Quantum mechanically guided design of mechanical properties and topology of metallic glasses

Evertz, Simon; Schneider, Jochen M. (Thesis advisor); Dehm, Gerhard (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020, Kumulative Dissertation

Abstract

Metallic glasses are promising structural materials due to their unique property combinations such as high fracture toughness and high strength. For structural applications and processing, the coefficient of thermal expansion is an important design parameter. Here, it is demonstrated that predictions of the coefficient of thermal expansion for metallic glasses by density functional theory based ab initio calculations are efficient both with respect to time and resources. The coefficient of thermal expansion is predicted by an ab initio based method utilising the Debye-Grüneisen model for a Pd-based metallic glass, which exhibits a pronounced medium range order. The predicted coefficient of thermal expansion of 3.4∙10−5 K−1 at room temperature iscritically appraised by in situ synchrotron X-ray diffraction and excellent agreement is observed. Through this combined theoretical and experimental research strategy, the feasibility to predict the coefficient of thermal expansion from the ground state structure of a metallic glass until the onset of structural changes is shown. This strategy provides a method to efficiently probe a potentially vast number of metallic glass alloying combinations regardingthermal expansion. For the application of metallic glasses as structural materials, high fracture toughness is crucialto avoid catastrophic failure of the material in a brittle manner. One fingerprint for fracture toughness in metallic glasses is the fraction of hybridized bonds, which is affected by alloyingPd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os, Ir, Pt, and Au. It is shown that experimental fracture toughness data is correlated to the fraction of hybridized bonds which scale with the localized bonds at the Fermi level. Thus, the localized bonds at the Fermi level are utilized quantitatively as a measure for fracture toughness. Based on ab initio calculations, the minimum fraction of hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3. According to the ansatz that the crystal orbital overlap population at the Fermi level scales with fracture toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95 ± 20 MPa·m0.5 is predicted quantitatively for the first time. Consistent with this prediction, in micro-mechanical beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films show pronounced plasticity and absence of crack growth. As the properties of metallic glasses depend on the electronic structure, which in turn is definedby chemical composition, the influence of metalloids such as B on glass transition, topology, magnetism, and bonding is investigated systematically for B concentrations x = 2 to 92 at.% inthe (Co6.8±3.9Ta)100-xBx system. From an electronic structure and coordination point of view, theB concentration range is divided into three regions: Below 39 ± 5 at.% B, the material is a metallic glass due to the dominance of metallic bonds. Above 69 ± 6 at.%, the presence of an icosahedra-like B network is observed. As the B concentration is increased above 39 ± 5 at.%,the B network evolves while the metallic coordination of the material decreases until the Bconcentration of 69 ± 9 at.% is reached. Hence, a composite is formed. It is evident that, based on the B concentration, the ratio of metallic bonding to icosahedral bonding in the compositecan be controlled. It is proposed that, by tuning the coordination in the composite region, glassy materials with defined plasticity and processability can be designed. While it is accepted that the plastic behaviour of metallic glasses is affected by their free volume content, the effect thereof on chemical bonding has not been investigated systematically. According to electronic structure analysis, the overall bond strength is not significantly affected by the free volume content. However, with increasing free volume content, the average coordination number decreases. Furthermore, the volume fraction of regions containing atoms with lower coordination number increases. As the local bonding character changes from bonding to anti-bonding with decreasing coordination number, bonding is weakened in the volume fraction of lower coordination number. During deformation, the number of strong, short-distance bonds decreases more for free volume containing samples than for samples without free volume, resulting in additional bond weakening. Thus, it is shown that the introduction of free volume causes the formation of volume fractions oflower coordination number resulting in weaker bonding and proposed that this is the electronic structure origin of the enhanced plastic behaviour reported for glasses containing free volume.

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