Bulk Metallic Glasses: Deformation and Design

Abstract

Bulk metallic glasses are a group of alloys with amorphous atomic structure. When these alloys cool sufficiently fast, the atoms will “freeze” in the chaotic structure of the melt, rather than form crystals like most metals. The resulting materials are often very strong, hard and corrosion resistant, but brittle compared to their crystalline counterparts.

Two key limitations to the applicability of bulk metallic glasses are the focus of this thesis. Firstly, the typically very limited plasticity in tension is studied by tensile testing metallic glass samples across a range of temperatures and strain rates. Secondly, a design tool for reducing the normally high cost of discovery of new alloys is tested.

Zr70Ni16Cu6Al8 bulk metallic glass samples were mechanically tested in tension at nominal strain rates and temperatures of 10-4, 10-3, 10-2 and 10-1 s-1 and 77, 150 and 295 K, respectively. A strengthening is observed with decreased temperature, with yield strength increasing by an average of 16 % from 1503 MPa at 295 K to 1746 MPa at 77 K. Plastic deformability is found highest at 150 K, intermediate at 77 K, and very low at 295 K. At room temperature only a single sample showed any plastic deformation, which occurred due to the intersection and mutual stabilization of several shear bands. At the two lower temperatures, large plastic deformation was accommodated by the stable sliding of a single shear band. Between 150 and 295 K a reversal of strain rate sensitivity is observed, with high strain rate favoring high strength and low plasticity at low temperature and vice versa at room temperature.

In-situ probing of heat generation during the stable sliding of a shear band is achieved by studying localized boiling along the shear band in a sample submerged in liquid nitrogen during testing. By measuring size and frequency of the bubbles, heat release is correlated to plastic deformation, showing that heating occurs even during stable flow at cryogenic temperature, but only becomes noticeable well after onset of deformation. Heat release is therefore considered to be a secondary effect of the shear band deformation.

In the final part of the thesis, a tool for design of new glassy alloys is tested. By combining thermodynamic modelling in the Thermo-Calc software with a topological mismatch factor based on constituent atomic size and compositional ratios, an area in the Fe-Nb-B phase diagram is pinpointed as favorable for glass formation. Both new experimental production and literature data fit well with the predictions of the model, making it worthy of further testing in other systems.