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In liquids of high glass-forming ability, in which crystal growth rates are low, the rates can be measured over the full range of supercooling from the liquidus temperature down to the glass transition. For systems of low glass-forming ability, growth rates are readily measured at small supercooling and at very large supercooling around the glass-transition temperature, but it is difficult to acquire data over the full range of intermediate supercooling, especially at the maximum in growth rate. Data at intermediate supercoolings are however of considerable interest for understanding glass formation in such systems as pure metals and chalcogenides for phase-change data storage. We will review the methods emerging for making such measurements, and will note that the fragility of the liquid (including possible crossover from ‘fragile’ to ‘strong’ liquid behaviour on cooling) is an important part of understanding fast crystal growth. We also note that there are deficiencies in existing theories of fast crystal growth.

It has recently been shown that metallic glass structures can be idealized as inter-penetrating solute-centered atomic clusters that are packed with essentially periodic symmetry. The present work applies the same methodology to explore whether experimental observations can be matched by inter-connected solute-centered clusters that are organized in space via dense random cluster packing, Bergman icosahedral cluster packing or Mackay icosahedral cluster packing. Idealized partial pair distribution functions are developed where the symmetry of the solute positions in the structure is derived from the cluster-packing symmetry and the solute concentration, which establishes occupation of inter-cluster sites, especially β structural sites enclosed by an octahedron of solute-centered clusters. While each of the three models matches major features of the measured solute–solute partial pair distribution functions, the arrangement of clusters with Mackay icosahedral ordering provides the best fit. However, this model is not able to match an essential feature in solute-lean glasses and does not provide the same overall agreement as does periodic cluster packing for solute-rich glasses. Strong similarities between the structure factors in the Mackay icosahedral and periodic cluster-packing models, along with expected deviations from the idealized solute positions studied here, are likely to hinder an unambiguous distinction between these two models.

We show room temperature plasticity in several ZrCu-based bulk metallic glasses (BMG) after dispersions of crystalline nanoparticles were generated prior to mechanical testing. BMGs are heated in synchrotron light in transmission such that annealing can be interrupted at the very first detection of nucleation of the crystallites. Effect of embedded nanocrystals on the mechanical properties of BMG-Composites was investigated by compressive testing. When nanocrystal dispersions were generated, zirconium–copper-based BMGs that initially showed no plastic deformation prior to fracture, exhibited ductile behavior in compression with about 10% deformation.

Possible mechanisms for nanocrystal formation in metallic glasses are discussed, focusing primarily on the Al–transition metal–rare earth glasses. The presented transmission electron microscopy (TEM) data prove that nanoscale phase separation occurs prior to crystallization in some Al–RE–Ni glasses. TEM observations and modeling studies of the devitrification kinetic data for some of these glasses are presented, which demonstrate preferential nucleation of nanocrystal α-Al grains near the boundaries of the phase separated regions. Preliminary studies show no evidence for phase separation in Al–RE–Fe glasses, which also crystallize to a nanoscale microstructure. A new model for homogeneous nucleation, coupling the interfacial and the long-range diffusion fluxes, is advanced to explain this.

ArangeofMg–Ni–Ndbulkmetallicglasseshasbeencast.AroundthecompositionMg 65 Ni 20 Nd 15 (at.%),butnotatneighbouringcompositions,energy-filteredtransmissionelectronmicroscopyprovidesevidenceforstrongcomposi-tionalinhomogeneitiesintheas-castglass.Inthisfullyglassymaterial,anapparenttwo-phasecellularmicrostructureisseen;onephaseisrichinNi,theotherinMgandNd.Annealingatatemperaturejustabovetheglasstransition,orevenholdingatroomtemperatureforseveralmonthscaneliminatetheinhomogeneities.Thediffusivityvaluesderivedfromthehomogenizationkineticsareingoodagreementwiththoseknownforotherbulkmetallicglasses.Thepresentresultssuggesttheexistenceofaclosed-loopmiscibilitygapintheglass-formingliquid. 2003ElsevierScienceB.V.Allrightsreserved. PACS:81.05.Kf;81.40.Ef;66.30. h;64.75.þg 1.IntroductionThestabilityofbulkmetallicglasseshasfacili-tatedfundamentalstudiesoftheglassyandsu-percooled-liquid states. Phase separation hasreceivedsomeattentioninexplainingthenano-metrescaleofdevitrificationseenparticularlyinZr-basedbulkglasses[1].Inconventionaltrans-missionelectronmicroscopy(TEM),thecontrastfrom glassy thin foils has contributions fromvariationsinboththicknessandcomposition,andtheevidenceforphaseseparationcanbeambigu-ous.Inthepresentworkenergy-filtered(EF)TEMisusedtoprovideclearevidenceforcompositionalvariationsinMg-basedbulkglassesinwhichphaseseparationhasnotsofarseenmuchattention.Theinhomogeneitiesinas-castglassescanbeannealedoutbothneartheglasstransitionandatroomtemperature;thenatureofthemiscibilitygapisconsidered.2.ExperimentalmethodsTheAlloyAMg

Primary crystallization of a solid solution is studied for metallic glasses in the series (Fe100−xNix)100−yBy and (Fe100−xNix)93−yZr7By (at.%). The conditions under which the primary phase is bcc or ccp are characterized and then analysed in terms of homogeneous nucleation. The preference for the bcc phase is much stronger than expected from its thermodynamic stability and this is consistent with the primary phase selection in the solidification of binary Fe–Ni droplets. In each case the identification of the primary phase can be hindered by subsequent transformations, though these can be stifled in the finest crystallites and droplets.

Bulkmetallicglasses(MGs)arecurrentlysubjectofintenseresearchduetotheiruniquemechanicalproperties:NewtonianviscousformabilitybetweentheglasstransitiontemperatureT g andcrystallisationtemperatureT x andelasticstrainrangeofwellover1%belowT g .However,someMGsarebrittle.Fe-basedMGsandsomeothersarepronetothermalembrittlementevenduringthequench.CompositionaladjustmentsinthedesignofFeB-basedalloyshaveallowedpreparationofmetalloidpoorFe 40 Co 5 Ni 35 Zr 5 B 15 thathasc-Feasaprimarycrystallisationproductforob-tainingductilityofmetallicglass-matrixtoughenedbyc-Fecrystallisation.Structuralcharacterisationusingdiffractionofhighenergysynchrotronlight,transmissionelectronmicroscopy,scanningelectronmicroscopyarepresentedtogetherwithhardnessandweardata. 2002ElsevierScienceB.V.Allrightsreserved. 1.IntroductionDuetotheirnearly2%elasticstrainrange(ascomparedto0.2%forcrystallinematerials),me-tallicglasses(MGs)currentlyhavethebestknownAshbyperformanceindexr 2 =E (wherer andEaretheyieldstrengthandYoung’smodulusrespec-tively).Largerindicesofthisperformanceindexforsprings(reversiblestoringofelasticenergy)minimisevolume.Inadditiontometallichardnessandtoughness,someMGshavetheformabilityofpolymersintheirsupercooledliquidtemperaturerangebetweentheglasstransition,T

Several melt-spun ribbons of different compositions in the Al-rich corner of the Al–Ni–Sm ternary system were analysed by means of X-ray diffraction (XRD) and differential scanning calorimetry (DSC) in order to determine their thermal stability and to understand the nanocrystallisation process. Out of all the compositions analysed, only three were found to be completely amorphous, whereas the others were partially crystallised. The activation energies of the α-Al nanocrystallisation were evaluated using the Kissinger method. Isothermal DSC scans revealed the existence of quenched-in nuclei in the more unstable amorphous alloys. These quenched-in nuclei are probably due to the melt-spinning process and the crystalline structure after devitrification is strongly dependent on them.

In an attempt to probe the effect of vacancy-related defects on the crystalline-to-amorphous phase transformation, positron annihilation lifetime measurements were performed on Nb 3 Sn with the A15 structure as a function of mechanical milling. The lifetime spectra were deconvoluted into two components. Both components increased during the initial stage of milling, consistent with an increase in the concentration of the vacancy-type defects and void-type defects formed as a result of milling. The formation of these types of defects plays an important role in the amorphization reaction of A15 Nb 3 Sn.

We extend to the non-isothermal case a numerical technique that was developed to treat transient homogeneous nucleation in a one-component system by modeling directly the reaction by which clusters are produced. Calculations are presented for the nucleation frequency during the quench and for the number of nuclei produced and the volume fraction transformed at the end of quench for different rates of cooling from the melt. Three model systems are considered: an alkali silicate which is a relatively good glass former, and two metallic glasses. These show a wide range of critical cooling rates for glass formation. In some systems transient effects are predicted to be critical for glass formation. A simple technique is presented for determining when transient effects are important based on a calculation using steady state nucleation frequencies and macroscopic growth velocities.

Results for the activation energy and kinetics of crystallisation of Fe 80 B 20 metallic glass reported by several authors differ appreciably. Possible sources of systematic variation are identified as: 1. (a) temperature lags in DSC; 2. (b) differences in structural state arising from relaxation; 3. (c) differences in the population of quenched-in nuclei, In a typical set of measurements temperature lags can introduce a systematic error of 15% in activation energies determined by the Kissinger method. Relaxation is known to affect many of the parameters important to nucleation and growth. Its progressive effect on isothermal and constant heating-rate experiments is considered. The applicability of the Arrhenius approximation is assessed. Some variations from batch to batch and through the thickness of a ribbon may be due to differences in quenching-rate leading to different populations of very small crystalline nuclei.

Interdiffusivities are measured in compositionally modulated amorphous thin films as a function of modulation wavelength. The sensitivity of this method is demonstrated by the measurement of very low interdiffusivities, and permits the observation of structural relaxation effects. From these results we are able to determine the general thermodynamic behavior of the alloy system, and to derive the bulk interdiffusivity. Our data suggest that, contrary to earlier reports, the Stokes-Einstein equation (relating diffusivity and viscosity) is not valid in metallic glasses.

Structural relaxation effects in metal-metalloid glasses are reviewed. Data on viscous flow and atomic diffusion are presented, and possible mechanisms of structural relaxation are discussed.

The structural relaxation of ferromagnetic glasses containing about 20at.% metalloid has been studied by using differential scanning calorimetry to measure the Curie temperature (T c ). The kinetics of the T c changes on annealing are shown to depend not only on the annealing temperature, but also on the direction of the change: the change in T c is slower in a structure with a lower fictive temperature. A series of isochronal anneals on FeB, FeNiB, FePB and FeNiPB glasses suggests that, whilst the addition of a second transition metal roughly doubles the rise of T c on annealing at a given T/T x , the metalloids broaden and shift to lower T/T x the temperature range over which relaxation occurs. Care has to be exercised in interpreting the T c changes, since they may be due, not only to structural relaxation, but also to primary crystallisation and phase separation: anomalous results are obtained from FeMoB and FePB glasses. In a FeB glass cold-rolling appears to have no effect on the T c changes induced by subsequent annealing. Interpretations of the T c changes are considered both in terms of specific mechanisms and in terms of glass relaxation in general.