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.     

Pd71.5Cu12Si16.5金属玻璃的氢相关性能

钯基金属玻璃在氢相关工业中具有潜在的应用价值。在本工作中,我们通过电弧熔炼、铜辊甩带的方法制备了Pd_71.5Cu₁₂Si_16.5 金属玻璃的宽带样品。通过常规X射线衍射仪和短波长X射线应力分析仪的X射线衍射谱确定了样品的完全非晶态结构。在室温、100kPa压力条件下,对样品进行了多次的吸、放氢循环实验。经过10次以上的循环后,样品没有发生破坏,表现出良好的抗氢脆性能。通过气体直接渗透的方法进一步测试了Pd_71.5Cu₁₂Si_16.5 金属玻璃及其同成分晶态合金的氢渗透性能。在金属玻璃的过冷液相区温度范围内,其氢渗透率明显高于晶态相,这一结果由金属玻璃在该区间内的等温保持引入了更多的自由体积进行解释。     

Glass formation,thermal and mechanical properties of ZrCuAlNi bulk metallic glasses

The glass forming ability, thermal and mechanical properties of some ZrCuAlNi bulk metallic glasses were analyzed. The compositions of the alloys were theoretically determined with the dense packing and kinetic fragility index models. Cylindrical and conical ingots were produced by copper mould suction-casting under Ar atmosphere. The conical ingots were characterized by means of X-ray diffraction in order to determine the glassy structure. It was found that both alloys have a critical glassy diameter, D_c, of 3 mm. Thermal behaviours were investigated by differential scanning calorimetry at heating rates of 0.5, 0.67 and 0.83 K/s. The gamma parameter γ, supercooled liquid region ΔT_x, and reduced glass transition temperature T_rg, of the experimentally obtained glasses indicated high glass forming ability. The glassy compositions showed a fragility index of ～40 GPa. The compression test of the investigated alloys was carried out at a strain rate of 0.016 s^?1, obtaining a elastic modulus of ～83 GPa, total deformation of ～5%, yield strength of 1.6 GPa and hardness of 4 GPa. It was concluded that the use of the dense packing and kinetic fragility index models helped to predict glass-forming compositions in the family alloy investigated.     

Structural and magnetic nanoclusters in Cu50Zr50−xGdx (x = 5 at.%) metallic glasses

It is shown that phase-separated metallic glasses on the nanoscale can be prepared by rapid quenching of Cu₅₀Zr_50−xGd_x melts with a low concentration of gadolinium (x = 5 at.%). Gd-enriched clusters of 2 nm size are formed as early stages of decomposition in the deeply undercooled melt. The key physical parameter to obtaining such a nanoclustered microstructure upon quenching is the critical temperature of liquid-liquid phase separation which has to be close to the glass transition temperature. Thus, the thermodynamic properties of the liquid phase even in the metastable deeply undercooled melt essentially determine the structure formation. Analysis of the spatial atomic arrangement by atom probe tomography after annealing in the supercooled liquid state provides direct evidence of the spinodal character of the decomposition by uphill diffusion. The Gd-enriched nanoclusters exhibit ferromagnetic ordering below 50 K and the cluster size regime derived from magnetization measurements is in good agreement with that obtained from atom probe tomography investigations. The first stage of crystallization of Cu₅₀Zr₄₅Gd₅ glass is observed to be Ostwald-type ripening on a nanoscale. The phase-separated glass acts as a precursor for the formation of a metastable nanocrystalline structure.     

Structural behaviour of Pd40Cu30Ni10P20 metallic glass under high pressure

The structural behaviour of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass as a function of hydrostatic pressure up to 47.4 GPa was investigated by means of in situ high energy synchrotron X-ray diffraction patterns. Monotonic changes are observed in the diffraction data without any indication of a phase transition. In real space all maxima of the atomic pair correlation function including the nearest neighbour distance are decreasing and scale with pressure. The volume as function of hydrostatic pressure is extracted from the diffraction data. For the largest hydrostatic pressure of 47.4 GPa the volume is reduced by 18%. The bulk modulus B₀ = 178 GPa was calculated from the diffraction data. The dependence of volume on pressure of Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass can be well described by the Birch–Murnaghan equation of state.     

过冷液态Zr48Cu36Ag8Al8块体非晶合金的相分离及组织演变

本文通过x射线衍射(XRD)、差示扫描量热法(DSC)和透射电子显微镜(TEM)研究了退火温度对Zr₄₈Cu₃₆Ag₈Al₈金属玻璃微观结构演化的影响。结果表明,快速凝固获得的样品为典型的非晶态结构。当样品在703K保温20分钟时,均一的非晶基体相分离成两种非晶合金,即,发生相分离。由于相分离结构与非晶基体在等温退火过程是竞争的关系,这个结构很容易向晶化态进行转变,形成AlZr₂ AlAg₃相。Zr₄₈Cu₃₆Ag₈Al₈金属玻璃的微观结构在过冷液相区等温退火过程中经历了的局部结构转变,相分离以及纳米晶转变,这个过程意味着Zr₄₈Cu₃₆Ag₈Al₈金属玻璃的微观结构对退火温度十分敏感。此外,相分离的形成可以加速纳米晶的形成。     

Crossover and normal structural relaxation in naturally aged glassy Pd40Cu30Ni10P20

Measurements of Young’s modulus relaxation at elevated temperatures in glassy Pd₄₀Cu₃₀Ni₁₀P₂₀ stored at room temperature for nine years are performed. It is found that testing in a wide range above room temperature leads to the crossover relaxation, which is absent in the as-prepared glass. At high testing temperatures, the crossover behavior changes into normal log-time relaxation kinetics. The crossover relaxation is accompanied by low temperature endothermal heat reaction, which is followed by exothermal heat flow characteristic of normal structural relaxation below the glass transition. Structural rejuvenation of samples by rapid quenching from the supercooled liquid state drastically changes the relaxation kinetics removing the crossover almost completely.It is revealed that the formation enthalpy of structural defects responsible for the crossover relaxation is very close to that derived in shear modulus relaxation experiments as well as to the formation enthalpy given by the Interstitialcy theory. This provides further support for the interpretation of structural relaxation behavior within the framework of this theory.     

Enthalpy recovery and free volume relaxation in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass

We report on the free volume of the Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅ bulk metallic glass in terms of its enthalpy recovery and volumetric relaxation below the glass transition temperature, T_g. Glassy samples are isothermally annealed below T_g using differential scanning calorimetry and the resulting enthalpy recovery, ΔH_r, is measured upon re-heating into the supercooled liquid region. Volumetric changes below T_g are measured isothermally using Thermo-Mechanical Analysis. The total changes in the relative free volume, Δν_f/ν_m, between the initially glassy state and the equilibrium liquid are calculated from the volumetric relaxation. The measured values of ΔH_r and Δν_f/ν_m correlate well within the framework of free volume theory and a linear relationship is found between the two.     

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.     

Temperature dependence of the short-range order of Cu65Zr35 metallic glass

The thermal behaviour of the short-range order of Cu₆₅Zr₃₅ metallic glass has been investigated by means of in situ high energy synchrotron X-ray diffraction. The temperature dependence of the X-ray structure factor and the atomic pair correlation function was analysed within the glassy state. In addition to the effect of volume expansion and thermal atomic oscillations changes occur in the short-range order. Local atomic rearrangements take place as a consequence of the structural heterogeneity in the glass. The structural transformations depend linearly on temperature. The changes of the short-range order of the glass by thermal expansion are found to be different from those under elastic tensile strain.     

Diffusion and viscous flow in bulk glass forming alloys

We review radiotracer diffusion and isotope measurements in bulk glass forming alloys from the glassy state to the equilibrium melt and compare diffusion and viscous flow. In the glassy as well as in the deeply supercooled state below the critical temperature T_c, where the mode coupling theory predicts a freezing-in of liquid-like motion, very small isotope effects indicate a highly collective hopping mechanism. Not only in the glassy state but also in the supercooled state below T_c the temperature dependence of diffusion is Arrhenius-like with an effective activation enthalpy. A clear decoupling takes place between the diffusivities of the individual components of the alloys and between time scales related to diffusive transport and viscous flow. While the component decoupling is small for the smaller components a vast decoupling of more than 4 orders of magnitude is observed in Pd–Cu–Ni–P alloys between the diffusivity of the large majority component Pd and of the smaller components at the glass transition temperature T_g. The diffusivities of all components merge close to the critical temperature T_c of mode coupling theory. Above T_c, the onset of liquid-like motion is directly evidenced by a gradual drop of the effective activation energy. This strongly supports the mode coupling scenario. The isotope effect measurements show atomic transport up to the equilibrium melt to be far away from the regime of uncorrelated binary collisions. For Pd, in contrast to the behavior of single component molecular glass formers, the Stokes–Einstein equation even holds in the entire temperature range below T_c over at least 14 orders of magnitude. Apparently, the majority component Pd forms a slow subsystem in which the other elements move fast. Rearrangement of the Pd atoms thus determines the viscous flow behavior. The decoupling of atomic mobility seems to arise from a complex interplay between chemical short order and atomic size effects that gets more pronounced on approaching the glass transition temperature. The ability of the bulk glass forming alloys to form a slow subsystem in the liquid state appears to be a key to the understanding of their excellent glass forming properties.     

Synthesis of Fe–Cr–Mo–C–B–P bulk metallic glasses with high corrosion resistance

Bulk metallic glasses with a maximum thickness (t_max) of 1.0–2.7 mm were synthesized in the Fe₄₃Cr₁₆Mo₁₆(C, B, P)₂₅ system over a wide composition range by copper mold casting. They exhibit a large supercooled liquid region (ΔT_x) of 40–90 K and a high reduced glass transition temperature (T_g/T_l) of 0.54–0.60, indicating high glass-forming ability (GFA) and high thermal stability of the supercooled liquid. The critical cooling rate for glass formation was evaluated to be of the order of 10² K s⁻¹. The bulk metallic glasses exhibited high corrosion resistance in aggressive HCl solutions. The alloying element P has a beneficial effect on corrosion resistance.     

Effect of ball-milling and shot-peening on Zr55Al10Ni5Cu30 alloys

Effect of ball-milling and shot-peening on a metallic glass Zr₅₅Al₁₀Ni₅Cu₃₀, which possesses a large supercooled liquid region, has been investigated by means of differential scanning calorimetry, x-ray diffractometry and transmission electron microscopy. Metallic glassy ribbons, powders and plates were prepared by melt-spinning, gas-atomizing and mold-clamp casting techniques, respectively. No structural changes were observed in both the ribbon and powder specimens by ball-milling for around 100 h; however, the powder specimens were crystallized by Fe contamination when they were ball-milled for 540 h. No structural evolution was also observed when the plate specimens were subjected to shot-peening, while crystallized plate specimens were easily amorphized by mild and short period shot-peening. These results imply high phase stability of the Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass against deformation.     

Structure relaxation and crystallization of Al83Ni10Ce7 metallic glass

Structure relaxation and crystallization of Al₈₃Ni₁₀Ce₇ metallic glass were studied by different scanning calorimetry (DSC) and X-ray diffraction (XRD). According to the DSC scan, it is interesting to find that the second exothermic peak changes with pre-annealing temperatures (below glass transition temperature), suggesting a change in the amorphous structure upon relaxation. Continuous heating crystallization and isothermal crystallization exhibit different crystallization mechanism of the present alloy. fcc-Al and a metastable phase precipitate simultaneously in the first stage crystallization during continuous heating; however, only a metastable phase precipitates during isothermal annealing below glass transition temperature (T_g).     

Anomalous structural evolution and liquid fragility signatures in Cu–Zr and Cu–Hf liquids and glasses

The results of high energy X-ray scattering studies of equilibrium and supercooled Cu_100?xZr_x (x = 46 and 54) and Cu_xHf_100?x (x = 55 and 60.8) liquids and the corresponding glasses are presented. The liquid data were obtained in a containerless environment using the beamline electrostatic levitation (BESL) technique. The total structure factor and total pair correlation functions were measured as a function of temperature for the liquids, and for the glasses at room temperature. A developing asymmetry in the peak of the first coordination shell in the total pair correlation function suggests chemical ordering in the liquids with cooling. This asymmetry takes the form of two prominent peaks, suggesting two prominent ordering length scales. When the magnitudes of these peaks are extrapolated to the glass transition temperature a discontinuity is observed, indicating that an abrupt increase in the magnitude is required to match the observed peak heights in the glass. This suggests that the structure of the supercooled liquid orders more rapidly near the glass transition temperature, a conclusion that is supported by molecular dynamics simulations. This observed structural evolution of the liquid indicates that the concept of fragility, typically defined from the behavior of viscosity with temperature, has a measurable structural signature as well, which can be observed in X-ray diffraction studies.     

The synthesis and properties of Zr-based metallic glasses and glass-matrix composites

In this article, the formation of metallic glass composites in the system Zr/Ti-Al-Cu-Ni by partial devitrification or by blending with second-phase particles through solid-state processing is discussed with respect to the effect of second phases on the thermal stability of the glassy matrix and on mechanical properties. The composites exhibit no significant reduction of the supercooled liquid region as compared to the particle-free metallic glass. The viscosity of the supercooled liquid increases with an increasing volume fraction of particles. The mechanical behavior was characterized by microhardness measurements and constant compression rate tests. At room temperature, there is a significant increase in yield strength with an increasing volume fraction of crystalline phases. At temperatures around the glass transition, the influence of these nanoscaled particles is of minor importance. Rather, the homogeneous flow of the composites is determined by Newtonian viscous flow of the amorphous matrix. This opens a promising route for easy shaping of complex parts of bulk metallic glasses at temperatures that are above T_g. For more information, contact J.H. Ecker, IFW Dresden, Institute of Metallic Materials, P.O., Box 27 00 16, Dresden D-01171, Germany; telephone 49-351-4659-602; fax 49-351-4659-541; e-mail j.eckert@ifw-dresden.de.     

Origin of enhanced glass-forming ability of Ce-containing Al–Fe alloy: Ab initio molecular dynamics study

Using ab initio molecular dynamics simulation, glass-forming abilities of Al₉₀Fe₁₀ and Al₉₀Fe₅Ce₅ alloys have been investigated successfully correlated with the atomic structure and composition. The origin of enhanced glass-forming ability for Al₉₀Fe₅Ce₅ alloy is interpreted by taking advantage of the calculated information. It is found that the enhanced glass-forming ability with the addition of Ce into Al₉₀Fe₁₀ alloy, in contrast to the transitional metallic glass, has nothing to do with the kinetic factor and stability of local atomic packing, but the atomic environment and medium-range order in the supercooled liquid state. A comparison of structure and composition between the supercooled liquid and potential crystalline phases demonstrates that the precipitation of the solid solution and compounds from the amorphous matrix for Al₉₀Fe₁₀ alloy becomes much more difficult with the addition of Ce. After glass transition, the resultant glassy state for Al₉₀Fe₅Ce₅ alloy is further stabilized by stable local atomic packings.     

Characterization of free volume in cold-rolled Zr55Cu30Ni5Al10 bulk metallic glasses

Cold-rolled Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses (BMGs) exhibited volume dilatation across the entire range of thickness reduction fractions from 5.7% to 63%. The dilatation was dominantly attributed to the free volume generated during cold-rolling, together with any open volumes, like the voids or microcracks, or both, which survived even after sufficiently annealing the rolled sample at the temperature in the supercooled liquid region. The relaxation of free volume formed during cold-rolling resulted in more heat evolution per unit volume than that of as-cast glass. The structural change during cold-rolling could be ascribed to the increased fluctuation of atomic-level hydrostatic stress, 〈p²〉, in the glass.     

A new glass-forming ability criterion for bulk metallic glasses

A new indicator of glass-forming ability (GFA) for bulk metallic glasses (BMGs) is proposed based on crystallization processes during cooling and reheating of the supercooled liquid. The interrelationship between this new parameter and the critical cooling rate or critical section thickness is elaborated and discussed in comparison with two other representatives, i.e. reduced glass transition temperature T_rg (=T_g/T_l, where T_g and T_l are the glass transition temperature and liquidus temperature, respectively) and supercooled liquid range ΔT_xg (=T_x−T_g, where T_x is the onset crystallization temperature and T_g the glass transition temperature). Our results have shown that ΔT_xg alone cannot infer relative GFA for BMGs while the new parameter γ, defined as T_x/(T_g+T_l), has a much better interrelationship with GFA than T_rg. An approximation of the critical cooling rate and critical section thickness for glass formation in bulk metallic glasses is also formulated and evaluated.     

Structure and thermal behavior of multicomponent Fe68−xNixZr15Nb5B12 (x = 5, 10, 15, 20) alloys

Multicomponent Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloy powders milled for 60 h were prepared by mechanical alloying (MA). The structure and crystallization behavior were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). Ni enhances the amorphisation of alloy powders. Particle size increases with increasing Ni content. Both onset crystallization temperature T_x and the first crystallization peak temperature T_p of the four alloys shift to a higher temperature with increasing heating rate while melting temperature (T_m) is just the opposite. Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloys all have a large supercooled liquid region ΔT_x. The supercooled liquid region ΔT_x increases and the crystallization activation energy E decreases with increasing Ni content.