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.     

Bulk Metallic Glasses VI

Symposium: Bulk Metallic Glasses VI

Bulk Metallic Glasses VI Foreword     

Nanocrystallization in Cu-Zr-Al-Sm Bulk Metallic Glasses

Metallurgical and Materials Transactions A - The effect of rare-earth element (Sm) microalloying on the thermal stability and crystallization kinetics of melt-spun ribbons and suction-cast rods of...     

Tough Hypoeutectic Zr-Based Bulk Metallic Glasses

Significant softening of Zr-based bulk metallic glasses (BMGs) can be seen in a hypoeutectic Zr-enriched composition, which brings about very high toughness (for the Zr₅₅Cu₃₅Al₁₀ and Zr₆₀Cu₃₀Al₁₀ BMGs) and tensile plasticity at room temperature (for the Zr₇₀Ni₁₆Cu₆Al₈ BMG). The unique features of such BMGs include the formation of multiple shear bands and harmonic alternating movements that can immediately accommodate concentrated stresses and avoid accidental catastrophic fracture.     

Self-organized Criticality Behavior in Bulk Metallic Glasses

Serrated flows are known as repeated yielding of bulk metallic glasses (BMGs)during plastic deformation under different loading conditions,which are associated with the operation of shear banding.According to the statis-tics of some parameters,the shear avalanches can display a self-organized critical state,suggesting a large ductility of BMGs.The emergence of the self-organized criticality (SOC)behavior in different BMGs is due to the tempera-ture,strain rate,and chemical compositions.The SOC behavior is accompanied with the following phenomena:the interactions occur in the shear bands;the incubation time is longer than the relaxation time;the time interval is lac-king of typical time scale;and the spatial or temporal parameters should display a power-law distribution.     

Relaxation Behavior of Ca-Based Bulk Metallic Glasses

The relaxation behavior of Ca₆₀Mg₂₀Zn₂₀, Ca₆₀Mg₂₀Cu₂₀, Ca₆₅Mg₁₅Zn₂₀, Ca₅₀Mg₂₀Cu₃₀, and Ca₅₅Mg₁₈Zn₁₁Cu₁₆ bulk metallic glasses was determined in the glass transition region using differential scanning calorimetry (DSC) with heating rates from 1 to 160 K/min. The activation enthalpy of structural relaxation and the fragility index m were found to be smaller in the glassy state (onset of the glass transition) than in the supercooled liquid state (end of glass transition). The Ca-based glass-forming liquids showed strong behavior of the relaxation time, with the fragility indexes m in the range of 33 to 40. The strong liquid behavior implies sluggish kinetics of crystallization in the supercooled liquid region and explains the very good glass-forming ability (GFA) of these alloys. The critical cooling rate for amorphization R _c of the Ca-based bulk metallic glasses was estimated to be in the range of 0.3 to 10 K/s, which is similar to R _c values for the best Pd- and Zr-based metallic glass-forming alloys discovered so far.     

Thermodynamics and Magnetostriction of Fe-Co-B-Si-Nb-C Bulk Metallic Glasses

Fe-Co-B-Si-Nb-C glassy alloy with the addition of C was prepared by arc melting and copper suck-casting. The thermodynamics and soft magnetic properties were investigated. The casted amorphous alloys were heat-treated at different temperatures. The differential scanning calorimeter (DSC) results and thermal expansion show complete thermodynamics of crystallization. Its magnetostriction properties were studied by capacitance method. Saturation magnetostriction increases to 38×10-6. The amorphous alloy exhibits good soft magnetic properties with low coercivity and high saturation magnetic induction. The results show that minor addition of C is beneficial to enhance the saturation magnetostriction λs, and do not deteriorate the ability of forming amorphous.     

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.     

Oxidation and Crystallization Behavior of Quinary Zr-Based Bulk Metallic Glasses

Non-isothermal and isothermal oxidation behavior of four Zr-based bulk metallic glasses (Zr₅₈Cu₂₂Co₄Ag₄Al₁₂, Zr₅₈Cu₂₂Co₂Ag₆Al₁₂, Zr₅₈Cu₂₂Fe₄Ag₄Al₁₂, and Zr₅₈Cu₂₂Fe₂Ag₆Al₁₂ (compositions are in at.%)) has been studied in oxygen environment. Non-isothermal oxidation has been performed at different heating rates up to 1,173 K to understand the effect of progressive crystallization on the oxidation behavior. In addition, crystallization behavior of the glassy alloys has been studied, and activation energies have been calculated in an inert and oxygen environment. Partial replacement of iron with silver and cobalt has a distinct effect on the oxidation and crystallization behavior of the alloys. Oxidation of the glassy alloys starts with the dissolution of oxygen in the amorphous matrix followed by rapid oxidation after crystallization.     

Ductile Ti-Based Bulk Metallic Glasses with High Specific Strength

Ti-based bulk metallic glasses (BMGs) with large compressive plasticity were developed in the Ti-rich part of Vitreloy series BMGs (Ti_65–x Zr _x Cu₉Ni₈Be₁₈ alloys with x = 0, 5, 10, 15, and 20). The current materials exhibit high fracture strength reaching ~2.3 GPa and plastic strains up to ~8.3 pct after partial substitution of Zr by Ti. The plasticity of the investigated alloys strongly depends on the Zr content, which affects the elastic constants, such as Poisson’s ratio and shear modulus. This, in turn, has an impact on the shear transformation zone (STZ) volume and, hence, on the shear banding of the glasses.     

Ti-Cu-Zr-Fe-Sn-Si-Ag-Pd Bulk Metallic Glasses with Potential for Biomedical Applications

Ti₄₇Cu_38−xZr_7.5Fe_2.5Sn₂Si₁Ag₂Pd_x (x = 1, 2, 3, and 4 atomic percent, at. pct) bulk metallic glasses (BMGs) with potential for biomedical applications were fabricated by copper-mold casting. The Ti-based BMGs exhibited high glass-forming ability (GFA) with critical diameters of 4 to 5 mm and a supercooled liquid region over 50 K, though the high contents of Pd slightly decreased the GFA. The additions of 2 and 3 at. pct Pd benefited the improvement of plasticity, and the resultant BMGs showed the relatively low Young’s modulus of about 100 GPa, high compressive strengths of 2174 to 2340 MPa, and compressive plastic strain of around 4 pct. The addition of Pd also decreased the passive current density and increased the pitting potential of the Ti-based BMGs in the Hank’s solution, leading to the enhanced bio-corrosion resistance of the BMGs. Furthermore, the cell adhesion, viability, and proliferation behaviors revealed that the present Ti-based BMGs possess as good biocompatibility as that of the Ti-6Al-4V alloy. These results demonstrated the potential of the Ti-Cu-Zr-Fe-Sn-Si-Ag-Pd BMGs as biomedical materials.

     

Crystallization Kinetics of Misch Metal Based Bulk Metallic Glasses

Bulk metallic glasses (BMGs) have been devel-opedrecentlyin many multi-component metal systems .These BMGs exhibit unique properties such as highstrength,high elastic li mit ,high corrosion resistanceand wear resistance , and low coercivity etc . Sincethe…     

Preparation and Mechanical Properties of Misch Metal Based Bulk Metallic Glasses

Bulk metallic glasses(BMGs)have been devel-oped in many alloy systems during past decade.Among these systems,rare-earth(RE)metal-basedBMGs are ones of those found earliest and with betterglass forming ability(GFA).For example,La-basedBMGs were the first a…     

Designing Zr-Cu-Co-Al Bulk Metallic Glasses with Phase Separation Mediated Plasticity

New Zr-based bulk metallic glasses (BMGs) with improved plasticity were developed in the Zr-Cu-Co-Al system by a combination of Zr₄₅Cu₅₀Al₅ and Zr₅₅Co₂₅Al₂₀ BMGs with a certain concentration ratio. The compressive plasticity of the investigated alloys depends strongly on the concentration ratio of the two BMGs. Because of the positive enthalpy of mixing between Cu and Co (??H_Cu-Co?=?+9?kJ/mol), a strong repulsive interaction between Cu and Co is introduced, whereas an attractive interaction exists among the other constituent elements in the liquid state. When two BMGs are combined at a 1:1 concentration ratio, a maximum compressive plasticity of ~12?pct is achieved for the Zr₅₀Cu₂₅Co_12.5Al_12.5 BMG. The plasticity enhancement is attributed to atomic-scale chemical/structural fluctuations achieved through liquid-phase separation.     

Biocompatibility Study of Zirconium-Based Bulk Metallic Glasses for Orthopedic Applications

Bulk metallic glasses (BMGs) represent an emerging class of materials that offer an attractive combination of properties, such as high strength, low modulus, good fatigue limit, and near-net-shape formability. The BMGs have been explored in mechanical, chemical, and magnetic applications. However, little research has been attracted in the biomedical field. In this work, we study the potential of BMGs for the orthopedic repair and replacement. We report the biocompatibility study of zirconium (Zr)–based solid BMGs using mouse osteoblast cells. Cell attachment, proliferation, and differentiation are compared to Ti-6Al-4V, a well-studied alloy biomaterial. Our in-vitro study has demonstrated that cells cultured on the Zr-based BMG substrate showed higher attachment, alkaline phosphatase activity, and bone matrix deposition compared to those grown on the control Ti alloy substrate. Cytotoxicity staining also revealed the remarkable viability of cells growing on the BMG substrates.     

Tensile Mechanical Behaviors of In Situ Metallic Glass Matrix Composites at Ambient Temperature and in Supercooled Liquid Region

In this study, new Ti-based metallic glass matrix composites (MGMCs) are fabricated, which contains ~41 vol pct of large dendrites with a size of ~0.8 to 1.2 μm, The newly developed Ti-based MGMCs exhibit excellent tensile strength of ~1650 MPa and a tensile strain of ~2.5 pct at room temperature. During tensile deformation, the work hardening is scarcely found in this alloy. Thus, the deformation of the in situ MGMC is simply described with two stages: (1) elastic and (2) softening deformation stages. Two simple models are adapted to simulate each stage. In the supercooled liquid region [at 613 K (340 °C)], superplastic homogeneous deformation, which is the feature of monolithic bulk metallic glasses, is not observed. The mechanical properties at 613 K (340 °C) are sensitive to the strain rates, the yield strength drops from 1390 to 960 MPa, when the strain rate decreases from 1 × 10^?2 to 1 × 10^?3/s, while the displacement is almost increased by twofold.     

The Density of Nickel in the Superheated and Supercooled Liquid States

Abstract

A.levitation melting. technique has been used to obtain the density of liquid nickel between 1149 and 1866°C. Goodagreement is found with recent measurements by an Archimedean method for superheated liquid metal, although the levitation results are higher than those obtained by apparently reliable maximum-bubble-pressure measurements.

Extensive supercooling occurs under levitation melting conditions and densities have been measured over a 300°C range below the melting temperature. The density varies linearly with temperature over the whole temperature range studied, showing no discontinuity at the melting temperature. The density is expressed by the relation:

ρ(g/cm²) = 9.966 – 12.000 × 10^?4T(°K)± 0.055 at the 68 per cent confidence level.     

Prediction of Glass-Forming Compositions in Metallic Systems: Copper-Based Bulk Metallic Glasses in the Cu-Mg-Ca System

A novel methodology for predicting specific compositions for glass-forming alloys based on efficiently packed atomic cluster selection, liquidus lines, and ab initio calculations is presented. This model has shown applicable adaptation to many known metallic and ceramic oxide glass-forming systems and has led to the discovery of soon to be reported Ag- and Zn-based bulk metallic glasses (BMGs). As a model system, glass formation in the Cu-Mg-Ca ternary system has been assessed using this alloy design methodology, which has led to the discovery of a number of Cu-based BMGs with compositions ranging from Cu-33 to 55 at. pct, Mg-18 to 45 at. pct, and Ca-18 to 36 at. pct. Included in this work are the calculated values of associated cluster binding energies and correlations between physical and thermal properties of these glassy compositions, which show significant physical evidence to support the likely existence of such clusters.