Faceted Dendrites Research Articles

Microstructural characterization and microscopy analysis of laser cladding Stellite12 and tungsten carbide

Stellite12 cobalt base alloys with different WC content were deposited on SK3-carbon tool steel by laser cladding. The behavior of WC particulates, including the dissolution, distribution, and microstructures of WC–Co–Cr–C composite coatings with rapidly solidifying were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Several significantly different solidified microstructures were characterized by dendrites, interdendritic eutectics, faceted dendrites (third phase) and the retaining of WC particles in the laser clads of Stellite12 + WC, under different laser energy densities. When WC was completely melted and fully dissolved into the Stellite12 melt pool, the basic solidification, characterized by the matrix and faceted dendrites (third phase) in various shapes, and the compositional evolution of the clad layers remained nearly identical, no matter the added WC was increased to 10%, 20% or 40% (wt.%). The faceted dendrites (third phase) contained the majority of W as well as some Cr, Co while more Cr and Co were located in the matrix. The X-ray diffraction analyses indicated the existence of σ-Co, M 23C 6, M 6C and M 7C 3 (M = W, Cr, Co) in the Stellite12 alloys with different WC contents when deposited on substrates by laser cladding.

Two-dimensional facet crystal growth of silicon from undercooled melt of Si–Ni alloy

The two-dimensional facet crystal growth of silicon from the undercooled melt of silicon–nickel alloys has been investigated using a phase-field model. The phase-field parameters derived at the thin interface limit and the anisotropic interface energy model proposed by Eggleston et al. have been used in the simulation. The calculated dendrite growth velocity depends on the interface width and the correct values are obtained when the interfacial Péclet number is sufficiently small. The growth velocity follows a power law relation to undercooling and the exponents are approximately two for Si–10 wt.% Ni alloy and approximately three for Si–20 wt.% Ni alloy. A dendrite grows maintaining its tip shape and a scaling law between the tip size of a dendrite and the growth velocity is confirmed. The phase-field simulations have been compared with the experimental data on the facet dendrite growth velocity in a thin liquid film of Si–6 wt.% Ni alloy and both are acceptable in agreement.

Laser cladding for wear‐resistant cobalt base alloy and tungsten carbide composite coatings

Stellite12 cobalt base alloys with different WC contents were deposited on SK3‐carbon tool steel by laser cladding. The behavior of WC particulates, including dissolution and distribution, and the microstructure of WC‐Co‐Cr‐C composite coatings with rapid solidification were investigated. Several significantly different solidified microstructures were characterized by dendrites, eutectics, faceted dendrites and the retained WC particles in the laser cladding WC + Stellite12 coatings, under different laser energy densities. When WC was melted and dissolved into the Stellite12 melt pool, the basic structure of solidification, characterized by the matrix and faceted dendrites in various shapes, and the contents of the faceted dendrites remained nearly identical. The faceted dendrites contained the majority of W as well as some Cr, Co while more Cr and Co were located in the matrix. The X‐ray diffraction analyses indicated the existence of σ‐Co, M23C6, M6C and M7C3 (M = W, Cr, Co) in the Stellite12 with different WC contents when deposited on substrates by laser cladding. The faceted dendrites provided the coatings with excellent resistance during dry sliding wear test. A higher content of WC gave higher volume fractions of faceted dendrites that imparted excellent wear resistance to the coating.

Faceted dendrite growth of silicon from undercooled melt of Si–Ni alloy

Two-dimensional faceted dendrite growth of silicon from undercooled melt of Si–6 wt%Ni alloy was experimentally investigated, in which molten alloy film from 10 to 20 mm in thickness was undercooled up to 115 K and growing dendrites were observed in situ. Both the in situ observation of dendrite morphology and the EBSP crystallographic analysis for solidified samples showed that both a ‹211› twin dendrite and a ‹100› twin-free dendrite grew in the range of undercooling from50 to 115 K. Dendrite growth velocity was also measured for different undercooling conditions. The growth velocity of ‹211› dendrites was slightly larger than that of ‹100› dendrites. It is concluded that the upper envelope of the data provide the correct dendrite growth velocity and it is compared with that obtained by phase-field simulations. Growth velocity in both follows power relationships to undercooling and the linear kinetic coefficient is estimated to be 0.01 m/s K.

Fragmentation of faceted dendrite in solidification of undercooled B-doped Si melts

The fragmentation of the faceted dendrite of B-doped Si solidified from the undercooled melt was investigated using an electromagnetic levitator. The 〈110〉 dendrites, which grew at ΔT ∼100 K, showing fourfold axial symmetry, broke up into small pieces at undercoolings of more than 200 K. It was suggested that the capillary force acts on the interface with a relatively high energy to break up the dendrite into small pieces, since the 〈100〉 dendrites are composed of {110} and {100} planes with interface energies larger than that of the {111} plane. Moreover, striations of concentric circles formed by the segregation of B revealed that the remaining melt solidifies from the surface toward the center to engulf the fragmented dendrites. This solidification process seems different from those of typical metallic materials, in which the fragmented dendrites are randomly distributed throughout the sample and the remaining liquid solidifies from the fragmented dendrites. This solidification characteristic was discussed in relation to the influence of electromagnetic force on the microstructure of Si.

Morphologies of silicon crystals solidified on a chill plate

Electromagnetically levitated liquid droplets of pure Si or a Si-Ge alloy were cooled to different temperatures and then dropped onto a chill plate of Cu. Droplet oscillations mark the solid/liquid interface during solidification and permit the different crystal morphologies of silicon to be observed on the quenched surface by scanning electron microscopy (SEM). A spherical morphology found on the quenched surface represents the initial stage of crystal growth. Further growth leads to octahedral crystals bounded by {111} faces near equilibrium and to other polyhedra or even faceted dendrites further from equilibrium. The spherical growth can be observed only when the initial melt undercooling is moderately high. The critical size at which spherical crystals start to develop dendritic growth is much bigger than that calculated from the Mullins and Sekerka model, and is bigger than the Coriell and Parker model when kinetic undercooling is taken into account.

Observation of rapid solidification of deeply undercooled Si melts using electrostatic levitation

Pure Si samples with diameters of about 1.7 mm were undercooled down to 317 K while electrostatically levitated. The crystal growth velocities were measured as a function of undercooling and the appearance of the solid/liquid interface was observed by means of a high-speed camera. The results were compared with those reported previously for larger Si droplets using an electromagnetic levitator. In this study, nucleation was triggered at a given undercooling using needle darting and particle interspersing. As a new finding, the transition of growth mechanism from a single plate to multi-plate crystal was observed at the undercooling of 80 K. Transitions from plate-like crystal to coarse faceted dendrite and from coarse to fine faceted dendrite hardly depended on the sample size.

Undercooling and solidification of Al-50 at. pct Si Alloy by electromagnetic levitation

Electromagnetic levitation is applied to achieve containerless solidification of 10-mm-diameter droplets of Al-50 at. pct Si. A maximum undercooling of 320 K is obtained. Phase morphologies on the droplet surfaces and on the deeply etched sections of the samples solidified at different undercoolings are examined by scanning electron microscopy. The primary silicon shows well-developed faceted dendrites at a small undercooling, but a fine granular form at a large undercooling. Stratified deposits of aluminum are found within the primary silicon plates, arising from solute pileup during growth. The microstructural refinement at a large undercooling has its origins in solute restriction of crystal growth and in fragmentation of the primary silicon dendrites. The form of the Al-Si eutectic is also found to be changed into an anomalous form at a large undercooling.

Novel criterion for splitting of plate-like crystal growing in undercooled silicon melts

An undercooled drop of silicon was grown to crystals in containerless states with electromagnetic and electrostatic levitators. A high-speed video camera was used to monitor the growth rate and observe the crystal–melt interface as a function of undercooling. The morphology of the growing crystal changed from a mono-plate crystal to a multi-plate crystal, and then to faceted dendrite with increasing undercooling. The mono-plate and multi-plate crystals observed at undercooling of less than 100 K were shaped by a faceted planer interface and wavy-edge plane. The critical undercooling for the transition from mono-plate to multi-plate depends on the sample size; it was 50 and 80 K for samples of 5 and 1.7 mm in diameter, respectively. A novel criterion for the transition from mono-plate to multi-plate based on the instability of the wavy-edge plane is proposed.

Phase-field modeling for facet dendrite growth of silicon

Dendrite growth of silicon from its undercooled melt was investigated by using the phase-field model for a faceted crystal with anisotropic interfacial energy. The phase-field parameters at the thin interface limit were derived and used in the simulation. The accuracy of the model was estimated from the calculated equilibrium interface shape. The errors in anisotropy and Gibbs-Thomson coefficient were within 1% and 10%, respectively. The growth of a silicon crystal from its undercooled melt has been analyzed and it is shown that the shape of growing crystal changes from square-like to dendritic with increase of undercooling. In a facet dendrite growth the tip grows keeping its shape and the shape is the same regardless of undercooling or growth velocity. It is also shown that there exists the scaling law between the characteristic length of the tip and growth velocity similar to that of a non-facet dendrite.

Microstructure formation and phase selection in the solidification of Al2O3–5 at% SiO2 melts by splat quenching

An Al2O3–5 at% SiO2 specimen was levitated in an Aero-Acoustic Levitation apparatus and then melted when a continuous-wave CO2 laser beam heating system was incorporated. The sample can be highly undercooled when decreasing the laser power. Rapid solidification by splat quenching can be realized at defined temperatures, using well-polished copper as chilling anvils. Microstructure transition from nonfaceted colony to strong faceted dendrites was observed when the melt was quenched at ΔT = 50 K, indicating that a kinetic contribution for roughening the microstructure may be significant for the morphology transition. The impacting, spreading, and solidifying processes were analyzed on the basis of microstructure observation. The additional undercooling was suggested to vary per an exponential relation with distance when the kinetic effect was taken into account. The nucleation behavior was also discussed according to the proposed additional undercoolings to demonstrate the difference in nucleation population at various regions. When the melt undercooling increases to 190 K, a double-phase structure with small polycrystalline inclusion embedded into amorphous matrix was obtained. The continuous cooling transformation profile was proposed to account for the phase selection upon quenching. The present observation and suggested model for acquiring high additional undercoolings are useful in elucidating the work of others.

Containerless solidification of highly undercooled mullite melts: Crystal growth behavior and microstructure formation

A mullite (3Al2O3·2SiO2) sample has been levitated and undercooled in an aero-acoustic levitator, so as to investigate the solidification behavior in a containerless condition. Crystal-growth velocities are measured as a function of melt undercoolings, which increase slowly with melt undercoolings up to 380 K and then increase quickly when undercoolings exceed 400 K. In order to elucidate the crystal growth and solidification behavior, the relationship of melt viscosities as a function of melt undercoolings is established on the basis of the fact that molten mullite melts are fragile, from which the atomic diffusivity is calculated via the Einstein-Stokes equation. The interface kinetics is analyzed when considering atomic diffusivities. The crystal-growth velocity vs melt undercooling is calculated based on the classical rate theory. Interestingly, two different microstructures are observed; one exhibits a straight, faceted rod without any branching with melt undercoolings up to 400 K, and the other is a feathery faceted dendrite when undercoolings exceed 400 K. The formation of these morphologies is discussed, taking into account the contributions of constitutional and kinetic undercoolings at different bulk undercoolings.

Microstructural evolution in high power laser cladding of Stellite 6+WC layers

The microstructural evolution in high power CO 2 laser cladding of steel with Stellite 6+WC powder mixture was investigated, among which the WC vol.% was, respectively, 0, 9, 18, 27, 36, 45, 54, 72 and 100% by dual powder feeding Stellite 6 and WC powders with different feeding rates. Two significantly different solidification characteristics were found in the microstructural evolution in laser clads of Stellite 6+WC. The first one was characterized by the dendrites and interdendritic eutectics from 0 to 36% WC, in which the added WC was completely melted into the melt pool and the re-solidified structure contained α-Co,σ-CoCr and various carbides such as M 7C 3. The second one was characterized by various faceted dendrites in block, flower, butterfly, star shapes and the matrix from 45 to 100% WC, in which most of the WC was melted, the microstructure contained re-solidified WC, Co and various Co–W–C/Fe–W–C complex carbides. Maximum 26 and 64 wt.% W were, respectively, dissolved into the matrix and the faceted dendrites. When laser cladding WC powder with severe dilution from the substrate, a microstructure with different composition appeared within the faceted dendrites with interior approximately 90 wt.% of W and exterior approximately 70 wt.% of W. Such a structure was probably the frozen result of the peritectic reaction during laser cladding rapid solidification.

Containerless crystallization of silicon

Crystallization from undercooled melt of silicon was carried out by means of electro-magnetic levitation method under controlled undercooling. The measured growth rate vs. undercooling was categorized into three regions, I, II and III, respectively, from the point of the interface morphology. Thin plate crystals whose interface consisted of both faceted (1 1 1) plane and wavy edge plane like saw-tooth were observed in the region I where the undercooling is less than 100 K. The growth rate of the wavy edge plane was well described by the dendrite growth model. The morphology of growing crystals was abruptly changed to faceted dendrite in the region II, though there was no abrupt change in the growth rate. Seeding at temperatures in the region I changes the drop to a mono-crystalline sphere, if the growth rate along the normal direction of the thin plate crystal is controlled by step-wise growth on the faceted plane. Actually, the sample of 5 mm in diameter seeded at undercooling of 26 K was a quasi-single crystal with large grain, except for a small area where twinning and cracking are observed. The result suggests that the single crystal could be grown, if a smaller sample, 1 or 2 mm in diameter, that is difficult to be levitated by electro-magnetic force were processed with other methods such as free fall in a drop tube.

High-power laser cladding Stellite 6+WC with various volume rates

Laser cladding of Stellite+WC with WC volume percentages of 0%, 9%, 18%, 27%, 36%, 45%, 54%, 72%, and 100% was investigated by dual powder feeding. The results demonstrated that no cracking happened until the WC volume percentage was over 18%. The average hardness and cracking rate increased while the height/width rate decreased with increased WC volume percentage. The microstructure evolution can be divided into two domains. The first domain is from 0% to 36% WC, in which the added WC was completely melted and dissolved into the melt pool and the resolidified structure dominated by Co-rich dendrites and interdendrite eutectic containing 9.7 and 21 wt % W, respectively. The second domain is for the WC volume percentage from 45% to 100%, in which most of the WC was melted, and the microstructure was dominated by the matrix and the various faceted dendrites. The dissolution of W into the matrix was limited to a maximum of about 26 wt % and reached 64.6 wt % in the faceted dendrites.

Stellite bearings by conventional PM and mechanical alloying

The microstructural evolution in high power CO2 laser cladding of steel with Stellite 6+WC powder mixture was investigated, among which the WC vol.% was, respectively, 0, 9, 18, 27, 36, 45, 54, 72 and 100% by dual powder feeding Stellite 6 and WC powders with different feeding rates. Two significantly different solidification characteristics were found in the microstructural evolution in laser clads of Stellite 6+WC. The first one was characterized by the dendrites and interdendritic eutectics from 0 to 36% WC, in which the added WC was completely melted into the melt pool and the re-solidified structure contained α-Co,σ-CoCr and various carbides such as M7C3. The second one was characterized by various faceted dendrites in block, flower, butterfly, star shapes and the matrix from 45 to 100% WC, in which most of the WC was melted, the microstructure contained re-solidified WC, Co and various Co–W–C/Fe–W–C complex carbides. Maximum 26 and 64 wt.% W were, respectively, dissolved into the matrix and the faceted dendrites. When laser cladding WC powder with severe dilution from the substrate, a microstructure with different composition appeared within the faceted dendrites with interior approximately 90 wt.% of W and exterior approximately 70 wt.% of W. Such a structure was probably the frozen result of the peritectic reaction during laser cladding rapid solidification.

The Growth Morphology and Nucleation Mechanism of Primary Ll<sub>2</sub> Al<sub>3</sub>Sc Particles in Al-Sc Alloys

The role of the nucleation mechanism and growth morphology of primary Ll 2 , Al 3 Sc, particles in the grain refinement of Al castings has been investigated using a simple hypereutectic, binary Al-Sc alloy. New evidence suggests a two-stage grain refining mechanism: the primary Al3Sc particles first nucleate on impurities in the melt (possibly Sc 2 O 3 ) before nucleating the α-Al grains. The primary particles grow as faceted dendrites that maintain an overall cubic morphology. High resolution EBSD analysis has shown that α-Al nucleates from Al 3 Sc particles with either an epitaxial, or a twin relationship below the eutectic temperature. α-Al then grows outwards as a single phase, pushing particles that did not act as nucleation centres to the grain boundaries. Some α-Al + Al 3 Sc eutectic is formed at the grain boundaries during the last stages of freezing.

The growth and characteristics of CVD SiCTiC in-situ composites

SiCTiC in-situ composites have been synthesized by low pressure chemical vapour deposition on graphite with SiCl 4, TiCl 4 C 3H 8 and H 2 reaction gases to improve the toughness of SiCTiC ceramics. The composite was deposited at various temperatures (1500~1600 °C), total pressure (40~300 torr) and reactant concentrations. The microstructure was investigated by scanning electron microscopy, optical microscopy and transmission electron microscopy. The morphologies of facet structure, nodular structure and dendrite structure appear depending on the experimental conditions. A dense SiCTiC deposit without porosity was obtained with a maximum growth rate of 1.6mm h −1 at C 3H 8 = 25 cm 3 min −1 and 200 torr. Only β-SiC and TiC phases have been identified in the composite with a dramatic change of the composition at the interface of SiC and TiC grains. The fracture toughness (K Ic) determined by the indentation method exhibits a value as high as 5.9 MPa m 1 2 that could be obtained by deposition at low pressure and low C 3H 8 concentration. Cracks propagate with more deflection in the SiCTiC composites than in monolithic SiC. The result is due to existing severe strain at the SiCTiC interface observed by transmission electron microscopy.

Faceted needle crystals: an analytical approach Mokhtar

Faceted dendrites bave been observed in some crystal growtb expenrnents. It bas previously been proposed that these diffusion-lirnited growing shapes obey the classical equations of dendritic growth with modified boundary conditions on trie interface. We analyse finis new set of equations when capillary elfects are neglected on the rougir parts. In this hmit, we find that it is non possible to require both a tangential rnatching of the rougir and faceted parts and tbe same melting temperature on trie front and trailing rougir interface. An exact solution is obtained when one of these physical constraints is relaxed. The consequences of this result for the fuit problem are considered and an approximate solution is proposed.

Growth and morphology of quasicrystals

Abstract The equilibrium shape problem and roughening transition in quasicrystals have been reviewed. Preliminary results on the edge rounding exponent in the binary Al-Mn system indicate a higher value than expected from current theory. It is pointed out that unstable quasicrystalline growth produces faceted dendrites, since growth always occur below the roughening transition temperature. The kinetic mechanism of growth is step controlled. Contrary to the case of growth along periodic potentials, where steps are of the dimension of the lattice parameter, growth of quasicrystals along the quasiperiodic growth directions can have large steps. This is borne out by experiments on the Al-Mn decagonal phase.