Structure And Mechanical Properties Of 3D-Printed Zr-based Bulk Metallic Glass And Composites By Selective Laser Melting

Bulk metallic glasses（BMGs）have received increasing attention owing to their outstanding properties.However,the limited size is the bottleneck restraining the applications of BMGs.In recent years,the emerging technology of selective laser melting（SLM）has provided a new approach to fabricating large-sized BMGs with complex geometries.Among various BMG systems,Zr-based BMGs have received extensive attention due to the excellent manufacturability in SLM.However,the Zr-based BMGs prepared by SLM usually displayed less plasticity and fracture toughness.Therefore,this dissertation aims to improve the plasticity and toughness of the SLMed Zr-based BMGs and reveal the relevant mechanisms.For this purpose,four Zr-based BMG systems are chosen,and the microstructures and mechanical properties of the BMGs and composites prepared by SLM are systematically investigated.Firstly,Zr_60.14Cu_22.31Fe_4.85Al_9.7Ag₃ BMG was studied.The defect engineering strategy,namely a proper combination of porosity and crystallization fraction,was employed to achieve a good strength-plasticity-toughness integration of the SLMed BMGs,as crystallization in heat affected zoons（HAZs）has a strengthening effect,while micropores have a toughening effect.The experimental results indicate that the compressive plasticity of the SLMed samples is improved when the crystallization fraction is low（<5%）and the porosity is high（>2.8%）.When the crystallization fraction and the porosity are both at moderate levels,the fracture toughness of the SLMed samples is improved.Subsequently,the BMG composite（BMGC）system of Zr_57.4Cu_16.4Ni_8.2Al₁₀Ta₈was selected and prepared by SLM,in which a small amount of ductile Ta particles are uniformly distributed on the amorphous matrix.The results show that the SLMed BMGC has a fracture strength of 1932 MPa,a plastic strain of 2.15%,and fracture toughness of60.8 MPa m^1/2.The excellent mechanical properties of the SLMed BMGC are because of Ta particles hindering the propagation of shear bands.Since nanocrystallization in HAZs cannot be completely avoided in SLMed Zr-based BMGs,we attempted to form a ductile crystalline phase in HAZs in the SLMed BMGs.Therefore,a binary BMG system of Zr₅₀Cu₅₀was selected as the ductile B2-Zr Cu phase could be preferentially precipitated during laser scanning.The results demonstrate that B2phase was indeed formed in HAZs and a fully amorphous structure retained in molten pools under SLM.This structure leads to a good combination of strength and plasticity in this SLMed BMGC.The fracture strength reaches 1841 MPa,and plastic strain reaches3.17%.More importantly,a significant work-hardening after yielding is achieved.To improve the stability of the amorphous phase and B2-Zr Cu,we modify the composition of the binary Zr Cu system with the minor addition of Al and Co elements,as these two elements are proved to be a stabilizer of the amorphous phase and B2 phase.Finally,we have designed a quaternary system of Zr_47.5Cu_45.5Al₅Co₂.Under SLM,nearly a single crystalline phase of B2-Zr Cu is precipitated in HAZs,while a fully amorphous phase occurs in molten pools.The SLMed BMG has a good combination with a fracture strength of 1851 MPa,a plastic strain of 4.65%,and fracture toughness of 53.9 MPa m^1/2.In addition,apparent work-hardening behavior is also observed.The enhancement of plasticity is mainly due to the"transformation-induced plasticity"effect.Furthermore,the enhancement of fracture toughness relies on the B2-Zr Cu near the notch undergoing martensitic transformation accompanied by energy absorption,which delays the initiation of cracks.In summary,by composition design and optimization of SLM processing,the microstructures,including porosity,crystallization phase in HAZs,and amorphous phase content of the SLMed Zr-based BMGs,could be extensively tailored,and a series SLMed BMGs and BMGCs with enhanced mechanical properties were obtained.The current work provides a guild for the future development of 3D-printed BMGs and BMGCs with desirable mechanical properties.