Microstructure transition from stable to metastable eutectic growth in Ni-25%Al alloy

The microstructural evolution during directional solidification of the Ni-25%Al(mole fraction) alloy was investigated in the range of growth velocity from 10 to 100 μm/s under a given thermal gradient of 10 K/mm. The solidification microstructures reveal a transition from γ'-β equilibrium eutectic to γ-β metastable eutectic plus β dendrites. A mixed microstructure of γ'-β and γ-β eutectics produced at a growth velocity of 25 μm/s illustrates that the transition occurs during the competitive growth between γ and γ' phases. The growth temperature for each phase was considered to understand the microstructure selection during solidification. The experimental results show that a phase or a microstructure solidifying with the highest temperature under a given growth condition is preferentially selected upon solidification. In addition, both stable eutectic and metastable eutectic are shown to coexist and simultaneously grow in the velocity range between 25 and 60 μm/s due to their similar growth temperatures.

Solidification process and microstructure evolution of bulk undercooled Co-Sn alloys

A series of Co-Sn alloys with Sn content ranging from 12% to 32%（mole fraction） were undercooled to different degrees below the equilibrium liquidus temperature and the solidification behaviors were investigated by monitoring the temperature recalescence and examing the solidification microstructures.A boundary clearly exists,which separates the coupled growth zone from the decoupled growth zone of eutectic phases for the alloys with Sn content ranging from 14% to 31%（mole fraction）.The other Co-Sn alloys out of this content range are hard to be undercooled into the coupled growth zone in the experiment.It is found that the so-called non-reciprocal nucleation phenomenon does not happen in the solidification of undercooled Co-Sn off-eutectic alloys.

Effect of nonlinear liquidus and solidus on dendrite growth in bulk undercooled melts

On the base of nonlinear liquidus and solidus,an extended model for dendrite growth in bulk undercooled melts was developed under local non-equilibrium conditions both at the interface and in the bulk liquid.In terms of thermodynamic calculations of the phase diagram,the model predictions are relatively realistic physically,since few fitting parameters are used in the model predictions.Adopting three characteristic velocities,i.e.the critical velocity of absolute solute stability(VC*),the velocity of maximal tip radius(VRm),and the velocity of bulk liquid diffusion(VD),a quantitative agreement is obtained between the model predictions and the experimental results in undercooled Ni-0.7%B and Ni-1%Zr(molar fraction) alloys,and the overall solidification process can be categorized.

Assessment of scaling factor in modified dendrite growth model

A model for dendrite growth during rapid solidification was established on the basis of BCT model and marginal stability criterion through modified Peclet numbers. Taking into account the interaction of diffusion fields, including solute diffusion field and thermal diffusion field around the dendrite tip, the model obtain a satisfactory results to predict the dendrite velocity and the tip radius, which agrees well with the experimental data from references in Cu Ni alloy.

快速加热循环热处理对TiAl基合金显微组织的影响

利用 Gleeble- 15 0 0热模拟机研究了快速加热循环热处理工艺对 Ti Al基合金显微组织的影响。结果表明 :利用该工艺可以使铸态 Ti Al基合金的晶团尺寸从约 80 0 μm细化至 5 0 μm以下 ,使加工态 Ti Al基合金的晶团尺寸从约5 0 μm细化至 2 0 μm以下。分析了晶界与相界发生的重结晶形核与长大 ,提出了在快速加热条件下 Ti Al基合金细小α2

TiAl基合金的组织演变

针对 TiAl 基合金的显微组织控制,综述了 TiAl 基合金中几种常见的组织演变,着重论述了变形 TiAl基合金在热处理过程中的晶拉长大及动力学分析,TiAl 基合盒在冷却时层状组织的形成和全层状 TiAl 基合盒在高温时的非连续粗化这3种组织演变的研究现状和面临的问题。

深过冷凝固TiAl基合金的研究进展

TiAl基合金由于诸多良好的性能而在许多领域广泛应用,而TiAl基合金非平衡凝固组织的多样性和复杂性往往影响其性能。电磁悬浮深过冷凝固技术作为一种分析和研究非平衡凝固行为的有效手段,可为研究非平衡凝固组织特征提供帮助。综述了深过冷凝固TiAl基合金近些年来的主要研究进展;从合金成分、相选择、组织演化以及相关理论等方面总结了TiAl基合金深过冷凝固研究的主要成果;提出了TiAl基合金深过冷凝固研究的研究方向和发展趋势,这对TiAl基合金成分设计和凝固组织控制具有重要意义。

Quantitative multi-phase-field modeling of non-isothermal solidification in hexagonal multicomponent alloys

A quantitative multi-phase-field model for non-isothermal and polycrystalline solidification was developed and applied to dilute multicomponent alloys with hexagonal close-packed structures.The effects of Lewis coefficient and undercooling on dendrite growth were investigated systematically.Results show that large Lewis coefficients facilitate the release of the latent heat,which can accelerate the dendrite growth while suppress the dendrite tip radius.The greater the initial undercooling,the stronger the driving force for dendrite growth,the faster the growth rate of dendrites,the higher the solid fraction,and the more serious the solute microsegregation.The simulated dendrite growth dynamics are consistent with predictions from the phenomenological theory but significantly deviate from the classical JMAK theory which neglects the soft collision effect and mutual blocking among dendrites.Finally,taking the Mg-6Gd-2Zn(wt.%)alloy as an example,the simulated dendrite morphology shows good agreement with experimental results.

GRAIN REFINEMENT INDUCED BY REMELTING INUNDERCOOLED Fe-30at.%Co ALLOYS

Fe-30at.%Co alloy is undereooled to investigate the effct of remelting on the grain size at certain undereooling rnnges. At low undereoolings, the primary stable fcc phase is remelted, which results in the equiaxed grains. When the melt is undercooled greater than the critical undereooling △T=204K the primary metastable bcc phase will alternatively nucleate in the melt. The primary bcc dendrite will be remelted into isolated dendrite cores dispersing in the remaining liquids. In the meanwhile, the isolated dendrite cores are remelted more or less due to the temperature fluctuation in the liquids. Different crystal structures between bcc and fcc phase make the subsequent crystal growth of fcc phase onto the Primary bcc phase non-epitaxial when the melt is undereooled greater than △T=204K. The primary metastable bcc phase may play an important role in coarsening grains in the systetn at larpe undereoolings.

Effects of β-Dendrite Growth Velocity on β→a Transformation of Hypoperitectic Ti–46Al–7Nb Alloy

Solidification characteristics of Ti–46Al–7Nb melts were studied by the electromagnetic levitation technique.A maximum melt undercooling up to 240 K has been achieved. When the undercooling is lower than the critical value DT＊ = 205 K, the alloy possesses typical hypoperitectic solidification characteristic which can be evidenced by a peritectic layer observed in the as-solidified microstructure. However, the Widmansta¨tten structure can be observed at large undercooling regime of DT C DT＊, where peritectic reaction cannot proceed and c lamellar precipitation within a plates is suppressed. Based on the BCT dendrite growth model, the dendrite growth velocities were calculated as a function of undercooling. Theoretical analysis indicates that the growth mechanism of the primary b phase transforms from solutaldiffusion-controlled to thermal-diffusion-controlled in the undercooling range of 188–205 K, which can be attributed to the onset of solute trapping at the critical undercooling. Meanwhile, with increasing undercooling, the solute trapping effect becomes more dominant as a consequence.

激光沉积修复DD5合金的枝晶外延生长控制与显微组织特征

采用DZ125高温合金粉末对DD5合金进行激光沉积修复,通过正交试验的方法,研究了激光功率、扫描速度和送粉量对单道单层沉积区枝晶外延生长的影响,实现了沉积区枝晶外延生长的控制。在此基础上,进行单道多层沉积修复试验,分析测量了单道多层沉积区的显微组织和硬度。结果表明:较低的热输入量和送粉量可显著提高沉积区枝晶外延占比;当激光功率为420 W、扫描速度为6 mm/s、送粉量为1.5 g/min时,单道单层沉积区枝晶外延占比约为100%。单道多层沉积区中下部为平面晶、沿沉积方向外延生长的柱状晶,顶部为等轴晶;沉积区γ′相不均匀地分布在γ相中,枝晶间区域的γ′相尺寸大于枝晶干区域的γ′相尺寸;沉积区底部短棒状MC碳化物沿枝晶间分布,且Ta元素含量较高;沉积区顶部的小块状以及八面体状MC碳化物随机分布。DD5合金基体平均显微硬度为425 HV_(0.5),沉积区平均显微硬度略高于基体,为449 HV_(0.5);与沉积区中部相比,沉积区底部和顶部的显微硬度略高,沉积区底部显微硬度最高。

Microstructure Evolution and Growth Orientation of Directionally Solidified Mg–4 wt% Zn Alloy with Different Growth Rates

The microstructure evolution and growth orientation of directionally solidified Mg-4 wt% Zn alloy in the growth rate range from 20 to 200μm/s were investigated. A typical cellular structure was observed with a growth rate of 20 μm/s, and the cellular spacing was 115 μm. When the growth rate increased to 60 μm/s, cellular structure with some developed perturbations was obtained and the cellular spacing was 145 μm, suggesting that the cell-to-dendrite transition happened at the growth rate lower than 60 μm/s. As the growth rate further increased, the microstructure was dendritic and the primary dendritic arm spacing decreased. The relationship between the primary dendritic arm spacings and the growth rates was in good agreement with Trivedi model during dendritic growth. Besides, X-ray diffraction and transmission electron microscopy analyses showed that the growth direction of directionally solidified Mg-4 wt% Zn alloy was （1120） lay in {0002} crystal plane, and the preferred orientation was explained with the lattice vibration model for one-dimensional monatomic chain.

Rapid solidification and dendrite growth of ternary Fe-Sn-Ge and Cu-Pb-Ge monotectic alloys

The phase separation and dendrite growth characteristics of ternary Fe-43.9%Sn- 10%Ge and Cu-35.5%Pb-5%Ge monotectic alloys were studied systematically by the glass fluxing method under substantial undercooling conditions. The maximum undercoolings obtained in this work are 245 and 257 K, respectively, for these two alloys. All of the solidified samples exhibit serious macrosegregation, indicating that the homogenous alloy melt is separated into two liquid phases prior to rapid solidification. The solidification structures consist of four phases including α-Fe, (Sn), FeSn and FeSn2 in Fe-43.9%Sn-10%Ge ternary alloy, whereas only (Cu) and (Pb) solid solution phases in Cu-35.5%Pb-5%Ge alloy under different undercool- ings. In the process of rapid monotectic solidification, α-Fe and (Cu) phases grow in a dendritic mode, and the transition "dendrite→monotectic cell" happens when alloy undercoolings become sufficiently large. The dendrite growth velocities of α-Fe and (Cu) phases are found to increase with undercooling according to an exponential relation.

Rapid dendrite growth in quaternary Ni-based alloys

The high undercooling and rapid solidification of Ni-10%Cu-10%Fe-10%Co quaternary alloy were achieved by electromagnetic levitation and glass fluxing techniques. The maximum undercooling of 276 K (0.16TL) was obtained in the experiments. All the solidified samples are determined to be α-Ni single-phase solid solutions by DSC thermal analysis and X-ray diffraction analysis. The microstructure of the α-Ni solid solution phase transfers from dendrite to equiaxed grain with an increase in undercooling, accompanied by the grain refinement effect. When the undercooling is very large, the solute trapping effect becomes quite significant and the microseg-regation is suppressed. The experimental measure-ment of α-Ni dendrite growth velocity indicates that it increases with undercooling according to the relation, V=8×10?2×?T1.2.

铝合金枝晶生长的数值模拟

提出一种新的模拟铝合金枝晶生长的随机性方法,建立了简化的枝晶形状的物理与数学模型,并提出了一种形状函数来描述晶粒的外部轮廓,基于简化的晶粒形状,采用坐标变换技术来描述过冷液相中晶粒的生长过程及其对周围节点的捕获过程.根据上述思想,开发了等轴晶生长的模拟程序,并进行了二维计算.为了验证模拟结果,浇注了金属型和砂型试样.模拟结果表明,金属型铸造晶粒尺寸较小,砂型铸造的较大.模拟结果与金相观察结果相符.

深过冷Ni-31.44%Pb偏晶合金快速凝固行为

采用熔融玻璃净化和循环过热相结合的方法研究了Ni 31.44%Pb偏晶合金宽过冷区间的凝固组织演化规律;从形核热力学和动力学两方面分析过冷熔体中稳定相(α)和亚稳相(L2)两相的竞争形核规律。研究结果表明:过冷Ni 31.44%Pb偏晶合金在快速凝固阶段本质上是以枝晶方式生长,首先形成α枝晶骨架,再辉重熔后分布于枝晶间的残余液相按照正常凝固模式进行分相/偏晶等后续反应。

深过冷快速凝固技术的研究进展

综述了近年来液态金属深过冷凝固研究的新进展,简述了液态金属深过冷技术、凝固过程及凝固行为,介绍了过 冷熔体中的枝晶生长理论以及深过冷技术在偏晶合金研究方面的应用。

过偏晶合金Ni-40%Pb深过冷凝固组织

采用熔融玻璃净化与循环过热相结合的方法研究过冷Ni 4 0 %Pb(质量分数 )过偏晶合金的组织演化规律。结果发现 :过偏晶合金在快速凝固阶段本质上是以枝晶方式生长 ;当ΔT <5 0K时 ,合金组织为粗大枝晶 +枝晶间Pb相 +团块状Pb相 ;当 10 0 <ΔT <198K时 ,合金组织宏观偏析严重 ;当ΔT =2 92K时 ,合金组织呈粒状晶 ,第二相均匀弥散分布。分析表明 :凝固组织宏观偏析与快速凝固阶段固液相变速率和体系残余液相分数有关 ;粒状晶粒化机制属于枝晶碎断再结晶机制。

深过冷Cu-37.4%Pb偏晶合金凝固行为和微观组织

用熔融玻璃净化与循环过热相结合的方法 ,研究Cu 3 7.4%Pb偏晶合金过冷熔体凝固组织的演化规律和熔体过冷度及形核率对相分离的影响。研究表明 :对于偏晶合金 ,当过冷度 <15 0K时 ,由于初生相L2 的长大被抑制 ,形成Pb分布在α枝晶间的均匀凝固组织 ;当过冷度 >15 0K时 ,随着过冷度的增大 ,由于Ostward熟化和凝并的加强 ,L2 液滴尺寸增加 ,导致L2的液相分离 ,凝固组织中的Pb颗粒尺寸经历由小变大的过程 。

1种新的模拟铝合金铸态组织的随机性方法

采用1种新的随机性模拟方法并与宏观传热过程相耦合,对铝合金的微观组织进行了模拟研究。计算中采用了简化的枝晶形状。建立了简化的枝晶形状的物理与数学模型,并提出了1种形状函数来描述晶粒的外部轮廓。基于简化的晶粒形状,采用坐标变换技术来描述过冷液相中晶粒的生长过程及其对周围节点的捕获过程。连续形核模型被用来处理异质形核现象,在生长模型中则考虑了枝晶尖端生长动力学和择优生长方向。开发了等轴晶生长的模拟程序,并进行了二维计算。进行了浇注金属型和砂型试样的模拟验证实验。结果表明,对两种不同的工艺,所得到的晶粒组织不同,金属型铸造时得到的晶粒尺寸较小,砂型铸造的较大。模拟结果与金相观察结果相符。