Bulk Metallic Glass-forming Alloys Research Articles

Yttrium Enhanced Glass-Forming Ability of Zr-Based Metallic Glasses

Thermodynamic behavior of Zr56-xCo28Al16Yx (0, 2, 7, 10) bulk metallic glass-forming alloys has been studied using an analytical approach where some of the key thermodynamic quantities such as ΔH, ΔS and ΔG has been estimated using a hyperbolic temperature dependence of specific heat difference ΔCp in the supercooled liquid region. The study is focused on understanding the effect of Yttrium-doping on the thermodynamics of the alloys in the supercooled region and on the glass-forming ability of these alloys. The analytical approach has been found to give estimates of ΔG in a wide supercooled liquid region in excellent agreement with the experimental results. Estimated ΔG values are found to be minimum for Y concentration of 7 and 10% which is consistent with the observed high GFA for these compositions. Fundamental elemental properties such as atomic size, electronegativity, the heat of mixing have been found to play an important role in governing the thermodynamics of the alloys in the supercooled liquid region.

Observation of an apparent first-order glass transition in ultrafragile Pt–Cu–P bulk metallic glasses

An experimental study of the configurational thermodynamics for a series of near-eutectic Pt80-x Cu x P20 bulk metallic glass-forming alloys is reported where 14 < x < 27. The undercooled liquid alloys exhibit very high fragility that increases as x decreases, resulting in an increasingly sharp glass transition. With decreasing x, the extrapolated Kauzmann temperature of the liquid, T K , becomes indistinguishable from the conventionally defined glass transition temperature, T g For x < 17, the observed liquid configurational enthalpy vs. T displays a marked discontinuous drop or latent heat at a well-defined freezing temperature, T gm The entropy drop for this first-order liquid/glass transition is approximately two-thirds of the entropy of fusion of the crystallized eutectic alloy. Below T gm , the configurational entropy of the frozen glass continues to fall rapidly, approaching that of the crystallized eutectic solid in the low T limit. The so-called Kauzmann paradox, with negative liquid entropy (vs. the crystalline state), is averted and the liquid configurational entropy appears to comply with the third law of thermodynamics. Despite their ultrafragile character, the liquids at x = 14 and 16 are bulk glass formers, yielding fully glassy rods up to 2- and 3-mm diameter on water quenching in thin-wall silica tubes. The low Cu content alloys are definitive examples of glasses that exhibit first-order melting.

Thermodynamics of polymorphism in a bulk metallic glass: Heat capacity measurements by fast differential scanning calorimetry

The Au-based bulk metallic glass-forming alloy Au70Cu5.5Ag7.5Si17 is the first atomic system in which monotropic polymorphism has been experimentally found. In this study, strategies for measuring thermodynamic quantities such as heat capacity, transformation enthalpy and entropy, and Gibbs free energy for stable and metastable phases are evaluated based on this system. The transformation enthalpies between phases are determined via differential scanning calorimetry (DSC); the corresponding heat capacities are measured via stochastic temperature-modulated DSC (TOPEM); and the equilibrium melting temperatures of metastable phases, only detectable at high rates, are determined via fast DSC (FDSC). Whereas the slow heating rate of conventional DSC limits the characterization of metastable phases, the combination of both DSC and FDSC allows for their detailed heat-capacity determination. Furthermore, various approximation functions that have previously been used to determine the temperature dependence of the thermodynamic driving force for nucleation and crystallization are compared with the experimental data.

Long-Range Mass Transport during Structural Transitions in Metallic Glass-Forming Melts

We investigate the structure and dynamics of the bulk metallic glass-forming alloys Zr_{41.2}Ti_{13.8}Cu_{12.5}Ni_{10}Be_{22.5} and Zr_{58.5}Cu_{15.6}Ni_{12.8}Al_{10.3}Nb_{2.8}. Combining insitu synchrotron x-ray diffraction and quasielastic neutron scattering with electrostatic levitation, we directly observe an abrupt change in the temperature dependence of the first structure factor maximum of these melts. We find that the kinetics of this liquid-liquid transition during cooling are on the order of tens of seconds, whereas its onset temperature depends only weakly on the applied cooling rate. Such slow transition kinetics require long-range mass transport, which is incompatible with a transition mechanism involving only local structural changes as in oxides or molecular liquids.

Assessing continuous casting of precious bulk metallic glasses

This study describes investigations of continuous casting for the bulk metallic glass-forming alloys Pd43Ni10Cu27P20 and Pt57.3Cu14.6Ni5.3P22.8, respectively. Continuous casting of the Pd-based alloy readily produced fully amorphous rods of 10 mm diameter and >500 mm length. In contrast, the Pt-based alloy always underwent crystallization during the casting process, even at smaller rod diameters down to 2 mm. In search of the reason for this difference, we simulated the process using ProCAST. Via adapting the heat transfer coefficients for all the relevant interfaces of the continuous casting setup we obtained good agreement between the experimental temperature data and the simulations. The calculations revealed that the maximum cooling rates achievable with this industrial setup ranged from 15 K/s to 17 K/s. This is well above the critical cooling rate for glass formation of the Pd-based alloy, but below that of the Pt-based alloy, as inferred from literature data and corroborated by fast differential scanning calorimetry. These findings illustrate how detailed casting simulations can predict the feasibility of semi-industrial bulk metallic glass production techniques.

Bioinspired nacre-like alumina with a bulk-metallic glass-forming alloy as a compliant phase

Bioinspired ceramics with micron-scale ceramic “bricks” bonded by a metallic “mortar” are projected to result in higher strength and toughness ceramics, but their processing is challenging as metals do not typically wet ceramics. To resolve this issue, we made alumina structures using rapid pressureless infiltration of a zirconium-based bulk-metallic glass mortar that reactively wets the surface of freeze-cast alumina preforms. The mechanical properties of the resulting Al2O3 with a glass-forming compliant-phase change with infiltration temperature and ceramic content, leading to a trade-off between flexural strength (varying from 89 to 800 MPa) and fracture toughness (varying from 4 to more than 9 MPa·m½). The high toughness levels are attributed to brick pull-out and crack deflection along the ceramic/metal interfaces. Since these mechanisms are enabled by interfacial failure rather than failure within the metallic mortar, the potential for optimizing these bioinspired materials for damage tolerance has still not been fully realized.

Indications for a fragile-to-strong transition in the high- and low-temperature viscosity of the Fe43Cr16Mo16C15B10 bulk metallic glass-forming alloy

Viscosity of the Fe-based bulk metallic glass-forming liquid Fe43Cr16Mo16C15B10 is measured around the glass transition and in the stable liquid. Low-temperature measurements are conducted using three-point beam bending in a thermomechanical analyzer, and high-temperature data are obtained from the damping behavior of an oscillating droplet which is electromagnetically levitated in microgravity on a reduced-gravity aircraft. The alloy displays an intermediately strong liquid behavior (D* = 15.1) at low temperatures and a fragile behavior (D* = 7.9) at high temperatures. Hence, the temperature dependence of viscosity changes drastically between the high- and the low-temperature regime, which suggests the existence of a fragile-to-strong liquid-liquid transition in the supercooled liquid. Furthermore, viscosity and fragility data are discussed with respect to the glass-forming ability of the alloy.

Viscosity and fragility of the supercooled liquids and melts from the Fe–Co–B–Si–Nb and Fe–Mo–P–C–B–Si glass-forming alloy systems

The dynamic viscosity of four Fe-based bulk metallic glass-forming alloys, [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 (alloy A), {[(Fe0.5Co0.5)0.75B0.2Si0.05]0.96Nb0.04}99.5Cu0.5 (alloy B), Fe74Mo4P10C7.5B2.5Si2 (alloy C) and (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 (alloy D), was investigated as a function of temperature in the supercooled liquid region, as well as above the melting point. The alloy B is Cu-added alloy A, while the alloy D was obtained upon fine-tuning the alloy C composition. All these alloys may form bulk metallic glasses upon copper mold casting. The viscosities in the supercooled liquid region were calculated using the data obtained upon parallel plate rheometry measurements, as well as upon differential scanning calorimetry (DSC). The values of the supercooled fragility parameter m, 61, 66, 52 and 60 for the alloys A, B, C and D respectively, indicate that these alloys are intermediate glass formers. The behavior of the same alloys, in the molten state, was studied using a high temperature torsional oscillation cup viscometer. The values of the corresponding fragility parameter M was calculated as 5.03, 5.91, 4.25, 4.93 for the alloys A, B, C and D, respectively. They confirm the supercooled liquid behavior and predict that the alloys A and C may form glasses easier than the fine-tuned compositions B and D. Angell plot is constructed for the entire range of viscosities and the values from both regions, i.e. above melting point and supercooled liquid region, fit well with the model.

Measurements of volume, thermal expansion, and specific heat in Zr57Cu15.4Ni12.6Al10Nb5 and Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 liquids and glasses

The temperature dependence of the specific volume, thermal expansion coefficient, and specific heat in the equilibrium, supercooled liquids, and glasses of the bulk metallic glass-forming alloys Zr57Cu15.4Ni12.6Al10Nb5 and Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 are determined using the containerless electrostatic levitation technique. Such measurements have not been possible thus far because of rapid crystallization from the supercooled metastable liquids. The results show a decrease in the thermal expansion coefficient with decreasing temperature before the onset of the glass transition. The difference in the rate of decrease between the two alloys indicates larger structural contributions to the thermal expansion for the stronger liquid at high temperatures.

방전플라즈마소결을 이용한 Ni계 비정질 복합재의 제조

A bulk metallic glass-forming alloy, <TEX>$Ni_{59}Zr_{20}Ti_{16}Si_2Sn_3$</TEX> metallic glass powders was used for good commercial availability and good formability in supercooled liquid region. In this study, the Ni-based metallic glass was synthesized using by high pressure gas atomized metallic glass powders. In order to create a bulk metallic glass sample, the <TEX>$Ni_{59}Zr_{20}Ti_{16}Si_2Sn_3$</TEX> metallic glass powders with ball-milled Ni-based amorphous powder with 40%vol brass powder and Cu powder for 20 hours. The composite specimens were prepared by Spark Plasma Sintering for the precursor. The SPS was performed at supercooled liquid region of Ni-based metallic glass. The amorphous structure of the final sample was characterized by SEM, X-ray diffraction and DSC analysis.

High temperature melt viscosity and fragile to strong transition in Zr–Cu–Ni–Al–Nb(Ti) and Cu47Ti34Zr11Ni8 bulk metallic glasses

The molten viscous behavior of Vitreloy 106, 106a, 105 and 101 bulk metallic glass-forming alloys was investigated at temperatures above Tliq. Couette concentric cylinder viscometry was used to determine the isothermal and continuous cooling melt viscosities. Isothermal three point beam bending measurements of the viscosity in the vicinity of Tg revealed a kinetically stronger liquid at low temperatures than at high temperatures, near Tliq. This behavior is revealed to be the result of a fragile to strong transition in the undercooled liquid, possibly due to polyamorphism.

Evidence for a simple monatomic ideal glass former: The thermodynamic glass transition from a stable liquid phase

Under cooling, a liquid can undergo a transition to the glassy state either as a result of a continuous slowing down or by a first-order polyamorphous phase transition. The second scenario has so far always been observed in a metastable liquid domain below the melting point where crystalline nucleation interfered with the glass formation. We report the first observation of the liquid-glass transition by a first-order polyamorphous phase transition from the equilibrium stable liquid phase. The observation was made in a molecular dynamics simulation of a one-component system with a model metallic pair potential. In this way, the model, demonstrating the thermodynamic glass transition from a stable liquid phase, may be regarded as a candidate for a simple monatomic ideal glass former. This observation is of conceptual importance in the context of continuing attempts to resolve the long-standing Kauzmann paradox. The possibility of a thermodynamic glass transition from an equilibrium melt in a metallic system also indicates a new strategy for the development of bulk metallic glass-forming alloys.

Enhanced Work Hardening of Cu-Based Bulk Metallic Glass Composites by &lt;i&gt;In Situ&lt;/i&gt; Formed Nano-Scale Heterogeneities

The work hardening ability under room temperature compression of ductile Cu47.5Zr47.5Al5 and Cu47Ti33Zr11Ni8Si1 bulk metallic glass-forming alloys has been studied and compared. Both alloys exhibit high fracture strength, distinct work hardening and large plastic strain. Systematic investigations on the microstructural evolution reveal the occurrence of nano-scale heterogeneities, of both structural and chemical nature, which enables multiplication, branching, and restriction of the shear bands, thus controlling the plastic instability of metallic glasses. Phase separation in the liquid state leading to chemical inhomogeneities has been revealed for as-cast Cu47.5Zr47.5Al5 samples. In the case of Cu47Ti33Zr11Ni8Si1, a composite-type microstructure with in-situ formed nano-scale precipitates embedded in a glassy matrix is responsible for the distinct work hardening recorded on the stress-strain curves. The present results support the important role of nano-scale heterogeneities for promoting efficient work hardening in Cu-based metallic glass composites.

Tailoring of microstructure and mechanical properties of a Ti-based bulk metallic glass-forming alloy

To achieve plasticity of Ti-based bulk metallic glasses (BMGs) without sacrificing their high strength, the microstructure of Ti50Cu20Ni24Sn3Si2B1 alloy was tailored to form in situ composites. The cast rods consist of micrometer-sized NiTi dendrites surrounded by a thin metallic glass network along with a fine CuTi3 phase. The rods show high strengths of ∼1900–2250 MPa and plastic strains of ∼4.5–7.8%, which are superior to the monolithic Ti-based BMGs. The microstructure–property correlations and the deformation behavior are investigated.

Liquid-to-glass transition in bulk glass-formingCu60Ti20Zr20alloy by molecular dynamics simulations

We report results from molecular dynamics studies concerning the microscopic structure and dynamics of the ternary, bulk metallic glass-forming Cu60Ti20Zr20 alloy. In detail we consider the partial radial distribution functions, nearest-neighbor numbers, specific heat, simulated glass temperature, diffusion coefficients, and incoherent intermediate scattering function (ISF). The applied atomic model reproduces well experimental x-ray data of the total radial distribution function. It provides for Cu60Ti20Zr20 a structure with marked intermediate-range order. The ISF is analyzed within an extension of mode-coupling theory, where the effective memory kernel is evaluated from the Laplace transform of the ISF. The dynamics of the system fulfills in most respects the predictions of mode-coupling theory (MCT), up to an absence of the algebraic t{-a} decay in the early beta range. Comparison with the calculated memory kernel shows that this absence can be traced back to deviations of the kernel from its approximate form analyzed in MCT. As by-product, our investigation provides a method to reconstruct around the critical temperature major parts of the memory kernel from lambda and the plateau value f{c} of the ISF, and it indicates why the critical dynamics predicted by mode-coupling theory can be observed in a temperature interval of more than 500 K.

New Fe–Cr–Mo–Ga–C composites with high compressive strength and large plasticity

A novel nanostructured Fe–Cr–Mo–Ga–C composite derived from a bulk metallic glass-forming alloy composition has been prepared with different carbon content to improve the ductility of this high-strength material. Excellent compressive mechanical properties were found for both samples, which essentially consist of soft gallium-enriched dendrites embedded in a hard chromium- and molybdenum-enriched matrix. The coexistence of the soft and hard phases leads to a composite material with a high fracture strength combined with large ductility. The obtained values of true strength and true strain are 2.9 GPa and 36% for the alloy with lower carbon content and 3.0 GPa and 16% for the alloy with higher carbon content, respectively. It is worth noting that such excellent compressive behavior has never been observed before for any crystalline or glassy structured material.

Correlation between the fragility of supercooled liquids and thermal expansion in the glassystate for Gd-based glass-forming alloys

Dilatometric measurements were performed to obtain the average thermal expansioncoefficients of a series of Gd-based bulk metallic glasses. The fragilities of these alloys weredetermined based on differential scanning calorimetry measurements. It was found thatthere is a linear correlation between the fragility parameter of supercooled liquids and theaverage thermal expansion coefficient in Gd-based bulk metallic glass-forming alloys.

Formation of bulk metallic glasses in Cu 45Zr 48− xAl 7RE x (RE = La, Ce, Nd, Gd and 0 ≤ x ≤ 5 at.%)

We report a series of bulk metallic glass-forming alloys of compositions (Cu 45Zr 48− x Al 7RE x , RE = La, Ce, Nd, Gd and 0 ≤ x ≤ 5 at.%). By using a conventional copper mold sucking method, alloys with diameters ranging from 5 to 10 mm can be readily solidified into an amorphous structure without detectable crystallites. The best glass-forming ability is obtained for the alloys Cu 45Zr 46Al 7RE 2. Possible effects of RE addition on the glass-forming ability are discussed. In addition, the compositional effect on mechanical properties of Zr–Cu–Al–Gd alloys is presented.

Chill-cast in situ composites in the pseudo-ternary Mg–(Cu,Ni)–Y glass-forming system: Microstructure and compressive properties

Starting with a bulk metallic glass-forming alloy Mg65Cu18Ni6Y11, we prepared in situ composites by increasing the Mg content in a series of alloys, Mgx(Cu0.51Ni0.17Y0.32)100−x (65 ≤ x ≤ 90), via copper mold casting of rods 4 mm in diameter. The fully glassy alloy at x = 65 showed a compressive fracture strength of 755 MPa but no observable macroscopic plasticity prior to failure. Metallic glass-based composites were formed when the Mg content was increased. For x &gt; 80, the glassy phase no longer existed in the as-cast rods. In the composition range of 80 ≤ x ≤ 85, needle-shaped Mg solution with a 14H-type long period stacking (LPS) structure appeared as the primary phase in the as-cast microstructure. On further increase of the Mg content up to x = 90, the solidified primary phase became 2H-Mg, coexisting with the remaining eutectic structure. The best combination of mechanical properties was obtained for the alloy at x = 81.5, which showed a fracture strength of 665 MPa and a compressive plastic strain of 11.6%. The specific strength of this alloy was 2.8 × 105 N m kg−1, much higher than conventional cast magnesium alloys. The mechanical properties are discussed in light of the phase selection and microstructural features uncovered in microscopy examinations.

Structural characterization and mechanical properties of nanocrystal-containing Cu–Ti-based bulk metallic glass-forming alloys

Cylindrical Cu42.5Ti41.5Ni7.5Zr2.5Hf5Si1 bulk metallic glass with a diameter of 2 mm was fabricated by copper-mold casting. X-ray diffraction and differential scanning calorimetry analysis of the material showed that the alloy has a homogenous amorphous structure and high glass-forming ability. However, detailed observation by transmission electron microscopy revealed that a kind of nanocrystal with size of about 20 nm is sparsely distributed in the glass matrix. Nanobeam electron diffraction experiments indicated that the nanocrystal has a face-centered cubic crystalline structure. Room-temperature compression tests revealed that the alloy has a high fracture strength of 2250 MPa and obvious plastic strain of about 5.3%. Nanoindentation tests revealed that the as-cast alloy exhibits obviously serrated flow over a wide range of loading rate from 0.5 to 10 mN/min.