Amorphous Bulk Metallic Glass 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.

Solid State Joining of a Cold Rolled Zr-Based Bulk Metallic Glass to a Wrought Aluminum Alloy by Power Ultrasonics.

Ultrasonic metal welding (UMW) enables joining in the solid state at relative low temperatures with short cycle times. This technique is of particular interest for joining metallic glasses to each other or to other materials, because crystallization of the amorphous structure can be prevented due to the low thermal loading and the rapidity of the process. In this work, UMW is applied to join one 1 mm thick sheet of a commercial wrought aluminum alloy (AA5754) and one 0.4 mm thick strip of a commercial Zr-based bulk metallic glass (AMZ4). The introduced heat of the welding process is detected with thermocouples and thermal imaging. To investigate the strength of the joint and the influence on the microstructure, mechanical tensile tests are carried out in combination with scanning electron microscopy and differential scanning calorimetry. The results show that ultrasonic metal welding is a suitable technique to join amorphous bulk metallic glasses to crystalline aluminum alloys. The metallic glass component retains its amorphous structure in the joint, and the joint strength is higher than the strength of the Al sheet. These findings will help to develop future applications of BMG-based multi-material components, including medical tools.

Atomic structure of an FeCrMoCBY metallic glass revealed by high energy x-ray diffraction

Amorphous bulk metallic glasses with the composition Fe48Cr15Mo14C15B6Y2 have been of interest due to their special mechanical and electronic properties, including corrosion resistance, high yield-strength, large elasticity, catalytic performance, and soft ferromagnetism. Here, we apply a reverse Monte Carlo technique to unravel the atomic structure of these glasses. The pair-distribution functions for various atomic pairs are computed based on the high-energy x-ray diffraction data we have taken from an amorphous sample. Monte Carlo cycles are used to move the atomic positions until the model reproduces the experimental pair-distribution function. The resulting fitted model is consistent with our ab initio simulations of the metallic glass. Our study contributes to the understanding of functional properties of Fe-based bulk metallic glasses driven by disorder effects.

Structural design of additively manufactured porous Zr‐based bulk metallic glass

AbstractRecently, designing light‐weight BMGs with high‐strength has attracted much attention. Additive manufacturing (3D‐printing) using selective laser melting (SLM) enables the production of porous bulk metallic glasses (BMGs), which is difficult to achieve by applying traditional manufacturing techniques. In this study, different porous structural prototypes of 3D‐printed Zr‐based BMGs are comparatively studied. Main focus is put on the strength optimization by varying pore arrangement and pore morphology. A constitutive model based on Mohr‐Coulomb theory with multi‐slips system is applied to describe the shear banding behaviors of 3D‐printed amorphous BMGs.

Analysis on specific cutting energy in micro milling of bulk metallic glass

Micro milling is a suitable micro cutting process to create micro features. This paper presents an analytical model of the specific cutting energy in micro cutting and analyses the specific cutting energy in micro milling of amorphous bulk metallic glass. The analytical results are compared with the experimental results, where the experimental work comprises of micro milling amorphous bulk metallic glass at different cutting speeds and a wide range of feed rates. The analytical model considers an isotropic and homogeneous work material that macroscopically resembles the bulk metallic glass. In the experimental study, the influence of the cutting parameters on the specific cutting energy and the surface roughness generated in micro end milling is analysed. Experimental results show appreciable dependence of specific cutting energy and surface roughness on the feed rate. Size effect phenomenon is exhibited in both the analytical and experimental values of the specific cutting energy. The micro mechanism behind the size effect in bulk metallic glasses is discussed in the article. Experimental results indicate that for feed per tooth higher than the cutting edge radius, increase in the feed rate results in the deterioration of the surface quality and reduced values of the specific cutting energy. Results of the experimental study show a decrease in the specific cutting energy and surface roughness values with the increase in the cutting speed.

3D pore structure characterization and hardness in a powder bed fusion-processed fully amorphous Zr-based bulk metallic glass

Recently, fabrication of bulk metallic glasses (BMGs) components with complex shapes has been realized using laser powder bed fusion (PBF). Initial defects such as porosity inherent to the PBF process can significantly degrade the mechanical integrity of the BMGs components. In spite of intensive studies on pore structures in 3D-printed crystalline alloys, pore structures in 3D-printed fully amorphous BMGs have not been characterized due to the difficulties in achieving large-scale fully amorphous structures. In the present work, the pore characteristics of PBF-processed fully amorphous Zr-based BMGs were systematically studied. The amorphous structure of the printed parts was first verified using X-ray diffraction (XRD). A non-destructive high-resolution X-ray micro-computed tomography (micro-CT) technique was then applied to evaluate the inner 3D pore structure, while microstructures below the resolution of X-ray micro-CT were examined in 2D using scanning electron microscopy (SEM). The printed amorphous parts can reach a total volumetric porosity as small as 0.45%. The distribution of pore size and pore sphericity were evaluated for the PBF-processed BMGs samples with different porosities, where two observed pore features were irregular large lack-of-fusion pores, and smaller gas-induced round pores. Furthermore, correlations were found between porosity, pore size and morphology. Finally, hardness tests at various loads revealed that while the intrinsic properties were constant irrespective of laser processing, the overall mechanical properties of the PBF-processed BMGs samples are dominated by pore characteristics. While the densest printed BMG sample had a comparable macroscale hardness to the as-cast material, an increase in porosity of ~8% led to a decrease in HV5 hardness of ~17%. The presented results provide a detailed analysis of pore characteristics, and their impact on mechanical properties of PBF-processed BMGs, for further experimental and computational studies of structure-property relationships in this materials class.

An experimental investigation on the formation of burrs in micro milling of Zr-based amorphous bulk metallic glass

Fabrication of precision micro components demands part quality having burr free edges and acceptable surface finish to comply with its requirements of proper functionality. Machining conditions influence process performances in different ways. The present study examines the effect of cutting parameters like cutting speed, feed per tooth (feed rate) and axial depth of cut on the formation of top burrs and surface finish produced in micro-channels made through full immersion micro end milling process carried out on a workpiece made of amorphous bulk metallic glass (BMG). The process modelling is done based on response surface methodology and parametric analysis is performed to examine the influence of the process parameters on the process responses, namely the burr size formed in up-milling and down-milling sides and the surface roughness of the bottom surface of the micro-channels. The study also undertakes multiple response optimisation analysis that determines the optimal values of the cutting parameters that yield lower burr sizes and lower surface roughness.

An experimental investigation on the formation of burrs in micro milling of Zr-based amorphous bulk metallic glass

Fabrication of precision micro components demands part quality having burr free edges and acceptable surface finish to comply with its requirements of proper functionality. Machining conditions influence process performances in different ways. The present study examines the effect of cutting parameters like cutting speed, feed per tooth (feed rate) and axial depth of cut on the formation of top burrs and surface finish produced in micro-channels made through full immersion micro end milling process carried out on a workpiece made of amorphous bulk metallic glass (BMG). The process modelling is done based on response surface methodology and parametric analysis is performed to examine the influence of the process parameters on the process responses, namely the burr size formed in up-milling and down-milling sides and the surface roughness of the bottom surface of the micro-channels. The study also undertakes multiple response optimisation analysis that determines the optimal values of the cutting parameters that yield lower burr sizes and lower surface roughness.

Investigations on surface microstructure in high-speed milling of Zr-based bulk metallic glass

Bulk metallic glasses (BMGs) are a relatively new group of metallic alloys with amorphous microstructure. BMGs have been increasingly used in industrial applications due to their high strength and high corrosion resistance. The microstructural evolution and the surface generation mechanisms in machining of BMGs and crystalline metal alloys are substantially different. This paper presents the investigations on surface microstructure and progressive tool wear in high-speed milling of Zr-based BMG. The crystallization of originally amorphous BMG material caused by the milling process was examined. The microstructure properties of BMG at amorphous and crystalline phases were determined. The effect of coolant on the progressive tool wear and the constituent element changes of BMG was investigated. The results provide new and comprehensive information on surface microstructure property in response to high-speed milling of Zr-BMG, and can guide the process planning in achieving desired surface property in milling of Zr-BMG components.

3D printing metals like thermoplastics: Fused filament fabrication of metallic glasses

Abstract Whereas 3D printing of thermoplastics is highly advanced and can readily create complex geometries, 3D printing of metals is still challenging and limited. The origin of this asymmetry in technological maturity is the continuous softening of thermoplastics with temperature into a readily formable state, which is absent in conventional metals. Unlike conventional metals, bulk metallic glasses (BMGs) demonstrate a supercooled liquid region and continuous softening upon heating, analogous to thermoplastics. Here we demonstrate that, in extension of this analogy, BMGs are also amenable to extrusion-based 3D printing through fused filament fabrication (FFF). When utilizing the BMGs’ supercooled liquid behavior, 3D printing can be realized under similar conditions to those in thermoplastics. Fully dense and amorphous BMG parts are 3D printed in ambient environmental conditions resulting in high-strength metal parts. Due to the similarity between FFF of thermoplastics and BMGs, this method may leverage the technology infrastructure built by the thermoplastic FFF community to rapidly realize and proliferate accessible and practical printing of metals.

Enhanced mechanical properties of additively manufactured bulk metallic glasses produced through laser foil printing from continuous sheetmetal feedstock

Metal additive manufacturing (AM) is a rapidly growing field aimed to produce high-performance net-shaped parts. Therefore, bulk metallic glasses (BMGs), known for their superlative mechanical properties, are of remarkable interest for integration with AM technology. In this study, we pioneer the utilization of commercially available BMG sheetmetal as feedstock for AM, using laser foil printing (LFP) technology. LFP and traditional casting were used to produce samples for four-point bending and Vickers hardness measurements to rigorously compare the mechanical performance of samples resulting from these two fabrication techniques. Through LFP, fully amorphous BMG samples with dimensions larger than the critical casting thickness of the same master alloy were successfully made, while exhibiting high yield strength and toughness in bending. This work exemplifies a potential method to fabricate high-value BMG commercial parts, like gears or mechanisms, where the parts are conventionally machined after printing, and greatly benefit from utilizing novel materials.

Composites of amorphous and nanocrystalline Zr–Cu–Al–Nb bulk materials synthesized by spark plasma sintering

The fabrication of Zr70Cu24Al4Nb2 bulk metallic glass composite samples by spark plasma sintering (SPS) process has been successfully realized. The unique characteristics of bulk metallic glasses could lead to the possibility of future applications as new structural and functional materials. The densification of an amorphous Zr70Cu24Al4Nb2 powder was realized in a systematic study changing the sintering temperature in the SPS process leading to stable composites characteristic of amorphous and nanocrystalline structures. X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) analysis, transmission electron microscopy (TEM) as well as hardness tests were applied to determine the structural and mechanical properties of the sintered materials. A stable amorphous bulk metallic glass based on Zr70Cu24Al4Nb2 with a low fraction of crystallites could be fabricated applying a nominal sintering temperature of 400 °C. Higher sintering temperatures lead to composites with high fractions of nanocrystalline material with porosities below 0.5%.

Biocorrosion and biocompatibility of Zr–Cu–Fe–Al bulk metallic glasses

Owing to their unique chemo‐physical and structural characteristics, amorphous bulk metallic glasses (BMGs) are of great demand for fabrication of variety of advanced and innovative products including surgical and biomedical tools and devices. In this study, a series of Ni‐free Zr‐based BMGs in Zr–Cu–Fe–Al system are fabricated using copper‐mold casting technique, and their biocorrosion and biocompatibility are evaluated with respect to their corrosion behavior in the phosphate buffered saline (pH = 7.4) solution. Anodic polarization curves, scanning electron microscopy combined with energy‐dispersive X‐ray, and wettability analyses are conducted to characterize the surfaces of BMG samples. The biocompatibility of the BMG and control samples is studied by comparing cell–substrate interactions among different samples. It is found that Zr60Cu20Fe10Al10 displays a higher passive region compared with that of Zr60Cu22.5Fe7.5Al10, but both BMGs exhibit lower corrosion resistance compared with Ti–6Al–4V alloy. By addition of titanium to Zr–Cu–Fe–Al system (Zr60Ti6Cu19Fe5Al10), a significant increase in the passive region of the polarization curve is detected. The cell culture experiments reveal that the number of attached and grown cells is significantly higher on the surface of the treated BMGs as compared with Ti–6Al–4V substrates and the culture plate as controls. There is no noticeable difference in cellular morphology among the BMG samples, and no cytotoxicity is detected. We speculate that the interaction of water molecules and matrix proteins with the surfaces of BMGs plays an important role in cell–substrate interactions and improved cell response. Copyright © 2013 John Wiley &amp; Sons, Ltd.

The Role of Applied Strain on Crystallisation Kinetics of an Amorphous Zr-Based Bulk Metallic Glass

The crystallisation behaviour of Vitreloy 105 during different thermo-mechanical exposure conditions has been studied in a Gleeble 3500 thermo-mechanical simulator (TMS). Strains up to 0.1 applied at rates of 0.001 – 0.01 s-1 in the supercooled liquid region (SLR) from 420 – 435 °C resulted in no change in the crystallisation kinetics compared with purely thermal exposure for the same times and temperatures. Separate deformation studies on the amorphous bulk metallic glass confirmed that the deformation conditions used corresponded to Newtonian flow conditions. It remains to be confirmed whether the lack of influence of permanent deformation persists for deformation at higher strain rates in the non-Newtonian regime.

Role of crystalline precipitates on the mechanical properties of (Cu 0.50Zr 0.50) 100− xAl x ( x = 4, 5, 7) bulk metallic glasses

The mechanical behaviour upon compression of (Cu 0.50Zr 0.50) 100− x Al x ( x = 4, 5, 7) rods with 2 and 3 mm diameter was systematically studied and compared with the literature data. Fully amorphous bulk metallic glasses ( x = 5, 7) show little permanent deformation and reproducible yield stress values. The remarkable fracture strain, observed for some apparently X-ray diffraction amorphous samples ( x = 4), was found to be due to significant amounts (at least 20%) of the B2-CuZr crystalline phase. The effect of possible flaws on the external surface of the rods was evaluated by mechanical testing of either as cast and machined samples.

Understanding the Problem of Fatigue in Bulk Metallic Glasses

Early studies suggested there was a severe problem with the fatigue resistance of some bulk metallic glasses (BMGs) and BMG matrix composites, while more recent studies begin to demonstrate a wide variety of fatigue behaviors may be possible for both fully amorphous BMGs and their composites. However, in order to truly understand and control the fatigue behavior of these materials, the role of such factors as thermomechanical processing, the corresponding glass structure, environment, and defects must be understood. Additionally, it is important to understand how these factors relate to the mechanisms of fatigue. This article reviews the current understanding in this regard, and identifies some of the challenges for the future development of fatigue-resistant BMG-based materials.

Glass Forming Ability and Magnetic Properties of (Fe&lt;sub&gt;68&lt;/sub&gt;B&lt;sub&gt;25&lt;/sub&gt;Nd&lt;sub&gt;7&lt;/sub&gt;)&lt;sub&gt;96&lt;/sub&gt;Nb&lt;sub&gt;4&lt;/sub&gt; Bulk Metallic Glass

Flakes with a size of 20×4×0.8mm3 and rods of (Fe68B25Nd7)96Nb4 bulk metallic glass (BMG) with a diameter of 1.5 and 3mm were cast. Rods a diameter of 1.5 mm were proven to be XRD amorphous. High vacuum environment during the fabrication will benefit the formation of fully amorphous BMG. The super-cooled liquid regions of (Fe68B25Nd7)96Nb4 is 59°C and the reduced glass transition temperature is 0.624 indicating good glass-forming ability. The critical cooling rate for glass formation for (Fe68B25Nd7)96Nb4 melt is about 294°C/s by computer simulation (Fe68B25Nd7)96Nb4 BMG shows soft magnetic property while partially crystallized sample shows hard magnetic property.

Near-threshold fatigue crack growth in bulk metallic glass composites

A major drawback in using bulk metallic glasses (BMGs) as structural materials is their extremely poor fatigue performance. One way to alleviate this problem is through the composite route, in which second phases are introduced into the glass to arrest crack growth. In this paper, the fatigue crack growth behavior of in situ reinforced BMGs with crystalline dendrites, which are tailored to impart significant ductility and toughness to the BMG, was investigated. Three composites, all with equal volume fraction of dendrite phases, were examined to assess the influence of chemical composition on the near-threshold fatigue crack growth characteristics. While the ductility is enhanced at the cost of yield strength vis-à-vis that of the fully amorphous BMG, the threshold stress intensity factor range for fatigue crack initiation in composites was found to be enhanced by more than 100%. Crack blunting and trapping by the dendritic phases and constraining of the shear bands within the interdendritic regions are the micromechanisms responsible for this enhanced fatigue crack growth resistance.

Unusual glass-forming ability induced by changes in the local atomic structure in Ti-basedbulk metallic glass

The effect of partial replacement of Cu by Be inTi50Cu32Ni15Sn3 alloy on the thermal properties, structure, and forming ability of an amorphous phase wereinvestigated by differential scanning calorimetry (DSC), x-ray diffraction (XRD), extended x-rayabsorption fine structure (EXAFS), and high-resolution transmission electron microscopy (HRTEM).Ti50Cu25Ni15Sn3Be7 alloy shows enhanced glass-forming ability, enabling one to fabricatea fully amorphous bulk metallic glass sample 2 mm in diameter byinjection casting. With the replacement, the supercooled liquid regionΔTx (= Tx−Tg, whereTx is the crystallizationtemperature and Tg is the glass transition temperature) decreased from 73 to 45 K and the reduced glass transition temperatureTrg (= Tg/T1,where T1 is the liquidus temperature) increased from 0.53 to 0.57. The amorphousTi50Cu25Ni15Sn3Be7 phase showed a formation of short-range-ordered clusters 1–2 nm in size, which isattributed to the strong interaction between Ti and Be. The results show thatΔTx can be used as a thermal parameter reflecting the glass-forming ability of the alloy onlywhen the phase formed during crystallization is the same as the phase competing with theglass transition during solidification.

Reciprocating wear mechanisms in a Zr-based bulk metallic glass

The dry sliding friction coefficient μ and the wear volume loss W, in a zirconium-based bulk metallic glass (BMG) under high-frequency (50 Hz) reciprocating conditions, were investigated with the objective of assessing the influence of free volume and crystallization on the wear behavior of amorphous metals. The BMG samples were annealed either below the glass transition temperature Tg to induce structural relaxation and hence reduce the free volume that controls plasticity through shear-band formation or above Tg to crystallize the amorphous BMG prior to wear testing. Results show that the wear behavior of both the as-cast and relaxed glasses was dominated by the oxidation of the surface layers. A sharp transition in the contact electrical resistance complemented by a marked increase in μ was noted. This was attributed to the formation of a thick tribo film with high oxygen concentration and its subsequent delamination. The μ values, before as well as after the transition, in the relaxed glasses were similar to those for the as-cast alloy. However, a gradual decrease in W with annealing temperature was observed. A good correlation between W and nanohardness was noted, implying that the intrinsic hardness in the BMGs controlled the wear rate.