Homogeneous Cu90Co10 alloys were prepared by rapid solidification using the meltspin technique. The decomposition process of this highly supersaturated unstable solid solution was investigated on a nanometer scale using a combination of atom probe field ion microscopy (AP/FIM) analyses and transmission electron microscopy.

Annealing of the Cu90Co10 alloys at 440°C for various times leads to the formation of a compositional modulated microstructure within the grains. The composition profiles determined by AP/FIM analyses clearly exclude a classical nucleation and growth behavior of Co-rich particles. The microstructure is modulated with two different wavelengths. In addition, chemical ordering perpendicular to [111] directions of the fee lattice is observed.

At the grain boundaries of this alloy, heterogeneous nucleation of pure Co particles is observed. This heterogeneous nucleation process competes with the decomposition within the grains.

The Zr terminal portion of the Zr-Ni phase diagram has been evaluated by means of a many-body tight-binding potential. The internal energy curves of the Zr(Ni) solid solutions at T=300 K have been calculated by Molecular Dynamics simulations. These curves exhibit positive values, contrary to former empirical phase diagram calculations. Implications of these results relevant to the problem of amorphization in metallic systems by solid-state reactions are discussed.

We study the influence of pressure on the temperature of order-disorder phase transition in metastable TiSi(B2) phase as well as the pressure dependence of thermodynamic functions describing the stability of this phase. The suggested model outlines the technique to calculate the values of the energy parameters of ordering process from the first principles.

Using a combination of x-ray diffraction and curvature measurements, the stress evolution during solid-state amorphization in a Ni-Hf diffusion couple has been monitored. In contrast to the Co-Hf system, no dissolution of Ni in Hf is observed. During interdiffusion, the growing amorphous layer develops a large tensile stress, which subsequently relaxes by creep. Irradiation of the diffusion couple leads to an increase in tensile stress, and a further increase following a subsequent anneal. Composition measurements by Rutherford backscattering spectrometry indicate absence of an effect of the stress variations on the effective interdiffusion coefficient.

The diffusion properties of a Ni-Zr metallic glass formed at the interface of a bulk diffusion couple have been studied in conditions far from a fully relaxed state. The growth kinetics of the interface film have been enhanced by both plastic deformation and high energy electron irradiation. Different results have been obtained in the two cases, since in the first case the film grows exponentially with time, while in the second case the usual square root dependence on time is observed. This behaviour has been interpreted as a consequence of the annihilation kinetics of the excess free volume introduced in the glass by the above methods. Two different mechanisms of free volume annihilation , namely exchange with a crystal vacancy at the glass-crystal interface and structural relaxation in the bulk glassy phase have been considered to be operative so that the nature of the growth kinetics has been found to depend on the mechanism predominant in each experimental condition.

The mechanism of growth as well as optical and microstructural data dealing with the process of melting of metallic (Pb, Sn) and semiconducting (Ge) particles in dielectric matrix are reported and discussed. Details are given on the growth of such systems based on a self organization method that allows a rather regular shape of the particles (truncated spheres) relative low size dispersion (≤ 20%), wide range of particle size variation and negligible effects due to stress or contamination. The features of these growth techniques allow a careful study of the melting behavior vs. size that has been investigated using optical reflectivity methods and transmission electron microscopy techniques. These experimental results have made possible a careful investigation on the thermodynamics of these systems and the deduction of interesting data concerning: a) conditions for the existence of surface melting and decrease of melting temperature; b) interfacial effects related to shape and curvature of the particles; c) onset of rotations and phase fluctuations in connection with surface melting.

Kinetic modelling, incorporating variation in the degree of chemical order, is developed for the solidification of intermetallic compounds, and qualitative comparisons are made with experiment. It is found that a disordered phase can be obtained other than by the process of disorder trapping, and that partitioning can increase or invert with increasing solidification velocity. Solid-state ordering and its effects on final microstructure are also considered.

In the process of inverse melting, a crystalline solid solution on cooling becomes less energetically stable than the isocompositional undercooled liquid or amorphous phase. Due to this, the crystal may transform polymorphously to the amorphous or liquid phase. A thermodynamic requirement for this process is the presence of two crossing points of the free energy curves G(T) of the crystalline and liquid or amorphous phases. Fulfillment of the thermodynamic requirement is however not a sufficient criterion for inverse melting to occur. In the present work the question posed in the title is addressed from both a thermodynamic as well as a kinetic standpoint. It is shown that in bcc phases which would otherwise undergo inverse melting, the presence of an energetically favorable B2 phase may prevent inverse melting due to its fast ordering kinetics. This imposes an e/a limit to the occurrence of inverse melting.

The formation of nanosized microstructures from controlled devitrification of amorphous alloys is reviewed. The driving force for copious nucleation of crystals is discussed in terms of thermodynamic models for the free energy of crystallization and for the interfacial free energy. After fast nucleation and growth of crystals to nanometric size, there are evidences of a change in diffusion mechanism to slow growth. Examples are provided of primary processes in Fe-Zr-B and Al-Sm with data of XRD, TEM, DSC. Cases of eutectic and polymorphic transformations are also at hand (Ni-P, Cu-Ti-Al, Cu-Ti-Ni, Ti-Zr-Co-Cu-Al) showing that the thermodynamic and kinetic conditions leading to nanostructured materials are not unique.

Finally chemical effects are discussed such as those of impurities in crystallization (e. g. Zr-based alloys), in leaching of Ni-Zr-Al and in deposition reaction of electroless Ni-P.

This paper reports recent atom probe analysis results of Fe-Zr-B(-Cu) and Al-Ni-Ce(-Cu) amorphous alloys, in which nanocrystalline microstructures develop by primary crystallization. In these alloy systems, enrichment of slow diffusing solute was found at the interfaces between primary crystals and amorphous matrix during the nucleation and growth stage. In the case of ternary Fe-Zr-B, no evidence for compositional heterogeneities were found prior to the onset of crystallization reaction. On the other hand, clustering of Cu atoms was observed in quaternary Fe-Zr-B-Cu alloy prior to the crystallization reaction. In the ternary Fe-Zr-B alloy, nucleation sites seem to be provided by the quenched-in nuclei which were observed as medium range ordered (MRO) domains by HREM. In the as-quenched Al-Ni-Ce(-Cu) alloy, compositional fluctuations were present from the as-quenched state. These observations suggest that nuclei for primary crystallization are provided in various forms such as MRO domains, solute clusters and compositional heterogeneities.

The crystallization process of Ti-Al amorphous alloy has been studied by X-ray diffraction, small-angle neutron scattering and high resolution electron microscopy. In the early stage of crystallization of a thick sample with 55at%Al, a metastable hcp(α, α2) phase appeares. For a thin sample with 52at%Al, a new metastable phase having primitive tetragonal symmetry is found in the early stage of crystallization. The lattice parameter of this tetragonal phase is a=c=0.69nm. The prototype structure which has the same atomic arrangement as this phase is only Cu4Pd type structure (P42/m) in the Peason's Handbook.

Thin films of Ti-aluminides have been sputter deposited using a binary γ-TiAl based target and a quaternary γ-TiAl based target containing alloying additions of Nb and Mn. The as-deposited binary film consisted of microcrystalline agglomerates of α-Ti(Al) embedded in an amorphous matrix whereas the as-deposited quaternary film was found to be amorphous. These films have been annealed in a furnace under a protective Ar atmosphere (referred to as ex situ annealing) as well as on a hot stage in a transmission electron microscope (TEM), referred to as in situ annealing, to study the crystallization behavior of these films and also the effect of Nb and Mn additions on the same. Ex situ annealing of both binary and quaternary films resulted in a nanocrystalline microstructure consisting of primarily γ-TiAl with a small amount of finely dispersed α2-Ti3Al. During in situ annealing of the binary film, at 753 K (stage temperature), growth of the α-Ti(Al) phase was observed together with crystallization of a second phase in relatively thicker regions of the specimen. The first crystalline phase to appear during in situ annealing of the quaternary film was a surface crystallized metastable tetragonal phase at 873 K. Prolonged annealing at the same temperature resulted in the transformation of the amorphous region between grains of the tetragonal phase into γ-TiAl. Formation of the tetragonal and intergranular γ-TiAl phases were also observed in the thin regions of the binary film when it was heated to 873 K.

The objective of this work is to fabricate fine grain size NiTi-based shape memory alloys through crystallization of amorphous materials. An amorphous material with a composition of Ti50Ni25Cu25 was used as a sample for this study. First, a systematic investigation was conducted to understand the kinetics of the crystallization process of amorphous materials under an isothermal mode and under a continuous heating mode, respectively. The characteristic parameters associated with the crystallization process, such as the starting and finishing times for the isothermal crystallization process, the peak temperature for the continuous heating crystallization process, and the activation energy of the material were determined. Based on the results of the kinetic study, a series of isothermal annealing experiments were conducted to characterize the microstructure of the fully crystallized materials. The TEM micrographs indicate that the mean grain size of the fully crystallized Ti50Ni25Cu25 alloy is independ of isothermal temperature. The mean grain size is around 400-500nm, which is 1/10 of the grain size of usual NiTi based shape memory alloys. In addition, the crystallization micro-mechanism of the amorphous material is discussed based on TEM observations.

A nanocrystalline material was synthesized by cryogenic ball milling of Metglas Fe78B13Si9 ribbons. The nanocrystalline structure consisted of α-Fe(Si) and Fe2B grains of 2-13 nm in diameter. In the early stages of milling, bending-induced shear bands were formed, followed by the initial stages of crystallization. As the milling proceeded, predominantly wearlike mechanisms caused further crystallization. The crystallization process was slowed considerably by the addition of 17 at.% Ni during cryogenic milling. The results indicate that the 17 at.% Ni did not form a supersaturated solid solution in the metallic glass, but impeded crystallization by reducing the efficiency of bending and wear-like mechanisms during milling.

The liquid-crystalline transformation during continuous cooling was examined for a Zr60Al10Ni10Cu15Pd5 alloy which was ejected into a wedge-shape cavity in a copper mold or solidified on a copper hearth by unidirectional arc melting. The structure consists of a glassy phase in the wedge angles smaller than 10 degrees and changes to glassy and crystalline phases in the higher angle range. The start (Cs) and termination (Ct) points for the transformation were determined and the continuous-cooling-transformation (C. C. T. ) curves were constructed. The nose temperature (Tn) and the time (tn) up to the nose point in the C. C. T. curves were 1018 K and 0.93 s. The critical cooling rate for glass formation defined by (Tm-Tn)/tn is 110 K/s. Furthermore, with the aim of clarifying a solidification condition for formation of a bulk glassy alloy by unidirectional arc melting, the relation between the formation of a glassy phase and the solidification parameters of moving velocity of liquid/solid interface (V), temperature gradient (G) and cooling rale (R) was examined. The glassy phase was obtained in the condition of V>3 mm/s, G>4 K/mm and R>40 K/s. The supercooling reaches 385 K at 40 K/s. The large supercooling ability is presumably due to the formation of a highly dense randomly packed structure where the nucleation of a crystalline phase and the atomic rearrangement for growth reaction are difficult.

After discussing various applications of nanocrystalline solids which are presently under development, this paper describes in more detail the potential use of nanocrystalline materials for the production of sodium chlorate, a paper bleaching agent. The technology and advantages of using nanocrystalline solids as cathode material for this electrochemical process are discussed. The paper concludes by presenting the world market and potential savings associated with the use of this new technology.

Titanium-based icosahedral phases constitute the second largest class of quasicrystals. In contrast with other Ti-based icosahedral phases (i-phases), Ti-Zr-Ni i-phases are well ordered and their formation is inhibited by the presence of Si and O, elements that stabilize the Ti-3d transition metal quasicrystals. We present x-ray and DSC data that suggest that Ti-Zr-Ni i-phases form a different class of titanium-based quasicrystals that are closely related to the MgZn2 Laves phase. The DSC data also suggest that the i-phase may be stable in these alloys. The ability of Ti-Zr-Ni i-phases to absorb up to 62 atomic % of hydrogen is presented and discussed. This opens new avenues of investigation of the structure and dynamics of quasiperiodic phases using elastic and inelastic neutron scattering and nuclear magnetic resonance and may point to potential uses for quasicrystals in hydrogen storage applications.

We have developed a theoretical concept for the design of novel superhard materials and verified it experimentally on several systems nc-MenN/a-Si3N4 (nc-MenN is a nanocrystalline transition metal nitride imbedded in a thin amorphous Si3N4 matrix). Hardness in excess of 5000 kg/mm2 (about 50 GPa) and elastic modulus of ≥550 GPa have been achieved [1-3]. Here we address the questions of the universality of the concept for the design of a variety of nc/a systems and the upper limit of the hardness which may be achieved.

High levels of Mg solid solution in Ti have been achieved by mechanical alloying in a clean atmosphere. The equilibrium solubility of magnesium in titanium is very low and processing of these alloys by conventional routes is not possible due to the high vapour pressure of Mg and alloys of Ti and Mg have only been previously achieved by vapour quenching.

Contamination during milling was avoided by processing under a high purity argon atmosphere in sealed vials. Lattice parameter measurements and SEM observations suggested that complete solid solutions were achieved in alloys up to 24wt% Mg after 48h milling. For one alloy this was confirmed by high resolution TEM which showed extremely fine Ti grains in which Mg was homogeneously distributed with no clustering or precipitation.

Nanocrystalline titanium aluminum powder, of composition Ti-48Al, was synthesized using a Fritsch planetary ball mill with a grain size of approximately 10nm. The powder was compacted into cylindrical specimens with a relative density of 79% and subjected to uniaxial compressive loading at 600°C in a mechanical testing machine. From these results, the deformation of nanocrystalline titanium aluminum is examined with relation to the applied load and the resulting grain structure. The results suggest a grain refining mechanism occurring during the deformation process.