Dislocation mechanism of Ni_(47)Co_(53) alloy during rapid solidification

Dislocations and other atomic-level defects play a crucial role in determining the macroscopic properties of crystalline materials,but it is extremely difficult to observe the evolution of dislocations due to the limitations of the most advanced experimental techniques.Therefore,in this work,the rapid solidification processes of Ni_(47)Co_(53) alloy at five cooling rates are studied by molecular dynamics simulation,and the evolutions of their microstructures and dislocations are investigated as well.The results show that face-centered cubic(FCC) structures are formed at the low cooling rate,and the crystalline and amorphous mixture appear at the critical cooling rate,and the amorphous are generated at the high cooling rate.The crystallization temperature and crystallinity decrease with cooling rate increasing.Dislocations are few at the cooling rates of 1×10^(11) K/s,5×10^(12) K/s,and 1×10^(13) K/s,and they are most abundant at the cooling rates of 5×10^(11) K/s and1 × 10^(12) K/s,in which their dislocation line lengths are both almost identical.There appear a large number of dislocation reactions at both cooling rates,in which the interconversion between perfect and partial dislocations is primary.The dislocation reactions are more intense at the cooling rate of 5×10^(11) K/s,and the slip of some dislocations leads to the interconversion between FCC structure and hexagonal close packed(HCP) structure,which causes the twin boundaries(TBs) to disappear.The FCC and HCP are in the same atomic layer,and dislocations are formed at the junction due to the existence of TBs at the cooling rate of 1 ×10^(12) K/s.The present research is important in understanding the dislocation mechanism and its influence on crystal structure at atomic scales.

Energy-storage Welding Connection Characteristics of Rapid Solidification AZ91D Mg Alloy Ribbons

Energy-storage welding connection characteristics of rapidly solidified AZ91D Mg alloy ribbons with 40-70 μm thickness are investigated using a microtype energy-storage welding machine. The microstructure and performance of the connection joints are analyzed and studied. The research results indicate that energy-storage welding is able to realize the spot welding connection of AZ9ID Mg alloy ribbons. The welding nugget consists of developed α-Mg equiaxed grains with the sizes of 1.2-2.7 μm and intergranular distributed β-Mg17Al12 compounds. The thickness of bond zone is about 4 μm and the solidification microstructure is characterized by the fine equiaxed grains with the sizes of 0.8-1.2μm and grain boundary has become coarsening. The columnar crystal in HAZ also becomes slightly coarsening and the grain boundary has broadened, however, there is no obvious change in its primitive morphology and crystallographic direction. When welding energy is about 2.0 J, the welding joints with higher shear strength and smaller electrical resistivity are obtained.

MICROSTRUCTURE OF Mg-6.4Zn-1.1Y ALLOY FABRICATED BY RAPID SOLIDIFICATION AND RECIPROCATING EXTRUSION

In order to explore the methods to prepare high-strength quasicrystal-reinforced magnesium alloys, the flakes of rapidly solidified Mg-6.4Zn-1.1 Y magnesium alloy with a thickness of 50-60μm were obtained by a melt spinning single-roller device, and the flakes were then processed into rods by reciprocating extrusion and direct extrusion. The microstructure of the alloy was analyzed by optical microscope and SEM, and the constituent phases were identified by XRD. Phase transformation and its onset temperature were determined by differential thermal analyzer （DTA）. The analysis result shows that rapid solidification for Mg-6.4Zn-I.IY alloy can inhibit the eutectic reactions, broaden the solid solubility of Zn in α-Mg solute solution, and impede the formation of Mg3 Y2Zn3 and MgZn2 compounds, and thus help the icosahedral Mg3 YZn6 quasicrystal formed directly from the melt. The microstructure of the flakes consists of the α-Mg solid solution and icosahedral Mg3 YZn6 quasicrystal. Dense rods can be made from the flakes by two-pass reciprocating extrusion and direct extrusion. The interfaces between flakes in the rods can be welded and jointed perfectly. During the reciprocating extrusion and direct extrusion process, more Mg3 YZn6 compounds are precipitated and distributed uniformly, whereas the rods possess fine microstructures inherited from rapidly solidified flakes. The rods contain only two phases： α- magnesium solid solution as matrix and fine icosahedral Mg3 YZn6 quasicrystal which disperses uniformly in the matrix.

Texture Evolutions in Fe-6.5%Si Produced by Rapid Solidification and Chemical Vapor Deposition

Fe-Si ribbons and thin sheets with 6.5%Si content were prepared by means of the single roller rapid solidification and chemical vapor deposition (CVD), respectively. The initial textures of rapidly solidified Fe-6.5%Si ribbons were characteristic of the {100} fiber-type, which became weakened during primary recrystallization in various atmospheres. At the stage of secondary recrystallization, the {100} texture formed in Ar and the {110} texture in hydrogen, while there occurred a texture transformation from the {100} type to the {110} type in vacuum with the increase of annealing temperature. For Fe-6.5%Si sheets prepared by Si deposition in cold-rolled Fe-3%Si matrix sheets, their textures were dominated by the η-fiber (<001>//RD) with the maximum density at the {120}<001> orientations. After homogenization annealing, the η-fiber could evolve into the {130}<001> type or become more concentrated on the {120}<001> orientations, depending on the cold rolling modes of Fe-3%Si matrix sheets.

Microstructure and Mechanical Property of 2024 Aluminium Alloy Prepared by Rapid Solidification and Mechanical Milling

Rapidly solidified 2024 aluminium alloy powders were mechanically milled, then consolidated to bulk form. The microstructural changes of the powders in mechanical milling (MM) and consolidation process were characterized by X-ray diffraction analyses and transmission electron microscopy observations. The results showed that mechanical milling reduced the grain size to nanometer, dissolved the Al2Cu intermetallic compound into the aluminium matrix and produced an aluminium supersaturated solid solution. During consolidation process. the grain size increased to submicrometer, and the Al2Cu and Al2(Cu, Mg, Si, Fe, Mn) compounds precipitated owing to heating. Increasing consolidation temperature and time results in obvious grain growth and coarsening of second phase particles. The tensile yield strength of the consolidated alloy with submicrometer size grains increases with decreasing grain size, and it follows the famous HallPetch relation

Investigation of Intermetallic Compound Formed from Rapid Solidification of Al-Ti-RE Alloy

Al-Ti alloy containing rare earth elements can produce fine, uniform dispersion intermetallic phase through rapid solidification (RS) technology. RS Al-Ti-RE alloy can be designed for applications at elevated-temperature since the intermetallic compound has good thermal stability. A transmission electron microscopy investigation shows the intermetallic phase has a diamond cubic structure (a=1.47736 nm), with space group Fd3m. The chemical stoichiometry is Al_(20)Ti_2La. The particle is formed from the melting directly, prior to other phases, and the nucleus is formed from icosahedrons composed with twenty tetrahedrons. Twin crystal structure plays an important role in the nucleation stage.

Effects of rapid solidification on the microstructure and microhardness of a lead-free Sn-3.5Ag solder

A lead-free Sn-3.5Ag solder was prepared by rapid solidification technology. The high solidification rate, obtained by rapid cooling, promotes nucleation, and suppresses the growth of Ag3Sn intermetallic compounds （IMCs） in Ag-rich zone, yielding fine Ag3Sn nanoparticulates with spherical morphology in the matrix of the solder. The large amount of tough homogeneously-dispersed IMCs helps to improve the surface area per unit volume and obstructs the dislocation lines passing through the solder, which fits with the dispersion-strengthening theory. Hence, the rapidly-solidified Sn-3.5Ag solder exhibits a higher rnicrohardness when compared with a slowly-solidified Sn-3.5Ag solder.

Microstructural evolution of Al-20Si-5Fe alloy during rapid solidification and hot consolidation

Al-20Si-5Fe melt was rapidly solidified into particles and ribbons and then consolidated to near full density by hot pressing at 400℃/250 MPa/1 h. According to the eutectic-growth and dendritic-growth velocity models, the solidification front velocity and the amount of undercooling were estimated for the particles with different sizes. Values of 0.43-1.2 cm/s and 15-28 K were obtained. The secondary dendrite arm spacing revealed a cooling rate of 6 × 10^5 K/s for the particles with an average size of 20 μm. Solidification models for the ribbons yielded a cooling rate of 5 × 10^7 K/s. As a result of the higher cooling rate, the melt-spun ribbons exhibited considerable microstructural refinement and modification. The size of the primary silicon decreased from approximately 1μm to 30 nm while the formation of iron-containing intermetallic compounds was suppressed. Supersaturation of the aluminum matrix in an amount of-7 at.% Si was noticed from the XRD patterns During the hot consolidation process, coarsening of the primary silicon particles and precipitation of β-Al5FeSi phase were observed. Evaluation of the compressive strength and hardness of the alloy indicated an improvement in mechanical properties due to the microstructural modification.

Metastable phase separation and rapid solidification of undercooled Co_(40)Fe_(40)Cu_(20) alloy

The metastable liquid phase separation and rapid solidification behaviors of Co_（40） Fe_（40） Cu_（20） alloy were investigated by using differential thermal analysis（DTA） in combination with glass fluxing and electromagnetic levitation（EML） techniques. The critical liquid phase separation undercooling for this alloy was determined by DTA to be 174 K. Macrosegregation morphologies are formed in the bulk samples processed by both DTA and EML. It is revealed that undercooling level, cooling rate, convection, and surface tension difference between the two separated phases play a dominant role in the coalescence and segregation of the separated phases. The growth velocity of the（Fe,Co） dendrite has been measured as a function of undercooling up to 275 K. The temperature rise resulting from recalescence increases linearly with the increase of undercooling because of the enhancement of recalescence. The slope change of the recalescence temperature rise versus undercooling at the critical undercooling also implies the occurrence of liquid demixing.

NEAR RAPID DIRECTIONAL SOLIDIFICATION AND ITS SUPERFINE MICROSTRUCTURE

This paper briefly reviews the recent research on the near rapid directional solidification and microstructure superfining. The morphology transitions and the corresponding mechanical properties are presented. The critical velocities relevant to the morphology transitions are comprehensively given. Meanwhile the solidification characteristics near absolute stability limit are studied.It can be clearly seen that the superfine microstructures possess extremely better properties compared with the conventional microstructures.

Solute distribution in KNbO 3 melt-solution and its effect on dendrite growth during rapid solidification

This paper reports that the rapid solidification of mixed Li2B4O7 and KNbO3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions： in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.

Recalescence Behavior, Solidification Characteristics and Microstructure Transformation of Rapidly Solidified Undercooled Cu-based Alloys

The undercooled solidification microstructures of Cu55Ni45,Cu55Ni43Co2,and Cu60Ni38Co2 Cu-base alloys were obtained by fluxing method.Using infrared temperature measuring device,the law of the change of the recalescence degree with the increase of the undercooling during rapid solidification was studied.At the same time,high-speed camera was used to capture and photograph the images of solid/liquid interface migration during rapid solidification of undercooled melt,and the morphology evolution of solidification front was discussed.Finally,the microstructure morphology and transformation process of the Cubased alloys were systematically analyzed.It is found that the microstructure morphology of the alloys goes through the same evolution process and appeared two grain refinement phenomena,that is,“coarse dendrite-equiaxed grain-oriented fine dendrite-equiaxed grain”.But its characteristics undercoolingΔT_(1),ΔT_(2),and critical undercoolingΔT^(*)varies.Electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM)were used to characterize the grain refinement structure with high undercooling.EBSD results show that the grain refinement structure with high undercooling presents a very high proportion of high angle grain boundaries,the grain orientation is random and there is no high strength texture,and a large number of annealing twins,which indicates that recrystallization occurs in the structure.TEM results show that dislocation network and stacking fault density are relatively low in most areas of grain refinement structure with high undercooling,which can confirm the theory that stress induces recrystallization of the structure.

Oxygen ion conductivity of La_(0.8)Sr_(0.2)Ga_(0.83)Mg_(0.17-x)Co_xO_(3-δ) synthesized by laser rapid solidification

Materials Lao.8Sro.2Gao.83Mgo.17_xCox03_6 with x = 0, 0.05, 0.085, 0.10, and 0.15 are synthesized by laser rapid solidification. It is shown that the samples prepared by laser rapid solidification give rise to unique spear-like or leaf-like microstructures which are orderly arranged and densely packed. Their electrical properties each show a general depen dence of the Co content and the total conductivities of Lao.8Sro.2Gao.83Mgo.085Coo.08503_6 prepared by laser rapid solidification are measured to be 0.067, 0.124, and 0.202 S.cm-1 at 600, 700, and 800 ℃, respectively, which are much higher than by conventional solid state reactions. Moreover, the electrical conductivities each as a function of the oxy gen partial pressure are also measured. It is shown that the samples with the Co content values 〈 8.5 mol% each exhibit basically ionic conduction while those for Co content values 〉 10 mol % each show ionic mixed electronic conduction under oxygen partial pressures from 10-16 atm （1 atm = 1.01325 x 105 Pa） to 0.98 atm. The improved ionic conductivity of Lao.sSro.2Gao.83Mgo.085Coo.08503 prepared by laser rapid solidification compared with by solid state reactions is attributed to the unique microstructure of the sample generated during laser rapid solidification.

RAPID SOLIDIFICATION OF HYPEREUTECTIC SILVER-COPPERALLOY IN DROP TUBE

The Ag-50wt%Cu hypereutectic alloy was solidified and investigated in a 3 m drop tube, and its nucleation and microstructure characteristics were explored also. The results showed that (1) with the decrease of droplet size, the structural morphology transforms from the dendrites with long trunk and thin short secondary arms to fine dendritic structures; (2) the dendritic growth is mainly controlled by solute diffusion within the whole undercooling regime, while the influence of thermal diffusion-controlled growth becomes stronger above a critical undercooling about 268 K; (3) the refinement of dendritic structure is caused by large amounts of nuclei within droplet; (4) when droplet diameter is smaller than 300 micron, the undercooling and growth velocity were estimated to be greater than 268 K and 290 mm/s respectively, while the solute trapping effect may occur.

Solute Redistribution Model for Multicomponent AHoys during Rapid Solidification

1.IntroductionThe solute redistribution models for binary alloys during the rapid solidification havebeen extensively studied in recent years[1-10],but up to now the solute redistribution modelfor multicomponent alloys has not been reported.In this paper the solute redistribution mod-el for the multicomponent alloys based on the Aziz model is established theoretically.

EUTECTIC GROWTH OF CAST IRON UNDER RAPID SOLIDIFICATION

The eutectic growth of cast iron during rapid solidification has been studied with laser remelt- ing and deep etching technology.A new mode of mushroom eutectic growth was observed oth- er than the usual honeycomb and lamellar ones.The mushroom eutectic growth may finally form the lamellar dendritic structure.

Flow field simulation of the thin metal ribbon formed by twin-belt rapid solidification

The flow field formed when an aluminum ribbon is produced by twin-belt rapid solidification is simulated with the solution algorithm （SOLA）-volume of fluid （VOF） method, and the influence of casting speed and cooling-belt velocity on the flow field is analyzed for a given ejection nozzle size. It has been found that the flow field can be filled well when the width of the ejection nozzle outlet is the same as that of the up slit of the cooling belt. When the ejection nozzle outlet is wider than the up slit of the cooling belt, the aluminum fluid can fill well if only the casting speed matches the cooling-belt velocity properly, otherwise under filling occurs in some areas in the cooling zone.

High Strength, Ductility and Superplasticity Mg-6Zn-1Y-0.6Ce-0.6Zr Alloy Prepared by Rapid Solidification and Reciprocating Extrusion

High strength, ductility, and superplasticity Mg-6.0%Zn-1.0%Y-0.6%Ce-0.6%Zr（wt%） alloy was prepared by sequentially applying rapid solidification, extrusion, and reciprocating extrusion（REX）. The microstructure of the alloy after 2-pass REX consisted of fine grains smaller than 0.7 μm and nanometer strengthening particles. The refined grains resulted from recrystallization during which the nanometer particles played an important role in restrain grain growth. The mechanical properties of the material were investigated at room and elevated temperatures. High tensile yield strength of 336 MPa and elongation of 27% were obtained at room temperature. At elevated temperatures, the highest elongation of 270% was obtained at 250 ℃ and an initial strain rate of 3.3×10^-3 s^-1, and LTS and HSRS were achieved. The high strength, ductility, and superplasticity were attributed to the refined unique microstructure.

Precipitation of multi-type nano-quasicrystals in a Mg-Zn-Y alloy

Mg-6Zn-1Y(at.%)ribbons with strengthening precipitates of multi-type nanoquasicrystals were prepared by melt-spinning followed by aging treatments.Microstructural evolution of the rapidly solidified ribbons during isothermal aging was comprehensively studied using various electron microscopy techniques.Two new kinds of decagonal quasicrystals were formed in aged ribbons,in addition to precipitation of nanometer icosahedral quasicrystals.Atomic-resolution observations reveal that both decagonal quasicrystals can be modeled by quasiperiodic tiling with decagonal clusters of 2.5 nm in diameter,but overlap of neighboring clusters in both decagonal quasicrystals is different from the Gummelt model observed in other quasicrystals.A shell composed of complex Laves Mg-Zn domains was formed surrounding each decagonal quasicrystal precipitate upon prolonged aging.In addition,all kinds of nanoprecipitates exhibit excellent structure and size stability at 573 K.Our findings may have implications for not only fundamental studies about quasicrystals,but also microstructural manipulation of high-performance Mg alloys.