Linking structural defects to the damage behaviour of 3D-printed metallic glasses

• Verknüpfung von Strukturdefekten mit dem Schadensverhalten von 3D-gedruckten metallischen Gläsern

Shi, Jianye; Markert, Bernd (Thesis advisor); Hartmann, Stefan (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Metallic glasses (MGs), a novel class of metallic alloys, exhibit extraordinary mechanical properties due to their amorphous structures. However, the broad applications of MGs are strongly limited by the bottlenecks in manufacturing large-scale components with fully amorphous structures and complex geometries. Selective laser melting (SLM), a recently developed additive manufacturing technique, enables the production of bulkmetallic glass (BMG) components with intricate geometries. Nevertheless, many issues associated with SLM of BMGs remain unresolved, such as uneven heating and cooling rates, residual stress, crystallization, poor surface quality and inherent structural defects. The present thesis focuses on addressing the structure-property relationships of a Zr-based SLM-processed BMG including the characterization of microstructures, the influence of printing defects on the mechanical properties and the related modelling and simulations .A detailed study was firstly conducted on the microstructural characterization of four Zr-based BMGs processed under different SLM conditions. The amorphous structure was verified using X-ray diffraction. The inner pore structure was evaluated using non-destructive X-ray micro-CT and scanning electron microscopy. Both irregular lack-of-fusion pores and gas-induced round pores were observed. Clear correlations were found between porosity, pore size and morphology. Hardness tests at various loads revealed that the intrinsic properties are nearly constant irrespective of processing parameters, whereas the macroscopic properties depend strongly on the global porosity. Moreover, a deeper structure-property relationship was established by correlating the structural defects to the ultimate tensile strength (UTS) both experimentally and numerically. Under uniaxial tension, 3D-printed Zr-based BMGs fracture in a brittle mode with almost zero macroscopic plasticity. The UTS exhibits a linear decrease with increasing porosity, while the Young’s modulus is insensitive in the low-porosity regime. Distinctive shear banding events captured by finite element simulations indicate different fracture mechanisms between ideal defect-free and defect-containing samples. Finally, a thermosdynamically-consistent non-local damage model was developed to describe the unique deformation and fracture behaviours of MGs. The proposed damage model accounts for finite deformation, damage-induced anisotropy and tension-compression asymmetry (TCA). The elastic-viscoplastic response, including the normal-stress-sensitivity and plastic-dilatancy, was characterized by the Mohr-Coulomb criterion. A multi-surface damage concept was adopted for the definition of the second-order damage tensor. The presented damage model reproduced the shear band patterns and the force-displacement curves in bending tests on both unnotched and notched micro-cantilever beams to excellent accuracy. Parameter studies on the degree of regularization and TCA were also carried out. Due to the gradient-type enhancement of the free-energy functional, the finite element computations deliver mesh-objective results.