Effects of Nitrogen on the Glass Formation and Mechanical Properties of a Ti-Based Metallic Glass

Effects of nitrogen addition on glass formation and mechanical properties of the Ti42.5Cu40Zr10Ni5Sn2.5metallic glass were systematically investigated. It was found that a small amount of nitrogen addition facilitated the glass formation by suppressing formation of the competing eutectic structure. Unlike large atomic size elements such as Hf and Pd which usually deteriorate specific strength, nitrogen can also increase the specific strength of the current Ti-based BMGs. The results are not only helpful for understanding glass-forming ability in general, but also useful in developing cost-effective, high-performance Ti-based bulk metallic glasses with enhanced glass-forming ability.     

Cu-based bulk amorphous alloy with larger glass-forming ability and supercooled liquid region

The glassy rod with a maximum sample thickness of 11 mm and larger supercooled liquid region of 108 K was successfully fabricated when substituting Cu with minor amount of Ag in the Cu–Zr–Al–Gd alloy system. The value of γ reaches a maximum of 0.418 for the Cu_45.5Zr₄₅Al₇Gd₂Ag_0.5 bulk metallic glass (BMG) alloy. The high glass-forming ability (GFA) and larger supercooled liquid region are discussed from atomic size, negative mixing heat among constituent elements and thermodynamics.     

Effect of similar elements on improving glass-forming ability of La–Ce-based alloys

To date the effect of unlike component elements on glass-forming ability (GFA) of alloys have been studied extensively, and it is generally recognized that the main consisting elements of the alloys with high GFA usually have large difference in atomic size and atomic interaction (large negative heat of mixing) among them. In our recent work, a series of rare earth metal-based alloy compositions with superior GFA were found through the approach of coexistence of similar constituent elements. The quinary (La_0.5Ce_0.5)₆₅Al₁₀(Co_0.6Cu_0.4)₂₅ bulk metallic glass (BMG) in a rod form with a diameter up to 32 mm was synthesized by tilt-pour casting, for which the glass-forming ability is significantly higher than that for ternary Ln–Al–TM alloys (Ln = La or Ce; TM = Co or Cu) with critical diameters for glass-formation of several millimeters. We suggest that the strong frustration of crystallization by utilizing the coexistence of La–Ce and Co–Cu to complicate competing crystalline phases is helpful to construct BMG component with superior GFA. The results of our present work indicate that similar elements (elements with similar atomic size and chemical properties) have significant effect on GFA of alloys.     

Effect of addition of Be on glass-forming ability,plasticity and structural change in Cu–Zr bulk metallic glasses

The present study reports the effect of the addition of Be in Cu–Zr bulk metallic glass (BMG) on glass-forming ability (GFA), plasticity and structural change. Although Be has a negative enthalpy of mixing with all the constituent elements of these glasses, Cu_47.5Zr₄₀Be_12.5 alloy exhibits apparent double glass transitions (T_g) and enhanced plasticity as well as improved GFA. Intensive structural analysis using extended X-ray absorption fine structure suggests that a large difference in the enthalpy of mixing between atom pairs in multi-component BMGs can cause atomic scale structural inhomogeneity and/or locally favored structures in the amorphous matrix, resulting in enhanced compressive strains, although the enthalpies of mixing for atom pairs are all negative. This concept may shed light on the development of BMGs with large plasticity as well as high GFA.     

Thermodynamics and structural relaxation in Ce-based bulk metallic glass-forming liquids

Thermodynamics, kinetics and structural relaxation of Ce-based bulk metallic glass-forming liquid were investigated in the glass transition region by calorimetric measurements. The differences in thermodynamic functions were calculated between the supercooled liquid and crystalline state of the Ce-based alloys. Structural relaxation was studied by heating rate dependence of glass transition temperature. In terms of fragility parameter m, the Ce-based alloys were stronger liquid than other metallic glass-forming liquids. The correlation of the excellent glass-forming ability of Ce-based alloys with the thermodynamic property (Gibbs free energy) and the kinetic property (m) was discussed. The structural relaxation from glass state to the equilibrium supercooled state was well described by Tool-Narayanaswamy-Moynihan (TNM) model using the parameters derived from the calorimetric measurements.     

Mechanical Relaxation of a Ti_(36.2)Zr_(30.3)Cu_(8.3)Fe_4Be_(21.2) Bulk Metallic Glass: Experiments and Theoretical Analysis

The dynamic mechanical relaxation behavior of Ti_(36.2)Zr_(30.3)Cu_(8.3)Fe_4Be_(21.2) bulk metallic glass with good glass-forming ability was investigated by mechanical spectroscopy. The mechanical relaxation behavior was analyzed in the framework of quasi-point defects model. The experimental results demonstrate that the atomic mobility of the metallic glass is closely associated with the correlation factor χ. The physical aging below the glass transition temperature T g shows a non-Debye relaxation behavior, which could be well described by stretched Kohlrausch exponential equation. The Kohlrausch exponent β_(aging) reflects the dynamic heterogeneities of the metallic glass. Both concentration of "defects" and atomic mobility decrease caused by the in situ successive heating during the mechanical spectroscopy experiments.     

Atomic packing in multicomponent aluminum-based metallic glasses

Three-dimensional atomic configurations have been established for Al-based multicomponent metallic glasses (MGs). This was achieved via computer simulations employing effective pair-potentials, which were derived from ab initio molecular dynamics simulation data using the inverse Monte Carlo (IMC) method. The ab initio and IMC structural models were validated using structure factors and extended X-ray absorption fine structures obtained from synchrotron X-ray experiments. The Al-based MGs are characterized by solute-centered quasi-equivalent clusters. These coordination polyhedra of different types and sizes intermix in the multicomponent alloy, resulting in improved glass-forming ability for the Al–La–Ni alloys when compared with binary Al–La and Al–Ni systems. Our survey of a large number of Al–solute systems using ab initio calculations demonstrates that the topological short-range order (cluster type, size and coordination number of the solute) correlates directly with the Al–solute bond length (or the effective atomic size ratio). The differences between our findings and previously proposed structural models are also discussed.     

Computational analysis of the atomic size effect in bulk metallic glasses and their liquid precursors

The atomic size effect and its consequences for the ability of multicomponent liquid alloys to form bulk metallic glasses are analyzed in terms of the generalized Bernal’s model for liquids, following the hypothesis that maximum density in the liquid state improves the glass-forming ability. The maximum density that can be achieved in the liquid state is studied in the 2(N-1) dimensional parameter space of N-component systems. Computer simulations reveal that the size ratio of largest to smallest atoms are most relevant for achieving the maximum packing for N = 3–5, whereas the number of components plays a minor role. At small size ratio, the maximum packing density can be achieved by different atomic size distributions, whereas for medium size ratios the maximum density is always correlated to a concave size distribution. The relationship of the results to Miracle´s efficient cluster packing model is also discussed.     

Mechanical heterogeneity and its relation with glass-forming ability in Zr-Cu and Zr-Cu-Al metallic glasses

Molecular dynamics simulations of nanoindentation using a spherical indenter were adopted to quantitatively probe the local mechanical heterogeneity (MH) in Zr-Cu and Zr-Cu-Al metallic glasses. Distinct MH at the nanometer scale has been revealed by statistically analyzing the first pop-in stress of different regions in the metallic glasses with an indenter of 4 nm in diameter. More interestingly, it is found that the degree of MH has a close relation to glass-forming ability of alloys, i.e., the smaller MH, the better glass-forming ability. Our findings not only shed light on the intrinsic feature of atomic structures, but also have important implications in understanding the structure-property relationship of MGs.     

Structural investigation upon the high glass-forming ability in Fe-doped ZrCuAl multicomponent alloys

Understanding the underlying mechanism of microalloying affecting the glass formation in multicomponent alloys is scientifically required, while addressing this issue remains a challenge. In this work, the structural mechanisms of extraordinarily high glass-forming ability (GFA) caused by Fe doping in ZrCuAl bulk metallic glasses was investigated via synchrotron radiation techniques combined with simulations. It is revealed that when minor Fe (5 at.%) are doped, the local structures (clusters) have relatively high values of some structural parameters, in terms of fivefold symmetry, geometrical regularity, atomic packing, and chemical interaction between heterogeneous atoms. The combination of these structural factors leads to stabilized amorphous structure and enhanced GFA. This work may extend our understanding on the sensitive dependence of GFA on the concentration of doping atoms in multicomponent metallic glasses.     

Efficient atomic packing-chemistry coupled model and glass formation in ternary Al-based metallic glasses

An efficient atomic packing-chemistry coupled model has been described for ternary Al-based metallic glasses based upon efficient atomic packing and electrochemical potential equalization principle. For Al-TM (transition metal)-RE (rare earth) system, RE-centered clusters are arranged in a mode of spherical periodic order, and TM atoms locate at the interstitial sites between RE-centered clusters to form an efficient atomic packing. The equalization of electrochemical potential between the RE-centered clusters and the TM-centered clusters accounts for the stability of amorphous structure. Further, complementary inverse structure is utilized to select efficient clusters and solute elements for the best glass-forming ability. The validity of this model was testified in the Al–Ni-RE (Y, Ce, La, Gd) systems. A new Al–Ni–Ho glass with a wedge thickness of about 718 μm has been discovered at the predicted composition.     

Recent progress in criterions for glass forming ability

The glass-forming ability(GFA) is an important factor in studying metallic glasses. So far, there are several criteria for evaluating the glass-forming ability. For predicting compositions for bulk metallic glasses, however, they show more or less accuracy and versatility for different cases. In this work, four types of criteria for the glass-forming ability are categorized and reviewed: 1) Indicators with characteristic temperatures; 2) Indicators involving structural factors; 3) Indicators based on Miedema's model; and 4) Indictors based on phase diagram. It is pointed out that a single indicator cannot be used to predict GFA of all the metallic glass systems correctly due to its limited theoretical framework, and the combination of multiple indicators shows more efficiency and accuracy. Though it is still very difficult to develop a universal indicator for GFA, recent indicators seem to be of more reliable physical meaning than those previously suggested.     

In Situ Scattering Studies of Crystallization Kinetics in a Phase-Separated Zr-Cu-Fe-Al Bulk Metallic Glass

Thermal stability and the crystallization kinetics of a phase-separated Zr-Cu-Fe-Al bulk metallic glass were investigated using in situ high-energy synchrotron X-ray and neutron diffraction, as well as small-angle synchrotron X-ray scattering. It was revealed that this glass with excellent glass-forming ability possesses a two-step crystallization behavior. The crystalline products and their evolution sequence are more complicated than a homogeneous Zr-Cu-Al glass with average glass-forming ability. The experimental results indicate that a finely distributed nanometer-sized cubic Zr₂Cu phase forms first and then transforms to a tetragonal Zr₂Cu phase, while the matrix transforms to an orthorhombic Zr₃Fe phase. The strength of the Zr-Cu-Fe-Al composite containing cubic Zr₂Cu phase and glass matrix increases, and the plasticity also improves compared to the as-cast Zr-Cu-Fe-Al bulk metallic glass. Our results suggest that the formation of multiple and complex crystalline products would be the characteristics of the Zr-Cu-Fe-Al glass with better glass-forming ability. Our study may shed light on the synthesis of bulk-sized glass-nanocrystals composites of high strength and good plasticity.     

一种具有优异玻璃形成能力的Zr50Ti5Cu27Ni10Al8大块非晶合金的开发

多组元的Zr基非晶合金成分的复杂性对开发具有优异玻璃形成能力的Zr基非晶合金提出巨大的挑战。另外,大部分Zr基非晶合金含有有毒元素Be或者贵金属。因此,采用一种简单有效的方法开发无毒无贵金属元素的多组元Zr基非晶合金十分必要。本文中采用二元共晶比例法和部分元素替代法快速的开发出了一种新的临界尺寸大于10mm的 Zr50Ti5Cu27Ni10Al8非晶合金。这个非晶合金的热稳定性和硬度也通过原位高温X射线衍射和纳米压痕方法测量得出。     

Influence of the molten quenching temperature on the thermal physical behavior of quenched Zr-based metallic glasses

Thermo-physical behavior of some Zr-based metallic glasses prepared by different molten quenching temperatures was studied by Differential scanning calorimetry (DSC) measurements. The characteristic thermo-physical properties are normally used for evaluating the glass-forming ability (GFA) of metallic glasses. Our results show that the glass transition temperature, crystallization temperature and supercooled liquid region of these metallic glasses increased with increasing the molten quenching temperature. Their glass-forming abilities were discussed in terms of the GFA criterion γ and the reduced glass transition temperature, T_rg, using these thermo-physical properties.     

Improvement of corrosion resistance in NaOH solution and glass forming ability of as-cast Mg-based bulk metallic glasses by microalloying

The influences of the addition of Ag on the glass forming ability (GFA) and corrosion behavior were investigated in the Mg-Ni-based alloy system by X-ray diffraction (XRD) and electrochemical polarization in 0.1 mol/L NaOH solution.Results shows that the GFA of the Mg-Ni-based BMGs can be improved dramatically by the addition of an appropriate amount of Ag;and the addition element Ag can improve the corrosion resistance of Mg-Ni-based bulk metallic glass.The large difference in atomic size and large negative mixing enthalpy in alloy system can contribute to the high GFA.The addition element Ag improves the forming speed and the stability of the passive film,which is helpful to decrease the passivation current density and to improve the corrosion resistance of Mg-Ni-based bulk metallic glass.     

High-Entropy Metallic Glasses

The high-entropy alloys are defined as solid-solution alloys containing five or more than five principal elements in equal or near-equal atomic percent. The concept of high mixing entropy introduces a new way for developing advanced metallic materials with unique physical and mechanical properties that cannot be achieved by the conventional microalloying approach based on only a single base element. The metallic glass (MG) is the metallic alloy rapidly quenched from the liquid state, and at room temperature it still shows an amorphous liquid-like structure. Bulk MGs represent a particular class of amorphous alloys usually with three or more than three components but based on a single principal element such as Zr, Cu, Ce, and Fe. These materials are very attractive for applications because of their excellent mechanical properties such as ultrahigh (near theoretical) strength, wear resistance, and hardness, and physical properties such as soft magnetic properties. In this article, we review the formation and properties of a series of high-mixing-entropy bulk MGs based on multiple major elements. It is found that the strategy and route for development of the high-entropy alloys can be applied to the development of the MGs with excellent glass-forming ability. The high-mixing-entropy bulk MGs are then loosely defined as metallic glassy alloys containing five or more than five elements in equal or near-equal atomic percent, which have relatively high mixing entropy compared with the conventional MGs based on a single principal element. The formation mechanism, especially the role of the mixing entropy in the formation of the high-entropy MGs, is discussed. The unique physical, mechanical, chemical, and biomedical properties of the high-entropy MGs in comparison with the conventional metallic alloys are introduced. We show that the high-mixing-entropy MGs, along the formation idea and strategy of the high-entropy alloys and based on multiple major elements, might provide a novel approach in search for new MG-forming systems with significances in scientific studies and potential applications.     

Effects of alloying elements on glass formation,mechanical and soft-magnetic properties of Fe-based metallic glasses

Effects of alloying additions on glass formation, mechanical and soft-magnetic properties of Fe-(Si,P,C,B)-based bulk metallic glasses (BMGs) were systemically studied in detail. It was found that the glass-forming ability (GFA) and the optimum doping content strongly depend on the electronegativity of the alloying elements, which are discussed in terms of liquid phase stability and crystallization resistance of the competing crystalline phases. These BMGs exhibit high fracture strength ranging from 2800 to 3800 MPa, which closely relates to the atomic size distribution in the alloys. Furthermore, appropriate additions of Co, Ga and Cu could improve not only the GFA but also the saturation magnetization due to different coupling mechanisms.     

The general effect of atomic size misfit on glass formation in conventional and high-entropy alloys

It is a longstanding notion that atomic size misfit plays an important role with regard to glass formation in multi-component alloys. In the previous studies, this atomic size effect was commonly modeled as an “inclusion-in-matrix” problem and glass formation was usually linked to a threshold volume strain in “matrix” or solvent atoms. However, it becomes difficult to directly apply this approach to high entropy alloys, which are in lack of a clear distinction between solvent and solute atoms. With the simple geometric model we recently developed, here we show that glass formation in over two hundred glass-forming alloys, including conventional and high-entropy alloys, can be correlated with the excessive fluctuation in the intrinsic residual strains that result from the atomic size misfit. This interesting behavior suggests that, in most glass-forming multicomponent alloys hitherto reported, the atomic size effect acts with the chemistry effect to promote glass formation. Furthermore, our findings also imply that glass formation in multi-component alloys, regardless of their compositional complexity, may be rationalized with the Lindamann's criterion that was long established for the instability of crystalline lattices.