Development and Characterization of Low-Density Ca-Based Bulk Metallic Glasses: An Overview

Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of BMGs. Many of them have excellent GFA, which can be related to their efficient atomic packing, low onset driving force for crystallization, and high viscosity (high relaxation time) of the supercooled liquid. The Ca-based glasses have the lowest density and elastic moduli among all BMGs discovered to date. Unfortunately, as many other glasses, Ca-based BMGs are brittle below the glass transition temperature, and they also have marginal oxidation and corrosion resistance. The latter can be improved by proper selection of alloying elements. In this article, we review recent work on the development of low-density Ca-based BMGs and discuss the effect of alloy composition on the thermal, physical, and chemical properties of these glasses. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Metastability and thermophysical properties of metallic bulk glass forming alloys

The absence of crystallization over a wide time/temperature window can be used to produce bulk metallic glass by relatively slow cooling of the melt. For a number of alloys, including several multicomponent Zr-based alloys, the relevant thermodynamic and thermomechanical properties of the metastable glassy and undercooled liquid states have been measured below and above the glass transition temperature. These measurements include specific heat, viscosity, volume, and elastic properties as a function of temperature. As a result, it becomes obvious that the maximum undercooling for these alloys is given by an isentropic condition before an enthalpic or isochoric instability is reached. Alternatively, these glasses can also be produced by mechanical alloying, thus replacing the thermal disorder by static disorder and resulting in the same thermodynamic glass state. During heating through the undercooled liquid, a nanoscale phase separation occurs for most glasses as a precursor of crystallization. This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory.     

Chemical Short-Range-Ordering Effect on Structural Relaxation in Metallic Glasses

A model is proposed to address chemical effects on structural relaxation in metallic glasses. The atomic short range ordering (SRO) is described under the quasi-chemical approximation (QCA). Local chemical deviations from the ideal SRO are considered as an excess enthalpy. The simplified analysis of a disordered region’s evolution is based on the notion of the collective bond exchange between neighboring atoms. The approach suggests a bimolecular mechanism with possibly large apparent activation energy for structural relaxation near the glass transitions. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Phase-Field Study of the Cellular Bifurcation in Dilute Binary Alloys

Phase-field simulations in both two and three dimensions are used to investigate the microstructures that form closely above the threshold of the Mullins Sekerka instability in the directional solidi fication of dilute binary alloys. It is found that in this regime of shallow cells the simulation results strongly depend on the thickness of the diffuse interfaces even for model parameters that yield quantitative results for deep cells. For the material parameters of a dilute Sn-Bi alloy, the bifurcation is found to be supercritical, whereas weakly nonlinear amplitude expansions predict a subcritical bifurcation. Furthermore, an oscillatory instability of the cell grooves is found, which leads to the pinch-off of liquid inclusions even for relatively shallow cells. Finally, in three dimensions, three different morphologies are found, in agreement with experiments and previous numerical studies: regular hexagons, elongated cells (“stripes”), and inverted hexagons (“node” or “pox” structure, a hexagonal array of local depressions of the solidification front). Nodes and stripes are stable steadystate solutions only very close to the bifurcation. This article is based on a presentation made in the symposium entitled ”Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13-15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Viscoplastic Forming of Mg Bulk Metallic Glasses in the Supercooled Liquid Region

The Mg-Cu-rare earth (RE) alloys are produced under the form of amorphous cylindrical rods and characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Their capacity to be deformed in the supercooled liquid regions (SLRs) is studied. In the SLRs, crystallization can occur and strongly affect the viscoplastic forming conditions. Consequently, the thermal stabilities of the glasses are studied, in particular in the case of a Mg-Cu-Gd glass for which the kinetics of crystallization are quantified and the associated populations of crystallites identified. By appropriate heat treatments, various fractions of crystallites are thus produced and the effects of crystallization on the viscoplastic properties in the SLR are discussed in relation to mechanical models developed for materials containing rigid inclusions dispersed in a viscous medium. Finally, the possible effect of deformation on crystallization kinetics is also considered. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Glass-Forming Ability for Mg-Cu-Nd Alloys

The glass-forming ability (GFA), thermal stability, and mechanical properties of ternary Mg-Cu-Nd alloys were investigated. The results show that the amorphous structure of about 3 mm in diameter can be obtained in the composition range of 53 to 59 at. pct Mg, 32 to 38 at. pct Cu, and 9 to 11 at. pct Nd. Differential scanning calorimeter (DSC) measurements show that the bulk metallic glasses (BMGs) exhibit distinct glass transition temperature and supercooled liquid region before crystallization. However, the GFA for the present alloys cannot be explained by the existing calculated parameters, while it can be better explained by the strategy for pinpointing the best glass-forming alloys in terms of microstructure evolution. Compared with Mg₅₇Cu₃₃Y₁₀ BMG, Mg-Cu-Nd BMGs show a better fracture strength, which is increased with the copper content for those Nd-containing BMGs. Viscous flow was observed on the fracture surfaces of compressive samples, showing that apparent strengths can be reproducible. The Mg-Cu-Nd BMGs are challengeable in potential application for engineering materials due to their high strength and low cost. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Motion of Crystal Particles by the Electromagnetic Vibrations in Mg-Cu-Y Bulk Metallic Glasses

The method for producing Mg-Cu-Y and Fe-Co-B-Si-Nb bulk metallic glasses using electromagnetic vibrations is effective in forming the metallic glass phase. Disappearance or decrement of clusters by the electromagnetic vibrations applied to the liquid state is considered to cause suppression of crystal nucleation, because the electromagnetic vibrations vibrate the clusters vigorously in the melt. The purpose of this study was to investigate motion of the crystal particles by the electromagnetic vibrations in Mg-Cu-Y bulk metallic glasses. The electromagnetic vibration force vibrated the crystal particles or the clusters that become crystal nuclei in the melt, because the electric current for the electromagnetic vibrations concentrates in those. Thus, the electromagnetic vibrations were found to select vibration particles from the melt. Moreover, it was considered that composites for which second phases or other compounds are dispersed into the metallic glass phase or a nanostructure phase can be produced by the electromagnetic vibration process. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Natural Convection and Columnar-to-Equiaxed Transition Prediction in a Front-Tracking Model of Alloy Solidification

A meso-scale front-tracking model (FTM) of nonequilibrium binary alloy dendritic solidification has been extended to incorporate Kurz, Giovanola, and Trivedi (KGT) dendrite kinetics and a Scheil solidification path. Model validation via comparison with thermocouple measurements from a solidification experiment, in which natural convection is limited by design, is presented. Via solution of the flow field due to natural thermal buoyancy, it is shown that resultant liquid-phase convection creates conditions in which equiaxed solidification is favored. Comparison with simulations in which casting solidification is diffusion controlled show that natural convection has greatest effect at intermediate times, but that at early and late stages of columnar solidification, the differences are relatively small. It is, however, during the time of greatest divergence between the simulations that the authors’ predictive index for equiaxed zone formation is enhanced most by convection. Finally, the columnar-to-equiaxed transition is directly simulated, in directional solidification controlled by diffusion. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Shear Band Patterns in Metallic Glasses under Static Indentation, Dynamic Indentation, and Scratch Processes

The deformation structure in terms of shear band patterns in bulk metallic glasses (BMGs) under static indentation, dynamic indentation, and dynamic scratch tests has been investigated. The evolved shear band patterns appear to be a strong function of loading rate, although the plastic regions beneath the loading surface have similarities in shape irrespective of loading type. Comparison of currently available modeling estimates with experimental measurements has revealed that these models predict the plastic zone size reasonably well at low loads but deviate considerably at higher loads. The variation in spacing of shear bands is rationalized on the basis of the shear displacement accommodated by the shear bands formed under different loading rates, which results from a proposed shear-band formation mechanism based on the momentum diffusion model. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

Associative adsorption on liquid iron

The depression of the surface tension of a liquid metal by a binary surface compound which is formed from a pair of solutes is considered. The necessary thermodynamic equations are developed for the case in which the solutes are not separately surface active. It is shown that the published experimental results for liquid Fe-Cr-C alloys are fully consistent with the formation of the surface compound “CrC” and that the results for Fe-Si-C alloys might indicate coverage by “SiC”.     

<Emphasis Type="Italic">In-Situ</Emphasis> High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition in a Ni<Subscript>2</Subscript>MnGa Ferromagnetic Shape-Memory Crystal

The full information on the changes in many crystallographic aspects, including the structural and microstructural characterizations, during the phase transformation is essential for understanding the phase transition and “memory” behavior in the ferromagnetic shape-memory alloys. In the present article, the defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a uniaxial pressure of 50 MPa applied along the [110] crystallographic direction were studied by the in-situ high-energy X-ray diffuse-scattering experiments. The analysis of the characteristics of diffuse-scattering patterns around different sharp Bragg spots suggests that the influences of some defect clusters on the pressure-induced phase-transition sequences of Ni₂MnGa are significant. Our experiments show that an intermediate phase is produced during the premartensitic transition in the Ni₂MnGa single crystal, which is favorable for the nucleation of a martensitic phase. The compression stress along the [110] direction of the Heusler phase can promote the premartensitic and martensitic transition of the Ni₂MnGa single crystal. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Geometrical Frustration and Glass Formation

The effect of geometrical frustration in the atomic structure on the formability of bulk metallic glasses is discussed from a general point of view. It is pointed out that there are two distinct and complementing pathways to easy glass formation: stabilizing the glass itself and destabilizing the corresponding crystalline state. While the discussions in the field tend to focus on the first one, the second in fact is a more effective approach. Examples of both will be discussed using soft-sphere, rather than hard-sphere, packing concepts. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Thermochemistry of binary liquid gold alloys: The systems Au-Ni,Au-Co,Au-Fe,and Au-Mn

In this paper we report enthalpy of mixing data for the liquid alloys of gold with manganese, iron, cobalt, and nickel obtained by a Calvet-type calorimeter at 1378 K. The enthalpies of mixing are compared with Gibbs energies calculated from earlier emf and vapor pressure studies to yield information on the excess entropies of mixing. The limiting enthalpies of solution of the liquid transition metals in liquid gold are compared with values predicted from the semi-empirical model of Miedemaet al. and with earlier data for the same transition metals in liquid copper. The calculated values of the excess entropy of solution in liquid gold are compared with the corresponding values in liquid copper near 1400 K. For Ni, Co, and Fe as solutes we observepositive shifts of 5 to 9 J K⁻¹ mol⁻¹ which are attributed to vibrational entropy terms. For Mn there is a strongnegative shift of about 35 J K⁻¹ mol⁻¹. This shift probably is due to “complex” or “associate” formation between gold and manganese atoms.     

Influence of Solidification Thermal Parameters on the Columnar-to-Equiaxed Transition of Aluminum-Zinc and Zinc-Aluminum Alloys

Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between –0.338 °C/mm and 0.167 °C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

The influence of a martensitic phase transformation on stress development in thermal barrier coating systems

Thermal barrier coatings (TBCs) provide thermal insulation and oxidation protection of Ni-base superalloys in elevated temperature turbine applications. Thermal barrier coating failure is caused by spallation, which is related to the development of internal stresses during thermal cycling. Recent microstructural observations have highlighted the occurrence of a martensitic bond coat transformation, and this finite-element analysis was conducted to clarify the influence of the martensite on the development of stresses and strains in the multilayered system during thermal cycling. Simulations incorporating the volume change associated with the transformation and experimentally measured coating properties indicate that out-of-plane top coat stresses are greatly influenced by the presence of the martensitic transformation, the temperature at which it occurs relative to the strength of the bond coat and attendant bond coat plasticity. Intermediate values of bond coat strength and transformation temperatures are shown to result in the highest top coat stresses. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.     

Pd-Cu-Ni-P Bulk Metallic Glass: A Very Low Damping Material

The present work addresses damping experiments performed in a Pd-Cu-Ni-P bulk metallic glass. After an appropriated thermal treatment, this material exhibits a very low damping coefficient, down to 10⁻⁶. This result is discussed considering the different possible origins of the damping phenomena: thermoelasticity, energy dissipation by electrons, phonons, defects, and residual stresses. Thermoelasticity and defects appear to be the most important sources of mechanical damping. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25−March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

A solidification diagram for Ni-Cr-Mo-Gd alloys estimated by quantitative microstructural characterization and thermal analysis

A γ-Gd solidification diagram is proposed as an aid to understanding solidification behavior of Ni-Cr-Mo-Gd alloys. In this system, the Ni-Cr-Mo solid solution γ primary austenite phase is treated as the “solvent” and Gd is treated as the solute. The proposed diagram, which has features characteristic of a binary “eutectic” system, was constructed by combining differential thermal analysis and quantitative microstructural analysis data. As a result of the partially divorced solidification microstructure in the ingots studied, determination of the fraction eutectic, and hence the eutectic composition, requires the use of advanced image analysis techniques. The diagram displays a number of features that are very similar to the Ni-Gd binary system and can be used to assess the influence of the Gd concentration on solidification behavior.     

Microsegregation in directionally solidified Pb-8.4 at. pct Au alloy

The dependence of microsegregation behavior on growth rate and thermal gradient has been examined in a Pb-8.4 at. pct Au alloy material partially directionally solidified and quenched. The composition of the quenched “liquid” at the dendrite tip (C _t), that of the eutectic-like solid phase freezing from the interdendritic liquid at the base of dendrite(C _se), the volume fraction of this eutectic-like region(f _e), and solute profiles in the interdendritic quenched liquid and ahead of the dendrite have been measured. Two dendritic growth models for solidification of a binary alloy melt in a positive thermal gradient at the liquid-solid interface, one for dendrites with “minimum undercooled dendrite tip” and the other for an Ivantsov type of dendrite with “marginally stable tip,” have been examined for a quantitative comparison with measured values ofC ₁, C_se, andf _e. Convection in the melt, possibly due to horizontal density gradients, is found to be a serious limitation for theoretical understanding of the observed experimental behavior and meaningful comparison of theories.     

Prediction of properties of intermetallics using a chemical bonding model

Materials with novel properties are needed for new technology developments. It is important to be able to predict which of the millions of multicomponent intermetallics might provide the desired properties. A single chemical bonding model, the Brewer-Engel, has provided reliable thermodynamic data for the metallic elements in a variety of crystal structures. The model can be extended to intermetallics. An illustration of the calculations is available for Al or Mg with transition metals forming intermetallics with binary CsCl structures. A number of publications will be available for similar calculations for a variety of crystal structures. The calculations will be extended to a variety of compositions and multicomponent systems. The present article will discuss the procedures that can be used to simplify the calculations, yet maintain reliable accuracy. This article is based on a presentation made at “The Milton Blander Symposium on Thermodynamic Predictions and Applications” at the TMS Annual Meeting in San Diego, California, on March 1–2, 1999, under the auspices of the TMS Extraction and Processing Division and the ASM Thermodynamics and Phase Equilibrium Committee.