Solidification microstructure of directionally solidified superalloy under high temperature gradient

The effect of solidification rate on the microstructure development of nickel-based superalloy under the temperature gradient of 500 K·cm-1 was studied. The results show that, with the increase of directional solidification rate from 50 to 800 μm·s-1, both the primary and the secondary dendrite arm spacings of the alloy decrease gradually, and the dendrite morphologies transform from coarse dendrite to superfine dendrite. The sizes of all precipitates in the superalloy decrease gradually. The morphology of γ' precipitate changes from cube to sphere shape and distributes uniformly in both dendrite core and interdendritic regions. MC carbide morphology changes from coarse block to fine-strip and then to Chinese-script and mainly consists of Ta, W, and Hf elements. The γ-γ' eutectic fraction increases firstly and then decreases, and similar regularity is also found for the variation of segregation ratio of elements.

Nucleation and growth of high purity aluminum grains in directional solidification bulk sample without electromagnetic stirring

A self-made directional solidification device was used to fabricate d 80 mm high purity aluminum ingots. SEM and AFM were used to detect the shape of grain boundaries. The orientation of the grain was studied by X-ray diffractometry. The results show that the nucleation points locate at the intersections of three adjacent grains. The lattice orientation of grains does not alter in the horizontal direction, but gradually approaches the optimum growth direction in the vertical direction during the growth process. All the grains suffer the competition and only the one whose orientation is closest to the preferred direction can occupy the final growth space.

Microstructural evolution and corrosion behavior of directionally solidified FeCoNiCrAl high entropy alloy

The FeCoNiCrAl alloys have many potential applications in the fields of structural materials, but few attempts were made to characterize the directional solidification of high entropy alloys. In the present research, the microstructure and corrosion behavior of FeCoNiCrAI high entropy alloy have been investigated under directional solidification. The results show that with increasing solidification rate, the interface morphology of the alloy evolves from planar to cellular and dendritic. The electrochemical experiment results demonstrate that the corrosion products of both non-directionally and directionally solidified FeCoNiCrAI alloys appear as rectangular blocks in phases which Cr and Fe are enriched, while AI and Ni are depleted, suggesting that AI and Ni are dissolved into the NaCl solution. Comparison of the potentiodynamic polarization behaviors between the two differently solidified FeCoNiCrAl high entropy alloys in a 3.5%NaCl solution shows that the corrosion resistance of directionally solidified FeCoNiCrAI alloy is superior to that of the non-directionally solidified FeCoNiCrAI alloy.

Microstructural evolution of directionally solidified DZ125 superalloy castings with different solidification methods

The properties of Ni-base superalloy castings microstructure, and different solidification methods have are closely related to the uniformity of their as-cast serious effect on microstructural uniformity. In this paper, the influences of high rate solidification （HRS） process （with or without superheating） and liquid metal cooling （LMC） process on the microstructure of DZ125 superalloy were investigated. Blade-shape castings were solidified at rates of 40 pm.s-1 to 110 tJm.s1 using HRS process and a comparative experiment was carried out at a rate of 70 IJm.s1 by LMC process. The optical microscope （OM）, scanning electron microscope （SEM） were used to observe the microstructure and the grain size was analyzed using electron back scattered diffraction （EBSD） technique. Results show that for the castings by either HRS or LMC process, the primary dendrite arm spacing and size of 7＇ precipitates decrease with increasing the withdrawal rate; the dendrites and 7＇ precipitates at the upper section of the blade are coarser than those in the middle, especially for the HRS castings without high superheating technique. When the withdrawal rate is 70 iJm.s1, the castings by HRS with high superheating technique have the smallest PDAS with fine 7＇ precipitates; while the size distribution of 7＇ precipitates is more homogenous in LMC castings, and the number of larger grains in LMC castings is smaller than that in the HRS castings. Moreover, high superheating technique yields smaller grains in the castings. Both the LMC method and HRS with high superheating technique can be used to prepare castings with reduced maximum grain size.

High Temperature Tensile Property and Fracture Behavior of Directionally Solidified Fe-Al-Ta Eutectic Composites

Fe-Al-Ta eutectic composites with solidification rates of 6,20,30,80 and 200μm/s were obtained by a modified Bridgman directional solidification technique and alloying.Moreover,tensile property and fracture behavior of Fe-Al-Ta eutectic composites were studied at 600℃.The relationship between mechanical property and microstructure at high temperature was studied.Microstructure of Fe-Al-Ta eutectic is composed of Fe_(2)Ta(Al)Laves phase and Fe(Al,Ta)matrix phase.In addition,the tensile strength at high temperatures is higher than that at room temperature.The tensile strength is increased with the increase of solidification rate.Moreover,fracture morphology transforms from cleavage fracture to dimple fracture as the solidification rate is increased at high temperatures.

HIGH-TEMPERATURE TENSILE FRACTURE BEHAVIOR OF DIRECTIONALLY SOLIDIFIED Ni,Cr,Al-TaC EUTECTIC SUPERALLOY

The high-temperature tensile fracture behavior of the Ni, Cr, Al-TaC eutectic superal-loy directionally solidified under high temperature gradient is investigated. The high-temperature tensile fracture of this in situ composite has ductile character with lots of ductile nests whose diameters decrease with the increasing solidification rates. The maximum σb and δ are respectively 668.5MPa and 19.6%. There is α TaC whisker in the center of each nest, and the deformation of γ' and TaC is uneven. The high-temperature tensile behavior cannot be explained by the rule of mixtures but is decided by the formation of the plastic deformation band. The crack extension model is given.

Mechanism of primary Si and Fe-segregation for a hypereutectic 90 wt.%Si-Ti alloy melt in directional solidification via electromagnetism

The electromagnetic directional solidification(DS)phase separation experiments of high silicon 90 wt.%Si–Ti alloy were performed under various pulling-down speeds.The results showed that Si enriched layer,Si+TiSi_(2)-rich layer and Si–Ti–Fe alloy layer appeared successively in axial direction of ingot after electromagnetic DS of 90 wt.%Si–Ti alloy melt at different pulling-down speeds.Separation of primary Si and segregation mechanism of metal impurities(Fe)during the electromagnetic DS process were controlled by pulling-down speed of ingot and electromagnetic stirring.When pulling-down speed was 5μm/s,minimum thickness of the Si enriched layer was 29.4 mm,and the highest content of primary Si in this layer was 92.46 wt.%;meanwhile,the highest removal rate of Fe as metal impurity was 92.90%.The type of inclusions in the Si enriched layer is determined by Fe content of segregated Si enriched layer.When the pulling-down speed was 5μm/s,the inclusions in the Si enriched layer were TiSi_(2).Finally,when the pulling-down speed reached greater than 5μm/s,the inclusions in the Si enriched layer evolved into TiSi_(2)+τ_(5).

Microstructure evolution, Cu segregation and tensile properties of CoCrFeNiCu high entropy alloy during directional solidification

CoCrFeNiCu(equiatomic ratio)samples(?8 mm)were directionally solidified at different velocities(10,30,60 and 100μm/s)to investigate the relationship between solidification velocity and microstructure formation,Cu micro-segregation as well as tensile properties.The results indicate that the morphology of the solid-liquid(S-L)interface evolves from convex to planar and then to concave with the increase of solidification velocity.Meanwhile,the primary and the secondary dendritic arm spacings decrease from100μm to 10μm and from 20μm to 5μm,respectively.They are mainly influenced by the axial heat transfer and grain competition growth.During directional solidification,element Cu is repelled from the FCC phase and accumulates in the liquid owe to its positive mixing enthalpy with other elements.Tensile testing results show that the ultimate tensile strength(UTS)gradually increases from 400 MPa to 450 MPa,and the strain of the specimen prepared at the velocity of 60μm/s is higher than those of others.The fracture mode of all specimens is the mixed fracture containing both ductile fracture and brittle fracture,in which ductile fracture plays a fundamental role.In addition,the brittle fracture is induced by Cu segregation.The improvement of UTS is resulted from columnar grain boundary strengthening.

TiAl金属间化合物定向技术研究进展

TiAl合金具有低密度和良好的高温性能,被视为航空航天领域极具应用潜力的一类新型结构材料。具有柱晶/单晶组织特征的全片层结构TiAl合金展现出极为优异的高温综合力学性能。本文首先介绍了定向TiAl合金的发展背景、定向组织、力学性能各向异性等基本概念;然后,基于工作原理、关键技术、组织特征三个层面,对传统Bridgman法、光学浮区法、电磁约束以及电磁冷坩埚等定向技术进行了论述;进一步,结合最新研究成果,对定向退火制备TiAl合金的工作进行了阐述;最后,展望了TiAl合金定向技术的研究方向,指出需要从优化成分设计、组织/相变可控、大尺度定向晶稳定制备等方面发展新原理与新技术。

型壳温度对DZ22B合金叶片枝晶组织的影响

以DZ22B镍基高温合金叶片为研究对象,采用ProCAST数值模拟及试验相结合的方法对不同型壳保温温度下铸造叶片定向凝固过程中的温度场、热裂倾向及组织进行研究。结果表明:随着型壳保温温度的增加,等温线整体呈下凹趋势,处于1 540℃温度下的糊状区下凹程度明显大于其他温度,且叶片HTI结果有所增加,容易产生热裂的区域集中在叶身上部,排气边位置容易产生热裂的区域比进气边更大;相同高度位置下的叶片排气边枝晶间距均小于进气边枝晶间距;随型壳保温温度升高,一次枝晶间距和二次枝晶间距均降低,对比模拟结果与试验统计得出的一次枝晶间距和二次枝晶间距,得出一致结论,模拟与试验吻合较好。

Microstructure evolution of peritectic Al–18 at.%Ni alloy directionally solidified in high magnetic fields

The effect of a high magnetic field on the microstructural evolution of a peritectic Al—18 at.%Ni alloy during directional solidification and its dependence on pulling speed were investigated.At a low pulling speed,the application of a 2 T magnetic field triggered the appearance of a primary Al_(3)Ni_(2)phase.At higher pulling speeds,a high magnetic field application induced primary Al_(3)Ni_(2)phase segregation that formed close to the central alloy regions.For all pulling speeds,the application of a high magnetic field induced bulk Al_(3)Ni/Al eutectic formation on the upper and lower parts of the alloys,and promoted elongated growth of the peritectic Al_3Ni phase along the magnetic field direction.Microstructural analysis indicated that microstructural evolution that was induced by high magnetic fields can be attributed to solute migration and melt flow that is regulated by magnetic,Lorentz,and thermoelectric magnetic forces and their coupling effects during peritectic solidification.

Effect of a High Magnetic Field on the Microstructure During Directionally Solidified Sn-20wt.%Pb Hypoeutectic Alloy

The microstructures of Sn-20wt.%Pb hypoeutectic alloy directionally solidified under a longitudinal magnetic field were investigated.The results show that the application of a high magnetic field has a great influence on the morphology of primary β-Sn phase at a temperature gradient of G_L=52 K/cm.At a certain growth speed,with the increase of magnetic field intensity,the magnetic field causes the primary β-Sn phase irregular and to be deformed,further,the magnetic field promotes the columnar to equaixed transition(CET).Further,the thermoelectric magnetic force(TEMF) imposed on the dendrite under a high magnetic field has been calculated and the results show that the numerical magnitude of the TEMF during directional solidification under a 10 T high magnetic field is about 10~4N/m^3 and this force should be responsible for the occurrence of the CET in the Sn-Pb alloy.This may act as an experimental proof that the coupling of temperature gradient and high magnetic field will induce the occurrence of the CET in Sn-Pb alloy.Above phenomena may be attributed to the thermoelectric magnetic force(TEMF)in solid.

Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5高熵合金定向凝固固液界面转变与力学性能

采用定向凝固在3μm/s、5μm/s、10μm/s和50μm/s抽拉速率下制备了Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5高熵合金,对其组织演变及力学性能进行了研究,揭示了该高熵合金定向凝固固液界面转变机制与强化机理。结果表明:Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5高熵合金为单一FCC固溶体相,随抽拉速率增大,固液界面发生由胞状枝晶、胞状树枝晶到树状枝晶的转变,一次间距及晶粒尺寸均减小。抗压强度和压缩应变均随抽拉速率增加而增大,在50μm/s下最大压缩应变达到35.11%,最大抗压强度达到2369.41 MPa。抽拉速率提高和迟滞扩散效应导致固液界面胞枝转变;树状枝晶中明显增多的晶界阻碍位错滑移,导致变形中位错在晶界处塞积,晶界强化在定向凝固Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5高熵合金中起到重要作用。此外,由于Ni、Co、Fe与其他原子半径差产生晶格畸变及固溶强化效应。

强磁场下Cu-50%Ag合金定向凝固过程中的组织及固液界面形貌演变

目的研究强磁场下Cu-50%(质量分数)Ag合金定向凝固过程中的组织演变、固液界面形貌变化及溶质迁移行为,分析强磁场对金属凝固过程的作用机制,为强磁场下的金属材料制备提供理论借鉴和指导。方法在不同的凝固速率与磁场条件下进行定向凝固和淬火实验,对合金的定向凝固组织、糊状区与固液界面形貌以及溶质分布行为进行考察。结果强磁场破坏了凝固组织的定向生长,使凝固组织转变为枝晶与等轴晶共存的形貌;强磁场诱发了熔体对流,减少了糊状区中溶质的含量;强磁场改变了固液界面处的溶质分布和固液界面形貌,破坏了固液界面的稳定性。结论强磁场通过洛伦兹力和热电磁力的共同作用,诱发了糊状区内液相的纵向环流,改变了固液界面及糊状区中的组织形貌与元素分布。

稀土元素Y和Ce对定向凝固镍基高温合金高温氧化行为的影响

用热重法研究稀土元素Y和Ce对定向凝固镍基高温合金DZ444高温氧化行为的影响,用X射线衍射仪和扫描电镜等观察和分析氧化膜组成和形貌。结果表明:合金在700、850和950℃下的恒温氧化动力学均符合抛物线规律,稀土元素的添加对其无影响;然而,添加稀土元素使其氧化激活能由257.6 kJ/mol降低至246.8kJ/molDZ444合金氧化膜分为3层:外层为疏松的Cr、TiO和(Cr2的混合物;中间层为Cr内氧化物层为Al。稀土元素未改变合金氧化膜的组成。稀土元素极易偏聚在合金表面,促进保护性Cr膜的形成,从而阻止合金的进一步氧化,并能有效抑制合金的内氧化。

高温金属间化合物的定向凝固特性

高温金属间化合物是一类重要的高温结构材料,其中Ti-Al及Ni-Al等许多合金系在凝固时都存在连续的包晶反应.本文就包晶合金凝固的特性从多方面加以总结和分析.凝固过程中的包晶相变对组成相的生长与取向有不同的影响.除此之外,定向凝固包晶反应过程中,相的竞争与选择及界面形态的演化也将同时对金属间化合物的凝固组织产生重要的影响.在考虑了界面温度对生长速率的响应后,可以发现生长速度超过某一值后亚稳相将析出,而根据凝固条件不同可以以不同的凝固方式进行长大,形成如带状、浅胞状和枝/胞双相共生等形式的组织,其中共生组织更有利于提高材料的单向力学性能.虽然定性的分析已获得某些规律。但作为工程应用的具有包晶反应特点的高温金属间化合物的定向凝固研究还极不充分,需进行大量的基础的理论与实验研究.

CMSX-2单晶高温合金高梯度定向凝固下过渡区的组织演化特征

用高温度梯度定向凝固装置研究了CMSX-2单晶高温合金初始过渡区的组织演化特征,获得了不同条件下过渡区单晶高温合金的平-胞-枝组织结构,以及相应的一次胞枝晶间距大小,发现初始过渡区的凝固过程影响到最终获得的凝固组织,采用的匀加速抽拉方式有利于提高单晶的引晶率和单晶凝固组织的完整性.

定向凝固的进展对高纯铝偏析法提纯工艺的影响

铝提纯工艺为高纯铝发展的关键。现在国内外通用的高纯铝制备工艺为三层液电解法和分步冷凝法。由于能耗高或提纯效率低 ,这两种工艺都不能完全满足当今工业发展的需要。针对当前广泛采用的凝固偏析法 ,结合定向凝固的新进展 ,讨论了改进结晶方式 ,提高提纯效率和生产效率的工艺方法。

高温共晶自生复合材料的研究进展

介绍了高温共晶自生复合材料的应用潜力,分析了自生复合材料的研究热点,指出了自生复合材料发展中存在的问题。

定向凝固Ni_3Al合金高温变形后的显微组织特征

利用金相、扫描电镜和透射电子显微镜研究了定向凝固Ni3Al合金高温变形后的显微组织特征研究结果表明，当变形速率较快时，原始柱状晶晶界无明显变化，晶内无明显亚结构存在；当变形速率较慢，合金呈现超塑变形时，变形初期柱状晶晶界呈现“锯齿状”，后期原始柱状晶界消失，代之以晶粒尺度约为15mm的晶粒带，晶粒带中既有小角度晶界，也有大角度晶界合金在超望变形时发生了连续回复与再结晶的过程.