Rapid Resolidification Research Articles

Rapid solidification of Al-Cu eutectic alloy by laser remelting

Rapid surface resolidification using a high powered CO 2-laser has been performed on eutectic Al-32.7 wt% Cu at speeds between 0.2 and 8 m/s. By means of longitudinal cuts through the centre of the laser trace, the local growth rate has been measured by observation of the orientation of the microstructure using transmission electron microscopy. The various microstructures as a function of growth rate, are: 1. (a) regular lamellar eutectic α-Al/θ-Al 2Cu structure for growth rates below 20 cm/s with interlamellar spacing as fine as 17 nm; 2. (b) a new wavy eutectic α-Al/θ'-Al 2Cu morphology for growth rates of between 20 and 50 cm/s; 3. (c) a banded structure formed by alternating supersaturated α-Al solid solution and the wavy eutectic for growth rates greater than 50 cm/s. A recent analytical model for eutectic growth under rapid solidification condition is compared to the experimental results. Contrary to the classical λ 2 V s = const. relationship, which predicts a continuous decrease in spacing as the growth rate increases, this new theoretical model clearly predicts a limit for the coupled eutectic growth which finds its analogy in single phase solidification in the limit of absolute stability.

Solute Trapping at a Rapidly Moving Solid/Liquid Interface for a Lennard-Jones Alloy

ABSTRACTNon-equilibrium Molecular Dynamics simulation methods have been used to study the trapping of “impurities” in an Ag5B15 Lennard-Jones alloy where the B atoms are 10% bigger in diameter than A. The observation of surface melting in this system is used to calculate an equilibrium interfacial segregation coefficient. Simulations of rapid melting and resolidification were performed for the (100) and (111) orientations at two different substrate temperatures (0.65 Tm and 0.95 Tm) for each orientation. Solute impurity atoms are shown to have been trapped at greater concentrations in the solid than under equilibrium conditions. Partitionless solidification is observed when the regrowth velocity greatly exceeds the diffusive velocity.

Laser annealing of silicon

In recent years laser processing has attracted much attention in view of its potential use in basic solid-state and material science research as well as in new processing technologies. The dominant feature of laser processing being the deposition of large amounts of energy (a few J/cm2) over very short time scales (a few tens of nanoseconds), it leads to melting of surface layers of solid followed by rapid resolidification. In this article, a few basic consequences of such laser-induced phenomena in silicon are reviewed.

Microstructural changes taking place during dynamic compaction of aluminium powders

The dynamic compaction of powders is characterized by a complex sequence of pressure loading and shear deformation, adiabatic temperature rise and structural annealing. The relative effects of these factors have been examined by studying the hardness and microstructure of compacted aluminium samples. Particle interiors are considerably shock-hardened, and then partially softened by dislocation recovery. The recovered structures appear to be relatively stable against further softening and recrystallization. The melted zones which occur as pore-filling between powder particles takes place are extremely hard and thermally stable. These zones possess a fine-grained structure after rapid resolidification, contain an oxide dispersion from the oxide films on the original powder surfaces, and are hardened by microvoids from the shrinkage occurring during solidification. The final material structures and properties are therefore variable throughout a given compacted samples, and depend sensitively on many aspects of the material, the powder and the consolidation conditions.

Amorphous Alloys from Microsecond Current Pulses

AbstractWe report the use of microsecond current pulses to produce an amorphous metal alloy. Starting materials consisted of electron beam deposited films of elemental crystalline zirconium and nickel layers with a composition modulation wavelength of 46 nm, a total thickness of about 2.5 μn, and an average composition of Zr38Ni62 deposited onto sapphire substrates. Current pulses had a total ring time of several microseconds and were produced with a low inductance capacitor and a spark gap switch. Integration of current traces showed that 85% of the energy was deposited into the sample within 2 microseconds. The energy in the pulses could be controlled in a range where their effect on the films went from no visible change to total vaporization. Pulses with a peak current in the kiloampere range produced melting in 1 cm wide samples. The cooling rate following this treatment was sufficient to produce amorphous material. We report structural changes resulting from rapid melting and resolidification as well as results of a search for solid state transformations resulting from current pulses below the melting threshold.

Laser surface alloying

Laser alloying is a material-processing method which utilizes the high power density available from focused laser sources to melt metal coatings and a portion of the underlying substrate. Since the melting occurs in a very short time and only at the surface, the bulk of the material remains cool, thus serving as an infinite heat sink. Large temperature gradients exist across the boundary between the melted surface region and the underlying solid substrate. This results in rapid self-quenching and resolidification. Quench rates as great as 1011 KS-l and concomitant resolidification velocities of 20m S-l have already been achieved. What makes laser surface alloying both attractive and interesting is the wide variety of chemical and microstructural states that can be retained because of the rapid quench from the liquid phase. These include chemical profiles where the ‘alloyed’ element is highly concentrated near the atomic surface and decreases in concentration over shallow depths (hundreds of nanometres), and uniform profiles where the concentration is the same throughout the entire melted region. The types of microstructures observed include extended solid solutions, metastable crystalline phases, and metallic glasses.

Laser surface alloying: a bibliography

Laser alloying is a material processing method which utilizes the high power density available from focused laser sources to melt metal coatings and a portion of the underlying substrate. Since the melting occurs in a very short time and only at the surface, the bulk of the material remains cool, thus serving as an intimate heat sink. Large temperature gradients exists across the boundary between the melted surface region and the underlying solid substrate. The result is rapid self-quenching and resolidification. Quench rates as great as 1011 K sec−1 and concomitant resolidification velocities of 20 m sec−1 have already been realized. What makes laser surface alloying both attractive and interesting is the wide variety of chemical and microstructural states that can be retained because of the rapid quench from the liquid phase. These include chemical profiles where the “alloyed” element is highly concentrated near the atomic surface and decreases in concentration over shallow depths (hundreds of nanometers), and uniform profiles where the concentration is the same throughout the entire melted region. The types of microstructures observed include extended solid solutions, metastable crystalline phases, and metallic glasses. This bibliography is a compilation of published work in this area.

Local microstructural modification in dynamically consolidated metal powders

Powders of 4330V steel, aluminum-6 pct silicon, and copper have been dynamically consolidated under well-characterized conditions using shock waves. Different regions in the final microstructures correlate well with the shock conditions during compaction, demonstrating the importance of the shock history in determining the final microstructure. Martensite is observed to form locally at powder particle surfaces in compacts of 4330V steel, and interparticle melting and rapid re-solidification are observed in compacts of aluminum-6 pct silicon. Microprobe analyses of locally melted regions in the aluminum alloy indicate a homogeneous distribution of 6 pct silicon, well above the maximum equilibrium solid solubility. Comparison with the structure of “splat caps,” found in the starting powder, suggests that locally melted regions experience a cooling rate comparable to that obtained in splat quenching. The extent of martensite formation and local melting are in good agreement with current models for energy deposition at powder particle surfaces during consolidation. The general implications of the analysis and observations are discussed.

Pulsed Laser Melting: The Effect of Implanted Solutes on the Resolidification Velocity

ABSTRACTTransient electrical conductance has been used to measure the resolidification velocity in silicon containing implanted solutes. Nonequilibrium segregation of the solutes occurs during the rapid resolidification following pulsed laser melting. The velocity of the liquid-solid interface is observed to depend on the type and concentration of the solute. A 25% reduction in solidification velocity is observed for an implanted indium concentration of three atomic percent. Implanted oxygen is also shown to reduce the solidification velocity. The dependence of the velocity on solute concentration impacts a variety of segregation, trapping and supersaturated solution studies.

Dynamics of melting, evaporation, and resolidification of materials exposed to plasma disruptions

First wall components exposed to a high energy flux during a plasma disruption experience a sequence of processes which consists of rapid heating, melting, intense evaporation, resolidification, and cool-down. The dynamics of all these processes has an impact on both the melt layer stability and the thermal stress cycle in the component of the first wall that stays solid. The detailed time history of the temperature distribution is accurately computed by solving a two-moving-boundary problem. The time behavior of the melt layer thickness for both stainless steel and molybdenum is calculated for different disruption energies and different energy fluxes. The duration of melting, which is an important factor in determining the melt layer stability under different forces, is calculated for both stainless steel and molybdenum. The duration of the melt layer may in fact be shorter for materials with thicker melt layers. The effect of vapor shielding (the stopping of the incoming plasma ions by the vaporized wall material) on the dynamics of melting is also investigated.

Processing/Microstructure Relationships in Surface Melting

ABSTRACTThe development of predictive models for rapid surface melting and resolidification requires coupling of realistic heat flow models to emerging theories of rapid solidification processing. Attainment of unique microstructures and phases,for example through plane-front solidification and solute trapping, can be correlated to solid/liquid interface velocity,temperature and temperature gradients, and to theories of morphological stability. However, there are important limitations on achievable solid/liquid interface velocity depending upon the heating mode and heat flux distribution,melt thickness and location of the interface within the molten zone.An overview is given of the emerging guidelines for prediction and control of rapid solidification conditions and microstructures. Homogenization of the liquid by convection and diffusion is also discussed. Electron beam surface melting of alloy substrates is used as an example of these processes.

Rapid oxidation via adsorption of oxygen in laser-induced amorphous silicon

Amorphous silicon has been produced on a single-crystal silicon surface that was exposed to intense pulsed UV-laser radiation at 266 nm. In addition, the formation of an oxide several tens of nanometers in thickness is observed when the irradiation takes place in an O2 or in an air ambient. Various experimental techniques including transmission electron microscopy, sputtered Auger electron spectroscopy, and differential Fourier-transform IR spectroscopy have been employed to characterize the laser-induced amorphous silicon and the oxide layer formed by this rapid melting and resolidification process. The present study suggests a new oxidation phenomena, namely, ’’laser-induced oxidation.’’

Vacancies in Al after pulsed electron beam melting

We have used transmission electron microscopy (TEM) to study the retention of vacancies in Al after rapid melting and resolidification of a thin (∼ 3 μm) surface layer using a pulsed (∼50 ns) electron beam. After pulsing and aging at room temperature, TEM examination showed dislocation loops, which are interpreted to be due to the coalescence of the quenched-in vacancies on {111} planes as is the case for the loops observed in earlier furnace quenching studies. Our results indicate that the rapid melting and resolidification leaves a high vacancy concentration (∼100 ppm) in the resolidified Al. Heat transport calculations show that cooling rates for the pulse heated samples (∼108 K/s) are much higher than those achieved by conventional quenching techniques (∼ 104 K/s).

A New Electron Beam Polishing Technique for Heteroepitaxial Thin Films

ABSTRACTWe have developed a novel technique for improving the morphology of as-grown monocrystalline thin films on foreign substrates. A pulsed electron beam briefly heats the surface of the film above its melting temperature, allowing surface tension in the liquid to erase irregular features. Crystallinity of the layer is preserved by rapid resolidification with the underlying material acting as a seed. Pulse polishing has been applied to two heteroepitaxial systems exhibiting rough as-deposited surfaces: germanium on silicon and cadmium telluride on mica. In both cases the surface morphology was dramatically improved while crystallinity and other critical properties of the layers were maintained. We believe this technique has potentially wide application to many thin film systems and should allow a greater degree of freedom in the growth conditions for heteroepitaxial thin films.

Nonequilibrium incorporation of impurities during rapid solidification

Laser annealing of silicon in the pulsed, high energy density mode results in melting and rapid epitaxial resolidification. The incorporation of dopant atoms in the resultant crystals in excess of the retrograde solubility maximum provides firm evidence for the occurence of nonequilibrium, impurity trapping processes at the solid-liquid interface during crystal growth.

Phase transformations in laser-irradiated Au-Si thin films

Laser-pulse-induced melting, interdiffusion, and rapid resolidification are applied to deposited Au-Si thin films of various compositions. It is found that, if 30-ns pulses are used, amorphous Au-Si films can be produced over a compositional range 9–91 at.% Au. The stability of the amorphous phases varies with their composition. Thermal decomposition involves the formation of a single-metastable silicide with a hexagonal structrue. Application of 300-μs laser pulses directly leads to formation of the same compound.

Adsorption of Oxygen in Laser-Induced Amorphous Silicon

ABSTRACTAmorphous silicon has been produced on a single crystalline silicon surface by intense UV laser radiation at 266 nm followed by rapid quenching. In addition, formation of oxide of several tens of nanometers has been observed when irradiation takes place in air or in O2 ambient. Various experimental techniques including Transmission Electron Microscopy (TEM), sputtered Auger Electron Spectroscopy (AES), and differential Fourier-transform IR spectroscopy (FT-IR) have been employed to study adsorption of oxygen during rapid melting and resolidification process. The present results suggest a new processing technique for forming thin oxide film, namely, “Laser-induced oxidation.”

Silicide Formation using Laser and Electron Beams

ABSTRACTThe reaction of thin metal films with single crystal or amorphous silicon to form silicide compounds has been successfully undertaken using both scanned-cw and pulsed beam processing. Similar to the case for beam annealing of ion-implanted semiconductors, both the basic mechanisms and the experimental results are found to depend on whether pulsed or scanned beams are used to promote the reactions. For both cases the experimental results are often found to differ sharply from those obtained with standard furnace processing. For the pulsed beam reaction, melting and rapid resolidification occurs in the metal silicon films. The principal result is that the reacted film is frozen in a mixed phase compound which can consist of several silicide compounds including silicon and metal precipitation. Compositions inaccessible by conventional furnace annealing can also be formed by this process. In contrast to the pulsed beam results, scanned-cw beam processing of metal silicon films is found to result in the formation of large area uniform silicide phases with essentially no silicon or metal precipitation. These films are generally found to be homogeneous with well defined silicon/silicide interfaces.

Laser-induced reactions of platinum and other metal films with silicon

We have reacted thin Pt, Pd, and Ni films with single-crystal Si using Q-switched Nd : YAG laser pulses of 100-nsec duration in the power range 18–50 MW cm−2. The layers are laterally very uniform in thickness but are not single phase. Average composition of the reaction product layer can be changed over a wide range by varying film thickness and laser power. The microstructure of the reacted layers indicates that reaction occurs via surface melting, mixing, and rapid resolidification.