Solidification Of Superalloys Research Articles

A Kinetic Model for Oxide–Carbonitride Inclusion Heterogeneous Nucleation and Precipitation during Superalloy Solidification

Complex oxide–carbonitrides (MgO-Ti(CN), Al2O3-Ti(CN), and MgO·Al2O3-Ti(CN)) are the most common non-metallic inclusions presented in cast and wrought superalloys. In this work, a coupled kinetics model was proposed to predict the complex oxide–carbonitride inclusion’s precipitation behavior during the solidification of superalloys. This model takes into account thermodynamics, micro-segregation, heterogeneous nucleation in the inter-dendritic liquid, and growth controlled by the diffusion of solute elements and kinetics of interfacial reaction. The results demonstrated that both the cooling rate and nitrogen content take significant effects on the final size of complex oxide–carbonitride inclusions, as the former controls the total growth time and the latter determines the initial precipitation temperature. In comparison, the particle size of primary oxides shows a negligible impact on the final size of complex inclusions. The practice of an industrial vacuum arc remelting confirmed that the inclusion size variation predicted by the present model is reasonably consistent with the experimental results.

Thermal-solutal convection-induced low-angle grain boundaries in single-crystal nickel-based superalloy solidification

Low-angle grain boundaries (LAGBs) are one of the solidification defects in single-crystal nickel-based superalloys and are detrimental to the mechanical properties. The formation of LAGBs is related to dendrite deformation, while the mechanism has not been fully understood at the mesoscale. In this work, a model coupling dendrite growth, thermal-solutal-fluid flow, thermal stress and flow-induced dendrite deformation via cellular automaton-finite volume method and finite element method is developed to study the formation of LAGBs in single crystal superalloys. Results reveal that the bending of dendrites is primarily attributed to the thermal-solutal convection-induced dendrite deformation. The mechanical stress of dendrite deformation develops and stabilises as solidification proceeds. As the width of the mushy zone gets stable, stresses are built up and then dendritic elastoplastic bending occurs at some thin primary dendrites with the wider inter-dendritic space. There are three characteristic zones of stress distribution along the solidification direction: (i) no stress concentration in the fully solidified regions; (ii) stress developing in the primary dendrite bridging region, and (iii) stress decrease in the inter-dendritic uncontacted zone. The stresses reach maximum near the initial dendrite bridging position. The lower temperature gradients, the finer primary dendritic trunks and sudden reductions in local dendritic trunk radius jointly promote the elastoplastic deformation of the dendrites. Corresponding measures are suggested to reduce LAGBs.

Understanding the metal-mold interfacial reactions during directional solidification of superalloys using Al2O3-rich ceramic mold

This study investigates the reactions occurring at the metal-mold interface during directional solidification of Ni-based superalloys, exerting a significant influence on both the surface quality and the desired composition of the alloy. The origin and nature of these reactions are systematically investigated by integrating thermodynamics and microstructural evolution of the alloy. It was observed that Hf from the melt reacts with the Al2O3-based ceramic mold, leading to the formation of HfO2 at the interface. This compound adheres to the surface of the ingot and also infiltrates into the melt near the interface. In order to validate these findings, an exploration of four alternative Ni-based superalloys, both with and without Hf content, was conducted, revealing an absence of comparable reactions at the interface in Hf-free alloys. Additionally, it was explored that the interface formed due to the metal-mold reaction may serve as a preferential nucleation site for MC-type carbides in C-containing Ni-based superalloys. This study concludes that the Hf content is the primary determinant influencing reactions at the metal-mold interface.

Mechanical properties and microstructure evolution of NbSiZr based alloy

To improve the mechanical properties of Nb-Si-Zr based superalloys at room temperature, the effect of a trace amount of Cr, C, Hf, Ta and Sc elements on the solidification processes, phase composition, mechanical properties and fracture behavior of Nb-Si-Zr based superalloys are studied by vacuum non-consumable arc-melting. The results show that the based alloy consists Nbss phase, γ-(Nb,X)5Si3 phase and Zr-rich γ-(Nb,X)5Si3 phase. However, there is no Zr-rich γ-(Nb,X)5Si3 phase in 20Zr-2C and 20Zr-4Hf alloys, indicating that a trace amount of C or Hf elements accelerates the diffusion of Zr element, and improves the segregation of the Nb-Si-Zr based alloys for the later stage of the superalloy solidification. The Nbss fraction in the matrix alloy (20Zr alloy) is 60%. C element increases the Nbss phase fraction by 4%, and Cr and Sc elements reduce the Nbss phase fraction by 4% and 6%, Hf and Ta elements reduce the Nbss phase by 1% and 2% compare with the matrix alloy (20Zr alloy). C element is dissolved in Nbss phase and γ-(Nb,X)5Si3 phase, while Cr element is more dissolved in Nbss phase and Sc element is more dissolved in Zr-rich γ-(Nb,X)5Si3 phase. The toughness of 20Zr-2 C alloy is the highest, and its KQ value reaches 14.05 MPa·m1/2, compared with the matrix alloy (20Zr alloy) increased by 23.46%. The enhancement of Nbss phase fraction and petal shaped microstructure make the crack propagation more difficult, thus increasing the KQ value of the 20Zr-2C alloy. The compressive properties of 20Zr-4Cr alloy are the best, and its compressive strength reaches 1807.3 MPa, compared with the matrix alloy (20Zr alloy) increased by 17.09%. The reason is that a trace amount of Cr element increases the fraction of γ-(Nb,X)5Si3 phase.

Numerical study on the formation of spurious grains and freckles during the directional solidification of superalloys

Freckles, a significant issue encountered during the directional solidification of superalloys, are recognised by a trail of equiaxed grains parallel to the direction of gravity accompanied by local eutectic enrichment. In the current study, a mixed-columnar-equiaxed multiphase volume-average solidification model was employed to study the formation of freckles in superalloy casting. Fragments produced via flow-driven and capillary-driven fragmentation mechanisms are considered as the source of spurious grains. The transport and the growth/remelting of the fragments are considered. According to the simulation results, some segregation channels develop at the corners of the casting. Flow-driven fragments are produced in/around the segregation channels, whereas capillary-driven fragments are produced at a certain depth of the mushy zone across the entire section of the casting. The fragmentation rate caused by the flow-driven mechanism is several orders of magnitude larger than that caused by the capillary-driven mechanism, i.e. the flow-driven fragmentation mechanism is dominant for the currently investigated sample. After the solidification process, four freckles formed at the casting corners on the shadowed side, whereas it was freckle-free on the bright side.

Study on the solutal convection during dendrite growth of superalloy under directional solidification condition

The solutal convection is able to affect the dendrite growth and solute distribution during directional solidification. The solutal convection is raised by the solute difference-caused buoyancy. When solutal convection becomes intensive, solute plumes will occur, which may cause freckle defects in directional solidification of superalloys. The onset of solutal plumes depends on the solidification condition. In this study, the dendrite growth with solutal convection during directional solidification of nickel-based superalloy was investigated by two-dimensional phase-field simulations. The lattice Boltzmann method was used to solve the flow field driven by solute difference caused-buoyancy force. The pseudo-binary alloy approximation was adopted for simplification of the multicomponent alloy. The effects of the temperature gradient, pulling velocity, and isotherm inclination angle on dendrite growth and solutal convection were investigated. The dendrite growth dynamics and solutal convection pattern under different solidification conditions were analyzed. Dendrite growth velocity oscillation was observed. The effect of pulling velocity on dendrite growth velocity oscillation was analyzed.

Computational fluid dynamics calculations of binary representation of Ni-base superalloy solidification in frustum geometries

CFD calculations were carried out on a number of frustum geometries from the experimental literature on freckling of superalloys. The effects of two imposed thermal fields were investigated, flat and inclined. Experiments document that when thermal fields are inclined the cold side is more prone to formation of freckles; our results conclude that rising liquid plumes in advance of the solidification front, which can correlate to freckles, are more abundant and intense on the cold side. Also, modelled plume activity is greater in inward sloping frustum geometries as opposed to outward sloping ones, which also agrees with established literature. Complex flow patterns were observed in inclined geometries and there is a clear interplay and competition between both the thermal and solutal aspects of flow, with flow being significantly changed, sometimes reversed in direction in some capacity. Our results also offer insight into the incubation length for plume and resultant freckle formation after the transition between frustums, with some critical distance of advance of the solidification front necessary to initiate and establish other plumes that feed into the main one circulating liquid flow which is locally intense enough to potentially create a freckle.

Breaking thermodynamic and kinetic barriers in superalloy homogenization process by electropulsing to improve mechanical properties

During the solidification of superalloy, dendrite segregation will be inevitably formed, causing uneven structure and performance. Segregation elimination and atomic diffusion in the alloy need to overcome kinetic and thermodynamic limitations. However, traditional thermal homogenization has disadvantages such as high energy consumption and low efficiency. Moreover, the coarse grains after the traditional treatment deteriorate the workability. Here it was found that the limitations of the thermal homogenization process can be broken through electropulsing. The electrical free energy can change the thermodynamic state of the system, reducing the dissolution temperature of η phases precipitated in the interdendritic space to a lower one of 1100 °C. Meanwhile, electromigration increases the atomic diffusion rate and significantly shortens the processing time. The time decrease effectively prevents the grain growth, which will further improve the workability of the ingot. Compared with the thermal homogenization process, the sample treated by pulsed electric current has better plasticity.

Phase-field–lattice Boltzmann simulation of dendrite growth under natural convection in multicomponent superalloy solidification

The thermosolutal convection can alter segregation pattern, change dendrite morphology and even cause freckles formation in alloy solidification. In this work, the multiphase-field model was coupled with lattice Boltzmann method to simulate the dendrite growth under melt convection in superalloy solidification. In the isothermal solidification simulations, zero and normal gravitational accelerations were applied to investigate the effects of gravity on the dendrite morphology and the magnitude of melt flow. The solute distribution of each alloy component along with the dendrite tip velocity during solidification was obtained, and the natural convection has been confirmed to affect the microsegregation pattern and the dendrite growth velocity. In the directional solidification simulations, two typical temperature gradients were applied, and the dendrite morphology and fluid velocity in the mushy zone during solidification were analyzed. It is found that the freckles will form when the average fluid velocity in the mushy zone exceeds the withdraw velocity.

Multiphase-field and experimental study of solidification behavior in a nickel-based single crystal superalloy

Understanding the solidification behavior in nickel-based single crystal superalloy solidification is essential for determining the following heat treatment process, while the experimental findings have led to contradictory results. On the one hand, the fine γ/γ′ structure in the interdendritic zone suggests a eutectic reaction. On the other hand, the coarse γ′ precipitates indicate a peritectic reaction. In this work, the CALPHAD-based multiphase-field model was used to investigate the superalloy solidification behavior and complex γ/γ′ morphology generation. The fine γ/γ′ and coarse γ′ patterns were reproduced in the γ/γ′ colony, which were compared with the interdendritic microstructure observed by experiments. And the eutectic and peritectic reactions were verified by analyzing the chemical driving force at the phase interfaces. The simulation results suggest that the coarsening of γ′ in the γ/γ′ colony was due to the fast decrease of chemical driving force at the γ/l and γ'/l interfaces. Besides, remelting of the γ dendrite was found near the bulk γ′, which can be mainly ascribe to the increasing of Mo concentration rejected by the growing γ'. The simulated microsegregation patterns of the alloy components were also obtained and the results were in good agreement with the experimental findings.

Study of dendrite growth with natural convection in superalloy directional solidification via a multiphase-field-lattice Boltzmann model

The natural convection is able to change dendrite morphology and cause freckles formation in alloy solidification. Though great progress has been made in in-situ observation techniques and numerical models, the interaction between the melt flow and the growing dendrite in directionally solidified superalloy has not been well understood. In this study, the dendrite growth with natural convection in directional solidified nickel-based superalloy was investigated using a developed multiphase-field-lattice Boltzmann model. The PanNickel thermodynamic database was used to present realistic data for the multicomponent superalloy solidification simulation. A range of temperature gradients and pulling velocities were applied to investigate the effects of solidification conditions on dendrite growth and melt flow. First, the microsegregation pattern of each alloy component was obtained, and the simulation results were compared with the experimental results. Then, the dendrite morphology change and the variation of fluid velocity in the mushy zone under different solidification conditions were analyzed. The normalized average fluid velocity in the mushy zone was obtained and can be used to predict freckle formation. Finally, the effects of grain orientation on the dendrite morphology and melt convection were studied and the results were analyzed.

Multi-scale coupling simulation in directional solidification of superalloy based on cellular automaton-finite difference method

Casting microstructure evolution is difficult to describe quantitatively by only a separate simulation of dendrite scale or grain scale, and the numerical simulation of these two scales is difficult to render compatible. A three-dimensional cellular automaton model couplling both dendritic scale and grain scale is developed to simulate the microstructure evolution of the nickel-based single crystal superalloy DD406. Besides, a macro–mesoscopic/microscopic coupling solution algorithm is proposed to improve computational efficiency. The simulation results of dendrite growth and grain growth of the alloy are obtained and compared with the results given in previous reports. The results show that the primary dendritic arm spacing and secondary dendritic arm spacing of the dendritic growth are consistent with the theoretical and experimental results. The mesoscopic grain simulation can be used to obtain results similar to those of microscopic dendrites simulation. It is indicated that the developed model is feasible and effective.

The Suppression of the Natural Convection in the Directional Solidification Processing of Superalloy by the Introduction of the Traveling Magnetic Field: 2D and 3D Simulation

AbstractIn this work, 2D and 3D, time-dependent flow simulations with relevance to the melt motion during directional solidification of superalloy under a traveling magnetic field (TMF) are presented. Numerical results indicate that the velocity at the front of solidification interface increases rapidly in the area-changeable transition region under the no TMF. However, the downward TMF is able to either stabilize or destabilize the flow in this transition region, which is determined by the effects of the strength of the magnetic field. Under the weak downward TMF, the natural convection is suppressed and the interface always maintains flat in the process of directional solidification, providing favorable conditions for the growth of crystal. Flow is deeply destabilizing with the increase of magnetic field intensity. Finally, in terms of thermal field, flow field and interface shape, their 2D and 3D simulation results are also compared respectively.

Effect of a Transverse Magnetic Field on Stray Grain Formation of Ni-Based Single Crystal Superalloy During Directional Solidification

The effect of a transverse magnetic field on stray grain formation during directional solidification of superalloy was investigated. Experimental results indicated that the transverse magnetic field effectively suppressed the stray grain formation on the side the primary dendrite diverges from the mold wall. Moreover, the quenched experimental results indicated that the solid/liquid interface shape was obviously changed in a transverse magnetic field. The effect of a transverse magnetic field on stray grain formation was discussed.

Low Melt Height Solidification of Superalloys

Effect of a reduced melt height in the directional solidification of a superalloy has been investigated by two methods: vertical Bridgman (VB) and vertical Bridgman with a submerged baffle (VBSB). The latter is a relatively new technique and provides a reduced melt height ahead of the solidifying interface. A low melt height leads to a larger primary dendrite arm spacing but a lower mushy length, melt-back transition length, and porosity. The VBSB technique yields up to 38 pct reduction in the porosity. This may improve a component’s mechanical strength especially in a creep-fatigue type dynamic loading.

Experimental Evidence of the Effect of a High Magnetic Field on the Stray Grains Formation in Cross-Section Change Region for Ni-Based Superalloy During Directional Solidification

The effect of a high magnetic field on the stray grains in cross-section change region was investigated experimentally during directional solidification of superalloy. The microstructures showed that the high magnetic field significantly suppressed the stray grains formation in cross-section change region. Meanwhile, the temperature curves indicated that the nucleation undercooling was significantly increased in a high magnetic field. The effect of a high magnetic field on the stray grains was discussed based on the nucleation mechanism.

A Concept for the Control of Pore Size in Superalloy Membranes

A new method to adjust the pore size in superalloy membranes is shown, utilizing controlled cooling from solution heat treatment of the solid superalloy. Hereby, the nucleation rate and, thus, the size of the γ'-precipitates can be varied to a large extent. This leads to a corresponding variation in the pore size once the membrane material is produced by directional coarsening of the γ'-phase to an interconnected network and subsequent selective extraction of the γ-phase. Furthermore, it was found that coherent and incoherent γ'-precipitates can be used alike to fabricate superalloy membranes, and yet, result in vastly different pore morphologies. The findings widen the application range of this novel material class.

Manufacture and Analysis of Directional Solidification Organization of CMSX-6 Nickel-Base Superalloy

In this paper, we use the High rate solidification to study process parameters and CMSX-6 super-alloy solidification structure. And then, it can provide more basis for the industrial directionally solidified casting of superalloys blades. The results show that: under certain process parameters, it can be obtained parallel columnar crystals directional solidification structure. The organization is composed of the matrix γ phase and the secondary precipitated γ ' phase; under an optical microscope, it can be observed the shrinkage porosity due to the volumes reduction arising solidification.

The effect of liquid metal cooling on thermal gradients in directional solidification of superalloys: Thermal analysis

Numerical methods were used to examine the influence of casting and baffle geometry, mold thickness as well as withdrawal speed on solidification conditions and resulting microstructure. Achievable thermal gradients, stability limits and primary dendrite arm spacings for Liquid Metal Cooling (LMC) and High Rate Solidification (HRS) process are reported. Calculations were compared with experimental results from the literature and good agreement was found. A thermal gradient almost 1.8 times higher was observed for the use of LMC in the case of simple cylindrical castings. In contrast, a thermal gradient up to three times higher was calculated with LMC compared to HRS for large section size castings. The numerical investigations indicate that the nature of the baffle has a stronger effect than the different mechanism of heat dissipation when HRS and LMC are compared.

Numerical modeling of crystal growth of a nickel-based superalloy with applied direct current

A cellular automaton-finite difference (CA-FD) model is developed to simulate crystal growth with applied direct current (DC) in this study. The simulated dendrite growth is controlled by the CA rules that involved the effects of the supercooling, interfacial curvature and crystal anisotropy. The electric current distribution, the temperature field and the electromagnetic flow caused by the Lorentz force are solved by the FD calculation. The directional solidification of a model nickel-based superalloy with DC field is simulated. The results show that Joule heat brought by electric current delays the dendrite growth and changes the primary dendrite arm spacing (PDAS). Excessive Joule heat will completely stop the crystal growth. The maximum allowable current intensity increases with the cooling efficiency of directional solidification. It is found that the dendrites growing with the DC field have a self-adjustment ability to achieve flat S/L interface in macroscale during directional solidification. The electromagnetic flow induced by the DC field may theoretically accelerate the dendrite growth, but usually the influence is negligible for the directional solidification of the superalloy.