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.     

Microstructural Evolution in Directionally Solidified Ni-Base Superalloy IN792+Hf

Microstructural evolution during directional solidification （DS） of Ni-base superalloy IN792＋Hf has been investigated with an emphasis on theγ′precipitates and MC-type carbides.The quantitative image analyses revealed that the increase in the solidification rate up to 100μm/s at constant thermal gradient of 178 K/cm resulted in a fine and uniform distribution ofγ′precipitates.The relationship between the as-castγ′size and cooling rate was also determined for DS IN792＋Hf.In the mean time,the MC carbide size was found to be dependent both on the solidification rate and the S/L interface morphology while the area fraction of MC carbide was significantly influenced by the S/L interface morphology.     

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.     

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.     

高温合金定向凝固技术研究进展

首先回顾了定向凝固的发展历史,重点分析了液态金属冷却定向凝固的技术特点。总结了高温度梯度下制备的定向凝固法单晶高温合金在组织和性能方面的研究现状,结合作者在本领域的研究,着重分析了定向凝固温度梯度、凝固速率、晶体取向、熔体超温处理、熔体对流控制对组织和性能的作用规律和机制,认为高温度梯度定向凝固是细化组织、减少缺陷、提高合金性能的重要途径。最后展望了高温合金定向凝固的发展趋势。     

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.     

Analysis of the high growth-rate transition in Al–Si eutectic solidification

The local solidification conditions and mechanisms associated with the flake-to-fiber growth mode transition in Al–Si eutectic alloys are investigated here using Bridgman-type gradient-zone directional solidification. Resulting microstructures are examined through quantitative image analysis of two-dimensional sections and observation of deep-etched sections, showing three-dimensional microstructural features. Several microstructural parameters were investigated in an attempt to quantify this transition, and it was found that the particle aspect ratio is effective in objectively identifying the onset and completion velocity of the flake-to-fiber transition, whereas traditional spacing parameters are not effective indicators of the transition. For a thermal gradient of 7–14 K/mm, the transition was found to occur in two stages, appearing over velocity regimes from 0.10 to 0.50 mm/s and from 0.50 to 0.95 mm/s. The initial stage is dominated by in-plane plate breakup and rod formation within the plane of the plate, whereas the second stage is characterized by the onset of out-of-plane silicon rod growth, leading to the formation of an irregular fibrous structure. The boundary between the two stages is marked by widespread fibrous growth and the disappearance of the remnant flake structure, indicating a transition in the structural feature that governs the relevant diffusion length, from inter-flake spacing to inter-rod spacing.     

The effect of Re and Ru on γ/γ′ microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys

A new in-house designed series of Ni based superalloys with stepwise increased Re and Ru additions has been investigated, to systematically determine the influence of Re and Ru on γ/γ′-microstructure and high temperature creep properties. Improved creep resistance and thus also a higher alloy temperature capability of up to 87 K/at.% was found for additions of Re. Additions of Ru revealed a lower temperature capability improvement of up to 38 K/at.% for low Re-containing second generation alloys. However, in third and fourth generation alloys with higher Re-contents, no significant influence of Ru on creep rupture strength was observed. The creep properties are discussed with respect to the γ′-volume fraction, γ′-size and γ′-coarsening rate, as well as the γ/γ′-lattice misfit and the γ/γ′ partitioning coefficient of the different Re and Ru containing alloys. The presented data shows, that these microstructure parameters are strongly influenced by additions of Re, but only marginally by additions of Ru. A further influence on creep rupture strength is given by the solid solution hardening of the γ-matrix, which is discussed based on solid solution hardener concentrations either experimentally derived or calculated from ThermoCalc data.     

高温度梯度定向凝固高温合金单晶初始生长过程非稳态自组织行为

用ＺＭＬＭＣ超高梯度定向凝固装置研究宽凝固速度范围内籽晶法制备含细亚结构的ＤＤ３单晶的起动生长段的组织演化与选择特征。试验获得了直径为７ｍｍ的ＤＤ３单晶合金，起动生长段组织演化对界面前沿液相中的温度梯度ＧＬ，系统稳定抽拉速率Ｖ，系统起动时的升速模式及系统干扰等因素的基本依赖关系，并对该非稳态自组织过程的内在控制因素及其形成机制进行了初步的分析。     

Investigation of the effect of solidification processing parameters on the rod spacings and variation of microhardness with the rod spacing in the Sn–Cu hypereutectic alloy

Sn–3 wt% Cu hypereutectic alloy was prepared in a graphite crucible under the vacuum atmosphere. The samples were directionally solidified upwards under argon atmosphere with different temperature gradients (G = 4.24–8.09 K/mm) at a constant growth rate (V = 7.64 μm/s) and with different growth rates (V = 2.24–133.33 μm/s) at a constant temperature gradient (G = 4.24 K/mm) by using a Bridgman type directional solidification apparatus. The microstructure of directional solidified Sn–3 wt% Cu alloy seems to be rod eutectic structure. The influence of the growth rate (V) and temperature gradient (G) on the rod spacing (λ) and undercooling (ΔT) were analysed. The values of λ²V, λ²G, ΔTλ, ΔTV^−0.5 and ΔTG^−0.5 were determined by using the Jackson–Hunt eutectic theory. The dependence of microhardness (HV) on the rod spacing (λ) was analyzed. According to present results, it has been found that the value of HV increases with the increasing the value of λ.     

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 ones 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 classifies the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Solidification Simulation of Single Crystal Investment Castings

The three dimensional solidification simulation of the single crystal investment castings at withdrawal rates of 2 mm/min ,4.5 mm/min and 7 mm/min was performed with the finite element thermal analysis method.The calculated result were in accordance with the experimental ones.The results showed that with the increase of with-drawal rate the concave curvature of the liquidus isotherm was bigger and bigger and the temperature gradient of the castings decreased.No effects of withdrawal rate on the distribution of the temperature gradient of the starter and helical grain selector of the castings were observed at withdrawal rates of 2 mm/min ,4.5 mm/min and 7 mm/min.The relatively high temperature gradient between 500℃/cm and 100℃/cm in the starter and helical grain selector was obtained at three withdrawal rates.The study indicates the three dimensional solidification simulation by finite element method is a powerful tool for understanding solidification and predicting defects in single crystal investment castings.     

Directional solidification of Cu-20Sn alloy at low speed: From peritectic coupled growth to banding

Bridgman-type directional solidification experiments have been carried out in Cu-20Sn peritectic alloy. Peritectic coupled growth and banding structures have been observed at low growth rates (1.5 and 2 μm/s) under a temperature gradient up to 40 K/mm. The peritectic coupled growth structure, containing rod dendrite primary α phase plus peritectic β phase, forms initially. As solidification proceeds, peritectic coupled growth is overgrown by banding or island banding structures. The formation of banding structure from coupled growth is explained by a model involving Sn concentration change at nucleation of the secondary phase ahead of the solid/liquid interface. It is found that the competitive growth between the α and β phases also plays a critical role on the formation of banding structures.     

Microstructure evolution and analysis of a single crystal nickel-based superalloy during compressive creep

During compressive creep, the cubical γ′ phase in [0 0 1] orientation single crystal nickel-based superalloy is transformed into the rafted structure along the direction parallel to the applied stress axis. By means of the elastic stress-strain finite element method (FEM), the von Mises stress distributions of the cubical γ′/γ phases are calculated for investigating the influence of the applied stress on the stress distribution and the directional coarsening regularity of γ′ phase. Results show that the stress distribution of the cubical γ/γ′ phases may be changed by the applied compressive stress, and the coarsening orientation of γ′ phase is related to the von Mises stress distribution of the γ matrix channel. Thereinto, under the action of applied compressive stress, the bigger von Mises stress produced on (0 0 1) plane of the cubical γ′ phase is thought to be a main reason of the microstructure evolution. The expression of the driving force for the elements diffusion and the directional growing of γ′ phase during compressive creep are also proposed.     

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。     

合金化对定向凝固Ti-AI-Nb合金微观结构与力学性能的影响

采用定向凝固方法制备了Cr/Mo微合金化高Nb Ti-Al合金,并考察了其微观结构和力学性能.结果表明,两种合金都呈现出层片状微观组织.尽管温度梯度受限,抽拉速度较高,但与Ti-16A1-12Nb相比,Ti-16A1-12Nb-3Cr-1Mo合金中的片层倾向于与抽拉方向呈更小角度生长,在室温和650℃时的拉伸强度和塑性...     

The Control of Solidification of Ni‐Based Superalloy Single‐Crystal Blade by Mold Design Modification using Inner Radiation Baffle

This work is focused on an investigation of the directional solidification process of CMSX‐4 single‐crystal blades in the mold modified by application of inner radiation baffle (IRB). Micro‐ and macrostructure examination is carried out along the height of single‐crystal blades which are manufactured using standard and modified mold, at the withdrawal rates of 3 and 5 mm min^?1. It is established that application of modified mold allows better control over liquidus isotherm and leads to increase in temperature gradient, as compared to the manufacturing of blades using standard mold. The average value of temperature gradient in airfoil increases from 14 up to approx. 30 K cm^?1 and from 16 up to approx. 40 K cm^?1, for withdrawal rate of 5 and 3 mm min^?1, respectively. The curvature of liquidus isotherm diminishes and attains near‐flat profile along the airfoil for both values of withdrawal rate. The application of proposed technique results in reduction of primary dendrites arm spacing (PDAS) particularly in the inner and middle part of the blade. PDAS reaches average value of approx. 360 μm for withdrawal rate of 5 mm min^?1 and is lower as compared to the standard mold casting withdrawn at the rate of 3 (433 μm) and 5 mm min^?1 (468 μm).

     

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 lowsegregation, 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 solidiication 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.     

Investigation of mechanical,electrical, and thermal properties of a Zn–1.26 wt% Al alloy

Zn–1.26 wt% Al alloy was directionally solidified upward with a constant growth rate (V = 16.6 μm/s) in a wide range of temperature gradients (1.94–5.15 K/mm) and with a constant temperature gradient (G = 5.15 K/mm) in a wide range of growth rates (8.3–500 μm/s) with a Bridgman-type directional solidification furnace. The microhardness (HV) and tensile strength (σ) of alloy were measured from directionally solidified samples. The dependency of the microhardness, tensile strength for directionally solidified Zn–1.26 wt% Al alloy on the solidification parameters (G, V) and microstructure parameters (λ₁, λ₂) were investigated and the relationships between them were experimentally obtained using regression analysis. According to present results, the microhardness and tensile strength of directionally solidified Zn–1.26 wt% Al alloy increase with increasing solidification processing parameters and decrease with the microstructure parameters. Variations of electrical resistivity (ρ) with the temperature in the range of 300–650 K were also measured using a standard dc four-point probe technique for cast samples. The enthalpy of fusion and specific heat for same alloy was also determined by means of differential scanning calorimeter (DSC) from heating trace during the transformation from solid to liquid.     

单晶高温合金定向凝固过程数值模拟

实测了不同抽拉速度单晶高温定向凝固过程的初始条件,边界条件及温度场,建立了数值模拟系统的实体模型,采用ＰｒｏＣＡＳＴ有限元模拟软件包计算了不同抽拉速度单晶合金试板的定向凝固过程温度场。