Critical Casting Thickness Research Articles

Stress related magnetic imaging of iron-based metallic glass produced with laser beam powder bed fusion

Additive manufacturing makes the production of bulk metallic glasses possible in thicknesses exceeding the critical casting thickness. However, a crucial challenge is the build-up of thermally induced stress, often resulting in printed parts suffering from cracking. In this study, the process parameters are optimised for printing soft-magnetic metallic glass samples of an Fe-based alloy (Fe73.8P10.6Mo4.2B2.3Si2.3C6.7), using laser beam powder bed fusion. In addition, the structural and magnetic properties of as-received and heat-treated powder are investigated and compared to those of the printed samples. Kerr microscopy is used for imaging the magnetic domains on single track cross-sections produced on top of a polished printed sample. This reveals the shape of the melt pool of a single laser track, as well as the magnetic domains around it and in other regions of the printed sample. The shape and size of the magnetic domains reflect the residual stress in the sample through the effect of magneto-elastic coupling. This magnetic contrast could be used to get further insights into how to control the development of stress during the printing process.

Gas atomization of fully-amorphous Ni62Nb38 powder

In this study, close-coupled gas atomization is explored to produce the binary metallic glass-former Ni62Nb38. The alloy is noteworthy due to its yield strength exceeding 3GPa, glass transition above 900 K, and considerable ductility when quenched from the melt. Ni62Nb38 is the best glass former in the Ni-Nb system, possessing an extraordinarily high glass-forming ability for a binary alloy. Yet, its critical casting thickness remains below 2 mm. Hence, Ni62Nb38 becomes a potential candidate for additive manufacturing to overcome size constraints for engineering applications. Our results reveal that spherical, fully amorphous powder with a high yield in the 20–63 µm size fraction and an oxygen level below 900wppm can be successfully produced by close-coupled gas atomization. This paves the way for using binary Ni-Nb alloy powders in additive manufacturing of large, intricate parts with very high strength.

Nanocrystallization of Cu46Zr33.5Hf13.5Al7 Metallic Glass

The recently discovered Cu46Zr33.5Hf13.5Al7 (at.%) bulk metallic glass (BMG) presents the highest glass-forming ability (GFA) among all known copper-based alloys, with a record-breaking critical casting thickness (or diameter) of 28.5 mm. At present, much remains to be explored about this new BMG that holds exceptional promise for engineering applications. Here, we report our study on the crystallization behavior of this new BMG, using isochronal and isothermal differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). With the calorimetric data, we determine the apparent activation energy of crystallization, the Avrami exponent, and the lower branch of the isothermal time–temperature–transformation (TTT) diagram. With XRD and TEM, we identify primary and secondary crystal phases utilizing samples crystallized to different degrees within the calorimeter. We also estimate the number density, nucleation rate, and growth rate of the primary crystals through TEM image analysis. Our results reveal that the crystallization in this BMG has a high activation energy of ≈360 kJ/mole and that the primary crystallization of this BMG produces a high number density (≈1021 m−3 at 475 °C) of slowly growing (growth rate < 0.5 nm/s at 475 °C) Cu10(Zr,Hf)7 nanocrystals dispersed in the glassy matrix, while the second crystallization event further produces a new phase, Cu(Zr,Hf)2. The results help us to understand the GFA and thermal stability of this new BMG and provide important guidance for its future engineering applications, including its usage as a precursor to glass–crystal composite or bulk nanocrystalline structures.

Modification of structural, mechanical, corrosion and biocompatibility properties of Ti40Zr10Cu36Pd14 metallic glass by minor Ga and Sn additions

Ti40Zr10Cu32Pd14Ga4 and Ti40Zr10Cu32Pd14Sn4 (in at%) bulk metallic glasses (BMGs) with different geometries (wedges, rods, ribbons and discs) were prepared by suction casting, melt spinning and splat quenching, respectively. For comparison, the reference Ti40Zr10Cu36Pd14 BMG was cast as a rod with 2 mm diameter and in wedge-shaped form. High-energy X-ray diffraction measurements yielded a critical casting thickness of 2.4, 2.1 and at least 4 mm for the reference, Ga-containing, and Sn-containing BMGs, respectively. The extension of the supercooled liquid region of about 50 K, measured for the glassy rods and ribbons by differential scanning calorimetry, is larger than that of only 20 K found for the splat-quenched discs. As to the alloys’ mechanical properties, the Ti40Zr10Cu36Pd14 glassy rods deform plastically in compression up to a strain of 3.8% and possess a Young’s modulus of 78 GPa. The Sn- and Ga- containing BMG rods reach respectively a plastic strain of 6.1% and 4.7%, and a Young’s modulus of 72 and 63 GPa. Corrosion tests were performed by electrochemical experiments, and the highest pitting resistance was observed for Ti40Zr10Cu32Pd14Sn4 (pitting overpotential ηpit = 446 mV) compared to Ti40Zr10Cu32Pd14Ga4 (379 mV) and Ti40Zr10Cu36Pd14 (183 mV). The results of live/dead assay and cell viability revealed excellent biocompatibility for the Ga-containing BMGs.

Corrosion behavior of new rare-earth free Cu-based metallic glasses in NaCl solution of different molarity

The present study reports the glass-forming ability, density change on crystallization and corrosion behavior of newly developed rare-earth free Cu-based metallic glasses (CBMGs). The critical casting thickness of these new CBMGs increased from 15 mm at 8 at.% Hf to 22 mm at 12 at.% Hf, reaching a maximum value of 28.5 mm at 13.5 at.% Hf. It dropped to 24 mm and 15 mm thereafter, for 15 and 20 at.% Hf, respectively. The alloys showed a monotonic increase in density going from 8 at.% Hf to 20 at.% Hf, with the density of amorphous samples increasing from 7.83 g/cm3 to 8.84 g/cm3 and crystallized samples increasing from 7.92 g/cm3 to 8.91 g/cm3. Potentiodynamic polarization tests carried out in 0.1, 0.2 and 0.5 M NaCl solution at 25 °C showed a direct correlation between corrosion resistance and Hf content in the order: 8 at.% < 13.5 at.% < 20 at.% Hf. Increase in NaCl concentration also had a significant impact on the corrosion intensity due to the chaotropic effect. Scanning electron microscopy of corroded sample surfaces showed formation and eventual merging of individual micro cracks resulting in a mud-cracking pattern. Energy-dispersive X-ray spectroscopy maps of pitted regions revealed reduction in Zr and Al concentration with increase in Hf and Cu percentage for all compositions. An increase in the O concentration along with the appearance of new peaks in the X-ray photoelectron spectroscopy results, measured post corrosion, confirmed the formation of metal oxides and oxyhydroxides in the passive layer.

Molecular dynamic characteristic temperatures for predicting metallic glass forming ability

We explore the use of characteristic temperatures derived from molecular dynamics to predict aspects of metallic Glass Forming Ability (GFA). Temperatures derived from cooling curves of self-diffusion, viscosity, and energy were used as features for machine learning models of GFA. Multiple target and model combinations with these features were explored. First, we use the logarithm of critical casting thickness, log10(Dmax), as the target and trained regression models on 21 compositions. Application of 3-fold cross-validation on the 21 log10(Dmax) alloys showed only weak correlation between the model predictions and the target values. Second, the GFA of alloys were quantified by melt-spinning or suction casting amorphization behavior, with alloys that showed crystalline phases after synthesis classified as Poor GFA and those with pure amorphous phases as Good GFA. Binary GFA classification was then modeled using decision tree-based methods (random forest and gradient boosting models) and were assessed with nested-cross validation. The maximum F1 score for the precision–recall with Good Glass Forming Ability as the positive class was 0.82±0.01 for the best model type. We also compared using simple functions of characteristic temperatures as features in place of the temperatures themselves and found no statistically significant difference in predictive abilities. Although the predictive ability of the models developed here are modest, this work demonstrates clearly that one can use molecular dynamics simulations and machine learning to predict metal glass forming ability.

Developing a multilateral-based neural network model for engineering of high entropy amorphous alloys

Due to a great number of composition-processing factors, it is very difficult to design high entropy amorphous alloys without performing manifold trial-and-error experimentations. To solve this problem, in this study we developed a machine learning-based approach, namely multilateral-based neural network, which is able to predict new high entropy amorphous compositions through estimating the highest glass forming ability and the critical casting thickness. In this approach, the entropy parameters were individually correlated to each input, which leads to the improvement of predictive model in evaluating the high entropy glassy alloys. As a case study, Ti20Zr20Hf20Be20Co20 high entropy metallic glass (MG) was considered and the effects of added elements such as Y, Ni, Cr and V and Cu on the glass formation and critical casting thickness were investigated. According to the results, it is determined that the Y addition acts as a microalloying process in the base composition, while other elements improve the configurational entropy and the total negative heat of mixing, which lead to the engineering of equi-atomic high entropy MGs.

Atomic structure of the Mg66Zn30Ca4 metallic glass

Amorphous alloys consisting of elements present in the human body, such as magnesium, zinc and calcium, are currently extensively studied in order to utilize them as a material for biodegradable orthopaedic implants. amongst all Mg-Zn-Ca alloys investigated up to date, the Mg66Zn30Ca4 composition has the greatest potential for applications. Its critical casting thickness reaches a value of 5 mm, the compressive strength (716–854 MPa) is about 4 times the limit of human cortical bone while elastic modulus is (31 GPa) is only 3 times higher than that of human bone. During dissolution the alloy shows only marginal hydrogen evolution. Here we present a detailed, experiment-based structural investigation of Mg66Zn30Ca4. Structural and topological analysis of its atomic structure reveals a high number of predominantly icosahedral densely packed Zn-centred clusters. It is believed that the existence of these structural units is responsible for the suppression of internal diffusion and thus greatly improves glass formability.

Fracture and fatigue behaviour of a laser additive manufactured Zr-based bulk metallic glass

Laser additive manufacturing of bulk metallic glass (BMG) provides an effective bypassing of the critical casting thickness constraints that limit the size of components that can be produced; however, open questions remain regarding the resulting mechanical properties. In this work, a Zr-based BMG known as AMZ4 with composition Zr59.3Cu28.8Nb1.5Al10.4 was printed using a laser powder bed fusion (LPBF) technique. Micro X-ray computed tomography results together with electron microscopy imaging revealed porous processing defects in LPBF produced AMZ4 that led to a loss in tensile strength. Fatigue crack growth studies revealed a fatigue threshold, ΔKth., of ∼1.33 MPa√m and a Paris law exponent of m = 1.14, which are relatively low values for metallic materials. A KIC fracture toughness of 24−29 MPa√m was found for the LPBF BMG samples, which is much lower than the KQ of 97−138 MPa√m and KJIC of 158−253 MPa√m measured for the cast alloy with the same composition. The lower fracture toughness of the laser processed AMZ4 was attributed to ∼7.5× higher dissolved oxygen in the structure when compared to the cast AMZ4. Despite the higher level of oxygen, the formation of oxide nanocrystals was not observed by transmission electron microscopy. Oxygen induced toughness loss was confirmed by dissolving elevated concentrations of oxygen into cast AMZ4 rods, which led to a reduction in bending ductility and changes in the short-range order of the glass structure, as revealed by synchrotron X-ray diffraction.

Bulk metallic glass formation in the (Ti,Zr)–(Ni,Cu)–S system

New bulk glass-forming alloy compositions, exceeding a critical casting thickness of 1 mm, are developed in the (quasi-ternary) (Ti,Zr)–(Ni,Cu)–S system. The ternary eutectic composition Ti65.5Ni22.5Cu12 is stepwise modified through additions of S (0–8 at%) and Zr (0–22.5 at%) at the expense of Ni and Ti, respectively. By increasing the plate thickness of the casted samples from 500 µm to 1.25 mm, the primary precipitating phases are identified which is for the best glass-formers (e.g. Ti58Zr7.5Ni18.5Cu12S4) an icosahedral phase. In calorimetric experiments, several exothermic crystallization events are observed upon heating glassy samples. The first exothermic event, obscuring the glass transition, is attributed to the formation of the icosahedral phase. As the icosahedral phase forms upon heating and cooling for the best glass-formers, the origin of the increased glass-forming ability might be attributed to a pronounced icosahedral short-range order in the liquid state, impeding the formation of the stable crystalline phases.

A novel Ni-based bulk metallic glass containing high amount of tungsten and boron

Glass forming ability (GFA) and thermal stability of (Ni51W31.6B17.4)100-xCox (x = 0–35) alloy system have been investigated. It is found that as the cobalt content is increased critical casting thicknesses improve significantly. Critical casting thickness of 20, 25 and 30 at. % cobalt alloys are determined as 0.5, 1, and 1 mm, respectively. However, further increase of cobalt content degrades the GFA. The Ni-based bulk metallic glasses discovered in this study are the first ones ever developed which contain more than 10 at. % boron. It is also determined that after annealing the fully amorphous samples above their first and second crystallization temperatures, nickel solid solution and CoWB phase precipitate. This is the first study where precipitation of ultra-hard CoWB phase in a bulk metallic glass is reported.

Structural periodicity in laser additive manufactured Zr-based bulk metallic glass

Additive manufacturing of bulk metallic glasses (BMGs) allows for effective bypassing of critical casting thickness constraints for glassy alloys, opening up this exciting material class to new applications. An open question is how the laser processing of such materials affects the short-range structural order, a critical mediating parameter for glass deformation. Synchrotron X-ray microdiffraction was used to understand structural heterogeneity across the build-planes of a selective laser melted Zr-based BMG. While negligible macroscopic heterogeneity in the structure was observed over a 10 mm build height for the X-ray amorphous material, small periodic variations were observed on the order of 40–80 μm. This dimensional scale was rationalized as a consequence of melt-pool solidification from laser processing, which imparts a calculated local strain variation of ±0.1%. It is anticipated that this structural insight will help to rationalize microscale deformation effects from the periodic structural variation of selective laser melting produced BMGs.

High-temperature bulk metallic glasses developed by combinatorial methods.

Since their discovery in 19601, metallic glasses based on a wide range of elements have been developed2. However, the theoretical prediction of glass-forming compositions is challenging and the discovery of alloys with specific properties has so far largely been the result of trial and error3-8. Bulk metallic glasses can exhibit strength and elasticity surpassing those of conventional structural alloys9-11, but the mechanical properties of these glasses are critically dependent on the glass transition temperature. At temperatures approaching the glass transition, bulk metallic glasses undergo plastic flow, resulting in a substantial decrease in quasi-static strength. Bulk metallic glasses with glass transition temperatures greater than 1,000kelvin have been developed, but the supercooled liquid region (between the glass transition and the crystallization temperature) is narrow, resulting in very little thermoplastic formability, which limits their practical applicability. Here we report the design of iridium/nickel/tantalum metallic glasses (and others also containing boron) with a glass transition temperature of up to 1,162kelvin and a supercooled liquid region of 136kelvin that is wider than that of most existing metallic glasses12. Our Ir-Ni-Ta-(B) glasses exhibit high strength at high temperatures compared to existing alloys: 3.7 gigapascals at 1,000kelvin9,13. Their glass-forming ability is characterized by a critical casting thickness of three millimetres, suggesting that small-scale components for applications at high temperatures or in harsh environments can readily be obtained by thermoplastic forming14. To identify alloys of interest, we used a simplified combinatorial approach6-8 harnessing a previously reported correlation between glass-forming ability and electrical resistivity15-17. This method is non-destructive, allowing subsequent testing of a range of physical properties on the same library of samples. The practicality of our design and discovery approach, exemplified by the identification of high-strength, high-temperature bulk metallic glasses, bodes well for enabling the discovery of other glassy alloys with exciting properties.

The role of Ga addition on the thermodynamics, kinetics, and tarnishing properties of the Au-Ag-Pd-Cu-Si bulk metallic glass forming system

Effective strategies to improve the tarnishing resistance of the 18 K (karat) gold-based bulk glass-forming composition Au49Ag5.5Pd2.3Cu26.9Si16.3 were recently found, with the addition of Ga at the expense of Cu and a sufficient reduction of Si. However, the modification of the alloy is accompanied by a reduction of the glass-forming ability (GFA) from 5 to 2 mm in terms of the critical casting thickness, and eventually collapses for Ga contents higher than about 9 at%. In this work, thermodynamic and kinetic studies of the newly discovered 18 K Au-Ag-Pd-Cu-Ga-Si system shed light on the reason for the loss in GFA with increasing Ga content. Investigations of the liquid kinetics in terms of viscosity and transition time, assessed by three-point beam bending and the Tg-shift method, reveal an unexpected change of the fragility to stronger liquid kinetics with increasing Ga concentration, which is usually attributed to an increase in the GFA. Thermodynamic considerations based on specific heat capacity measurements reveal, in contrast, an ascending driving force for crystallization with an increasing Ga-content, accountable for the drop in GFA. Therefore, a Ga-rich melt beyond the threshold concentration of 9 at% is not desirable for the production of bulk metallic glasses (BMGs) from the liquid state. With the intention to outrun this barrier, while preserving the amorphous structure, Ga-ion implantation with a liquid metal focused ion beam source is used as a post-processing routine for a cast Ga-containing 18 K Au-BMG. Hence, the thermodynamic borderline concentration is successfully crossed with additional tremendous improvement of the tarnishing resistance in the Ga-enriched areas.

Additive manufacturing of an iron-based bulk metallic glass larger than the critical casting thickness

Fe-based bulk metallic glasses (BMG) are of increasing research interest, driven in part by a unique combination of mechanical, magnetic and chemical properties. However, the maximum thickness and geometry of BMGs achievable in traditional manufacturing processes is limited. This work examines the capabilities of laser based powder bed additive manufacturing (AM) to produce relatively large Fe-based bulk metallic glass specimens. AM fabricated specimens exceed the critical casting thickness of the material by a factor of 15 or more in all dimensions. Resulting microstructural and mechanical properties are reported. Despite decreasing quench effect with increasing build thickness, X-ray diffraction analysis suggests that a fully amorphous structure was maintained throughout the build. However, a low concentration of sparsely distributed nano-grain clusters was discovered using a high-resolution electron backscatter diffraction scan. The results pave the way for novel applications of metallic glasses achievable through appropriate material design and optimization of existing additive manufacturing processes.

Electronic structure and glass forming ability in early and late transition metal alloys

A correlation between the change in magnetic susceptibility (Δχexp) upon crystallisation of Cu–Zr and Hf metallic glasses (MG) with their glass forming ability (GFA) observed recently, is found to apply to Cu–Ti and Zr–Ni alloys, too. In particular, small Δχexp, which reflects similar electronic structures, ES, of glassy and corresponding crystalline alloys, corresponds to high GFA. Here, we studied Δχexp for five Cu–Ti and four Cu–Zr and Ni–Zr MGs. The fully crystalline final state of all alloys was verified from X-ray diffraction patterns. The variation of GFA with composition in Cu–Ti, Cu–Zr and Cu–Hf MGs was established from the variation of the corresponding critical casting thickness, dc. Due to the absence of data for dc in Ni–Zr MGs their GFA was described using empirical criteria, such as the reduced glass transition temperature. A very good correlation between Δχexp and dc (and/or other criteria for GFA) was observed for all alloys studied. The correlation between the ES and GFA showed up best for Cu–Zr and NiZr2 alloys where direct data for the change in ES (ΔES) upon crystallisation are available. The applicability of the Δχexp (ΔES) criterion for high GFA (which provides a simple way to select the compositions with high GFA) to other metal-metal MGs (including ternary and multicomponent bulk MGs) is briefly discussed.

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.

The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–Fe–Ni–Ta–Nb–B–Si Bulk Metallic Glass

In this study, change in glass forming ability (GFA) and thermal stability of Co–Fe-based bulk metallic glasses were investigated as a function of iron content. Cylindrical samples of alloys with diameters of up to 4 mm were synthesized by a suction casting method in an arc furnace. Structures and thermal properties of the as-cast samples were determined by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. It was found that the critical casting thickness of the alloys reduced as iron content was increased and cobalt content was decreased. It was determined that GFA parameters, reduced glass transition temperature (Tg/Tl) and δ (= Tx/(Tl − Tg)), show a very good correlation with critical casting thickness values. It was also observed that changing iron content did not effect thermal properties of the alloys.

Softening of phonon spectra in metallic glasses

AbstractThe vibrational spectra of a series of MgZnCa amorphous alloys were computed using density functional theory and implementing the small displacement method. The atomic structures of the alloys were obtained by ab initio molecular dynamics simulations. The vibrational thermodynamic properties were calculated as a function of temperature and, in particular, the specific heat at low temperature was approximated by temperature cubed based on the Debye model. We computed the contribution of Mg vibrations to the specific heat and investigated the softening of Mg phonon spectra, where the maximum allowed vibrational frequency is lowered and highly collective diffusion processes are promoted. The statistical correlation between the reported critical casting thickness of the alloys and softening of Mg phonons was obtained. Similar calculations were performed for two distinctively different amorphous ZrTiCuAl alloys with large and small reported critical casting thickness, respectively. The findings were consistent with those of the MgZnCa alloys.

Thermo-physical characterization of the Fe67Mo6Ni3.5Cr3.5P12C5.5B2.5 bulk metallic glass forming alloy

The iron-phosphorus based bulk metallic glass forming alloy Fe67Mo6Ni3.5Cr3.5P12C5.5B2.5 is characterized with respect to its thermophysical properties, crystallization and relaxation behavior, as well as its viscosity. The alloy provides a high critical casting thickness of 13 mm, thus allowing for the casting of amorphous parts with a considerable size. Calorimetric measurements reveal the characteristic transformation temperatures, transformation enthalpies, and the specific heat capacity. The analyses show that no stable supercooled liquid region exists upon heating. The specific heat capacity data are used to calculate the enthalpy, entropy, and Gibbs free energy differences between the crystalline and the supercooled liquid state. The crystallization behavior of amorphous samples upon heating is analyzed by differential scanning calorimetry and X-ray diffraction, and a time-temperature-transformation diagram is constructed. Dilatometry is used to determine the thermal expansion behavior. The equilibrium viscosity below the glass transition as well as volume relaxation behavior are measured by three-point beam bending and dilatometry, respectively, to assess the kinetic fragility. With a kinetic fragility parameter of D* = 21.3, the alloy displays a rather strong liquid behavior. Viscosity above the melting point is determined using electromagnetic levitation in microgravity on a reduced gravity aircraft in cooperation with the German Aerospace Center (DLR). These high-temperature viscosity data are compared with the low-temperature three-point beam bending measurements. The alloy displays a strong liquid behavior at low temperatures and a fragile behavior at high temperatures. These results are analogous to the ones observed in several Zr-based bulk metallic glass forming liquids, indicating a strong to fragile liquid-liquid transition in the undercooled liquid, which is obscured by crystallization.