Effect of Co on Solidification Characteristics and Microstructural Transformation of Non-equilibrium Solidified Cu-Ni Alloys

Non-equilibrium solidification structures of Cu55Ni45 and Cu55Ni43Co2 alloys were prepared by the molten glass purification cycle superheating method.The variation of the recalescence phenomenon with the degree of undercooling in the rapid solidification process was investigated using an infrared thermometer.The addition of the Co element affected the evolution of the recalescence phenomenon in Cu-Ni alloys.The images of the solid-liquid interface migration during the rapid solidification of supercooled melts were captured by using a high-speed camera.The solidification rate of Cu-Ni alloys,with the addition of Co elements,was explored.Finally,the grain refinement structure with low supercooling was characterised using electron backscatter diffraction(EBSD).The effect of Co on the microstructural evolution during nonequilibrium solidification of Cu-Ni alloys under conditions of small supercooling is investigated by comparing the microstructures of Cu55Ni45 and Cu55Ni43Co2 alloys.The experimental results show that the addition of a small amount of Co weakens the recalescence behaviour of the Cu55Ni45 alloy and significantly reduces the thermal strain in the rapid solidification phase.In the rapid solidification phase,the thermal strain is greatly reduced,and there is a significant increase in the characteristic undercooling degree.Furthermore,the addition of Co and the reduction of Cu not only result in a lower solidification rate of the alloy,but also contribute to the homogenisation of the grain size.

Preparation of Laser Cladding Coating Undercooling Cu-based Alloy and Co on Non-equilibrium Solidification Structure

The effect of the gradient content of Co element on the solidification process of Cu-based alloy under deep under cooling conditions was explored.The non-equilibrium solidification structure of the under cooled alloy samples were analyzed.It is found that the rapidly solidified alloy has undergone twice grain refinement during the undercooling process.Characterization and significance of the maximum undercooling refinement structure of Cu60Ni35Co5 at T=253 K were analyzed.High-density defects were observed,such as dislocations,stacking faults networks,and twinning structures.The standard FCC diffraction pattern represents that it is still a single-phase structure.Based on the metallographic diagram,EBSD and TEM data analysis,it is illustrated that the occurrence of grain refinement under high undercooling is due to stress induced recrystallization.In addition,the laser cladding technology is used to coat Co-based alloy(Stellite12) coating on 304 stainless steel substrate;the microstructure of the coating cross-section was analyzed.It was found that the microstructure of the cross-section is presented as columnar crystals,planar crystals,and disordered growth direction,so that the coating has better hardness and wear resistance.By electrochemical corrosion of the substrate and coating,it can be seen that the Co and Cr elements present in the coating are more likely to form a dense passivation film,which improved the corrosion resistance of the coating.

Effect of melt superheat on structural uniformity of lotus-type porous metals prepared by unidirectional solidification

Structural uniformity is an important parameter influencing physical and mechanical properties of lotus-type porous metals prepared by directional solidification of metal-gas eutectic (Gasar). The effect of superheat on structural uniformity as well as average porosity, pore morphology of porous metals was studied. The experimental results show that, when the superheat is higher than a critical value (ΔTc), the bubbling or boiling phenomenon will occur and the gas bubbles will form in the melt and float out of the melt. As a result, the final porosity will decrease. In addition, a higher superheat will simultaneously cause a non-uniform porous structure due to the pores coalescence and bubbling phenomenon. Finally, a theoretical model was developed to predict the critical superheat for the hydrogen to escape from the melt and the corresponding escapement ratio of hydrogen content. Considering the escapement of hydrogen, the predicted porosities are in good agreement with the experimental results.

Hydrogen diffusion coefficient in liquid metals evaluated by solid-gas eutectic unidirectional solidification

Based on the solid-gas eutectic unidirectional solidification technique and the principle of unidirectional solidification of single-phase alloy, a new method for evaluating the diffusion coefficient of hydrogen in liquid metals was proposed. Taking Cu-H2 system for example, the influences of argon partial pressure and superheat degree of melt on the diffusion coefficient of hydrogen in liquid metal were studied and the predicted values were similar to each other. The obtained temperature-dependent equation for diffusion coefficient of hydrogen in liquid copper is comparable with experimental data in literature, which validates the effectiveness of this method. The temperature-dependent equations for diffusion coefficient of hydrogen in liquid Mg, Si and Cu-34.6%Mn alloy were also evaluated by this method, along with the values at the melting point of each metal and alloy.

Directional solidification of metal-gas eutectic and fabrication of regular porous metals

Directional solidification of metal-gas eutectic (Gasar) is a novel process for making regular porous metals.This process is based on a solid-gas eutectic reaction involving a gaseous medium and a metal or a ceramic phase, andallows an easy control of the porosity, such as its pore size, pore orientation and morphology in a wide range by properlyadjusting its melting and solidification conditions. The latest progress and our research work in this field are reviewed inthis paper.

MSWI Fly Ash Based Novel Solidification/Stabilization Matrices for Heavy Metals

The possibilities of MSWI fly ash as a major constituent of novel solidification/stabilization matrices for secure landfill were investigated by mixing MSWI fly ash with rich aluminum components, which was added as bauxite cement or metakaolinite instead, to form Friedel and Ettringite phases with high fixing capacities for heavy metals. The physical properties, heavy metals-fixing capacity, mineral phases and its vibration bands in the novel matrices were characterized by compressive strength, TCLP（toxic characteristic leaching procedure）, XRD （x-ray diffraction） , DTG （derivative thermogravimetry）, and FTIR （fourier transform infrared spectroscopy）, respectively. The Tessier＇s five-step sequential extraction procedure was used to analyze the fractions of chemical speciation for Pb, Cd and Zn ions. The experimental results indicate that Friedel-Ettringite based novel solidification/stabilization matrices can incorporate Pb, Cd and Zn ions effectively by physical encapsulation and chemical fixation, and it exhibits a great potential in co-landfill treatment of MSWI fly ash with some heavy metals-bearing hazardous wastes.

Solidification crack susceptibility of aluminum alloy weld metals

The susceptibilities of the three aluminum alloys to solidification crack were studied with trans-varestraint tests and tensile tests at elevated temperature. Their metallurgical characteristics, morphologies of the fractured surface and dynamic cracking behaviors at elevated temperature were analyzed with a series of micro-analysis methods. The results show that dynamic cracking models can be classified into three types. The first model has the healing effect which is called type A. The second is the one with deformation and breaking down of metal bridge, called type B. The last one is with the separation of liquid film along grain boundary, called type C. Moreover, the strain rate has different effects on crack susceptibility of aluminum alloys with different cracking models. ZL101 and 5083 alloys belong to type A and type C cracking model respectively, in which strain rate has greater effect on eutectic healing and plastic deformation of metal bridge. 6082 alloy is type B cracking model in which the strain rate has little effect on the deformation ability of the liquid film.

Cooling rate dependence of polymorph selection during rapid solidification of liquid metal zinc

The polymorph selection during rapid solidification of zinc melt was investigated by molecular dynamics simulation. Several methods including g（r）, energy, CNS, basic cluster and visualization were used to analyze the results. The results reveal that the cooling rate has no observable effect on the microstructure as TTc（Tc is the onset temperature of crystallization）; and at the first stage of crystallization, although microstructures are different, the morphologies of nucleus are similar, which are composed of HCP and FCC layers; the polymorph selection of cooling rate finally takes place at the second stage of crystallization： at a high cooling rate, the rapid increase of FCC atoms leads to a FCC crystal mixed with less HCP structures; while at a low cooling rate, HCP atoms grow at the expense of FCC atoms, resulting in an almost perfect HCP phase. The results reveal that the cooling rate is one of the important factors for polymorph selection.

Directional Solidification Assisted by Liquid Metal Cooling

An overview of the development and current status of the directional solidification process assisted by liquid metal cooling （LMC） has been presented in this paper. The driving force of the rapid development of the LMC process has been analyzed by considering the demands of （1） newer technologies that can provide higher thermal gradients for alleviated segregation in advanced alloy systems, and （2） better production yield of the large directionally solidified superalloy components. The brief history of the industrialization of the LMC process has been reviewed, followed by the discussion on the LMC parameters including selection of the cooling media, using of the dynamic baffle, and the influence of withdrawal rates and so on. The microstructure and mechanical properties of the traditional superalloys processed by LMC, as well as the new alloys particularly developed for LMC process were then described. Finally, future aspects concerning the LMC process have been summarized.

Directional solidification casting technology of heavy-duty gas turbine blade with liquid metal cooling(LMC) process

In this work, some important factors such as ceramic shell strength, heat preservation temperature, standing time and withdrawal rate, which influence the formability of directionally solidified large-size blades of heavy-duty gas turbine with the liquid metal cooling(LMC) process, were studied through the method of microstructure analysis combining. The results show that the ceramic shell with medium strength(the high temperature flexural strength is 8 MPa, the flexural strength after thermal shock resistance is 12 MPa and the residual flexural strength is 20 MPa) can prevent the rupture and runout of the blade. The appropriate temperature(1,520 ℃ for upper region and 1,500 ℃ for lower region) of the heating furnace can eliminate the wide-angle grain boundary, the deviation of grain and the run-out caused by the shell crack. The holding time after pouring(3-5 min) can promote the growth of competitive grains and avoid a great deviation of columnar grains along the crystal orientation <001>, resulting in a straight and uniform grain structure. In addition, to avoid the formation of wrinkles and to ensure a smooth blade surface, the withdrawal rate should be no greater than the growth rate of grain. It is also found that the dendritic space of the blade decreases with the rise of solidification rate, and increases with the enlarging distance between the solidification position and the chill plate.

Recent progress on apparatus development and in situ observation of metal solidification processes via synchrotron radiation X-ray imaging: A review

The solidification process of metals plays a critical role in their final microstructure and, correspondingly, in their performance. It is therefore important to probe the solidification behavior of metals using advanced in situ techniques. Synchrotron radiation X-ray imaging is one of the most powerful techniques to observe the solidification process of metals directly. Here, we review the development of the solidification apparatus, including the directional solidification device, resistance furnace, multi-field coupling device, semisolid forming device, aerodynamic levitation apparatus, and laser additive manufacturing apparatus. We highlight the recent research progress on the use of synchrotron radiation X-ray imaging to reveal the solidification behavior of metals in the above circumstances. The future perspectives of synchrotron radiation X-ray imaging in metal research are discussed. Further development of this technique will contribute to improve the understanding of the solidification process of metals and other types of materials at different scales.

Fabrication of directional solidification components of nickel-base superalloys by laser metal forming

Straight plates, hollow columns, ear-like blade tips, twist plates withdirectional solidification microstructure made of Rene 95 superalloys were successfully fabricatedon Nickel-base superalloy and DD3 substrates, respectively. The processing conditions for productionof the parts with corresponding shapes were obtained. The fabrication precision was high and thecomponents were compact. The solidification microstructure of the parts was analyzed by opticalmicroscopy. The results show that the solidification microstructure is composed of columnardendrites, by epitaxial growth onto the directional solidification substrates. The crystallographyorientation of the parts was parallel to that of the substrates. The primary arm spacing was about10 mum, which is in the range of superfine dendrites, and the secondary arm was small or evendegenerated. It is concluded that the laser metal forming technique provides a method to manufacturedirectional solidification components.

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.

Liquid−liquid structure transition in metallic melt and its impact on solidification:A review

Understanding the nature of liquid structures and properties has always been a hot field in condensed matter physics and metallic materials science.The liquid is not homogeneous and the local structures inside change discontinuously with temperature,pressure,etc.The liquid will experience liquid−liquid structure transition under a certain condition.Liquid−liquid structure transition widely exists in many metals and alloys and plays an important role in the final microstructure and the properties of the solid alloys.This work provides a comprehensive review on this unique structure transition in the metallic liquid together with the recent progress of its impact on the following microstructure and properties after solidification.These effects are discussed by integrating them into different experimental results and theoretical considerations.The application of liquid−liquid structure transition as a strategy to tailor the properties of metals and alloys is proven to be practical and efficient.

Construction and analysis of dynamic solidification curves for non-equilibrium solidification process in lost-foam casting hypoeutectic gray cast iron

Most lost-foam casting processes involve non-equilibrium solidification dominated by kinetic factors, while construction of a common dynamic solidification curve is based on pure thermodynamics, not applicable for analyses and research of non-equilibrium macro-solidification processes, and the construction mode can not be applied to nonequilibrium solidification process. In this study, the construction of the dynamic solidification curve(DSC) for the nonequilibrium macro-solidification process included: a modified method to determine the start temperature of primary austenite precipitation(T_(AL)) and the start temperature of eutectic solidification(T_(ES)); double curves method to determine the temperature of the dendrite coherency point of primary austenite(T-(AC)) and the temperature of eutectic cells collision point(T_(EC)); the "technical solidus" method to determine the end temperature of eutectic reaction(T_(EN)). For this purpose, a comparative testing of the non-equilibrium solidification temperature fields in lost-foam casting and green sand mold casting hypoeutectic gray iron was carried out. The thermal analysis results were used to construct the DSCs of both these casting methods under non-equilibrium solidification conditions. The results show that the transformation rate of non-equilibrium solidification in hypoeutectic gray cast iron is greater than that of equilibrium solidification. The eutectic solidification region presents a typical mushy solidification mode. The results also indicate that the primary austenite precipitation zone of lost-foam casting is slightly larger than that of green sand casting. At the same time, the solid fraction(f_s) of the dendrite coherency points in lost-foam casting is greater than that in the green sand casting. Therefore, from these two points, lost-foam casting is more preferable for reduction of shrinkage and mechanical burntin sand tendency of the hypoeutectic gray cast iron. Due to the fact that the solidification process(from the surface to center) at primary austenite growth area in the lost-foam cylinder sample lags behind that in the green sand casting, the mushy solidification tendency of lost-foam casting is greater and the solidification time is longer.

Stress Field of Non-equilibrium Grain Boundaries in Nano-crystalline Metals

Introducing the stress distribution near grain boundaries to improve the dislocation pileup model for the Halt-Petch (H-P) relation, the continuous distribution of dislocations in the pileup could be solved by means of Tschebysheff polynomials for the Hilbert transformation. An analytical formula of the stress intensity factor for the dislocation pileup is obtained. The reverse H-P relation may be explained by the modified dislocation-pileup-model.

Simulation of Solidification Parameters during Zr Based Bulk Metallic Glass Matrix Composite’s (BMGMCs) Additive Manufacturing

After a silence of three decades, bulk metallic glasses and their composites have re-emerged as a competent engineering material owing to their excellent mechanical properties not observed in any other engineering material known till date. However, they exhibit poor ductility and little or no toughness which make them brittle and they fail catastrophically under tensile loading. Exact explanation of this behaviour is difficult, and a lot of expensive experimentation is needed before conclusive results could be drawn. In present study, a theoretical approach has been presented aimed at solving this problem. A detailed mathematical model has been developed to describe solidification phenomena in zirconium based bulk metallic glass matrix composites during additive manufacturing. It precisely models and predicts solidification parameters related to microscale solute diffusion (mass transfer) and capillary action in these rapidly solidifying sluggish slurries. Programming and simulation of model is performed in MATLAB&reg;. Results show that the use of temperature dependent thermophysical properties yields a synergic effect for multitude improvement and refinement simulation results. Simulated values proved out to be in good agreement with prior simulated and experimental results.

Parallel algorithm of solidification process simulation for large-sized system of liquid metal atoms

A parallel arithmetic program for the molecular dynamics (MD) simulation study of a large sized system consisting of 50 000100 000 atoms of liquid metals is reformed, based on the cascade arithmetic program used for the molecular dynamics simulation study of a small sized system consisting of 5001 000 atoms. The program is used to simulate the rapid solidification processes of liquid metal Al system. Some new results, such as larger clusters composed of more than 36 smaller clusters (icosahedra or defect icosahedra) obtained in the system of 50 000 atoms, however, the larger clusters can not be seen in the small sized system of 5001 000 atoms. On the other hand, the results from this simulation study would be more closed to the real situation of the system under consideration because the influence of boundary conditions is decreased remarkably. It can be expected that from the parallel algorithm combined with the higher performance super computer, the total number of atoms in simulation system can be enlarged again up to tens, even hundreds times in the near future.

Stabilization/solidification mechanisms of tin tailings and fuming slag-based geopolymers for different heavy metals

Tin mine tailings(TMT)and fuming slag(FS)contain many heavy metals(As,Cr,Cu,Zn and Mn)that cause severe pollution to the environment.Herein,geopolymers were prepared using TMT,FS and flue gas desulfurization gypsum(FGDG)to immobilize heavy metals,and their compressive strength and heavy metal leaching toxicity were investigated.It was first determined that T4F5(TMT:FS=4:5)sample exhibited the highest compressive strength(7.83 MPa).T4F5 achieved 95%immobilization efficiency for As and Cr,and nearly 100%for Cu,Zn and Mn,showing good immobilization performance.A series of characterization analyses showed that heavy metal cations can balance the charge in the geopolymer and replace Al in the geopolymer structure to form covalent bonds.In addition,about 2%–20%of heavy metal Fe was immobilized in hydration products,heavy metal hydroxides and non-bridging Si–O and Al–O coordination with silica-aluminate matrices.AsO_(3)^(3−) was oxidized into AsO_(4)^(3−),which may form Ca–As or Fe–As precipitates.Cr_(2)O_(7)^(2−)was converted to CrO_(4)^(2−)under alkaline environment and then combined with OH−to form Cr(OH)3 precipitates.Mn^(2+)may react directly with dissolved silicate to form Mn_(2)SiO_(4) and also form Mn(OH)_(2) precipitates.The unstable Mn(OH)_(2) can be further oxidized to MnO_(2).The heavy metal cations were immobilized in the silicoaluminate lattice,while the anions tended to form insoluble precipitates.These results may benefit the industry and government for better handling of TMT,FS and solid wastes containing the abovementioned five heavy metals.

Melting Reconstruction Features and Solidification Mechanism of Heavy Metal-containing Slag

The relationships between microstructure and melting temperature of slag containing different heavy metals （Zn, Cu, Pb and Cr） were studied. Furthermore, the corresponding solidification mechanism and rule of heavy metals were analyzed by microscopic tests during melting and reconstructing process. Based on preliminary results, three conclusions were derived. Firstly, pure slag would begin to melt when the temperature reached 1 180℃; however, Zn did not play any fluxing action. Secondly, upon adding Cu and Pb, the initial melting temperature of slag decreased by 5-8℃ and their fluxing effect was observed. Thirdly, the initial melting temperature and the reaction time for slag decreased by 22℃ and 6 s respectively after adding Cr; the fluxing action was significant under Cr. The results of X-ray diffraction （XRD） and Fourier transform infrared spectroscope （FTIR） analyses showed that the above heavy metals had little influence on the reconstruction of slag. Toxicity characteristic leaching procedure （TCLP） leaching tests showed a good solidification effect of the heavy metals with melting slag, fixation rate of Zn, Cu, Pb and Cr was 36.3%, 24.6%, 9.2% and 93.2%, respectively. The leaching toxicity of the heavy metals met the requirements for environmental emission after solidification treatment.