Glass formation and nanocrystallization in Zr based alloys

In the present paper bulk glass forming behaviour of some of Zr based Ni, Cu, Al and Ti bearing alloys has been investigated. Examination of the as solidified microstructure of these alloys in the partially crystalline state has shown that the predominance presence of the Zr₂Ni or its derivatives phases as the phases competing with glass formation whereas in the fully amorphous microstructure, quenched-in nuclei and the atomic short range order existing in the amorphous phase were observed. The aim of the microstructural examinations of the fully amorphous phase was to ascertain the nature and morphology of the quench-in nuclei. In the partially crystalline microstructures, study of the crystalline phases competing with glass formation has helped in better understanding of the solidification process during BMG formation. The kinetics of crystallization of the as solidified Zr based bulk metallic glasses were studied by differential scanning calorimetry (DSC) and crystallized microstructures were examined by conventional and high-resolution electron microscopy. The activation energies of crystallization and the Avrami exponent have been evaluated. The Avrami exponent values have been rationalized in terms of the observed nucleation and growth behaviour of the phases forming on crystallisation. Conditions of crystallization leading to the formation of nanocrystals have been identified.     

Ball-milling-induced crystallization and ball-milling effect on thermal crystallization kinetics in an amorphous FeMoSiB alloy

Microstructure evolution in a melt-spun amorphous Fe_77.2Mo_0.8Si₉B₁₃ alloy subjected to high-energy ball milling was investigated by means of X-ray diffraction (XRD), a transmission electron microscope (TEM), and a differential scanning calorimeter (DSC). It was found that during ball milling, crystallization occurs in the amorphous ribbon sample with precipitation of an α-Fe solid solution, and the amorphous sample crystallizes completely into a single α-Fe nanostructure (rather than α-Fe and borides as in the usual thermal crystallization products) when the milling time exceeds 135 hours. The volume fraction of material crystallized was found to be approximately proportional to the milling time. The fully crystallized sample with a single α-Fe nanophase exhibits an intrinsic thermal stability against phase separation upon annealing at high temperatures. The ball-milling effect on the subsequent thermal crystallization of the amorphous phase in an as-milled sample was studied by comparison of the crystallization products and kinetic parameters between the as-quenched amorphous sample and the as-milled partially crystallized samples. The crystallization temperatures and activation energies for the crystallization processes of the residual amorphous phase were considerably decreased due to ball milling, indicating that ball milling has a significant effect on the depression of thermal stability of the residual amorphous phase.     

Kinetics study on non-isothermal crystallization of Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> metallic glass

In this paper, the crystallization kinetics of melt-spun Cu₅₀Zr₅₀ amorphous alloy ribbons has been investigated using differential scanning calorimetry. Moreover, the Kissinger, Ozawa and isoconversional approaches have been used to obtain the crystallization kinetic parameters. As shown in the results, the onset crystallization activation energy E _x is less than crystallization peak activation energy E _p. The local activation energy E _α increases at the crystallized volume fraction α < 0.2 and decreases at the rest, which suggests that crystallization process is increasingly hard (α < 0.2) at first, after which it become increasingly easy (α > 0.2). The nucleation activation energy E _nucleation is greater than grain growth activation energy E _growth, indicating that the nucleation is harder than growth. In terms of the local Avrami exponent n(α), it lies between 1.27 and 8, which means that crystallization mechanism in the non-isothermal crystallization is interface-controlled one- two- or three-dimensional growth with different nucleation rates.     

Primary Transformation Kinetics in Zr-Al-Ni-Cu-Pd Bulk Metallic Glass Correlated with Relaxation State

The primary transformation kinetics of nanoicosahedral quasicrystalline (QC) phase formation were investigated in Zr₆₅Al_7.5Ni₁₀Cu_12.5Pd₅ bulk metallic glass (BMG) in various relaxation states. A less relaxed (unrelaxed) BMG exhibited higher activation energy for atomic diffusion in the glassy structure than that of a relaxed one, which represents a change in the nucleation and grain growth kinetics of the primary phase with the relaxation state. Actually, the grain growth rate of a QC particle near the crystallization temperature was approximately 1 × 10^?9 m/s in the less relaxed BMGs, which was less than half of that in the relaxed BMGs. In contrast, the calculated homogeneous nucleation rate significantly increased in the less relaxed samples. It increased with the volume fraction transformed in the early stage. It is concluded that the relaxation state of glassy alloys markedly affects the primary transformation kinetics. The current study also indicates a necessity of development of the relaxation state for structure controlling in industrial applications of BMGs.     

The crystallization of a Pd60Ti20Si20 metallic glass

The kinetics and mechanism of crystallization have been followed in a Pd₆₀Ti₂₀Si₂₀ metallic glass by means of a combination of experimental techniques. As quenched alloys contain a variable population of crystals and/or nuclei. Oxide islands were found mostly on the dull (wheel-side) surface of the ribbon, favouring partial crystallization around 710 K. The glass transition temperature, T_g, is manifest in DSC and low-frequency internal friction measurements from heating rates of 1 K/min upwards, prior to the main crystallization event (761 and 772 K at 20 K/min heating rate, respectively). Isothermal crystallization in the temperature range from 739.0 to 758.7 K occurred in two steps because of a different population of quenched-in nuclei in various parts of the ribbon. A Johnson-Mehl-Avrami analysis of the DSC isothermal output gave an Avrami exponent of 3, indicating growth on preexisting nuclei, as shown by microscopy observations. The activation energy and the preexponential factor for growth are very high: 566 ± 20 kJ/mol and 10³⁶ms⁻¹ respectively. This finding is discussed with reference to analogous data for diffusion in metallic glasses.     

Co68.5Fe4Si10B17.5非晶合金晶化动力学行为研究

采用示差热分析(DSC)研究了旋铸急冷法在大气环境中制备出的Co68.5Fe4Si10B17.5非晶合金的非等温晶化的动力学行为。研究发现:在连续加热条件下,随升温速率的加快,合金的特征温度Tg,Tx,Tp均向高温区移动。当分别采用晶化开始温度和峰值温度时,所得非晶合金的激活能并不是稳定值。用Doyle-Ozawa方法计算出的激活能值比Kissinger法计算出的激活能值要大。非晶态Co68.5Fe4Si10B17.5合金的晶化百分比与退火温度和退火时间的关系曲线均呈S型。随加热速度的增加,非晶合金的晶化百分比与温度的关系曲线向高温处移动。     

Phase separation and crystallization of La_2O_3 doped ZnO-B_2O_3-SiO_2 glass

In order to investigate the effect of the La_2O_3 on the phase separation and crystallization of ZnO-B_2O_3-SiO_2 glass, after the occurence of the phase separation and crystallization of glasses by heat treatment, the microstructure morphology and distribution of elements in different sample areas were characterized by the scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS); the non-isothermal crystallization kinetics of the glass samples was studied by using a differential scanning calorimeter(DSC) and the precipitated crystals of crystallized glass were determined by the X-ray diffraction(XRD). The results suggest that the phase separation and crystallization of 60ZnO-30 B_2O_3-10SiO_2 glass occur at glass surface, and the incorporation of small amount(4 mol%) of La_2O_3 significantly inhibits the glass phase separation and consequently improves the thermal stability of glass.Doping of La_2O_3 accelerates the glass crystallization at the elevated temperature(660 ℃), making the depth of crystal layer thicker and diffraction intensity in XRD patterns stronger. However, due to the precipitation of several crystals that occur simultaneously when La_2O_3 doping amount is 4 mol%, crystallization of the 60ZnO-30B_2O_3-10SiO_2 glass is obviously depressed, the crystallization activation energy Ec and the relative crystallinity X_c of the glass reach the maximum and the minimum values, respectively.Although transition from one-dimensional growth of crystals to two-dimensional growth of crystals results from La_2O_3 addition, the one-dimensional growth of crystals remains dominant in crystallization process. This work can provide some useful information for preparing glass ceramics with nano-crystals precipitated in the glass surface.     

The effect of hydrogen on the crystallization behavior of amorphous PdSi alloys

Differential scanning calorimetry, X-ray diffractometry and Optical metallography were used to study the effect of hydrogen atmosphere and surface treatment on the crystallization process in PdSi metallic glasses. In Pd₈₀Si₂₀ metallic glasses, the surface treatment by mechanical polishing with 1200 SiC induces surface crystallization in which preferred nucleation at the surface occurs, while homogenous crystallization occurs in as-quenched samples without surface grinding. In amorphous Pd₈₅Si₁₅, the crystallization occurs simultaneously from the sample surface and in the bulk. Surface treatment by mechanical polishing does not change the crystallization morphology. These results were not affected by the atmosphere of argon or hydrogen. It is shown that the surface crystallization, which is dominant in the initial stages of the transformation, is revealed by the low temperature shoulder of the main peak of crystallization in a DSC trace. The process of crystallization of amorphous Pd₈₀Si₂₀ and Pd₈₅Si₁₅ is found to be impeded by absorbed hydrogen, this having been measured by the increase of both crystallization temperature and activation energy for crystallization process in a hydrogen environment. Such crystallization behavior can be explained by assuming that hydrogen reduces the free volume available for rearrangements of host atoms of the metallic glasses.     

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…     

Correlation between the structure and internal friction of metallic glass Cu45Ti55

The structural change of metallic glass Cu₄₅Ti₅₅ from room temperature to 800 K is investigated by combining internal friction using the torsion penulum method with X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The internal friction curve exhibits a point of inflection at about 640 K and an internal friction peak between 640 and 780 K, which corresponds to the precipitation of CuTi₂ (tetragonal) and a metastable phase I (tetragonal) and the formation of CuTi (tetragonal), respectively. The result shows that internal friction is closely related to the phase transition during crystallization, and there is much correspondence between internal friction and DSC in describing the crystallization behavior of metallic glasses.     

Salient shear bands and second-phase addition interactions of bulk metallic glass matrix composite

This article discusses Charpy impact testing and fracture morphology of the Zr_41.25Ti_13.75Cu_12.5Ni₁₀Be_22.5 bulk metallic glass matrix composite with long tungsten fibers. Energy to failure was measured via the impact test as well as by integrating the compressive stress-strain curves, and compared for various fiber fractions. Failure energy increased with fiber volume fraction by both measures. Observation of fracture surfaces was made by using scanning electron microscopy. The results show that the fracture surface of the unreinforced bulk metallic glass (BMG) exhibits three different regions, i.e., the impact zone, the transition zone, and the ridged zone, which have different morphology. The composites present uneven or jagged morphology on macroscopic scale, while the microstructure exhibits salient shear bands and second-phase addition interactions. Bridge formation between tungsten fibers is interpreted as evidence that the shear band propagation in the matrix is suppressed by the fibers. Furthermore, shear lips were observed for the composites containing over 50 pct, fiber volume fraction, showing a great improvement in toughness.     

Cu基块体非晶合金的高温均匀塑性流变行为研究

本文选用非晶形成能力高,且在玻璃转变区和过冷液相区有高热稳定性的Cu46Zr47-x Al7块体非晶合金为研究对象,利用差示扫描量热分析（DSC）、高温真空压缩实验等方法,对其高温均匀塑性流变行为进行研究。结果表明,Cu46Zr47Al7块体非晶合金的高温均匀塑性形变行为依赖于温度和应变速率,随着温度的降低或应变速率的增大呈现出牛顿流变向非牛顿流变的转变,该行为可以用基于自由体积的过渡态理论来进行定量描述。根据过渡态理论拟合出来的驱动体积大致相当于25~50个原子,驱动能为551kJ/mol,说明控制合金高温均匀形变的原子不会脱离周围原子而单独跃迁,其扩散涉及到多原子的协同运动。     

Kinetics of Glass Transition and Crystallization in Carbon Nanotube Reinforced Mg-Cu-Gd Bulk Metallic Glass

Mg65Cu25Gd10 bulk metallic glass and its carbon nanotube reinforced composite were prepared. Differential scanning calorimeter （DSC） was used to investigate the kinetics of glass transition and crystallization processes. The influence of CNTs addition to the glass matrix on the glass transition and crystallization kinetics was studied. It is shown that the kinetic effect on glass transition and crystallization are preserved for both the monothetic glass and its glass composite. Adding CNTs in to the glass matrix reduces the influence of the heating rate on the crystallization process. In addition, the CNTs increase the energetic barrier for the glass transition. This results in the decrease of GFA. The mechanism of the GFA decrease was also discussed.     

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.     

Influence of Cooling Rate on the Enthalpy Relaxation and Fragility of a Metallic Glass

Structural relaxation behavior of a rapidly quenched (RQ) and a slowly cooled Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass was investigated and compared. Differential scanning calorimetry was employed to monitor the relaxation enthalpies at the glass transition temperature, T _g , and the Kolrausch–Williams–Watts (KWW) stretched exponential function was used to describe its variation with annealing time. It was found that the rate of enthalpy recovery is higher in the ribbon, implying that the bulk is more resistant to relaxation at low temperatures of annealing. This was attributed to the possibility of cooling rate affecting the locations where the glasses get trapped within the potential energy landscape. The RQ process traps a larger amount of free volume, resulting in higher fragility, and in turn relaxes at the slightest thermal excitation (annealing). The slowly cooled bulk metallic glass (BMG), on the other hand, entraps lower free volume and has more short-range ordering, hence requiring a large amount of perturbation to access lower energy basins. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.     

Influence of the Crystalline Phase on Strain-Rate Sensitivity of a Zr-Cu-Ni-Al Bulk Metallic Glass at Room Temperature

In this work, Zr₅₃Cu_18.7Ni₁₂Al_16.3 alloy has been cast into rod samples with different diameters. Glassy composites with various volume fractions of quenched-in crystalline are obtained. Their mechanical behaviors and fracture mechanisms have been investigated upon both quasistatic and dynamic loading. As the volume fraction of crystalline phase increases, the increase in the strain-rate sensitivity exponent could be attributed to the combination of the reduction of the shear band-related deformability and the enhancement of the dislocation-related deformability. These results may shed more insight on optimizing the microstructure and performance of bulk metallic glass composites in the future.     

Fatigue Softening of (Zr58Ni13.6Cu18Al10.4)99Nb1 Bulk Metallic Glass

The fatigue study was conducted on the (Zr₅₈Ni_13.6Cu₁₈Al_10.4)₉₉Nb₁ bulk metallic glass (BMG) under four-point bending with a maximum stress of 600 MPa. After the fatigue experiment, compression tests were performed with samples cut from three different sections, including sections a: within the inner pins, b: between inner and outer pins, and c: outside the outer pins. It is concluded that, as the fatigue damage increases, the compression flow stress decreases, the ductility increases, and the deformation becomes more homogeneous.     

Comparative Studies of Different Methods for Determining Crystallization Kinetics of Metallic Glass

Different non-isothermal and isothermal techniques are used to find out the phase transformation kinetic parameters (in this case, activation energy [E] of crystallization, Avrami exponent [n] and frequency factor [k ₀]) for the crystallization of a newly developed four component Zr₅₈Cu₂₂Al₁₂Ag₈ glassy alloy using differential scanning calorimetry (DSC) data. A comparative study is carried out to understand the effectiveness of different methods of evaluation of crystallization kinetic parameters from the DSC traces. n for the alloy varies in the range of 2.5?C4 suggesting that growth varies from one to three dimensional with growth mechanism varying from diffusion controlled 3D growth to an interface controlled 1D growth.     

Spray forming of bulk Al85Y8Ni5Co2 with co-existing amorphous, nano- and micro-crystalline structures

A 12 mm thick Al₈₅Y₈Ni₅Co₂ plate was spray deposited on a 30 mm thick pre-heated copper substrate. The deposit was characterized using optical, high resolution scanning and transmission electron microscopy. Differential scanning calorimetry was performed to assess the crystallization behaviour of the deposit compared to melt spun ribbons of the same composition. The deposit shows an amorphous phase fraction of 83 and 56 vol.% (based on the total crystallization energy), in the bottom and top regions of the deposit. The deposit consists of amorphous and nanoscale structures along with microcrystalline intermetallic phases. The large amorphous phase fraction in the deposit is attributed to the chilling effect upon deposition of highly undercooled/partially crystallized droplets onto the pre-heated substrate and rapid heat extraction thereof due to bonding at the deposit/substrate interface.