High rate directional solidification and its application in single crystal superalloys

In the present paper there are two parts contributing to the discussion of high rate directional solidification and its application. The first part aims to characterize the high rate directional solidification of various kinds of alloys. It was found that the relevant cooling rate of the high rate directional solidification is defined to be within 1–10³ K/s (solidification rate is 10⁻⁴–10⁻¹ m/s as G_L=100 K/cm) and that it is located in the region between the near-equilibrium slow growth rate and the rapid solidification rate beyond the equilibrium condition, whilst at the same time there occurs a series of turning effects of interface stability and morphologies. With the increase in the growth velocity the interface with the plane front evolves to cells and dendrites at the stage of near-equilibrium and with a further increase in growth rate they transformed reversibly from dendrites to cell structure and then to the absolute stability of a planar interface. The change of solute segregation reveals the same from a low segregation, then increased and finally reduced again. An explanation based on effective constitutional supercooling about the evolution of interface morphologies with respect to the changes of growth rate is proposed.The second part is devoted to introducing experimental results for single crystal superalloys using the rate directional solidification principle. It is shown that the single crystal superalloys CMSX-2 and NASAIR 100 exhibit significant improvement in microstructure segregation and mechanical properties at high temperature both in the as-cast and after-heat-treatment conditions with the high rate directional solidification technique.     

Characteristics of S/L Interface Evolution during High Rate Directional Solidification

The present paper aims to the characterization of high rate direction solidification on AI-Mn and AI-Cu alloys. It is indicated that the relevant cooling rate of high rate directional solidification is defined within 10(0)～10(3) K/s that is located in the region between near-equilibrium slow growth rate and rapid solidification rate beyond equilibrium condition, and at the meantime there occurred a series of turning effect of interface stability and morphologies. With the increase of growth velocity the interface with planar front evolved to cells and dendrites at the stage of near-equilibrium and with further increase of growth rate they transformed reversely from dendrites to cell structure and then to absolute stability of planar interface. An explanation based on effective constitutional supercooling about the evolution of interface morphologies with the change of growth rate was proposed.     

Microstructure and stress rupture properties of single crystal superalloy CMSX-2 under high thermal gradient directional solidification

The interface morphologies of single crystal superalloy CMSX-2 were studied over a range of cooling rate with large variations in withdrawal speeds in directional solidification. A superfine cellular structure was obtained under both high thermal gradient up to 1000 K/cm and fast withdrawal rate up to 1 mm/s. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of γ′ phase, reduced microsegregation of alloying elements and smaller size of γ-γ′ eutectics. The rupture life and plasticity of fine structure samples produced in high thermal gradient directional solidification increase significantly than that in conventional directional solidification process at 1323 K.     

A model for distribution of aluminum in silicon refined by vacuum directional solidification

We propose a simple numerical model for distribution of Aluminum (Al) in silicon ingot during vacuum directional solidification, including segregation from silicon crystal to silicon melt as well as evaporation from silicon melt to vacuum atmosphere. According to the model, the effective segregation K_eff and the total evaporation coefficient k_T(Al)of Al under the experimental conditions are 0.0137 and 2.6755 × 10⁻⁶ m·s⁻¹, respectively. In comparison with experimental results, the segregation of Al is dominated at the beginning of solidification, whereas at the last stage of solidification the removal of Al is mainly depended on the evaporation. It is also found that the critical influences on aluminum removal during vacuum directional solidification are temperature and solidification rate.     

Microstructure evolution and interfacial reaction of TiAl–Si alloy solidified in alumina crucible

The microstructure evolution of Ti–43Al–3Si (at-%) alloy solidified in alumina crucible was investigated by directional solidification technology. After directional solidification, the microstructure of the alloy is consisted of γ-TiAl, α₂-Ti₃Al, ξ-Ti₅Si₃ phases and Al₂O₃ particles. There are three morphologies of ξ phases formed in the alloy, namely, long rod-like, cluster-like with eutectic morphology, and needle-like shape. The volume fraction of ξ phases decreases with increasing growth rates. Al₂O₃ particles broke from the crucible and enter into the melt by the thermal physical erosion. Al₂O₃ particles enrich in the liquid phase with the moving of solid-liquid interface, and are captured or entrapped by dendrites during solidification. The Al₂O₃ particles mainly distributed in the interdendritic region, and some particles exist in dendrites.     

Phase-field simulation of directional solidification of a binary alloy under different boundary heat flux conditions

Complicated morphologies of directional solidification structures attract a lot of theoretical studies and commercial uses. As known, the boundary heat flux has an important significance to the microstructures of directional solidification. In this article, the interface evolution of directional solidification with different boundary heat fluxes is discussed. In this study, only one interface has heat flow, and Neumann boundary conditions are imposed at the other three interfaces. From the calculated results, it is found that different heat fluxes cause different microstructures in the directional solidification. When the heat flux equal to 18 W/cm², the growth of lengthways side branches is accelerated and the growth of transverse side branches is restrained. At the same time, there is dendritic remelting in the calculating domain. When the heat flux equal to 36 W/cm², the growth of the transverse side branches and the growth of the lengthways side branches compete with each other. When the heat flux equal to 90 or 180 W/cm², the growth of transverse side branches absolutely dominates. The temperature field of dendritic growth is also analyzed and the relation between heat flux and temperature field is found.     

Effect of Solidification Rate on Microstructure and Solid/Liquid Interface Morphology of Nie11.5 wt% Si Eutectic Alloy

In this study NieN i3 Si eutectic in situ composites are obtained by Bridgman directional solidification technique when the solidification rate varies from 6.0 mm/s to 40.0 mm/s. At the low solidification rates the lamellar spacing is decreased with increasing the solidification rate. When the solidification rate is higher than 25 mm/s, the lamellar spacing tends to be increased, because the higher undercooling makes the mass transport less effective. The adjustments of lamellar spacing are also observed during the directional solidification process, which is consistent with the minimum undercooling criterion. Moreover, the transitions from planar interface to cellular, then to dendritic interface, and finally to cellular interface morphologies with increasing velocity are observed by sudden quenching when the crystal growth tends to be stable.     

Growth morphology and mechanism of MC carbide under quasi-rapid solidification conditions

The carbide of group IVB and group VB elements, i.e. MC carbide, is an important constitution and strengthening phase for many alloy tool steels and cast nickel-base superalloys. Since the as-solidified growth morphology, size and distribution have an important influence on both the mechanical properties and hot workability, research on the solidification behavior of MC carbide is an important subject for cast superalloys and many high alloy tool steels. The growth morphology and mechanisms of MC carbide, under slow-cooling and rapid solidification conditions, has been studied intensively as functions of the solidification cooling rate. The solidification behavior of MC carbide under quasi-rapid solidification conditions has not been reported in open literature. In this paper, the growth morphology and mechanism of an MC carbide (TiC type) under quasi-rapid solidification conditions is studied in a laser surface alloyed coating on a titanium aluminide alloy Ti–48Al–2Cr–2Nb (at.%). The growth morphology of the quasi-rapidly solidified MC carbide with a cooling rate of 4×10²°C is found to be dendritic with strong faceted, double zigzag brick-stacking growth characteristics on the dendrite arms. The growth mechanism of the MC carbide is found to be a brick-stacking/double zigzag micro-branching lateral growth from steps on the intersecting {111} planes.     

Effect of cooling rate on the morphology of γ′ precipitates in a nickel–base superalloy under directional solidification

The morphological evolution of γ′ precipitates in a nickel-based superalloy K5 was studied by zone melting directional solidification under vacuum conditions. The results show that at the lower cooling rate of 12.42 K s^–1, γ′ precipitates remand big cuboids, γ′ particles become smaller at the cooling rate ranges from 12.42 to 38.80 K s^–1. For a rather fast cooling rate of 50.16 K s^–1, γ′ particles retain a spherical shape. The experiments show that big cuboids will become unstable and split into several small one at the lower cooling rate of 1.1 K s^–1. The mechanism of the evolution of the γ′ morphologies is also analyzed by introducing a new parameter-shape factor which classifie the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Effect of cooling rates on the dendritic morphology transition of Mg–6Gd alloy by in situ X-ray radiography

The effect of cooling rate on the transition of dendrite morphology of a Mg-6 Gd(wt%) alloy was semiquantitatively analyzed under a constant temperature gradient by using synchrotron X-ray radiographic technique. Results show that equiaxed dendrites, including exotic 'butterfly-shaped' dendrite morphology, dominate at high cooling rate(1 K/s). When the cooling rate decreases in the range of 0.5–1 K/s, the equiaxed-to-columnar transition takes place, and solute segregates at the center of two long dendrite arms(LDA) of the 'butterfly-shaped' dendrite. When the cooling rate is lower than 0.3 K/s, directional solidification occurs and the columnar dendritic growth direction gradually rotates from the crystalline axis to the thermal gradient direction with an increase in cooling rate. Meanwhile, interface moves faster but the dendrite arm spacing decreases. Floating, collision and rotation of dendrites under convection were also studied in this work.     

Numerical Simulation of Morphology and Microsegregation Evolution during Solidification of Al-Si Alloy

A stochastic model coupled with transient calculations for the distributions of temperature, solute and velocity during the solidification of binary alloy is presented. The model can directly describe the evolution of both morphology and segregation during dendritic crystal growth. The model takes into account the curvature and growth anisotropy of dendritic crystals. Finite difference method is used to explicitly track the sharp solid liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations are performed to simulate the evolution of columnar and equiaxed dendritic morphologies of an AI-7 wt pct Si alloy. The effects of heat transfer coefficient on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys are analyzed. This model is applied to the solidification of small casting. Columnar-to-equiaxed transition is analyzed in detail. The effects of heat transfer coefficient on final casting structures are also studi     

亚快速单向凝固晶体生长的非稳态演化

晶体生长中的非稳态演化过程一直是凝固领域人们很少涉及的课题，尤其在胞枝转变之后相当宽范围的亚快速凝固更是少人问津，而非稳态过程对材料最终的组织往往产生在影响，本文采和有机物模拟合金研究了低速及亚快速凝固范围界面形态与一次间距的演化规律，并初步探讨了其演化机制。     

Phase-field simulation for the evolution of solid/liquid interface front in directional solidification process

In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode, and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.     

Growth morphology and mechanism of MC carbide under quasi-rapid solidification conditions

The carbide of group IVB and group VB elements, i.e. MC carbide, is an important constitution and strengthening phase for many alloy tool steels and cast nickel-base superalloys. Since the as-solidified growth morphology, size and distribution have an important influence on both the mechanical properties and hot workability, research on the solidification behavior of MC carbide is an important subject for cast superalloys and many high alloy tool steels. The growth morphology and mechanisms of MC carbide, under slow-cooling and rapid solidiication conditions, has been studied intensively as functions of the solidiication cooling rate. The solidiication behavior of MC carbide under quasi-rapid solidification conditions has not been reported in open literature. In this paper, the growth morphology and mechanism of an MC carbide (TiC type) under quasi-rapid solidification conditions is studied in a laser surface alloyed coating on a titanium aluminide alloy Ti–48Al–2Cr–2Nb (at.%). The growth morphology of the quasi-rapidly solidified MC carbide with a cooling rate of 4 × 10²°C is found to be dendritic with strong faceted, double zigzag brick-stacking growth characteristics on the dendrite arms. The growth mechanism of the MC carbide is found to be a brick-stacking/double zigzag micro-branching lateral growth from steps on the intersecting {111} planes.     

Effect of Solidification Interface Morphologies on Microstructures of a Ni-base Superalloy

A cast Ni-base superalloy K5 wasdirectionally solidified and various solidification in-terfaces including plane front,cellular,cellular-dendritic and dendritic were obtained in awider range of G/R ratio by using improved highwithdrawal device and liquid metal cooling experi-mental sets.The precipitation pattern of some prin-cipal phases of the alloy and correlation of the vari-ous interfaces with microstructure were studied sys-tematically.It was indicated that the morphology ofsolidification interface of superalloy K5 varied withG/R ratio and that the solidification interfacemorphologies have a considerable effect on the fea-tures of phases both precipitated duringsolidification and post-solidification.Plane frontand cellular directional solidification of superalloyK5 lead to a substantial decrease of MC carbideand elimination of γ-γ'eutectic,but makeneedle-shape M_6C carbide precipitate easily duringageing treatment.The finer dendritic structuressolidified under the condition of higher cooling ratehave less dendritic segregation and idealmicrostructure.     

Simulation of microsegregation and microstructural evolution in directionally solidified superalloys

Abstract

A two-dimensional model for solidification and secondary phase precipitation in directionally solidified superalloys is presented, which makes use of a shape function approximation for the isothermal cross-section of the primary dendrites. The phase boundary is represented by a diffuse interface on a finite difference grid, as in phasefield methods. Thus, two-dimensional multicomponent diffusion can be calculated for all phases. Via a Fortran interface, the model is coupled to thermodynamic databases assessed using the ‘calculation of phase diagrams’ (Calphad) approach. In this way, for each time step actual data are available for partitioning, dendritic growth, and nucleation of secondary phases. The model is applied to Ni–Al–Cr as a ternary model system and to the commercial superalloy IN706.     

Influence of directional solidification variables on the microstructure and crystal orientation of AM3 under high thermal gradient

Solid–liquid interface morphologies of a nickel-base single crystal superalloy AM3 were investigated under high thermal gradient. The critical velocities of planar–cellular and cellular–dendritic transition were greatly increased by high thermal gradients. A high thermal gradient was of great benefit to dendrite refinement. Experimental results showed that the primary and secondary dendrite arm spacings decreased with increasing cooling rate. As expected, the segregation of elements was suppressed and the size of the gamma prime (γ′) phase decreased significantly with increasing withdrawal rates. The shape of γ′ in interdendritic region kept cuboidal at higher withdrawal rate. It was found that the withdrawal rates had little influence on the crystallographic orientation in high thermal gradient directional solidification.     

The effect of silicon on the microstructure and segregation of directionally solidified IN738 superalloy

Th e effect of silicon on the microstructure and solidification segregation of directionally solidified IN738 nickel-based superalloy was studied. Directional solidification at various solidification rates and partial directional solidification plus rapid quenching were applied. Metallographican alysis and an electron microp robe were mainly used to observe and measure the micro structure and elemental segregation of the alloy, respectively. It was found that silicon affected the morphology of the liquid-solid interface of the alloy during solidification and gave the alloy a tendency to form well-developed dendrites. The addition of siliconen larged the solid us-liquidus temperature interval. and the solidification rate also greatly influenced the interval. The interval increased with increasing solidification rate. Silicon promoted the precipitation of the γ/γ' eutectic, and also affected its precipit ation temperature. Silicon segregated mainly in interd endritic regions, and promoted the segregation of other elements. All of the effects of silicon on the alloy related to the solidification rate.     

Al-Cu 合金高梯度定向凝固过程中的形态转变

本文利用 ZMLMC 定向凝固装置,研究了 Al-Cu 合金系在不同温度梯度下定向凝固时凝固界面和组织形态的变化。发现存在着两种树枝状生长和胞状生长之间的转变,即在低速范围内胞状向树枝状转变,在高速范围内树枝状向胞状转变;当温度梯度足够高时,可以在整个生长速率范围内不出现树枝状生长,获得高度细化的胞状组织;合金的结晶温度间隔越宽,完全消除树枝状生长所需的温度梯度越高。