Industrial gas turbines (IGTs) are the key equipment to achieving energy strategy, such as energy conservation and clean power generation. When the large and complex IGT blades are fabricated by the conventional Bridgman directional solidification process, the thermal gradients at the solidification front are low and unstable, resulting in some disadvantages: the coarse dendrite structure with severe dendritic segregation, the increased occurrence of casting defects and the poor performance of mechanical properties. These disadvantages provide a good opportunity for rapid development of the directional solidification with high thermal gradient (HG), such as the liquid metal cooling (LMC). In the present work, the physical basis of HG process, the microstructure, mechanical properties, solution heat treatment, and casting defects of the superalloys processed by HG process, have been reviewed. The HG process increases the thermal gradient and the cooling rate, thus permitting microstructural improvements including a more homogeneous fine-dendrite structure with lower elemental segregation and shrinkage porosity, and refinement of carbide, γ′ phase and eutectic, reducing the volume fraction of eutectic and shrinkage porosity. During the solution heat treatment, the HG process increases the incipient melting temperature and reduces the residual segregation as well as the content of solution pore. The HG process could effectively inhibit the formation of freckle chains, increase the critical withdrawal rate of the stray grain formation, and decrease the degree of the misorientation of the <001> grain orientation from the casting axis. Moreover, the HG process could improve the mechanical properties including the stress rupture life, low-cycle fatigue (LCF), high-cycle fatigue properties and short-term strength, but the improvement might be reduced at higher temperature or under the oxidation condition.

Solidification of deeply undercooled alloy melts proceeds with obvious temperature recalescence, during which part of primary solid is inevitably remelted, and the microstructural morphology is inevitably changed. During past decades, great improvement was achieved in modelling crystal growth in undercooled alloy melts, making it possible to quantitatively evaluate the remelting degree of primary solid at different undercoolings. In this paper, the progress in modelling the remelting of primary solid was introduced, and the variation of remelted fraction of primary solid as a function of the alloy feature and undercooling was presented. In combination with experimental results of crystal growth pattern and solidification structure in selected alloys, the mechanisms for grain refinement in undercooled single-phase alloys and anomalous eutectic formation in undercooled eutectic alloys were then discussed.

The researches on development, application and solidification microstructures of high performance magnesium (Mg) alloys have received considerable interest recently. The solidification microstructures of Mg alloys can be effectively controlled by using directional solidification technology, rapid solidification technology and the application of external field during solidification, and thus the enhanced comprehensive mechanical properties of the materials are obtained. The current researches on solidification microstructure controlling of high performance Mg alloys by using directional solidification technology, rapid solidification technology and electromagnetic stirring were reviewed. Finally, the development trend on the controlling of solidification microstructure was proposed.

TiAl-based alloys will be potentially used as light-weight high temperature structural materials in aerospace industry. The comprehensive mechanical properties of TiAl-based alloys can be improved significantly when lamellar orientation is aligned parallel to principle stress. In this paper, the development of seeding technique in directionally solidified TiAl-based alloys is reviewed, including the traditional Ti-43Al-3Si seeding method and some novel seeding methods. Those methods mainly include the second directional solidification method, self-seeding technique, quasi-seeding technique and high-melting metal seeding technique. Those newly developed methods will promote the engineering applications of the lamellar structure controlling technology for TiAl-based alloys. However, the stable growth of different leading phase in its designed direction depends on the coupling of the seed and growth dynamic parameteres. How to discover the influence of the growth dynamic parameteres on the designed growth direction is a key problem.

Competitive growth between different structures including phases, dendrites and grains is a common phenomenon existing in various microstructure evolution processes. The overgrowth outcome of competitive growth has a paramount influence on final solidification microstructures and mechanical behaviors of materials. The competitive grain growth during directional solidification is a key factor for microstructures controlling, especially for the preparation of single crystal turbine blades. In recent years, the competitive grain growth during directional solidification becomes a hot spot due to an increasing demand for the single crystal preparation and inconsistent experimental results with the classical Walton-Chalmers model. In this paper, the mechanism of competitive grain growth based on the classical Walton-Chalmers model and its challenges were firstly discussed, and then some recent research progresses in converging growth and diverging growth in two dimensional spaces, and non-uniplanar growth in three dimensional spaces were reviewed. Furthermore, the recent works of our group on competitive grain growth during directional solidification by using the phase field method were introduced. Finally, the outlooks of future studies on competitive grain growth during directional solidification are presented.

This paper reviews the study of our group on controlled solidification and its applications in recent 20 years, including melt heat treatment before solidification, semi-solid processing between liquidus and solidus, rapid progressive solidification of nonequilibrium materials, semi-solid progressive solidification, and directional heat treatment in solid phase transformation. Furthermore, a new technique of material preparation is proposed with the combination of directional solidification and directional heat treatment. In addition, an outlook of controlled solidification technology and its applications are provided.

Monotectic alloys or alloys with a miscibility gap in the liquid state are a broad kind of materials. Many of them have great potential applications in industry. However, these alloys have an essential drawback that the miscibility gap poses problems during solidification. When a homogeneous single phase liquid is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition begins with the nucleation of the minority phase droplets. These droplets grow and coarsen then. They can also settle or float due to the specific gravity differences between phases and migrate due to the temperature gradient or concentration gradient. The motions of the droplets cause the formation of a microstructure with serious phase segregation. Researchs have been carried out to investigate the solidification process of monotectic alloys on ground as well as under the microgravity conditions in space. The feasibility of controlling the microstructures of monotectic alloys by using electric field, magnetic field, microalloying, etc. has been investigated. Meanwhile, plenty of efforts have been made to model and simulate the microstructure evolution of monotectic alloy during the L-L phase transformation. This article will review the research work in this field during the last few decades and propose some perspectives for future studies on the solidification process of monotectic alloys.

Considering both phase transition and material hot-working or material design, unified analysis of non-equilibrium solidification and solid-state phase transformations has profound significance of science and great prospect of engineering application. Here, non-equilibrium solidification and solid state transformation behavior, correlated with peritectic reaction, massive transformation, metastable-stable transformation, precipitation, recrystallization and grain growth, for single phase solid solution alloy, eutectic alloy, peritectic alloy, multi-component super alloy and aluminum alloy, have been systematically reviewed. Regarding further the influence of non-equilibrium solidification on subsequent solid-state transformations, the new transformation mechanism induced by non-equilibrium solidification and the integrated microstructure regulation, the physical correlation between non-equilibrium solidification and subsequent solid-state transformations were discussed, and eventually, the microstructure control under the joint action of non-equilibrium solidification and solid phase transformations was realized. This review is expected to provide theoretical support for quantitative characterization of non-equilibrium solidification effect and microstructure prediction under the joint action of non-equilibrium solidification and solid-state transformations.

Microporosity and inverse segregation are two common casting defects mainly caused by solidification shrinkage, which are detrimental to the mechanical properties of components, especially to the fatigue performance and ductility. Numerous efforts have been put into the investigation on microporosity and inverse segregation independently. However, few work has been done to establish a theoretical model for predicting the two defects simultaneously, whereas they often interact with each other and the formation of microporosity may exert a beneficial effect on inverse segregation. In this review, the coupling models for prediction of microporosity and inverse segregation were introduced. Firstly, the mechanisms and the predicting models for the two defects were summarized separately. Microporosity is a resultant of solidification shrinkage and gas segregation. Therefore, the porosity was previously categorized into two types: shrinkage porosity and gas porosity. More recent porosity models have combined the effect of pressure drop induced by feeding, the evolution of pores radius, the decrease of gases solubility in the liquid and the gas rejection at the solid/liquid interface, which provide rather good approximation to experimental results. As for inverse segregation, it is mainly caused by the suction of interdendritic liquid which is generally rich in solute. Therefore, determination of the feeding velocity is crucial for most inverse segregation models. Then, through the analysis of the underlying interaction between microporosity and interdendritic feeding flow, the coupling methods for prediction of the two defects were reviewed. Most of the models have added porosity into the continuity equation to amend the feeding velocity and utilized the “local solute redistribution equation” to get the solute concentration profiles. A new coupling model recently proposed by the present authors, based on analyses of the redistribution of gases element as well as the alloying element, is also in this route. The result shows that for Al-4.5%Cu (mass fraction) alloy solidified in a columnar dendrites structure, the predicted fraction of microporosity is a little smaller than that of Poirier's model, and the increase of initially dissolved hydrogen in the melt will decrease the solute enrichment in the interdendritic liquid. Microporosity seems to reduce the flow needed to compensate the solidification shrinkage, thus the solute segregation gets reduced. Finally, several suggestions were proposed, including the treatment of pore radius, eutectic shrinkage and gas porosity precipitated during eutectic reaction, etc.

This paper reviews the recent development of porous metals with directional pores, from the aspects of the solidification principle, fabrication method, properties and applications. This kind of porous metals is fabricated by a directional solidification process in pressurized gas atmosphere, utilizing a metal/gas eutectic reaction (Gasar). By controlling solidification direction, not only lotus-type porous structure but also radial-type porous structure can be produced. The coupled growth of solid/gas phases is discussed by applying a solution procedure similar to that in the classical Jackson-Hunt eutectic growth model. The working window considering hydrogen escape and the formation of directional solidification porous structure has been given. Three fabrication techniques including mold casting, continuous casting techniques and Bridgman-type directional solidification method are introduced. Two new progresses about the fabrication of directionally solidified porous structure are described in details: porous alloy with uniform directional pores and high-porosity directionally solidified porous aluminum. Since directionally solidified porous metals exhibit peculiar physical and mechanical properties such as light-weight, air and water permeability, and anisotropy of thermal and mechanical properties, they are suitable for applications in heat sinks, filters, biomaterials and so on.

In recent years, the research on materials processing under high magnetic fields has developed rapidly. This paper reviews the progress of solidification process control of metal materials and the preparation of new materials under high magnetic fields. The influences of Lorentz force, thermoelectromagnetic force and magnetic force on the melt flow, solute distribution and microstructure evolution in the alloy, the effects of magnetic moment on the crystal orientation of magnetic phase, and the effects of magnetic dipole-dipole interactions on phase arrangement in alloys were mainly introduced. At the same time, this paper also summarizes the progress of preparing new functional materials such as gradient MnSb/MnSb-Sb composites, gradient magnetostrictive materials, and materials which have anisotropy of crystal orientation by the solidification method under high magnetic fields. The high magnetic fields control the solidification process of the metals to improve the microstructure of the materials and further improve the material properties. This provides a new way for the development of new functional materials.

With the development of metallurgical technology, some requirements such as high homogenization, high purification, superfine crystallization and low consumption have been put forward for the properties of metal materials. The pulse electromagnetic field has become the development direction of the new generation of metallurgy due to its energy saving, convenient application and refinement effect. The technology of pulse electromagnetic field in solidification includes electric current pulse, pulsed magnetic field and pulsed magneto oscillation (PMO). Based on the theories of pulsed current and electromagnetic stirring, the pulse current is applied to the induction coil in PMO technique. The melt is induced by magnetic field to produce electromagnetic force, and thereby, the non-contact treatment can be achieved to avoid dirtying the melt. In this paper, PMO technique is introduced, including invention process, theoretical basis, refinement mechanism, research status and its applications.

Solid-liquid interfacial energy (SLIE) plays a crucial role in accurately evaluating solidification characteristics and effectively tuning the solidification process of crystals, which determines the structures and properties of crystals. This paper is based on the investigations of the authors on SLIE of melt-crystal in the past decade, and concentrates on reviewing the recent developments on the experimental and theoretical results of SLIE of melt-crystal. It draws several important conclusions by comparing various experimental results of SLIE under different temperatures. Firstly, the SLIE of melt-crystal decreases with the decrease of temperature. Secondly, the reason for different SLIE respectively obtained by Spaepen model and experimental measurement is revealed. Eventually, a model and method based on the structure of the solid-liquid interface for predicting the SLIE are proposed, and the results provided by this model are in line with the experimental results and the simulated results at the melting temperature, as well as the experimental results and the simulated results of the undercooled state.

The inhomogeneity in large ingots not only decides the final properties of the product, but also restricts downstream hot working processing severely. It is very important to improve the homogeneity of ingots for saving energy, improving material utilization ratio, increasing performance of component, and the construction of key equipment. In this paper, the general inhomogeneity problem in large ingots, such as macrosegregation, inclusion, shrinkage porosity, and large crystal have been introduced. The evolutions of this inhomogeneity in the subsequent hot working processing have also been discussed, based on which the concept of homogeneity window for large ingots has been proposed. The research progress of numerical simulation of macrosegregation in large ingots and some new methods for improving the homogeneity of large ingot have also been introduced and analyzed. Three fundamental reasons for the inhomogeneity of ingots were concluded, i.e. the uneven cooling rate, the uncontrollable multiphase flow, and the solute redistribution during solidification. Aiming at these three fundamental reasons, a novel casting method called layer casting (LC), which has been proposed by our team recently, was introduced to modify the serious inhomogeneity problem in large ingots. In this method, molten alloy was poured into the mold separately and layer upon layer. As soon as the poured molten alloy solidified to a critical volume fraction range, the next layer amount of molten alloy was poured into the mold. For each layer, the mass, composition, and pouring temperature of poured molten alloy could be artificially designed and controlled based on the target homogeneity window. Both experiment and numerical simulated results shown that, in comparison with conventional ingot fabrication method, the LC method can significantly decrease the uncontrollable multiphase flow, uniform the cooling rate, and improve the solute redistribution, subsequently, improve the homogeneity of ingots. For large ingots fabrication, the LC method has the potential to substantially decrease the energy consumption, materials consumption, and the investment of large equipment. Its wide application prospect for high quality large ingots is also expected.

Dendrites are the most frequently observed solidification microstructures of metallic alloys. In most solidification processes at low and moderate cooling rates, dendrite coarsening in mushy zones has been recognized as an unavoidable phenomenon that significantly influences microstructures and thereby the properties of the final products. The behavior of dendrite coarsening has received persistent scientific interests owing to its importance in both academic value and practical application. During the last five decades, extensive efforts have been made through theoretical analyses, experimental techniques and numerical simulations for fundamentally understanding the mechanisms of dendrite coarsening during solidification under continuously cooling or isothermal conditions. This paper first gives a brief overview of the progress in the studies of dendrite coarsening. Then, a cellular automaton (CA) model recently proposed by the authors is presented, which involves the mechanisms of both solidification and melting. The model is applied to simulate the microstructural evolution of columnar dendrites of SCN-ACE alloys during isothermal holding in a mushy zone. The CA simulations reproduce the typical dendrite coarsening features as observed in experiments. The role of melting for dendrite coarsening is quantified by comparing the simulation results using the new CA model and a previous CA model that does not include the melting effect. The mechanisms of dendrite coarsening are investigated in detail by comparing the local equilibrium and actual liquid compositions at solid/liquid interfaces. The CA simulations render visualizing how local solidification and melting stimulate each other through the complicated interactions between phase transformation, interface shape variation and solute diffusion.

Columnar to equiaxed transition (CET) generating a fine-grain structure of GCr15 bearing steel with the homogeneity of the solute contents and the rather small amount of internal defects is often desired in solidification processes. In recent years much attention has been paid to the effect of static magnetic fields on the CET of Al base alloys, Pb-Sn alloys and Ni base superalloys. However, there are few papers to investigate the effect of static magnetic fields on the CET of GCr15 bearing steel. The present work investigates how longitudinal static magnetic fields affect the CET in directionally solidified GCr15 bearing steel. Experimental results show that columnar dendrites degenerate and transform into equiaxed dendrites at the edge of the sample as the longitudinal static magnetic field increases at pulling rate of 20 μm/s and temperature gradient of 104 K/cm. The dendritic morphology without the longitudinal static magnetic field is regular and columnar at pulling rate of 5 and 50 μm/s and temperature gradient of 104 K/cm. When the 4 T longitudinal static magnetic field is applied, the dendritic morphology is still regular and columnar at pulling rate of 50 μm/s and temperature gradient of 104 K/cm. However, the CET occurs at low pulling rate of 5 μm/s and temperature gradient of 104 K/cm. This phenomenon is simultaneously accompanied by more uniformly distributed alloying elements. The corresponding numerical simulations verify that the thermoelectric (TE) magnetic force is induced by the interaction between the longitudinal static magnetic field and TE current. Owing to TE magnetic force localized into the root of the dendrite, the dendritic fragments detach from the primary dendrites. Then the TE magnetic convection induced by TE magnetic force acting on the melt transports the fragments from the interdendritic spacing to the region ahead of columnar dendrites. It can be deduced from above phenomena that the TE magnetic force leads to the CET under the longitudinal static magnetic field.

Intermetallic compounds (including quasicrystals) have been widely employed as reinforced phases in many alloys due to their high strength, high hardness and good thermal stability. The growth behavior and growth pattern of these intermetallic compounds affect the mechanical properties of materials significantly. However, the intermetallic compound, which exhibits complex crystal structures and directional bonding usually shows a faceted growth pattern with strong anisotropy and forms crystals with a wide range of morphologies and coarse grains during solidification. The inappropriate morphology and size of the intermetallic compound will destroy the integrity of the matrix and thus deteriorate the mechanical properties of materials. In this work, the microstructural evolution, morphology evolution of intermetallic compounds and mechanical properties have been investigated in directionally solidified Al-3Mn-7Be (atomic fraction, %) alloy with a wide pulling rates of 1~1500 μm/s. The addition of Be results in the shift of Al-Mn binary phase diagram toward the Mn-rich side, the appearance of intermetallic compounds, namely λ-phase, T-phase, Be₄AlMn, and icosahedral quasicrystal (I-phase) and significantly refines the microstructures of the as-cast and directionally solidified samples. With increasing pulling rates, a transition of primary phase is observed from λ-phase to T-phase, and then I-phase, accompanied by the formation of the primary Be₄AlMn phase, which can be attributed to the increase of supersaturation and supercooling near the solid/liquid interface. Meanwhile, the morphology, size and growth pattern of primary phases vary with the increase of pulling rates. The mechanical properties of directionally solidified Al-3Mn-7Be alloy have been investigated. It is indicated that the room-temperature strength of this alloy decreases first and then increases as the pulling rates increase, and a larger elongation is presented at the lowest and highest pulling rates, which can be attributed to the microstructures of alloys, properties of strengthening phases and the interfaces between matrix and strengthening phase.

Especially in the past decades, Ti-6Al-4V alloy has received much attention, not only due to its high melting temperature, good corrosion resistance, low density and high hardness, but also because of the diverse and complicated microstructures formed under different conditions. This makes Ti-6Al-4V a potential candidate in both aerospace industries and fundamental research. It is well known that the solidified microstructures of alloy have a great influence on their mechanical properties. Therefore, it is crucial to investigate the mechanical properties of Ti-6Al-4V solidified under different conditions, in particular in the undercooling conditions. However, it is noted that most research on the solidification of Ti-6Al-4V alloy was carried out under equilibrium condition. With respect to Ti-6Al-4V alloy solidified under substantial undercooling conditions, few studies could be found. Thus, it is interesting to study two points: (1) the feature of the microstructure of Ti-6Al-4V alloy solidified under highly undercooled conditions and large cooling rate, (2) the influence of undercooling and cooling rate on the mechanical property of Ti-6Al-4V alloy. To address these two problems, Ti-6Al-4V alloy was rapidly solidified in a drop tube. The main results are summarized as follows. The microstructure of the Ti-6Al-4V alloy solidified under free fall condition displays "lamellar α+β→α dendrites→basket-weave α'+β→ needle-like α'→ needle-like α'+ anomalous β " transformation with decreasing the droplets diameter. And the needle-like α' phase in the original boundaries of equiaxed β grains is transformed into a continuous distribution and anomalous structure of β phase when the droplet size is less than about 400 μm. The microhardness of this alloy ranges from 506 kg/mm² to 785 kg/mm² when the droplet diameter decreases from 1420 μm to 88 μm, which is much higher than that of the master alloy. For "lamellar structure of α+β phases", "needle-like α' phase" and "needle-like α' phase+ anomalous β phase", the microhardness increases with the decrease of droplet diameter. But for 'basket-weave' microstructure, the microhardness diminishes with the decrease of droplet diameter.