Bulk metallic glasses (BMGs) are known to have various advantageous chemical and physical properties. However, the condition of producing BMGs is critical. From a melt to congealing into a glass, the nucleation and growth of crystals has to be suppressed, which requires a fast removal of the heat. Such high cooling rates inevitably confine the casting dimensions (so-called critical casting thickness). To overcome this shortcoming, additive manufacturing proves to be an interesting method for fabricating metastable alloys, such as bulk metallic glasses.
Selective laser melting (SLM), one widely used additive manufacturing technique, is based on locally melting powder deposited on the powder bed layer by layer. During the SLM process, the interaction between laser beam and alloys is completed with a high energy density (105 - 107 W/cm2) in very short duration (10-3 - 10-2 s), which results in a high cooling rate (103 - 108 K/s). Such high cooling rates favour vitrification and to date, various glass-forming alloys have been prepared. The approach to prepare bulk metallic glasses (BMGs) by SLM bears the indisputable advantage that the size of the additively manufactured glassy components can exceed the typical dimensions of cast bulk metallic glasses. Simultaneously, also delicate and complex geometries can be obtained, which are otherwise inaccessible to conventional melt quenching techniques. By using such advantages of SLM, Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 (at.%) and Zr52.5Cu17.9Ni14.6Al10Ti5 (at.%) BMGs have been successfully fabricated via SLM in the current work. The SLM process yields material with very few and small defects (pores or cracks) while the conditions still have to render possible vitrification of the molten pool. This confines the processing window of the fully amorphous SLM samples. By additively manufacturing different BMG systems, it is revealed that the non-linear interrelation is differently pronounced for varied compositions. The only way to obtain glassy and dense products is optimizing all the process parameters. However, it is difficult to obtain fully dense sample (100%). The relative density of the additively manufactured BMGs can reach 98.5% (Archimedean method) in current work. The residual porosity acts as structural heterogeneities in the additively manufactured BMGs.
The structures of BMGs are sensitive to the thermal history, i.e. to the cooling rate and to the thermal treatment. During SLM process, the laser beam not only melts the topmost powder, but also the adjacent already solidified parts. Such complicated thermal history may lead to locally more/less relaxed structure of the additively manufactured BMGs. Thus, systematic and extensive calorimetric measurements and nanoindentation tests were carried out to detect these structural heterogeneities. The relaxation enthalpies, which can reveal the free volume content and average atomic packing density in the additively manufactured BMGs are much higher than that in the as-cast samples, indicating an insufficient duration for structural relaxation. The nanoindentation tests indicate that the structure of additively manufactured BMG is more heterogeneous than that of as-cast sample. Nevertheless, no obvious heat-affected zone which corresponds to the more/less relaxed structure is visible in the hardness map. In order to reveal the origin of such heterogeneity, the thermal field of the additively manufactured BMGs was simulated via finite volume method (FVM). Owing to the different process parameters and varied thermophysical properties of Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 and Zr52.5Cu17.9Ni14.6Al10Ti5 BMGs, the heat-affected zone (HAZ) is differently pronounced, resulting in the varied heterogeneities of both additively manufactured BMGs.
Afterwards, the physical and chemical properties of the additively manufactured BMGs were systematically studied. The additively manufactured BMGs tend to fail in a premature manner. The heterogeneities (defects, crystalline phases and relaxed/rejuvenated regions) can determine the mechanical and chemical properties of the BMGs. In the current work, the additively manufactured BMGs are fully amorphous. Thus, the effects of crystalline phases can be ruled out. The effect of residual porosity and more/less relaxed state on the deformation of additively manufactured and as-cast BMGs has been studied. The analysis of the observed serrations during compressive loading implies that the shear-band dynamics in the additively manufactured samples distinctly differ from those of the as-cast glass. This phenomenon appears to originate from the presence of uniformly dispersed spherical pores as well as from the more pronounced heterogeneity of the glass itself as revealed by instrumented indentation. Despite these heterogeneities, the shear bands are straight and form in the plane of maximum shear stress. Additive manufacturing, hence, might not only allow for producing large BMG samples with complex geometries but also to manipulate their deformation behaviour through tailoring porosity and microstructural heterogeneity. Different from the compressive tests, the heterogeneities of additively manufactured BMGs have no significant effect on the tribological and corrosion properties. The similar specific wear rate and the worn surfaces demonstrate that similar wear mechanisms are active in the additively manufactured and the as-cast samples. The same holds for the corrosion tests. The anodic polarization curves of SLM samples and as-cast samples illustrate a similar corrosion behaviour. However, the SLM samples have a slightly reduced susceptibility to pitting corrosion and reveal an improved surface healing ability, which might be attributed to an improved chemical homogeneity of the additively manufactured BMGs.
In order to improve plasticity, bulk metallic glasses composites (BMGCs) have been developed, in which crystals precipitate in a glassy matrix. The crystalline phases can alter the local stress state under loading, thereby, impacting the initiation and propagation of the shear bands. However, it is difficult to control the crystalline volume fraction as well as the size and spacing between the crystals by using the traditional melt-quenching method. One approach is to mix glass-forming powder with conventional alloy powder. In this way, a large degree of freedom for designing the microstructure can be gained. Thus, SLM was chosen to prepare such “ideal” BMGCs in the present work. The β-phase stabilizer Nb powder was mixed with Zr52.5Cu17.9Ni14.6Al10Ti5 powder. After SLM processing, the irregular-shaped Nb particles are distributed uniformly within the glassy matrix and bond well to it. At the higher Nb content, diffusion of Nb during processing locally deteriorates the glass-forming ability of the matrix and results in the formation of several brittle intermetallic phases around the Nb particles. The size of these precipitates covers a wide range from nanometres to micrometres. Despite the fact that the soft Nb particles increase the heterogeneity of the glassy matrix, none of the samples deforms plastically. This is attributed to the network-like distribution of the intermetallic phases, which strongly affects the fracture process. Besides the ex-situ method of mixing powders, designing in-situ ductile phases and controlling the fraction of the crystalline phases by altering process parameters can also prepare optimized BMGCs. Cu46Zr46Al8 (at.%) was processed via SLM to produce in-situ BMGCs. It is revealed that the microstructure of the nearly fully dense additively manufactured BMGs is strongly affected by the energy input. By increasing the energy input, the amount of the crystalline phases was raised. By optimizing the energy input, the B2 CuZr phase was particularly deliberately introduced. Due to the residual porosity and brittle phases, no plasticity is visible in the additively manufactured samples. Generally, selective laser melting opens a gateway to design the microstructure of the BMG matrix composites.:Abstract I
Kurzfassung IV
Symbols and abbreviations VIII
Aims and objectives VIII
CHAPTER 1 Metallic glasses and selective laser melting 1
1.1 Formation of metallic glasses from the melt 1
1.2 Mechanical properties of BMGs and their composites 4
1.2.1 Shear banding in metallic glasses 4
1.2.2 Effect of structural heterogeneities on plastic deformation 7
1.2.2.1 Nanoscale heterogeneities 8
1.2.2.2 Microscale heterogeneities 11
1.2.3 Shear band dynamics 13
1.2.4 Tribological properties of BMGs 15
1.3 Corrosion behaviour of bulk metallic glasses 16
1.4 Selective laser melting (SLM) 20
1.4.1 The SLM process 20
1.4.1.1 Powder properties 21
1.4.1.2 Process parameters 22
1.4.2 Solidification and thermal history 25
1.5 Selectively laser-melted glass formers 28
1.5.1 Selective laser melting of a single alloy powder 28
1.5.2 Heterogeneities and mechanical properties of additively manufactured BMGs 32
CHAPTER 2 Experimental 36
2.1 Sample preparation 36
2.1.1 Arc melting 36
2.1.2 Suction casting 36
2.1.3 Gas atomization 37
2.1.4 Powder mixtures 37
2.1.5 Selective laser melting (SLM) 38
2.1.5 Heat treatment 39
2.2 Sample characterization methods 39
2.2.1 Composition analysis 40
2.2.2 X-ray diffraction 40
2.2.3 Calorimetry 40
2.2.4 Density measurements (Archimedean method) 41
2.2.5 µ-CT 41
2.2.6 Scanning electron microscopy (SEM) 41
2.2.7 Transmission electron microscopy (TEM) 42
2.2.8 Hardness measurements 42
2.2.9 Compression tests 43
2.2.10 Sliding wear tests 43
2.2.11 Corrosion tests 44
2.2.12 Finite volume method modelling 45
CHAPTER 3 Selective laser melting of glass-forming alloys 46
3.1 Selective laser melting of a Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 BMG 46
3.1.1 Powder analysis 47
3.1.2 Parameter optimization and microstructural characterization 48
3.1.3 Mechanical properties 55
3.1.3.1 Compression tests 55
3.1.3.2 Microhardness and structural relaxation 57
3.1.3.3 Nanoindentation 59
3.1.4 Corrosion properties 61
3.2 Selective laser melting of a Zr52.5Cu17.9Ni14.6Al10Ti5 BMG 62
3.2.1 Powder analysis 62
3.2.2 Microstructural characterization 63
3.2.3 Mechanical properties 66
3.2.3.1 Compression tests 66
3.2.3.2 Microhardness and structural relaxation 68
3.2.3.3 Nanoindentation 71
3.2.4 Shear band dynamics and shear band propagation 74
3.2.5 Tribological and corrosion properties 80
3.3 Structural heterogeneities of BMGs produced by SLM 87
CHAPTER 4 Selective laser melting of ex-situ Zr-based BMG matrix composites 97
4.1 Phase formation 97
4.2 Microstructures 101
4.3 Mechanical properties 110
CHAPTER 5 Selective laser melting of in-situ CuZr-based BMG matrix composites 115
5.1 Powder analysis 115
5.2 Parameter optimization 116
5.3 Microstructure 120
5.4 Mechanical properties 124
5.4.1 Compression tests 124
5.4.2 Microhardness and structural relaxation 127
5.4.3 Nanoindentation 129
CHAPTER 6 Summary 132
CHAPTER 7 Outlook 132
Acknowledgements 137
Bibliography 139
Publications 163
Eidesstattliche Erklärung 164
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:71756 |
| Date | 28 August 2020 |
| Creators | Deng, Liang |
| Contributors | Zimmermann, Martina, Leyens, Christoph, Lin, Xin, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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