Impurity-driven nanocrystallization of Zr-based bulk amorphous alloys

The effect of oxygen content and Ti addition on the glass forming ability (GFA) and crystallization kinetic of Zr-based bulk glass forming alloys have been studied by means of thermal analysis and X-ray diffraction techniques. Presence of oxygen triggers the formation of a nanocrystalline metastable f.c.c. Zr2 Ni-type phase which can act as heterogenous nucleation sites for the formation of dendrites during solidification. An increase in oxygen content changes the crystallization behaviour from a single- to a double-step process and triggers the crystallization of stable Zr2(Cu,Al) besides metastable Zr2 Ni-type phase. Oxygen-triggered nucleation of a nanocrystalline metastable Zr2 Ni-type phase is found to be the initial step of crystallization. The important parameters of GFA such as glass transition temperatures, Tg, the crystallization temperatures, Tx, and crystallization enthalpies, deltaH, were determined by using DSC. It was observed that the presence of oxygen, even in a very small amount, and Ti addition cause a drastic reduction of the supercooled liquid region, deltaTx, accompanied by a change of the crystallization kinetic. This leads to the decrease in stability of the supercooled liquid, consequently results in a deterioration of the glass forming ability of the alloy.     

添加AlN对机械合金化Zr65Al7.5CU17.5Ni10非晶态合金热稳定性的影响

以成分为Zr65Al7.5Cu17.5Ni10的元素的粉末混合物及AIN颗粒为起始材料,经机械合金化形成非晶态合金为基体的复合材料,AIN添加量为5％－30％（体积分数,下同）,利用X射线衍射（XRD）,透射电子 显微镜（TEM）和差示扫描量热计（DSC）分析了含AIN复合材料的结构特性,玻璃转变与晶化行为,TEM观察表明,AIN第二相粒子弥散分布在晶Zr基合金基体上,粒子尺寸为20－200nm,仍为初始的晶体结构,与未添加AIN的Zr基非晶态合金相比,含5％－10％AIN的复合材料仍表现出较宽的过冷液态温度区域,玻璃转变温度（Tg)和晶化激活能（Ex)没有显著变化,但晶化起始温度（Tx)向高温移动大约10K,导致过冷液态温度区域的扩宽,AIN含量增至30％,明显的玻璃转变消失,Tx升高的20K。     

Liquid dynamics and glass formation of Gd55Co20Al25 metallic glass with minor Si addition

Liquid dynamics plays an essential role in glass formation.Here we observed a distinct change of liquid dynamics in Gd55Co20Al25 metallic glass induced by microalloying Si element.In the equilibrium melt,minor Si(0.5 at.％)addition leads to a more fragile liquid behavior and a smaller strength of liquid-liquid transition with the transition strength(ΔF)decreasing from 0.76 to 0.35.However,in the supercooled liquid,Si-doped liquid exhibits a remarkable enhanced fragile-to-strong transition(FST),and the value of FST factor f increases sharply from 1.63 to 3.84,resulting in a stronger liquid behavior and more sluggish crystallization kinetics for Gd55Co20Al24.5Si0.5 metallic glass.Moreover,minor Si addition promotes the formation of a crystal-like structure with a size of 1-2 nm.The interactions between the crystal-like structures and other local favored clusters frustrate the further growth of crystal-like phases,thus sta-bilizes the amorphous structure.As a result,the glass-forming ability(GFA)was largely improved.The critical diameter of Gd55Co20Al25 metallic glass increased from 2 to 7 mm with 0.5 at.％Si addition with-out deterioration of the magnetocaloric effect.This study provides valuable insight for understanding the distinct effect of microalloying on GFA of metallic glasses from the aspect of the evolution of the liquid.     

The evolution of Cu-rich domains in the Cu60Zr30Ti10 metallic glass by ASAXS

The structure of the melt-spun Cu₆₀Zr₃₀Ti₁₀ metallic glass is investigated by anomalous small angle X-ray scattering (ASAXS). It is confirmed that the compositional segregation in the diameter range of 30–50 nm exists in the as-quenched state. ASAXS results have convincingly shown the aggregation of Cu atoms and the formation of Cu-rich domains in the amorphous matrix. The Cu-rich domains grow slightly with low growth rate below the glass transition temperature, while the sizes of these domains drastically increase with high growth rate in the supercooled liquid temperature region. The Cu-rich domains are presumed to be associated with nucleation for the primary crystallization process.     

High formability of glass plus fcc-Al phases in rapidly solidified Al-based multicomponent alloy

A multicomponent Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ alloy was found to keep a mixed glassy + Al phases in the relatively large ribbon thickness range up to about 200 μm for the melt-spun ribbon and in the diameter range up to about 1100 μm for the wedge-shaped cone rod prepared by injection copper mold casting. The glassy phase in the Al-based alloy has a unique crystallization process of glass transition, followed by supercooled liquid region, fcc-Al + glass, and then Al + Al₃Y + Al₉ (Co, Fe)₂ + unknown phase. It is also noticed that the primary precipitation phase from supercooled liquid is composed of an Al phase instead of coexistent Al + compound phases, being different from the crystallization mode from supercooled liquid for ordinary Al-based glassy alloys. In addition, it is noticed that the mixed Al and glassy phases are extended in a wide heating temperature range of 588–703 K, which is favorable for the development of high-strength nanostructure Al-based bulk alloys obtained by warm extrusion of mixed Al + amorphous phases. The Vickers hardness is about 415 for the glassy phase and increases significantly to about 580 for the mixed Al and glassy phases. The knowledge of forming Al + glassy phases with high hardness in the wide solidification and annealing conditions through high stability up to complete crystallization for the multicomponent alloy is promising for future development of a high-strength Al-based bulk alloy.     

Ti-based amorphous alloys with a wide supercooled liquid region

Ti-Cu-Ni-Co quaternary amorphous alloys produce by melt spinning were found to have a wide supercooled liquid region before crystallization, though no glass transition was observed in Ti-Cu binary amorphous alloys. The largest temperature interval of the supercooled liquid region (ΔT_x) is as large as 90 K for Ti₅₀Cu₂₅Ni₂₀Co₅.There is a tendency for ΔT_x to increase with an increase in storage modulus and with a decrease in loss modulus. It is therefore presumed that the increase in ΔT_x for the multicomponent amorphous alloy is due to the suppression of crystallization for the supercooled liquid resulting from the increase in viscosity.     

宽过冷液相区铁基非晶合金的形成和磁性

用熔体急冷法制备出具有明显的玻璃转变和较宽的过冷液相区的Fe-Co-(Nb)-Zr-B非晶合金,研究了热稳定性和软磁性能。结果表明,在Fe-Co-Zr-B四元合金中添加适量的Nb可以显著扩大过冷液相区,提高合金的热稳定性。当冷却速率降低时,急冷合金具有非晶和纳米晶的复相结构。非晶合金的饱和磁化强度随Nb含量的增加而减小。不同Nb含量的非晶合金的饱和磁致伸缩系数均较低。在低于晶化温度的温度下退火可以有效地降低矫顽力,改善软磁性能。晶化导致软磁性能降低。     

Formation of metallic glass nanowires by gas atomization

Gas atomization which is a conventional technique in powder metallurgy is adapted for the formation of metallic glass nanowires. This approach is able to produce a large quantity of nanowires with diameters in the 50-2000 nm range. Experiments performed with different conditions and alloy compositions confirm that the key mechanism of the nanowire formation is the spinnability which increases exponentially when the melt stream is supercooled from the liquid state.     

Supercooled liquid foaming of a Zr-Al-Cu-Ag bulk metallic glass containing pressurized helium pores

Foaming of a Zr-based metallic glass in the supercooled liquid is successfully performed by introducing pressurized pores and subsequent isochronal annealing. Melting of a Zr₄₈Cu₃₆Al₈Ag₈ powder under 12 MPa pressurized helium atmosphere followed by water quenching introduces spherical helium pores, whose average diameter and volume fraction are estimated respectively to be 30 μm and 7%, into a fully glassy bulk Zr₄₈Cu₃₆Al₈Ag₈ alloy. The isochronal annealing of the porous alloy below the crystallization temperature under atmospheric pressure of argon enables the expansion of pores by viscous flow deformation of the supercooled liquid, resulting in a high porosity structure up to 70% with a uniform cell size and cell distribution.     

Pd-Ni-P metallic glass film fabricated by electroless alloy plating

In the present study a Pd-Ni-P film has been fabricated by electroless alloy plating. The fabricated Pd-Ni-P film was found to be a metallic glass on the basis of two features, namely, an amorphous structure and a glass transition followed by crystallization during heating. The thermal stability of the supercooled liquid region, however, was lower than that of bulk Pd-Ni-P metallic glass. And unlike the conventional metallic glasses, the fabricated Pd-Ni-P film did not have a uniform microstructure. The non-uniform microstructure of this film resulted from the inhomogeneous distribution of the free volume accompanying the electroless alloy plating reaction.     

Reproducibility of thermal effects within the glass transition region

The sensitivity of DSC-traces to controlled ageing in successive heating/cooling cycles within the glass transition region was studied. The effects of cooling rate both of the melt and of the supercooled liquid, as well as of the heating rate, were considered. It was concluded that within the transformation region the cooling rate of supercooled liquid exerts the predominant effect on the reproducibility of the DSC-curves. It was demonstrated that the thermal history of a sample during glass formation may be erased by only one heating/cooling cycle within the transformation region.     

Deformation behavior of a TiZr-based metallic glass composite containing dendrites in the supercooled liquid region

The deformation behavior of a TiZr-based bulk metallic glass composite(BMGC) was characterized in the supercooled liquid region(SLR) from 623 K to 693 K. It was observed that the alloy exhibits the deformation behavior from work softening at low temperatures to work hardening at high temperatures.The yield stress and overshoot stress decrease remarkably with the increase of temperature, accompanied by superplasticity. The results showed that the crystallization occurred in the amorphous matrix for the post-deformation samples and the volume fraction of the corresponding crystallization products increased with increasing testing temperature. It is implied that the work hardening behavior was closely associated with the crystallization of the amorphous matrix. The tensile stress can accelerate the crystallization of amorphous matrix and the martensitic transformation of dendrite phases, which implies that the thermal stability of the alloy decreases under tension. These findings shed light on designing new BMGCs with high mechanical performance as well as the good SLR formability.     

Glass-Forming Ability for Nd_(70-x)Fe_(20)Al_(10)Y_x and Nd_(60-x)Fe_(30)Al_(10)Y_x Alloys

The glass-forming ability (GFA) of Nd70-xFe2oAl10Yx and Nd60-xFe30Al10Yx (0< x <15) alloys produced by Cu mold casting was investigated. Except Y=5 at. pct, bulk amorphous Nd70-xFe20Al10Yx alloys up to 2 mm in diameter were obtained. The GFA for Nd60-xFe30Al10Yx alloys, however, was found to decrease with increase of Y due to the increasing compositional deviation from the original eutectic point of Nd60Fe30Al10 alloy. The Nd60Fe20Al10Y10 and Nd60Fe30Al10 alloy exhibit the largest GFA and can be cast into bulk amorphous cylindrical specimens of 3 mm in diameter. The melting temperature or/and the reduced crystallization temperature is closely related to the GFA of Y-containing alloys. The bulk amorphous cylinder for the Nd55Fe20Al10Y15 alloy shows a distinct glass transition temperature and a wide supercooled liquid region before crystallization. The crystallization temperature, Tg, and the supercooled liquid region, TX, are 776 K and 58 K, respectively. The GFA and thermal stability of the Nd-Fe-AI-Y a     

Physical Limit of Stability in Supercooled Liquids

The kinetic spinodal (KS) in supercooled liquids, similar to the KS in superheated and stretched liquids, has been introduced as a locus where the mean time of formation of a critical nucleus becomes shorter than a relaxation time to local equilibrium. If the surface tension of the solid–liquid interface is known, the kinetic spinodal is completely determined by the equation of state of the supercooled liquid. The theory was tested against experimental data for the surface tension and the homogeneous nucleation limit for supercooled water. Reasonably good agreement between theoretical predictions and experimental data was observed. A prediction of the high-temperature limit for glass transitions is also discussed.     

Glass-Forming Ability for Nd70-xFe20AlioYx and Nd60-xFe30Al10Yx Alloys

The glass-forming ability (FGA) of Nd70-xFe20Al10Yx and Nd60-xFe30Al10Yx(0≤x≤15) alloys produced by Cu mold casting was investigated.Except Y=5 at.pct,bulk amorphous Nd70-xFe20Al10Yx alloys up to 2mm in diameter were obtained.The GFA for Nd60-xFe30Al10Yx alloys,however,was found to decrease with increase of Y due to the increasing compositional deviation from the original eutectic point of Nd60Fe30Al10 alloy.The Nd60Fe20Al10Y10 and Nd60Fe30Al10 alloy exhibit the largest GFA and can be cast into bulk amorphous cylindrical specimens of 3mmm in diameter.The melting temperature or /and the reduced crystallization temperature is closely related to the GFA of Y-containing alloys.The bulk amorphous c ylinder for the Nd55Fe20Al10Y15 alloy shows a distinct glass transition temperature and a wide supercooled liquid region before crystallization.The crystallization temperature,Tg,and the supercooled liquid region,ΔTx,are 776K and 58K,respectively,The GFA and thermal stability of the Nd-Fe-Al-Y alloys were discussed.     

Enthalpy landscapes and the glass transition

A fundamental understanding of the glass transition is essential for enabling future breakthroughs in glass science and technology. In this paper, we review recent advances in the modeling of glass transition range behavior based on the enthalpy landscape approach. We also give an overview of new simulation techniques for implementation of enthalpy landscape models, including techniques for mapping the landscape and computing the long-time dynamics of the system. When combined with these new computational techniques, the enthalpy landscape approach can provide for the predictive modeling of glass transition and relaxation behavior on a laboratory time scale. We also discuss new insights from the enthalpy landscape approach into the nature of the supercooled liquid and glassy states. In particular, the enthalpy landscape approach provides for natural resolutions of both the Kauzmann paradox and the question of residual entropy of glass at absolute zero. We further show that the glassy state cannot be described in terms of a mixture of equilibrium liquid states, indicating that there is no microscopic basis for the concept of a fictive temperature distribution and that the glass and liquid are two fundamentally different states. We also discuss the connection between supercooled liquid fragility and the ideal glass transition.     

Dynamics and microstructural evolution of bulk amorphous Cu12.5Ni10Zr41Ti14Be22.5 during phase separations: a small-angle neutron scattering approach

Small-angle neutron scattering has been applied to investigate in detail the structural relaxation of bulk amorphous Cu_12.5Ni₁₀Zr₄₁Ti₁₄Be_22.5 alloy in the supercooled liquid range (620–673 K) and its effect on subsequent crystallization around the crystallization point (673 K). The interference events from typical phase separation were recorded as the alloy was annealed in the supercooled liquid range. It was revealed that the crystallization in the alloy which was previously relaxed in the supercooled liquid range was significantly prohibited, and further phase separation was observed. Considerable temperature dependence of these phase separations was observed. It has been demonstrated that the phase separation developed via the spinodal mode and the achieved microstructure consisted of one droplet-like supercooled liquid phase embedded in the similarly disordered matrix. The droplets showed a bar-like pattern and distributed in a relatively regular form in the matrix. The phase separations exhibited sluggish coarsening kinetics with much smaller exponent than the Lifshitz–Slyozov–Wagner value. However, the dynamic scaling property of the phase separations at different temperatures has been approved by our scaling analysis. This revised version was published online in November 2006 with corrections to the Cover Date.     

High-temperature deformation and crystallization behavior of a Cu<Subscript>36</Subscript>Zr<Subscript>48</Subscript>Al<Subscript>8</Subscript>Ag<Subscript>8</Subscript> bulk metallic glass

The influence of annealing on the crystallization behavior of a Cu₃₆Zr₄₈Al₈Ag₈ (at.%) bulk metallic glass (BMG) was investigated. In both isochronal and isothermal annealing processes, the effective activation energies of the primary crystallizations were obtained as 295.8 ± 13.4 and 302.7 ± 14.5 kJ/mol by applying the Kissinger and Ozawa methods, respectively. Using the isothermal transformation kinetics described by the Johnson–Mehl–Avrami model, the Avrami exponent n was found to range between 2.56 and 3.25, which indicates that the primary crystallization behavior was three-dimensional diffusion-controlled growth with an increasing nucleation rate. The high-temperature deformation behavior of a Cu₃₆Zr₄₈Al₈Ag₈ BMG was then investigated by performing a series of compression tests after rapid heating within a supercooled liquid region. It was found that at least 14–17 dense randomly packed atoms are necessary to produce a unit local flow when the present BMG is subjected to non-Newtonian homogeneous deformation, as described by the transition state equation. Deformation and processing maps were also constructed based on the dynamic materials model to predict optimum bulk formability in a Cu₃₆Zr₄₈Al₈Ag₈ BMG taking warm deformation-induced crystallization within a supercooled liquid into account.     

Fe61Co10Zr5W4B20块体非晶合金的形成及其力学性能

采用铜模吸铸法获得直径为2 mm的Fe61Co10Zr5W4B20块体非晶合金.采用X射线衍射、扫描电镜、示差扫描量热仪、微显硬度及压缩实验等研究了非晶合金的结构、热稳定性、显微硬度与压缩性能.结果表明:Mo的引入不利于非晶合金的形成;Fe61Co10Zr5W4B20块体非晶合金表现为二级晶化,玻璃转变温度为561.1℃,晶化起始温度为619.0℃,第一晶化峰值温度为632.6℃,第二晶化峰值温度为747.0℃,过冷液相区为57.9℃;该非晶合金的显微硬度为1207HV0.2,抗压强度σbc为1707.6 MPa.     

Structural relaxation and crystallisation of bulk metallic glasses Zr41Ti14Cu12.5Ni10−xBe22.5Fex (x = 0 or 2) studied by mechanical spectroscopy

The measurements of the internal friction and dynamic shear modulus as well as differential scanning calorimetry have been performed in order to investigate the structural relaxation and crystallization of Zr₄₁Ti₁₄Cu_12.5Ni_10−xBe_22.5Fe_x (x=0 or 2) bulk metallic glasses. It is found that the glass transition is retarded and the thermal stability of supercooled liquid is increased by the Fe addition. The experimental results are well analyzed using a physical model, which can characterize atomic mobility and mechanical response of disordered condensed materials.