Metallic Glass System Research Articles

Secondary Relaxation in Metallic Glass Formers: Its Correlation with the Genuine Johari−Goldstein Relaxation

We report experimental evidence for the existence of Johari−Goldstein (JG) relaxation in metallic glass formers. By using the hyperquenching−annealing−calorimetric approach, we study the dynamics of the secondary (β) relaxation in La55Al25Ni20 metallic glasses. The observed β relaxation exhibits a typical feature of the genuine JG relaxation, i.e., the variation of its activation energy (Eβ) with the glass transition temperature (Tg) obeys the relation Eβ = 26.8RTg. The correlative degree between the β and the primary α relaxations is closely associated with the liquid fragility. By a survey of the sub-Tg relaxation data of other metallic glass systems, we have determined the Eβ values that find the correlation Eβ = 26.1RTg, indicating that JG relaxations are intrinsic in metallic glass formers. By analyzing the primitive relaxation time of the JG motion, τ0,JG, and the crossover time in the Coupling model, tc, we discuss why the excess wing rather than the JG peak (or shoulder) is present in metallic glass systems.

Bulk metallic glass formation near intermetallic composition through liquid quenching

Metallic glass is typically formed either by liquid quenching near deep-eutectics or by solid-state reaction near the intermetallic in center of the phase diagram. In our research, two optimum bulk glass formers are synthesized in the vicinity of the intermetallic compound of Cu–Zr system through liquid quenching. It is shown that such a pair of unique glass formation is only feasible in a narrow composition range close to but clearly separated by the intermetallic composition where both the kinetic and thermodynamic conditions for the amorphous formation are matched. The discovery of this “intermetallic glass” pair through quenching allows us to find a potentially applicable host of metallic glass systems. This is followed by a discussion on its impact on understanding the formation and physics of bulk metallic glasses.

Atomistic modeling of the Cu–Zr–Ag bulk metallic glass system

In order to investigate the phase separation behavior in Cu–Zr–Ag bulk metallic glasses (BMGs) on an atomic level, a modified embedded-atom interatomic method potential for the Cu–Zr–Ag system has been newly developed. A clear tendency of phase separation of Ag-rich phases could be observed in the supercooled liquid, in reasonable agreement with experimental information. The potential can be used for atomistic investigations of the effects of alloying element Ag on a wide range of amorphous properties of Cu–Zr BMG.

Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses

The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of approximately 2 mum, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these "designed" composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems.

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.

Inherent Shear-Dilatation Coexistence in Metallic Glass

Shear deformation can induce normal stress or hydrostatic stress in metallic glasses [Nature Mater. 2 (2003) 449, Intermetallics 14 (2006) 1033]. We perform the bulk deformation of three-dimensional Cu46Zr54 metallic glass (MG) and Cu single crystal model systems using molecular dynamics simulation. The results indicate that hydrostatic stress can incur shear stress in MG, but not in crystal. The resultant pronounced asymmetry between tension and compression originates from this inherent shear-dilatation coexistence in MG.

Formation of Ni–Nb–Zr–X (X = Ti, Ta, Fe, Cu, Co) bulk metallic glasses

The effect of the fourth element on glass-forming ability (GFA) in Ni–Nb–Zr–X (X = Ti, Ta, Fe, Cu, Co, V, Y, Mo, Sn, Al, Si) bulk metallic glass (BMG) system has been studied. Among these elements, Co addition can increase GFA of the Ni–Nb–Zr BMG system. The best glass former is Ni 57Nb 33Zr 5Co 5 alloy with a critical diameter of 3.5 mm. The corrosion rate of this alloy is approximately one order of magnitude less than that of the stainless steel in aggressive 6 M HCl open to air at room temperature. This alloy exhibits high T g of 866 K, supercooled liquid region Δ T x of 45 K, compressive fracture strength of 2.9 GPa, 1% compressive plasticity, and excellent corrosion resistance as compared to stainless steel. The combination of high thermal stability, high strength with compressive plasticity and excellent corrosion resistance enables these alloys to have potential industrial applications. Furthermore, the addition of Sn to Ni–Nb–Zr–Co alloy is effective for enhancing the stability of supercooled liquid. The maximum supercooled liquid region of 85 K is obtained in Ni 57Co 5(Nb 33/38Zr 5/38) 33Sn 5 BMG.

Electronic structure of Fe-based amorphous alloys studied using electron-energy-loss spectroscopy

The local atomic electronic structures of Fe-Mo-C-B metallic glasses are investigated using electron energy-loss spectroscopy (EELS). The fracture behavior of this Fe-based amorphous alloy system undergoes the transition from being ductile to exhibiting brittleness when alloyed with Cr or Er atoms. In addition, the glass-forming ability is also enhanced. This plastic-to-brittle transition is suggested to correlate with the change of local atomic short-range order or bonding configurations. Therefore, the bonding configuration of Fe-Mo-C-B-Er(Cr) amorphous alloys is investigated by studying the electronic structure of Fe and C atoms using electron energy-loss spectroscopy. It is shown that the normalized EELS white line intensities of $\mathrm{Fe}\text{\ensuremath{-}}{L}_{2,3}$ edges decrease slightly with an increasing amount of Er additions, while no noticeable difference is obtained with Cr additions. As for the C $K$ edge, a prominent change of edge shape is observed for both alloy systems, where the first peak corresponding to a $1s\ensuremath{\rightarrow}1{\ensuremath{\pi}}^{*}$ transition increases with increasing Er and Cr additions. Accordingly, it is concluded that changes in the local atomic and electronic structure occur around Fe and C atoms when Er and Cr are introduced into the alloys. Furthermore, it is pointed out that the formation of Er-C and Cr-C carbide like local order inferred from the observed C $K$ edge spectra can provide a plausible explanation for the plastic-to-brittle transition observed in these Fe-based amorphous alloys. In spite of the complexity of electronic and atomic structure in this multicomponent Fe-based metallic glass system, this study could serve as a starting point for providing a qualitative interpretation between electronic structure and plasticity in the Fe-Mo-C-B amorphous alloy system. Complimentary techniques, such as x-ray diffraction and high-resolution transmission electron microscope are also employed, providing a more complete structural characterization.

Melting enthalpy ΔHm for describing glass forming ability of bulk metallic glasses

A new melting enthalpy ΔHm criterion for the prediction of glass forming ability (GFA) of alloys is proposed and five Zr–Al–Ni–Cu bulk metallic glasses (BMG) with critical dimension Zmax up to ∅ 7.5mm are also developed by us in the light of the optimum ΔHm of Zr–Al–Ni–Cu alloy system. And then, we researched the relationships between ΔHm and two GFA parameters (critical cooling rate Rc and Zmax) of five bulk metallic glass (BMG) systems, such as Mg–Ni–Nd, Pd–Cu–Si, La–Al–Ni–Cu, Zr–Al–Ni–Cu and Zr–Ti–Ni–Cu–Be, respectively. The results show that the relationships between ΔHm and Rc are all concave upward parabolas, and the optimum ΔHms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.3960kJmol−1, 21.2202kJmol−1, 19.7146kJmol−1, 18.1455kJmol−1 and 13.1558kJmol−1, respectively. On the contrary, the relationships between ΔHm and Zmax are all concave downward parabolas, and the optimum ΔHms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.5530kJmol−1, 21.0830kJmol−1, 19.6603kJmol−1, 19.7231kJmol−1 and 13.1173kJmol−1, respectively. Furthermore, other BMGs’ Rcs or Zmaxs predicted by above-mentioned relationships satisfactorily agree with the tested results, which indicates that these relationships are reliable. However, the predicted results are reliable only if the main components are similar with the fitted BMGs or the additive is sparkle enough that the alloy’s character does not change. On the whole, the ΔHm can act as a criterion for quickly predicting the alloy’s GFA and be helpful for the development of new BMGs.

Theoretical and experimental studies of devitrification pathways in the Zr 2Cu 1−xPd x metallic glass system

Using a model amorphous alloy series, Zr 2Cu 1− x Pd x ( x = 0, 0.25, 0.5, 0.75 and 1), we demonstrate that ab initio calculations can predict likely metastable phase formation during devitrification by comparing these with time-resolved X-ray scattering studies. All compositions share the same equilibrium C11 b phase, yet they follow different devitrification pathways. Only x = 0.5 leads to a metastable C16 phase formation. This corresponds precisely to calculations showing the C16 phase is closest in energy to the stable C11 b phase. The competition is shown to be dominated by electronic structure rather than size effects, with the favored composition for the C16 phase forming a pseudo-gap at the Fermi energy. All Pd-containing compounds devitrify first into a quasicrystalline phase. Zr 2Cu 1− x Pd x compounds based on the NiTi 2 O h 5 structure are higher in energy relative to the C16 and C11 b structures for all compositions, and the calculations show no increase in stability with Pd concentration.

Glass transition temperature of bulk metallic glasses: A linear connection with the mixing enthalpy

A linear relationship is found between the glass transition temperature Tg and the absolute value of the mixing enthalpy, |ΔHchem|, for bulk metallic glass systems. The increasing (or lowering) of Tg with an admixture of metals or other elements manifests itself in the larger (or smaller) of |ΔHchem| in a given system. The results indicate that the composition dependence of Tg results from the change of excess entropy (Sex) during thermal excitation. The |ΔHchem|, which relates to the strength of interaction among different atoms, corresponds to part of the Sex at Tg [Sex(Tg)]. The glass transition temperatures for Cu-Zr (Hf)-, Zr-Cu-, and La-Al-based glassy alloys are correlated with the interaction intensity between their based binary eutectic compositions, respectively.

Relationship between melting enthalpy Δ Hm and critical cooling rate Rc for bulk metallic glasses

In present work, we researched the relationships between Δ H m and critical cooling rate R c of five bulk metallic glass (BMG) systems, such as Mg–Ni–Nd, Pd–Cu–Si, La–Al–Ni–Cu, Zr–Al–Ni–Cu and Zr–Ti–Ni–Cu–Be, respectively, and five Zr–Al–Ni–Cu bulk metallic glasses with critical dimension Z max up to Ø7.5 mm are also developed by us in the light of the optimum Δ H m of Zr–Al–Ni–Cu alloy system. The results show that the relationships between Δ H m and R c are all concave upward parabolas, and the optimum Δ H ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.3960, 21.2202, 19.7146, 18.1455 and 13.1558 KJ mol −1, respectively. Furthermore, other BMG’ R cs predicted by above-mentioned relationships satisfactorily agree with the tested results, which indicates that these relationships are reliable. However, the predicted results are reliable only if the main components are similar with the fitted BMG system or the additive is sparkle enough that the alloy's characteristic is not changed. On the whole, the Δ H m can be used for quickly predicting the alloy's GFA and helpful for the development of new BMGs.

New criteria of glass forming ability, thermal stability and characteristic temperatures for various bulk metallic glass systems

Six mathematical criteria used for quantitatively measuring their glass forming ability, thermal stability and characteristic temperatures (temperature of glass transition T g, crystallization onset T x and liquidus T l) of various bulk metallic glasses (BMGs) have been proposed in this article. It is found that the criteria are suitable for evaluating these quantities for BMGs. The estimated results are better than those calculated by Fang et al.'s models and Lu et al.'s γ criterion, but our analyses indicate that these criteria, defined by the data (Pauling electronegativity x i , atomic radius r i , electron concentration n i and melting temperature T m i ) easily found in literatures, still need to be further refined in future research for shortcomings of the parameters.

A Simple Model for Examining Composition Effects in Eutectic Nucleation

We present a simple thermodynamic calculation for a strongly partitioning eutectic system, to examine how the critical nucleus energy changes, depending upon assumptions of the chemical diffusion. The calculations show that for strongly partitioning systems, the maximum undercooling may occur at a composition significantly different than the eutectic composition, particularly if the rate of diffusion is slow in the undercooled state. These simple calculations emphasize the role that partitioning and composition may play in determining optimal compositions in metallic glass systems, which typically occur near (but not at) deep eutectic compositions.

高強度長寿命ピーニング用金属ガラスショットの開発

Application of bulk metallic glasses was examined as the shot for the peening, which, as a reforming technology of the material, much attention is paid to in these days. As the strength of target products of peening is becoming higher, the shot is also demanded to be made of high hardness. The conventional shot has low toughness and is easy to be crushed. It has led to problems of increase in cost and waste. Therefore, we have tried to solve these problems by applying the bulk metallic glasses as shot. We have paid attention to Fe-base metallic glass system, and have searched for and found the best composition with high hardness and high toughness. Spheroidal particles made with the Fe-base metallic glass were used as shot for peening, and it was proved that this is of high hardness together with a long-life. Thus, we could develop new shot material for peening. We report details of development, and the characteristic when Fe-base metallic glass is applied to the shot for peening.

Study of serrated flow and plastic deformation in metallic glasses through instrumented indentation

The plastic deformation behavior and serrated flow in seven bulk metallic glass (BMG) systems were investigated through instrumented indentation. These materials include Ce 65Al 10Ni 10Cu 10Nb 5, Mg 65Cu 25Gd 10, Pd 43Ni 10Cu 27P 20, Cu 60Zr 20Hf 10Ti 10, Pt 57.5Cu 14.7Ni 5.3P 22.5, Ni 60Nb 37Sn 3 and Fe 43Cr 16Mo 16C 15B 10 BMGs, which show a glass transition temperature ( T g) ranging from 360 to 908 K at a heating rate of 0.33 K/s. Remarkable difference in deformation behavior was found among these BMGs in the load–depth curves during nanoindentation. Prominent serrations are observed in Mg-, Pt- and Pd-based BMGs with medium T g during the loading process, whereas no distinct serrated flow was found in Ce-, Ni- and Fe-based BMGs with quite low or high T g. The subsurface plastic deformation regions after indentation were investigated using depth-sensing microindentation to characterize the shear band feature developed in various BMG systems. The size of the shear band upset is found to be larger in the alloys with lower T g. The effect of T g on the operation of shear bands and the serrated flow behavior in various BMG systems were discussed.

Effect of strain rate on compressive behavior of Ti 45Zr 16Ni 9Cu 10Be 20 bulk metallic glass

Compressive behavior of Ti 45Zr 16Ni 9Cu 10Be 20 (numbers indicate at.%) bulk metallic glasses was investigated at strain rates ranging from 10 −4 to 10 −1 s −1. Little effect of strain rate on the mechanical properties is observed in an amorphous specimen when the strain rates are below 10 −3 s −1. An appreciable total strain of about 5% is observed and the plastic flow is serrated. However, a positive strain rate dependence of yield strength is found when the strain rate reaches 10 −1 s −1, which is reported for the first time in metallic glasses under a compressive loading condition. Further, the macroscopic plastic strain distinctly decreases and flow serration vanishes at a high strain rate. A comparison of the significantly different mechanical behavior of Ti 45Zr 16Ni 9Cu 10Be 20 bulk metallic glass with other bulk metallic glass systems is also presented.

The characterization of plastic flow in three different bulk metallic glass systems

Plastic deformation behaviors of Zr 52.5Al 10Ni 10Cu 15Be 12.5, Mg 65Cu 25Gd 10 and Pd 43Ni 10Cu 27P 20 bulk metallic glasses (BMGs) are studied by using the depth-sensing nanoindentation, macroindentation and uniaxial compression. The significant difference in plastic deformation behavior cannot be correlated to the Poisson's ratio or the ratio of shear modulus to bulk modulus of the three BMGs, but can be explained by the free volume model. It is shown that the nucleation of local shear band is easy and multiple shear bands can be activated in the Zr 52.5Al 10Ni 10Cu 15Be 12.5 alloy, which exhibits a distinct plastic strain during uniaxial compression and less serrated flow during nanoindentation.

A new criterion for glass forming ability of bulk metallic glasses

The relationship between Rc and the ratio d/e of Δd to Δe is discussed based on five bulk metallic glass (BMG) systems, which show all concave upward parabola relations. The second derivative value of the fitted curve reflects glass forming ability (GFA) and the sensitivity to composition of GFA. The greater the second derivative value, the higher is the GFA and the stronger is the sensitivity to composition. Furthermore, the parabola relations are demonstrated comparing the predicted Rc with the practical Rc in a Mg–Ni–Nd system, and indirectly reasoned by using a Pd–Cu–Si system and a Zr–Ti–Ni–Cu–Be system. Based on the above-mentioned results, the Rcs of other Zr–Al–Ni–Cu alloys are predicted by using the fitted formula. The Rcs are among 4–20 K s−1, which is coherent to the practical preparation condition and indirectly shows reliability of the related parabola relations. The fitted formula can be applied to the cases of the invariability of the primary components of the alloy system. In addition, Trg and ΔTx parameters are not suitable for the criteria for GFA of BMGs.

Thermal stability and its prediction of bulk metallic glass systems

We report a linear relationship between the thermal stability (ΔTx) and the amorphous formation enthalpy (ΔHam) of typical bulk metallic glasses (BMGs). The value of the slope of ΔTx vs ΔHam reflects the sensitivity of thermal stability to composition and the sign of the slope implies whether the local atomic structure of amorphous phase is similar to that of primary crystalline phase. A positive slope suggests a similarity between them and vice versa. We also demonstrate that the thermal stability can be fastly assessed and predicted by ΔHam at less cost using Ti53Cu15Ni18.5Al7M3Si3B0.5 (M=Hf,Sc,Ta,Nb) BMGs.