metal d-band filling and s-d hybridization was hence identified to be a key materials design criterion. Furthermore, an extended diagonal relationship between the B- and P-containing amorphous alloys (in coherence to the known relationship between elements of the second and third period) was revealed.
This thesis indicates that systematic quantum mechanics simulations enable the identification of composition-induced changes in short range order, charge transfer and bonding nature of metallic glasses. These characteristics are correlated with density, elasticity and magnetism. The here identified property-electronic structure correlations may thus provide the basis for future knowledge based design of glassy materials.