Short range ordering and microstructure property relationship in amorphous alloys

Abstract

A novel algorithm, "Next Neighbourhood Evaluation (NNE)", is enunciated during the course of this work, to elucidate the next neighbourhood atomic vicinity from the data, analysed using tomographic atom probe (TAP) that allows specifying atom positions and chemical identities of the next neighbouring atoms for multicomponent amorphous materials in real space. The NNE of the Pd55Cu23P22 bulk amorphous alloy reveals that the Pd atoms have the highest probability to be the next neighbours to each other. Moreover, P-P correlation corroborates earlier investigations with scattering techniques that P is not a direct next neighbour to another P atom. Normalizing the distances by the first next neighbour distance yields a ratio similar for all elemental correlations showing that there exists a certain order among the atoms, irrespective of the chemical nature of the atoms. Analogous investigations on the Fe40Ni40B20 metallic glass ribbons, in the as quenched state and for a state heat treated at 350ºC for 1 hour insinuate a pronounced elemental inhomogeneity for the annealed state, though, it also depicts glimpse of a slight inhomogeneity for B distribution even for the as quenched sample. A comparison of the NN atomic distributions for as quenched and annealed state shows accumulation of Ni and B. It also reveals the tendency of Fe and B to move slightly away from each other, an incipient step to Ni rich boride formation.Moreover, a comprehensive microstructural investigation has been carried out on the Zr53Co23.5Al23.5 glassy system. TEM and TAP investigations evince that the as cast bulk samples constitutes a composite structure of an amorphous phase and crystalline phase(s). The crystallization is essentially triggered at the mould walls due to heterogeneous nucleation. The crystalline phase(s) on the edge of the bulk rod lead to the magnetic anisotropy and two exothermic crystallization peaks in the DSC scans. The three dimensional atomic reconstruction maps of the volume analysed by TAP reveal a complex stereological interconnected network of two phases. The phase that is rich in Zr and Al concentration is depleted in Co concentration while the phase that is rich in Co concentration is depleted both in Zr and Al. Zr53Co23.5Al23.5 glassy splat samples exhibit a single exothermic crystallization peak contrary to the as cast bulk sample with a different Tg temperature. A single homogeneous amorphous phase revealed by TEM investigations depicts that the faster cooling rate during splat quenching is sufficient for the vitrification of this alloy system. Zr53Co23.5Al23.5 bulk samples and splat samples, both exhibit very soft ferromagnetic properties. The heat treatment of as cast bulk sample results in an increase in net magnetization. The crystallization kinetics during the heat treatment, effects both the remanent magnetization, Mr and coercivity, Hc accordingly. Intriguingly, a salient increase in soft ferromagnetic properties is recorded for the heat treated splat samples. Hence, the magnetic properties of the glassy Zr53Co23.5Al23.5 alloys especially for the splat quenched samples can be enhanced by a controlled heat treatment.