Susceptibility of magnesium alloys to solidification cracking

Solidification cracking of Mg welds has been reported frequently, but the crack susceptibility itself has not been studied much. In the present investigation the widely used Mg alloys AZ31, AZ61, AZ91 and ZK61 were selected for the study. The crack susceptibility was predicted based on the maximum │dT/d(f_S)^1/2│ up to (f_S)^1/2?=?0.99 as the crack susceptibility index (T: temperature; f_S: fraction of solid). The predicted crack susceptibility decreased in the order of ZK61?>?AZ31?>?AZ61?>?AZ91. Since no reported data were available for comparison with the prediction, the transverse motion weldability (TMW) test was conducted. The tested crack susceptibility decreased in the order of ZK61?>?AZ31?>?AZ61?>?AZ91, thus verifying the prediction based on the index │dT/d(f_S)^1/2│. The present study demonstrated that the crack susceptibility index and the TMW test can be useful tools for studying solidification cracking of Mg welds.     

Advanced microstructural characterization of plasma-sprayed zirconia coatings over extended length scales

Achieving control of the microstructure of plasma-sprayed thermal barrier coating (TBC) systems offers an opportunity to tailor coating properties to demanding applications. Accomplishing this requires a fundamental understanding of the correlations among processing, microstructure development, and related TBC properties. This article describes the quantitative characterization of the microstructure of plasma-sprayed partially stabilized zirconia (PSZ) coatings by means of x-ray and neutron-scattering imaging techniques. Small-angle neutron scattering, ultra-small-angle x-ray scattering, and x-ray microtomography were used to characterize and visualize the nature and structure of the features in these material systems. In addition, the influence of processing parameters on microstructure development is discussed along with thermal cycling effects on the pore morphology, and their resultant influence of the porosity on the thermal conductivity and elastic modulus of plasma-sprayed PSZ TBCs.     

定向凝固Sn-36%Ni包晶合金凝固组织特征长度的表征

通过对Sn?36%Ni包晶合金在恒定温度梯度(G=20 K/mm)下进行的一定速度范围内(v=2~200μm/s)的定向凝固实验,研究凝固组织特征尺度随生长速度v的变化;测量包括一次与高次枝晶间距及枝晶尖端半径在内的凝固组织特征尺度。通过实验结果与理论模型的对比,发现凝固组织特征尺度随生长速度的变化关系为：对于一次枝晶间距有λ1=335.882v?0.21,且与 Kurz?Fisher 模型吻合;对于二次枝晶间距有λ2=44.957v?0.277,且与 Bouchard?Kirkaldy模型吻合;对于三次枝晶间距有λ3=40.512v?0.274;对于枝晶尖端半径有R=22.7v?0.36。实验结果表明,λ1/λ2随着生长速度的增加而增加,λ1/λ3的变化明显较λ1/λ2的小,表明三次枝晶具有与一次枝晶类似的生长特征;而λ1/R的比值随着生长速度的增加而由2增加到2.3,变化很小。     

Growth restriction effects during solidification of aluminium alloys

The effects of solute elements during solidification on the grain size are very important and can be quantified by the growth-restriction parameter Q, and Q possesses the better correlation with the grain size. Based on the constitutional undercooling generated by the growth of an adjacent grain during the initial solidification, the growth-restriction parameter Q is deduced and a comprehensive physical basis of Q is obtained by using an initial solute distributing equation. For the alloys with more potent nucleants, Q is a suitable predictor of the grain size. For less potent nucleants, the relative grain size(RGS) is a more accurate prediction of the grain size. This prediction coincides with the experimental behaviors for Al-Ti and Al-Cu alloys with lower solute content.     

Microstructure evolution of Cu-Pb monotectic alloys during directional solidification

1 Introduction Monotectic alloy is an important class of alloy whose binary phase diagram has a miscibility gap, in which the original single liquid will decompose into two distinct immiscible liquids within a few seconds. In the normal gravity field, a …     

Effects of various Mg-Sr master alloys on microstructural refinement of ZK60 magnesium alloy

The effects of various Mg-Sr master alloys(conventional as-cast,rapidly-solidified,rolled and solutionized) on microstructural refinement of ZK60 magnesium alloy were investigated.The results indicate that the refinement efficiency of various Mg-Sr master alloys in ZK60 alloy is different.The rolled Mg-Srmaster alloy is found to have relatively higher refinement efficiency than the conventional as-cast,solutionized and rapidly-solidified Mg-Sr master alloys.After being treated with the rolled Mg-Sr maste...     

Dynamical solidification behaviors and microstructural evolution during vibrating wavelike sloping plate process

A vibrating wavelike sloping plate process (VWSP) was proposed. Heat and solute transformations, nucleation mechanism and grain growth as well as microstructure evolution were investigated. It is shown that the sloping plate can provide strong undercooling, and a large quantity of heterogonous nuclei appear on the sloping plate surface, wavelike flow and vibration can enable heterogonous nucleus to escape off the plate, which lead to nucleus multiplication. Moreover, wavelike flow and vibration improve solute diffusion coefficient and cause uniform solute and temperature field, which lead to eruptive nucleation. Grain growth has two typical ways, direct globular growth and dendritic growth. Under relative uniform temperature and solute fields, some grains can keep stable growth surface, go on growing with the round surface and finally maintain their globular structure. However, there are always some grains that grow along a certain preferred direction, but under wavelike flow and vibration their dendritic arms break and transform into near spherical structure. During the casting process, microstructural evolution from globular/dendritic structure to globular/equiaxed structure and to globular structure was observed.     

镁合金铸轧凝固层焊合点控制及工艺优化

结合镁合金铸轧工艺特点,分析镁合金板坯铸轧过程中凝固层焊合点位置与板坯缺陷的影响规律;针对板坯厚度、铸轧区长度及铸轧速度等关键参数,简化凝壳径向生长及凝固前沿周向转动过程,确定铸轧速度匹配范围,建立凝固层焊合点位置控制模型,并通过工艺试验对控制模型进行验证.结果表明通过理论模型确定工艺匹配范围,可稳定并优化镁合金铸轧工艺,大幅度降低铸轧板坯宏观缺陷,获得表面光洁、质量良好的镁合金铸轧板坯.     

Pb-Bi包晶合金定向凝固过程中带状组织的形成(英文)

采用Bridgman定向凝固技术对Pb-Bi包晶合金进行定向凝固实验,生长速度为0.5μm/s,温度梯度为35K/mm。在亚包晶和过包晶成分的Pb-Bi合金中(Pb-xBi,x=26%,28%,30%和 34%)均观察到带状组织。由于熔体对流的影响,在试样中心形成树状初生α相,周围被包晶β相基体包围。带状组织出现在树状组织后,并且该带状组织为过渡性的。测定带状组织中的成分分布,从而确定α和β两相的形核过冷度。在有限长试样中,熔体对流是形成过渡性带状组织的主要原因,该过渡性带状组织表现为有限带数,带宽不恒定,且在Pb-Bi合金的两相包晶区很宽的成分范围内出现。     

相场法模拟Cu-Ag合金时效过程中组织演变

基于相场方法,模拟研究了Cu-20和40 at%Ag合金在等温时效过程中相分解与微观组织的演变.系统自由能中耦合了体化学能、浓度梯度能和共格错配应变能,化学自由能是直接利用相图热力学数据计算得到的,因此,计算的微结构变化与真实合金系统是相对应的.通过模拟,获得了合金的形态和浓度分布随时间、温度和组分的变化规律.模拟结果表明,对称合金(Cu-40at%Ag)分解和粗化的速度均高于相应的非对称合金,其相分离的早期阶段形成规则的、相互连接的微观形态.在Cu-20at% Ag合金中,沉淀相Ag经历了调幅分解和粗化过程,其形态呈立方状,沿[110]方向排列.     

Numerical simulation on microstructural evolution during multipass hot-rolling of aluminum alloys

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Theoretical analysis of diffusion of solutes during the solidification of alloys

When an alloy solidifies and the composition of the solid differs from the composition of the liquid, atoms of the alloying elements rejected at the solid-liquid interface have to diffuse toward the bulk liquid. Diffusion may be supported by convection. Theoretical calculations have been made for a liquid without convection, for a liquid involving natural convection, and for solidification of a liquid alloy at the surface of a rotating disk as an example of forced convection.     

Controlling microstructures of AZ31 magnesium alloys by an electromagnetic vibration technique during solidification: From experimental observation to theoretical understanding

This paper examines the microstructure formation of magnesium-based AZ31 alloys solidified in a magnetic field with the imposition of an alternating current, from which an electromagnetic vibration force is yielded. The grain structure was characterized, revealing that refined equiaxed grains could be yielded at a frequency range from ∼500 Hz to less than 2000 Hz. When the vibration frequency was too low or too high, coarse structures could be obtained. In the mushy zone, a significant difference in electronic resistivity between a solid and a liquid drives the solid to move much faster than the surrounding liquid, thus yielding relative velocity and relative displacement, due to which a dendrite may be segmented into pieces. This motion generates agitation in the semisolid stage, thus making the microstructure more random and resulting in deformation twins. At low frequencies, the coarse structure may be due to the suppression of macrofluid flow by a high magnetic field. Grain refinement occurs at the frequency interval where the mobile leading solid is vibrated beyond the solute operating region, in which the relative displacement covered by the solid is larger than the thickness of solute equivalent boundary layer. At high frequencies, the relative displacement is so small that it is always less than the thickness of the solute boundary layer and the vibration cannot alter the solute pile-up ahead of the solid/liquid interface of the growing crystals. Thus, it is similar to that in normal casting and always produces very coarse structures.     

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al-Mg alloys

The mechanisms involved in the grain refinement of Al-Mg alloys through varying the Mg content and applying intensive melt shearing were investigated. It was found that the oxide formed in Al-Mg alloys under normal melting conditions is MgAl₂O₄, which displays an equiaxed and faceted morphology with {1 1 1} planes exposed as its natural surfaces. Depending on the Mg content, MgAl₂O₄ particles exist either as oxide films in dilute Al-Mg alloys (Mg < 1 wt.%) or as naturally dispersed discrete particles in more concentrated Al-Mg alloys (Mg > 1 wt.%). Such MgAl₂O₄ particles can act as potent sites for nucleation of α-Al grains, which is evidenced by the well-defined cube-on-cube orientation relationship between MgAl₂O₄ and α-Al. Enhanced heterogeneous nucleation in Al-Mg alloys can be attributed to the high potency of MgAl₂O₄ particles with a lattice misfit of 1.4% and the increased number density of MgAl₂O₄ particles due to either natural dispersion by the increased Mg content or forced dispersion through intensive melt shearing. It was also found that intensive melt shearing leads to significant grain refinement of dilute Al-Mg alloys by effective dispersion of the MgAl₂O₄ particles entrapped in oxide films, but it has marginal effect on the grain refinement of concentrated Al-Mg alloys, where MgAl₂O₄ particles have been naturally dispersed into individual particles by the increased Mg content.     

Directional solidification of TiAl-base alloys

Mechanical properties of the lamellar microstructure of TiAl-base alloys are extremely anisotropic with respect to the lamellar orientation. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of strength and ductility. Unfortunately, simple casting operations often lead to a solidification texture with the lamellar boundaries all perpendicular to the heat flow direction. This difficulty can be overcome by directionally solidifying TiAl-base alloys. We have been performing directional solidification experiments with and without using a seeding technique. The current status of directional solidification of TiAl-base alloys is reviewed.     

镁合金热变形的研究现状

综述了镁合金热变形的研究现状，比较系统地介绍了各种塑性变形工艺（挤压、轧制、冲版等）和热处理工艺对镁合金微观组织及力学性能的影响。通过变形工艺与热处理工艺相结合，可以获得高强度、良好延展性和更多样化性能的镁合金结构材料，因此变形镁合金有望成为本世纪重要的轻金属结构材料。     

Evolution of specific surface area with solid fraction during solidification

The specific surface area varies with solid fraction during phase transformation from liquid to solid. The few measurements available show a non-linear dependence of the specific surface area on the solid fraction, with an initial increase as the amount of solid increases, followed by a decrease as the system moves toward complete transformation. We derive a simple model for this behaviour assuming a combination of growth and coalescence. We obtain a relation exhibiting an increase with the square root of fraction solid at low volume fractions, independent of a growth law, and a decrease at higher volume fractions which depends on the model chosen to describe the coalescence of dendrites. By choosing an appropriate constant, the model accurately describes recent data presented by Limodin and co-workers.