Abstract
Additive manufacturing of bulk metallic glasses (BMGs) has opened this material class to an exciting new range of potential applications, as bulk-scale, net-shaped amorphous components can be fabricated in a single step. However, there exists a critical need to understand the structural details of additive manufactured BMGs and how the glassy structure is linked to the mechanical properties. Here, we present a study of structure and property variations along the build height for a laser powder bed fusion (LPBF) processed Zr-based BMG with composition Zr59.3Cu28.8Nb1.5Al10.4 commercially termed AMZ4, using hardness testing, calorimetry, positron annihilation spectroscopy, synchrotron X-ray diffraction, and transmission electron microscopy. A lower hardness, more rejuvenated glassy structure was found at the bottom of the build compared to the middle region of the build, with the structure and properties of the top region between the two. Such differences could not be attributed to variability in chemical composition or crystallisation; rather, the softer bottom region was found to have a larger medium range order cluster size, attributed to heat dissipation into the build plate during processing, which gave faster cooling rates and less reheating compared to the steady-state middle of the build. However, at the top of the build less reheating occurs compared to the middle, leading to a somewhat softer and less relaxed state.Graphical abstract
Highlights
Bulk metallic glasses (BMGs) are alloys with no longrange atomic order but with varying degrees of shortand medium-range order (SRO and MRO)
In this work the atomic-scale chemical and structural features were studied at distinct locations along the 18 mm build height of a laser powder bed fusion (LPBF)-processed bulk metallic glasses (BMGs) alloy (AMZ4—Zr59.3Cu28.8Nb1.5Al10.4) using microhardness mapping, differential scanning calorimetry (DSC), Doppler-broadening positron annihilation spectroscopy (DB-PAS), synchrotron X-ray diffraction, and transmission electron microscopy (TEM) with nanobeam electron diffraction (NEBD). The combination of these analyses demonstrated that no noticeable structural changes in porosity, chemical composition, and nearest neighbour atomic spacing in the glassy structure were observed along the build height on thin samples cut from the bulk free of residual stresses
Analysis of other samples produced by the same LPBF-processing parameters have further confirmed the uniformity in chemistry and porosity [11], and no chemical variations were found after doing further energydispersive spectroscopy (EDS) in this study
Summary
Bulk metallic glasses (BMGs) are alloys with no longrange atomic order but with varying degrees of shortand medium-range order (SRO and MRO). A significant bottleneck for the widespread utilisation of BMGs, is the rapid cooling rates required during processing to ‘freeze-in’ the amorphous structure [4] This requirement has traditionally led to difficulties in producing components with large enough dimensions to be industrially relevant for a range of targeted applications (e.g. for aerospace, transportation, biomedical implants, etc.). There is a growing body of evidence that LPBF-processed parts generally have reduced ductility and damage tolerance when compared with cast alloys of the same chemical composition [10–16] Since those properties are often seen as limiting factors in the application of BMGs [2, 17], such work highlights the importance of understanding the chemical and structural aspects that control the mechanical properties, so that future improvements can be made to LPBF-manufactured BMGs
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