Mechanical Properties Of Bulk Metallic Glasses Research Articles

Superior mechanical properties of a Zr-based bulk metallic glass via laser powder bed fusion process control

Additive manufacturing has made the fabrication of large-dimensioned bulk metallic glasses (BMGs) achievable; however, questions remain regarding how to control the processing parameters to obtain dense and fully amorphous BMGs with desirable mechanical properties. Here, laser powder bed fusion (LPBF) was used to produce dense and fully amorphous Zr59.3Cu28.8Nb1.5Al10.4 BMG samples from two different starting powders within a large processing window of laser powers and scanning speeds. X-ray diffraction (XRD) revealed that fully amorphous XRD patterns were obtained for samples with high relative densities (>99 %) when the LPBF energy density ranged from ∼20 J/mm3 up to ∼33 J/mm3 for coarse powder with low oxygen and up to ∼30 J/mm3 for fine powder with higher oxygen. For the fully XRD amorphous samples, strength and hardness increased with increasing energy density while the relaxation enthalpy and ductility decreased. Transmission electron microscopy revealed that the softer samples contain larger medium range order clusters within the amorphous matrix. With higher LPBF energy density, high relative density was still achieved along with devitrification and embrittlement. While lower energy densities below ∼20 J/mm3 could retain a fully XRD amorphous structure, such samples had relative densities <99 %. When comparing the two powders, the coarse powder with four times lower oxygen content gave better glass forming ability, compression ductility up to 6 % plastic strain, and fracture toughness up to ∼38 MPa√m. These findings demonstrate that it is possible to tailor the structure and mechanical properties of BMGs by tuning the LPBF process parameters within a wide processing window.

Applicability of Pre-Plastic Deformation Method for Improving Mechanical Properties of Bulk Metallic Glasses.

Pre-plastic deformation (PPD) treatments on bulk metallic glasses (BMGs) have previously been shown to be helpful in producing multiple shear bands. In this work, the applicability of the PPD approach on BMGs with different Poisson's ratios was validated based on experimental and simulation observations. It was found that for BMGs with high Poisson's ratios (HBMGs, e.g., Zr56Co28Al16 and Zr46Cu46Al8), the PPD treatment can easily trigger a pair of large plastic deformation zones consisting of multiple shear bands. These PPD-treated HBMGs clearly display improved strength and compressive plasticity. On the other hand, the mechanical properties of BMGs with low Poisson's ratios (LBMG, e.g., Fe48Cr15Mo14Y2C15B6) become worse due to a few shear bands and micro-cracks in extremely small plastic deformation zones. Additionally, for the PPD-treated HBMGs with similar high Poisson's ratios, the Zr56Co28Al16 BMG exhibits much larger plasticity than the Zr46Cu46Al8 BMG. This phenomenon is mainly due to more defective icosahedral clusters in the Zr56Co28Al16 BMG, which can serve as nucleation sites for shear transformation zones (STZs) during subsequent deformation. The present study may provide a basis for understanding the plastic deformation mechanism of BMGs.

Effect of pre-straining on the structure and nano-mechanical properties of a CuZrAl bulk metallic glass

Pre-straining was carried out on a CuZrAl bulk metallic glass (BMG), in order to understand the physical correlation between its structural heterogeneity and nano-mechanical performance. As the pre-strain increases, the studied glass remains its amorphous structure whereas the degree of local order firstly decreases and then increases, yielding a minimum value at 10 % pre-strain, and the Helmholtz activation energy (ΔF*), shear transformation zone (STZ) volume, number of atoms in the STZ (N), and the nanohardness value exhibits a similar trend. The variation in the structure and thus mechanical performance induced by pre-straining has been interpreted from the viewpoint of free volume concept. The results obtained here can be beneficial for tailoring the structure and the corresponding mechanical properties of BMGs.

Controlling the relaxation versus rejuvenation behavior in Zr-based bulk metallic glasses induced by elastostatic compression

Elastostatic compression (ESC) has received considerable research attention as a tool to study rejuvenation and relaxation processes for bulk metallic glasses (BMGs). However, little is understood about the conditions that control whether rejuvenation or relaxation will occur, and whether conditions exist that can give structural stability. We address these questions by applying ESC at 90% of the yield stress to both cast and laser powder bed fusion (LPBF) manufactured Zr-based BMG samples in the as-cast, as-built, and different annealed states. The structural state and mechanical properties for each material condition were characterized by differential scanning calorimetry and microhardness, respectively, and two representative groups were also used for compression testing. Initial relaxation or rejuvenation was observed for elastostatically compressed as-cast samples, and the behavior reversed over 72 h of ESC. In contrast, no ESC effect was observed for the as-built LPBF samples. It was found that the onset of either relaxation or rejuvenation by ESC could be better predicted if samples were annealed into a controlled initial state. Five different types of initial response to ESC were observed, corresponding to different initial energy state ranges. Materials in the highest and lowest initial energy states were stable against structural changes by ESC. Close to the highest energy state, rejuvenation was dominant, while relaxation took place close to the lowest energy state. At intermediate initial energy states, both relaxation and rejuvenation were observed after ESC loading, suggesting that the glass structure easily finds different local minima in the potential energy landscape. In all cases, relaxation was associated with BMG hardening and rejuvenation was associated with softening. Overall, the results of this study provide new insights into how ESC impacts the structural state and mechanical properties of BMGs.

Effect of Annealing on Strain Rate Sensitivity of Metallic Glass under Nanoindentation

The influence of isothermal annealing on the strain rate sensitivity (SRS) of a Zr-based bulk metallic glass (BMG) was investigated by nanoindentation. A more positive SRS is observed with a decrease in the content of the free volume (FV) of the sample. Furthermore, the SRS becomes nearly constant with increasing annealing time when the FV is annealed out. By taking into consideration the FV-assisted activation and combination of the shear transformation zones (STZs), the underlying mechanism is well understood. The current work may offer useful insights into the correlation between the microstructure and mechanical properties of BMGs.

Structural origins for the generation of strength, ductility and toughness in bulk-metallic glasses using hydrogen microalloying

A vital requirement for bulk-metallic glasses (BMGs) as structural materials is the attainment of both strength and toughness, yet invariably, as in most materials, these properties are mutually exclusive. However, by utilizing a hydrogen microalloying technology, involving alloying with a gas mixture of hydrogen/argon, we have converted “strong-yet-brittle” bulk-metallic glasses into “stronger-and-tough” ones. We combine experiments with molecular dynamics simulations to systematically analyze the atomic-scale details on how trace hydrogen additions can induce internal changes in the amorphous structure of Zr-Cu-based glassy alloys, with the aim of discerning the structural origin of the combined high strength, ductility and toughness of these materials. Our results, from both relaxation spectrum analysis and calculations of the atomic configurations, indicate that minor additions of hydrogen, instead of causing embrittlement, can have a positive influence on the mechanical properties of BMGs. Specifically, they generate more highly activated “soft spots” to promote multiple shear bands to enhance deformability, yet at the same time engender the formation of a more strengthened structural matrix to delay their initiation, a factor which can serve to elevate strength. Accordingly, our H-alloyed samples display a larger yield strength and fracture strain than the H-free ones. The current findings not only show how the strength-toughness trade-off can be successfully overcome in bulk-metallic glasses, but also we regard this understanding as a step forward to decoding the salient underlying mechanisms for the correlating structure, relaxation behavior and mechanical properties of these materials.

Ductile Fe-Based BMGs with High Glass Forming Ability and High Strength

Fe-based bulk metallic glasses (BMGs) with high glass forming ability (GFA) and excellent mechanical properties were synthesized in Fe-Ni-Mo-P-C-B alloy system by copper mold casting. Results show that the glass forming ability of Fe74−xNixMo6P10C7.5B2.5 alloys increases first and then decreases as Ni content, x, increases from 0 to 11.1 at%, with its climax being reached at x is between 3.7 and 5.0. Analyses indicate that either ΔTx and ΔHendo or Trg and γ can not illustrate the GFA of obtained alloys solely. With increasing Ni element in Fe74−xNixMo6P10C7.5B2.5 alloys, the yield strength and Vicker’s mircrohardness decline, while the plasticity increases, which implies that enhancing plasticity by adjusting the composition of alloys is followed with the loss of some strength. Serrated flow characteristics on the compressive stress-strain curves is observed for Fe74−xNixMo6P10C7.5B2.5 as x=11.1 at%, which is considered to relate to its lowest glass transition temperature. These mechanical properties of BMGs are illustrated with bonding nature between the constituent elements.

Effects of Size and Volume Fraction of Precipitated Crystalline Phase Induced by Friction Stir Processing on Hardness in Zr&amp;ndash;Al&amp;ndash;Ni&amp;ndash;Cu Bulk Metallic Glass

The effects of the size and volume fraction of the precipitated crystalline phase induced by friction stir processing (FSP) on the hardness in Zr–Al–Ni–Cu bulk metallic glass (BMG) was examined. The microstructure in the friction zone (FZ) exhibits an amorphous ‘‘band-like’’ structure with a small number of nanoscale crystalline particles. On the other hand, the microstructure in the FZ near the finishing point of the process exhibits a large number of nanocrystalline particles with a size of less than 20 nm and amorphous phase. The difference in the microstructures of FSP regions is explained in terms of differences in the heat input by FSP. The hardness in both FSP regions is greater than that of heat-treated specimens with almost the same volume fraction of the crystallized phase. Control of the size and volume fraction of precipitated crystalline phase induced by FSP offers the possibility of further improvement in the mechanical properties of BMGs. [doi:10.2320/matertrans.MAW200733]