Microstructure Evolution in Directionally Solidified Fe-Ni Alloys in Diffusive Regime

Directional solidification experiments have been carried out in Fe-Ni peritectic alloys to study microstructure evolution in diffusive regime. A numerical modeling of melt convection was developed and discussed to investigate the convective velocity in samples of different diameters. The simulation results show that convection effects can be reduced by decreasing the sample diameter, and diffusion-controlled growth can be achieved if the sample diameter is smaller than about 1 mm. Based on the simulation results, experiments were performed in thin samples of 1 mm diameter to obtain microstructures in diffusive regime. Different kinds of microstructure evolutions were observed in the directionally solidified Fe-Ni alloys. The time-dependent microstructure evolution implies that the solidification process seemed to be in non-steady state rather than in steady state. Based on the transient model, solute distributions in the liquid ahead of the primary and peritectic phases were discussed to reveal the microstructure evolution. Since the solute partition coefficients of the primary and peritectic phases are different, the magnitudes of solute element rejected by the two solid phases are different in two-phase growth conditions. And within the boundary layer, the solute fields ahead of the primary and peritectic phases are different owing to the weak effect of solute mixing in diffusion-controlled regime. Furthermore, microstructure evolution in peritectic alloys was discussed based on the analysis of solute redistribution.     

Microstructure Formation in AlSi7Mg Alloys Directionally Solidified in a Rotating Magnetic Field

To produce high stressed automotive components like engine frames and cylinder heads in foundry industry often AlSi7Mg alloys are used. During mould filling and casting melt flow affects the development of the microstructure, which defines the mechanical properties. In this paper the microstructure formation in AlSi7Mg0.3 and AlSi7Mg0.6 alloys during directional solidification is investigated. To induce a forced melt flow a rotating magnetic field is applied. For that purpose a Bridgman‐type gradient furnace is equipped with a rotary ring magnet. For detailed investigation of the shape of the solid‐liquid interface and the primary dendrite spacing a decanting device is used. As a result, the forced melt flow substantially changes the dendritic solidification microstructure. The rotating magnetic field generates a radial secondary flow in and ahead of the mushy zone, which causes an enrichment of eutectics in the centre of the samples. At lower solidification velocities this locally leads to the transition to mixed columnar‐equiaxed or even to equiaxed growth. In that case the solid‐liquid interfaces of the decanted samples show a significant depression in the centre part. In the out‐of‐centre region columnar growth still exists and the primary dendrite spacing decreases with increasing melt flow.     

Effect of Ag Content on the Microstructure and Crystallization of Coupled Eutectic Growth in Directionally Solidified Al-Cu-Ag Alloys

A series of coupled eutectic growths along the univariant eutectic groove in the ternary Al-Cu-Ag alloy was studied to investigate the effect of Ag on the microstructure and crystallization of directionally solidified Al-Cu-Ag alloys. The results indicated that the eutectic morphology and orientation relationship (OR) between eutectic phases were modified as the Ag content in the Al-Cu-Ag alloys increased. At a lower growth velocity (R ≤ 1 μm/s), a banded structure formed and the interlamellar spacing decreased with the increasing Ag content. At a higher growth velocity (R ≥ 3 μm/s), the eutectic cell spacing decreased with increasing Ag content. Increasing the Ag content in the Al-Cu-Ag alloys enhanced the enrichment of the Ag solute in the liquid ahead of the quenched liquid/solid interface. In addition, increasing the Ag content in the Al-Cu-Ag alloys promoted the transformation from a “Beta 6” OR to an “Alpha 4” OR between eutectic phases. Modifications of the eutectic morphology and the OR during directional solidification were attributed to the enrichment of Ag content at the solid/liquid interface and the changes in the interfacial energy due to the increase in Ag solubility in the α-Al phase.

     

快速凝固偏晶合金的显微结构

采用雾化快速凝固技术制备了ＡｌＰｂ二元偏晶合金。对合金的显微结构分析结果表明，在快速凝固条件下，偏晶合金中的第二相形态从带条状转变为颗粒状。随冷速的增加，第二相在基体中的分布更趋均匀、细化；随冷速的降低和第二相含量的增加，第二相趋于分布于材料的表面。本文从凝固过程的凝固速度和固／液界面对第二相液滴的界面排斥对上述现象进行了解释。     

Effect of Multi-Scale Thermoelectric Magnetic Convection on Solidification Microstructure in Directionally Solidified Al-Si Alloys Under a Transverse Magnetic Field

The influence of a transverse magnetic field (B < 1 T) on the solidification structure in directionally solidified Al-Si alloys was investigated. Experimental results indicate that the magnetic field caused macrosegregation, dendrite refinement, and a decrease in the length of the mushy zone in both Al-7 wt pct Si alloy and Al-7 wt pct Si-1 wt pct Fe alloys. Moreover, the application of the magnetic field is capable of separating the Fe-rich intermetallic phases from Al-7 wt pct Si-1 wt pct Fe alloy. Thermoelectric magnetic convection (TEMC) was numerically simulated during the directional solidification of Al-Si alloys. The results reveal that the TEMC increases to a maximum ( \( u_{\rm{max} } \) ) when the magnetic field reaches a critical magnetic field strength ( \( B_{\rm{max} } \) ), and then decreases as the magnetic field strength increases further. The TEMC exhibits the multi-scales effects: the \( u_{\rm{max} } \) and \( B_{\rm{max} } \) values are different at various scales, with \( u_{\rm{max} } \) decreasing and \( B_{\rm{max} } \) increasing as the scale decreases. The modification of the solidification structure under the magnetic field should be attributed to the TEMC on the sample and dendrite scales.     

The Influence of Surface Roughness on Freckle Formation in Directionally Solidified Superalloy Samples

A series of directional solidification experiments with superalloy CMSX-4 was carried out with various surface profiles. The results obtained clearly demonstrate the influence of surface features on the occurrence of freckles. The rough surfaces exhibit less freckling tendencies than smooth surfaces. For a notch depth larger than the critical layer beneath the surface, the freckle formation can be completely suppressed. The freckle formation is not only a surface defect but can initiate the occurrence of internal equiaxed and stray grains. The results of the present study can be applied to completely prevent freckle formation and the accompanying internal grain defects by using oversize with an appropriately roughened surface.     

An Evaluation of Primary Dendrite Trunk Diameters in Directionally Solidified Al-Si Alloys

The primary dendrite trunk diameters of Al-Si alloys that were directionally solidified over a range of processing conditions have been evaluated. The empirical data are analyzed with a model that is an extension of one used to describe the ripening of dendrite arms. The analysis, based primarily on an assessment of secondary dendrite arm dissolution in the mushy zone, fits well with the experimental data. It is suggested that the primary dendrite trunk diameter is a useful metric that correlates well with the actual solidification processing parameters, and complements the conventionally used primary, secondary, and tertiary arm spacings.     

Macrosegregation during steady-state arrayed Growth of Dendrites in Directionally Solidified Pb-Sn Alloys

Macrosegregation along the length of the directionally solidified samples is produced when Pb-Sn alloys (10 to 58 wt pct Sn) are directionally solidified in a positive thermal gradient (melt on top, solid below, and gravity pointing down) with steady-state dendritic arrayed morphology (the length of the mushy zone, much smaller than the initial length of the melt column, re- maining nearly constant during growth). The extent of the macrosegregation increases with increasing tin content, becomes maximum for 33.3 wt pct Sn, and decreases with further in- crease in tin content. The intensity of the interdendritic thermosolutal convection responsible for the longitudinal macrosegregation can be represented by the effective partition coefficient (k _ε), anempirical parameter obtained from the dependence of the longitudinal macrosegregation on fraction distance solidified. The extent of the macrosegregation appears to be related to a parameter, {λ ²₁ ƒ_E(C _E -C _t)}, where λ₁, is the primary dendrite spacing,f _E is the volume fraction of the interdendritic melt, andC _E andC _t, are the eutectic composition and the melt composition ahead of the dendrite tips, respectively.

     

定向凝固Fe-6.5%Si合金显微组织的EBSD分析

采用定向凝固方法制备了Fe-6.5%Si实验合金板,并进行了金相观察、X射线衍射分析,EBSD分析和磁性测定。结果表明,合金板由沿定向凝固方向（100）生长的粗大柱状晶所组成,柱状晶的平均截面直径约为3mm,柱状晶之间绝大部分为大角晶界,其余为少量小角晶界和重位点阵晶界。合金板具有强的立主织构并显示出优良的磁性能,其矫顽力只有3．45A／m,小于其他方法制备的Fe-6.5%Si合金。     

Effects of Growth Rates and Compositions on Dendrite Arm Spacings in Directionally Solidified Al-Zn Alloys

Dendritic spacing can affect microsegregation profiles and also the formation of secondary phases within interdendritic regions, which influences the mechanical properties of cast structures. To understand dendritic spacings, it is important to understand the effects of growth rate and composition on primary dendrite arm spacing (λ ₁) and secondary dendrite arm spacing (λ ₂). In this study, aluminum alloys with concentrations of (1, 3, and 5 wt pct) Zn were directionally solidified upwards using a Bridgman-type directional solidification apparatus under a constant temperature gradient (10.3 K/mm), resulting in a wide range of growth rates (8.3–165.0 μm/s). Microstructural parameters, λ ₁ and λ ₂ were measured and expressed as functions of growth rate and composition using a linear regression analysis method. The values of λ ₁ and λ ₂ decreased with increasing growth rates. However, the values of λ ₁ increased with increasing concentration of Zn in the Al-Zn alloy, but the values of λ ₂ decreased systematically with an increased Zn concentration. In addition, a transition from a cellular to a dendritic structure was observed at a relatively low growth rate (16.5 μm/s) in this study of binary alloys. The experimental results were compared with predictive theoretical models as well as experimental works for dendritic spacing.     

Preparation and Microstructure Evaluation of Directionally Solidified Nd-Fe-B Ingots by Electromagnetic Cold Crucible

In this paper Nd-Fe-B ingots with hyper-peritectic composition were prepared through continuous and directional solidification by a novel electromagnetic cold crucible approach.A group of experiments were carried out in order to investigate the effects of input power and withdrawal velocity on the microstructure and growth orientation of Nd-Fe-B phases.It was found that the peritectic Nd₂Fe₁₄B phase grows with a planar interface at lower withdrawal velocity and changed into the dendritic interface at higher withdrawal velocity.The result was explained by the theory of constitutional supercooling.Meanwhile the volume fraction of ferromagnetic T₁ phase was found to be increased first and then decreased with the increasing of withdrawal velocity during the growth process of the ingots.     

The Effect of Long-Term Thermal Exposure on the Microstructure and Stress Rupture Property of a Directionally Solidified Ni-Based Superalloy

Microstructural degradation and microstructure-property relationship during long-term thermal exposure in a directionally solidified Ni-based superalloy are systematically studied. The coarsening kinetics of γ′ precipitation conforms well to the LSW model during the long-term thermal exposure. The detailed time dependence of MC decomposition during the long-term thermal exposure is revealed. Grain boundary coarsening was mainly facilitated by γ′ and M₂₃C₆ precipitates coarsening in GBs region, and the GB coarsening kinetics conforms well to the JMAK theory. During different stages of the thermal exposure, dominant factors for the decrease of stress-rupture lifetime vary due to the evolution of multiple microstructures (γ′ coarsening, MC decomposition, and grain boundary coarsening).     

NiAl-Cr(Mo)-Hf共晶合金定向凝固组织的特性及力学性能

采用高温度梯度定向凝固装置制备NiAl-Cr(Mo)-Hf共晶合金,系统地研究了凝固速率对合金凝固组织和力学性能的影响.随凝固速率的增大,固液界面依次呈现平、胞、枝的形貌,而且共晶胞的尺寸和层片间距随之减小.同时发现当凝固速率在3.33μm/s到16.7μm/s范围内变化时,合金的力学性能随凝固速率的增大而增大.(Mo)-Hf;定向凝固;显微组织;力学性能