Rare earth based bulk metallic glasses

Recently, the rare earth based bulk metallic glasses (REBMGs) have attracted increasing interest due to their unique properties and potential applications as functional glassy materials. These REBMGs display many fascinating properties such as heavy fermion behavior, thermoplastic properties near room temperature, excellent magnetocaloric effect, hard magnetism, and polyamorphism, all of which are of interest not only for basic research but also for metallurgy and technology. These characteristics and properties are ascribed to the unique electronic, magnetic and atomic structures of the REBMGs. In this review paper, the fabrication, glass-forming ability, polyamorphism, elastic, thermal, and physical properties are summarized and discussed. Owing to the unique electronic structure of rare earth elements, the electric and magnetic properties of the REBMGs are especially addressed. The works have implications for seeking novel metallic glasses with controllable properties and for understanding the nature of glass formation. The development of REBMGs as functional materials might promote and extend the commercial applications of metallic glasses.     

Formation and properties of RE55Al25Co20 (RE = Y,Ce, La,Pr, Nd,Gd, Tb,Dy, Ho and Er) bulk metallic glasses

We report that a series of ternary RE₅₅Al₂₅Co₂₀ (RE = Y, Ce, La, Pr, Nd, Gd, Tb, Dy, Ho and Er) alloys can be readily cast into bulk glasses by a conventional casting method. The characteristics and properties of these new bulk metallic glasses (BMGs) are studied and compared. Due to the chemical comparability and well-regulated variety in atomic size, properties and elastic constants of these rare earth elements, the RE₅₅Al₂₅Co₂₀ BMGs could be regarded as a model system to investigate the glass-forming ability, thermal stability, glass transition, crystallization behavior, liquid fragility, elastic and mechanical properties as well as their relationships. An attempt is made to highlight commonality and contrasts of the effects of various factors on the metallic glasses formation and properties.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

Elastic properties of some bulk metallic glasses

The relationships between the elastic moduli, glass forming ability and response to deformation of bulk metallic glasses are investigated. Five bulk metallic glasses are prepared from high purity elements via suction casting. The results confirm that there exists a correlation between energy absorbed to failure during compression testing and the bulk to shear modulus ratio. This finding is developed such that it corresponds only to the elastic component of energy absorption, and that the bulk modulus dominates this. Plastic deformation appears to be favored by a reduced shear modulus, although it shows greater dependence on structural features that are frozen in during the glass transition, and so may well be dependent on the liquid fragility.     

Bulk metallic glasses based on rare-earth elements in lanthanum series

A series of bulk metallic glasses (BMGs) based on the rare-earth (RE) elements in lanthanum family have been obtained by a copper mold casting method. These chemical comparable RE elements with ‘continuous’ range of atomic size, electron structure and elastic constants may provide a good system for systematically exploring the BMG-forming characteristics. The glass-forming ability, elastic properties, thermal stability and their correlations in the RE-based BMGs have been investigated.     

Dissoluble and degradable CaLi-based metallic glasses

We study the degradable and dissoluble features of a Ca-Li-Mg-Zn bulk metallic glass in pure water at room temperature. A remarkable degradable feature of the metallic glasses is that the degradation is controllable by changing the composition and components. The degradable metallic glasses with superior combined properties of polymer-like thermal plasticity at low temperature (40-70 °C), the ultralow elastic moduli comparable to that of human bones, and ultralow density (< 2 g/cm³) in known metallic glasses to date, and good machinability at a lower temperature in the supercooled liquid region could have potential applications. The metallic glasses also provide a model system to study the corrosion behavior in glasses.     

Properties of as-cast and structurally relaxed Zr-based bulk metallic glasses

We have studied the influence of the arc melting procedure of Nb-containing Zr-based bulk metallic glasses (BMGs) on their thermal and mechanical properties. We found that the strength and plastic strain to failure, determined at room temperature, increases by the addition of a few percentage of Nb into the Zr-based BMGs. High-resolution transmission-electron microscopy results show that the addition of 2 at.% Nb introduces the formation of as-cast nanocrystals. At the same time, thermal analyses indicate an increase in glass forming ability with the small addition of Nb. Contrary to the reported results in other amorphous alloys, we found that the plastic strain increases further after heat-induced structural relaxation in the Zr-based BMGs.     

Improved plasticity of iron-based high-strength bulk metallic glasses by copper-induced nanocrystallization

A significant enhancement in plasticity was achieved by the micro-addition of Cu in Fe-based bulk metallic glasses (BMGs) which have high strength at room-temperature. It was found that the nanocrystallization caused by the formation of the α-Fe phase is responsible for the improvement in plasticity of the Fe-based BMGs. Our results suggest that the micro-addition of Cu is a promising approach to enhancing the plasticity of Fe-based BMGs through compositional and structural design by microalloying.     

High mixing entropy bulk metallic glasses

Bulk metallic glasses (BMGs) are usually based on a single principal element such as Zr, Cu, Mg and Fe. In this work, we report the formation of a series of high mixing entropy BMGs based on multiple major elements, which have unique characteristics of excellent glass-forming ability and mechanical properties compared with conventional BMGs. The high mixing entropy BMGs based on multiple major elements might be of significance in scientific studies, potential applications, and providing a novel approach in search for new metallic glass-forming systems.     

Bulk metallic glasses based on ytterbium and calcium

We report the formation of a family of bulk metallic glasses (BMGs) based on rare earth element of ytterbium and alkaline earth element of calcium. The glass-forming ability, atomic packing density and corrosion behaviors of the BMGs show an extremum around the eutectic point with the change of the concentration of Yb and Ca.     

Oxynitride glasses, their properties and crystallisation - a review

Oxynitride glasses were first discovered as intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of such oxynitride glass phases determine the properties of the material. In particular they have been shown to control high temperature mechanical properties. The desire to understand the nature of these grain boundary phases has resulted in a number of investigations on oxynitride glass formation and properties which have shown oxynitride glasses to possess refractory behavior and higher, elastic modulus, viscosity and hardness compared to the corresponding oxide glasses. This paper provides a review of the preparation and characterisation of oxynitride glasses and outlines the effect of composition, especially nitrogen content, on properties such as glass transition temperature, hardness, Young’s modulus and viscosity.     

Elastic constants,hardness and their implications to flow properties of metallic glasses

The longitudinal and transverse sound velocities and Vickers hardness of metallic glasses (Pd_{1 ? x}Ni_x)_0.80P_0.20, (Pd_{1 ? x}Fe_x)_0.80P_0.20 and (Pt_{1 ? x}Ni_x)_0.75P_0.25 have been measured. The elastic constants at room temperature exhibit a positive deviation with composition χ from linearity whereas the hardness shows a negative deviation. The increase in elastic constants has been attributed to a denser packing of the alloys on mixing. The reduced hardness H_rH/μ versus χ exhibits a remarkable similarity to a T_g versus χ relationship. This seems to indicate that flow mechanisms involved in metallic glasses above and below the glass-transition temperature are of similar origins. It is the excess entropy of disorder associated with alloying which lowers the hardness as well as the viscosity of metallic glasses. The metallic glasses possess in general a relatively high Poisson's ratio ν ≈ 0.40 and a shear modulus approaching that of the noble metals Cu, Ag and Au. Among the metallic glasses observed, the PtP glasses exhibit the highest ν = 0.42, whereas the glasses containing Fe tend to have lower values. The phenomenon that the conduction electrons in the glassy alloys behave as in the noble metals may be partly attributed to the filling of d shell orbitals of the transition metals in the PtP, PdP and NiP alloys. The high ν of metallic glasses is believed to be responsible for the ductile behavior of these glasses. Poisson's ratio ν of metallic glasses was observed to decrease with decreasing temperature. It is suggested that the decreasing ν with falling temperature causes the rapid increase in the fracture strength of Fe-based glasses.     

Pressure dependence of elastic properties of low-silica calcium alumino-silicate glasses

The hydrostatic and uni-axial pressure dependence of elastic properties of a low-silica calcium alumino-silicate glass (LSCAS) is determined by ultrasonic pulse-echo techniques at room temperature. The experimental results are used to obtain the third-order elastic constants (TOECs) of these glasses. The pressure dependence of fractal bond connectivity of these glasses is discussed. The normal behavior of positive pressure dependence of ultrasonic velocities was observed for the glass. The pressure dependence of both shear modulus and bulk modulus are positive for these glasses.     

Bulk metallic glasses based on binary rare earth elements

Bulk metallic glasses (BMGs) are usually based on single element such as Zr, Cu, and Fe. In this work, we report the formation of a series of BMGs based on arbitrarily selected binary rare earth elements. Compared with single-base BMGs, the binary-base BMGs have unique characteristics of excellent glass-forming ability, tunable physical, chemical and mechanical properties. The binary-base BMGs might be of significance in scientific studies and have potential applications, and the approach for formation of metallic glasses has implications in the search for new BMG systems.     

Development of quaternary Fe-based bulk metallic glasses with high saturation magnetization above 1.6 T

Quaternary Fe-based ferromagnetic bulk metallic glasses (BMGs) with saturation magnetization above 1.6 T were successfully fabricated in Fe–Si–B–P alloy system by copper mold casting. These BMGs exhibit low coercive force of 1.6–1.9 A/m, high effective permeability of 16,500–17,200 and low core loss. In additional, these BMGs exhibit good mechanical properties as well, i.e., high strength of 3200 MPa and plasticity of 1.1%. They are promising to be used as magnetic functional and structural materials in the future.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

Minor addition induced enhancement of strength of Mg-based bulk metallic glass

We report that the fracture strength of Mg-based bulk metallic glasses (BMGs) can be dramatically enhanced up to 1.10 GPa by minor Gd addition. The Poisson’s ratio v of the BMG also decreases to 0.261 close to that of brittle oxide glasses when 1 at.% Gd was added. Such significant enhancement in strength which approaches the theoretical strength value and dramatically decrease in the Poisson’s ratio are attributed to the structural change of the BMGs induced by the Gd minor addition.     

Static heterogeneity in metallic glasses and its correlation to physical properties

Glasses exhibit intriguing physical and mechanical properties resulting from their structure. We have investigated metal-glass dynamics using inelastic X-ray scattering and ultrasonic techniques for several Pd-, Pt-, and Zr-based glasses with varying fragility. In some cases we have observed a faster phase velocity at short wavelengths than long wavelengths, or positive dispersion. Here we apply elastic wave scattering theory to suggest that the behavior of acoustic phonons can be understood by considering the presence of intrinsic nanoscale elastic inhomogeneity with a certain correlation length, i.e., “static heterogeneity”. Furthermore, we suggest that such an elastic inhomogeneity could be the origin of many of the interesting physical and mechanical properties of metallic glasses.     

Mechanical properties of NiFe based alloy glasses

The ductility and tensile strength of NiFe based alloy glasses prepared by the ‘roller quenching’ technique were investigated. These alloy glasses are intrinsically ductile and possess a high tensile strength. However, the mechanical properties of glasses which are predominantly Fe are very susceptible to the quenching conditions, and such glasses tend to be brittle. It is suggested that the hot rolling of the alloy during quenching is responsible for the brittle behavior. Tensile strengths as high as 230 000 psi were obtained for the NiFe based alloy glasses.     

MeV electron irradiation induced crystallization in metallic glasses: Atomic structure,crystallization mechanism and stability of an amorphous phase under the irradiation

MeV electron irradiation via high voltage electron microscopy can lead to amorphous-to-crystal transition (i.e., crystallization) as well as crystal-to-amorphous transition (i.e., solid-state amorphization). Irradiation-induced crystallization can be observed in various alloy systems such as Co-, Fe-, Ni-, Pd-, and Zr-based metallic glasses, indicating that this phenomenon has wide generality in metallic materials. Irradiation-induced crystallization mechanism was discussed based on the following factors; (1) an increase in free energy for an amorphous phase, (2) the formation of crystalline clusters through modification of the atomic configuration near radiation induced defects, and (3) enhanced diffusion. The stability of an amorphous phase against irradiation-induced crystallization can be estimated from the thermal crystallization temperature (T_x), and Ni–Nb based metallic glasses have a tendency for high stability against irradiation because of high T_x.