Nd-Fe-B包晶合金高梯度定向凝固试验研究

在定向凝固试验中,采用全熔和区熔两种加热方式分别获得了180 K/cm和250～360 k/cm的温度梯度,在5～500μm/s的抽拉速率范围内对过包晶Nd13.5Fe79.75 B6.75合金进行了定向凝固试验,考察了温度梯度和抽拉速率对此合金凝固组织及相含量的影响.结果表明,合金的凝固组织包含初生a-Fe枝晶、包晶Nd2Fe14B相及少量的富Nd相.随着抽拉速率的增加,包晶相含量先增加后减少;而在同一抽拉速率下,提高温度梯度可增加铁磁性Nd2Fe14B相的含量.当抽拉速率为50 μm/s时,区熔试样中(G=300 K/cm)Nd2Fe14B相的含量可达约90%,而相同速率下全熔试样(G=180 K/cm)中其含量约为60%.     

定向凝固Al-4.5%Cu合金枝晶组织与抽拉速率的关系

用自制下拉式定向凝固设备,在一定的温度梯度下,在20-220μm/s的抽拉速率范围制备定向凝固Al-4.5%Cu合金,并对其微观组织、特别是一次枝晶间距随抽拉速率的变化规律进行研究。结果表明：定向凝固微观组织随抽拉速率的增大呈细化趋势,其一次枝晶间距减小;当抽拉速率小于100μm/s时,枝晶间距随抽拉速率而减小的幅度较大;当抽拉速率大于100μm/s时,枝晶间距减小幅度较为平缓。在综合分析抽拉速率、界面生长速率、温度梯度等影响因素的基础上,推导出界面局域平衡条件下预测定向凝固次枝晶间距的理论模型,该模型能够较为准确地反映定向凝固一次枝晶间距随抽拉速率在100-220μm/s范围的变化规律,为定向凝固工艺获得特定组织而预先选配合适的工艺参数提供理论参考。     

STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY II. Microstructure Evolution in the Steady--State Growth Region

分析了定向凝固Ti--43Al--3Si(原子分数, %) 合金在3---90 μm/s 的生长速度下的稳态生长区组织. 在定向凝固过程中经历下列反应: L→Ti₅Si₃, L→α+Ti₅Si₃, α→α₂(Ti₃Al)+γ(TiAl), α₂→γ+Ti₅Si₃, 其中, α 与Ti₅Si₃ 共晶是合金最显著的凝固行为. 当生长速度大于20 μm/s 时, 还出现L→γ+Ti₅Si₃. 随着生长速度增大, 稳态组织逐渐由粗胞晶向细胞晶、胞状枝晶及枝晶转变, 起稳定α相作用的Ti₅Si₃ 相由低速时分布于α相中逐渐向高速时分布于凝固γ 相中转变, 不利于该合金的引晶. 选择10 μm/s 的初始生长速度, 既能减少到达稳态生长的距离, 又能保证引晶效果.     

STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY I. Microstructure Evolution in the Transition Region

对Ti--43Al--3Si (原子分数, %) 合金在3---100 μm/s 的生长速度下进行了系统的定向凝固实验. 研究了生长速度对固/液界面形态及初始过渡区组织演化规律的影响. 合金在3---60 μm/s 的生长速度范围内均以胞晶形态生长, 胞晶间距随着生长速度的增大而减小; 当生长速度达到90 μm/s 时, 开始出现枝晶生长. 在定向凝固初始启动阶段, 存在清晰的热过渡区, 热过渡区内Ti₅Si₃ 相分布及过渡区组织与定向凝固区组织的关联性对于籽晶材料的引晶效果有重要影响. 生长速度在10 μm/s 以内时, 热过渡区内Ti₅Si₃ 相分布连续, 且热过渡区组织与定向凝固区组织的关联性好, 有利于该合金的引晶.     

EFFECTS OF WITHDRAWAL RATE ON MICROSTRUCTURE AND STRESS RUPTURE PROPERTIES OF A Ni3Al–BASED SINGLE CRYSTAL SUPERALLOY IC6SX

采用螺旋选晶法制备Ni₃Al基单晶合金IC6SX试棒, 研究了不同拉晶速率对凝固组织和高温持久性能的影响. 结果表明, Ni₃Al基单晶合金IC6SX凝固组织为树枝状, 随着拉晶速率从1.5 mm/min增加到6 mm/min, 一次枝晶间距逐渐减小. 与普通镍基高温合金不同, 位于枝晶干处的次生γ'相尺寸比枝晶间处的大. 随着拉晶速率的增加, 枝晶干和枝晶间处的γ'相尺寸都逐渐减小, 由不规则形状逐渐立方化, 枝晶间处的初生γ'相的数量逐渐增多. 一次枝晶间距和γ'相尺寸对高温持久性能影响显著, 随着拉晶速率的增加, 组织细化, 铸态IC6SX单晶合金的高温持久寿命增加.     

纵向静磁场对定向凝固GCr15轴承钢柱状晶向等轴晶转变的影响

进行了外加纵向静磁场下GCr15轴承钢的定向凝固实验,考察了纵向静磁场对试样凝固过程中柱状枝晶向等轴枝晶转变(columnar to equiaxed transition,CET)的影响。结果表明,在温度梯度(104 K/cm)和抽拉速率(20μm/s)一定时,随着磁场强度的增加(0~5 T),试样棒边缘柱状枝晶的生长逐渐地遭到破坏,从而发生不同程度的CET;当磁场强度和温度梯度分别为4 T和104 K/cm时,在较低抽拉速率(5μm/s)下,试样的凝固组织发生了完全CET;在试样发生完全CET后,其合金元素分布趋于均匀。结合数值模拟,可将这些现象归结为纵向静磁场与热电流相互作用产生的热电磁力对枝晶和熔体的作用所致。     

Mg-14.61Gd合金的定向凝固组织及生长取向

采用电子背散射衍射(EBSD)和元胞自动机有限元(CAFE)方法研究了Mg-14.61Gd合金在温度梯度G=30 K/mm和抽拉速率v=10～200μm/s条件下的定向凝固组织和生长取向。研究发现,Mg-14.61Gd合金纵向凝固组织呈单一方向的α-Mg枝晶生长形貌,随着v的增加,枝晶界面生长方式由凸前生长向平齐生长转变,枝晶间距减小。当v从10μm/s增至100μm/s时,α-Mg枝晶的生长取向由1120和1010转变为1120,其与凝固热流的偏离角(θ)由11.0°减小至5.7°,热流是影响生长取向的主导因素;当v从100μm/s增至200μm/s时,α-Mg枝晶的生长取向仍为1120,但θ却逐渐增大至10.6°,此时,晶体的各向异性占主导。研究表明,CAFE模型可以合理预测定向凝固镁合金的晶粒组织和生长取向。     

液态金属冷却法制备DD403合金过程温度场和晶粒组织的数值模拟

采用ProCAST和CAFE模型模拟了镍基单晶高温合金DD403定向凝固过程中的温度场及晶粒组织。研究了抽拉速率对变截面单晶铸件杂晶形成和铸件板身固液界面形状和位置的影响规律,得到了单晶铸件不出现杂晶的最大抽拉速率——临界抽拉速率(V c)。结果表明,当采用150μm/s的抽拉速率时,对于液态金属冷却(LMC)技术,铸件平台的凝固顺序是从中心到两边,杂晶形成倾向较小;而在高速凝固(HRS)条件下,铸件平台的边缘首先冷却,平台边缘容易出现大的过冷而产生杂晶。在本实验条件下,采用HRS技术,临界抽拉速率不得高于125μm/s;采用LMC技术,最大抽拉速率不宜超过150μm/s,否则可能会在螺旋段或平台处形成杂晶。当抽拉速率为150μm/s时,采用LMC法获得的板身部位的轴向温度梯度(G a)是HRS法的2倍多;一次枝晶臂间距(PDAS)减小了1/3~1/2,且沿铸件轴向的轴向温度梯度和一次枝晶臂间距均较HRS均匀。当抽拉速率在50~200μm/s范围内增大时,采用LMC技术,铸件板身的固液界面始终保持平直且逐渐下移至隔热挡板中部;而HRS条件下,固液界面逐渐下凹并下移至挡板下方。     

高温度梯度定向凝固镍基高温合金DZ125的组织演化

采用液态金属冷却(LMC)定向凝固结合液淬法,在温度梯度(G)为250 K/cm,凝固速率(V)为2-400 μm/s的条件下系统研究了镍基高温合金DZ125的固/液界面形态、胞/枝晶间距及MC碳化物形态.随着V的增大,界面形态经历了浅胞状→深胞状→粗枝晶→细枝晶的转变.一次胞晶间距随着V的增大而增大,当V=5 μm/...     

纵向强静磁场对定向凝固DZ417G合金枝晶形态和数目的影响

纵向强静磁场可明显影响高温合金DZ417G的定向凝固组织.为控制定向组织树枝晶数目提供了一种新手段.在抽拉速率为5μm/s时,强磁场影响了该合金组织的定向凝固生长特性,影响程度随磁场强度的加大而增加.当抽拉速率达到40μm/s及其以上时,施加强磁场使得单位面积上的枝晶数目增加,枝晶数目随磁场强度的增大而增大,增大幅度最大达到一倍左右.当温度梯度增大时,强磁场在低抽拉速率(5μm/s)下影响该合金定向凝固的效应加剧,在抽拉速率40μm/s及其以上时增加枝晶数目的效应也加大.从磁抑制对流和热电磁效应方面分析了上述现象,提出了作用机理.     

Influence of directional solidification variables on primary dendrite arm spacing of Ni-based superalloy DZ125

The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm. The results reveal that the primary dendrite arm spacing decreases from 94 μm to 35.8 μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s. The regression equation of the primary dendrite arm spacings λ1 versus cooling rate is λ1=0.013(GV)-0.32. The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data. The influence of directional solidification rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.     

Tailoring Microstructure and Microsegregation in a Directionally Solidified Ni-Based SX Superalloy by a Weak Transverse Static Magnetic Field

A weak transverse static magnetic field (WTSMF, 0-0.5 T) is applied to the directional solidification process of a DD3 Ni-based SX superalloy, aiming to tailor the microstructure and microsegregation of alloys. The mechanisms of microstructural refinement and microsegregation distribution caused by a WTSMF during directional solidification are discussed. It is shown that the primary dendrite arm spacing is rapidly reduced from 181 to 143 μm, and the average size of γ′ phase is significantly refined from 0.85 to 0.25 μm as the magnetic field increases from 0 to 0.5 T. At the same time, the volume fractions of γ/γ′ eutectic and the segregation coefficient are also gradually decreased. The 3D numerical simulations of the multiscale convection in liquid phase show that the modifications of the microstructure and microsegregation in DD3 are mainly attributed to the enhanced liquid flow caused by thermoelectric magnetic convection (TEMC) at dendrite/sample scale under the WTSMF. The maximum of the TEMC increases with increasing the magnetic field intensity. This work paves a simple way to optimize the microstructure and microsegregation in directionally solidified Ni-based SX superalloys without changing the processing parameters and composition.     

High thermal gradient directional solidification and its application in the processing of nickel-based superalloys

The heat transfer during directional solidification by Bridgman-type directional solidification has been analyzed and a relationship has been established that reflects the effect of alloy properties, process parameters and equipment characteristics on thermal gradients. Based on this relationship, some methods for obtaining high thermal gradients have been developed. By using zone-intensified overheating and liquid-metal cooling, high thermal gradients of up to 800 K/cm were achieved. Application of these methods in the processing of single crystal superalloys indicated that high thermal gradient directional solidification produced more uniform microstructures and optimized mechanical properties.     

Microstructure and microsegregation in directionally solidified Mg–4Al alloy

Directional solidification (DS) of a binary Mg–4Al (wt.%) alloy was carried out to investigate the microstructures and microsegregation under controlled solidification conditions. In directional solidification the microstructure depends on the growth rate V because the cooling rate, which governs the solidification microstructure, is the product of the growth rate and the temperature gradient. The ability to produce simple and uniform microstructures in directional solidification enables us to correlate the formation of the microstructure and its characteristic length scales quantitatively with processing parameters. The morphology of the solid–liquid interface and the microstructure of both the mushy zone and the steady-state region were characterized at different levels of growth rates. With the help of an electron microprobe, microsegregation was determined in a specimen directionally solidified with cooling rates ranging from 0.06 to 0.8 K/s. The calculated microsegregation results based on the Scheil model deviated significantly from the experimental data, which is anticipated since back diffusion was not included due to the lack of diffusivity data.     

The evolution of the growth morphology in Mg–Al alloys depending on the cooling rate during solidification

The comprehensive microstructural evolution of Mg–3, 6 and 9 wt.% Al alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (G·V) and solute (Al) content were investigated in the present study. Various solidification techniques, including directional solidification, wedge casting, sand and graphite mould casting, gravity casting in a Cu mould and water quenching, were employed in order to obtain wide ranges of cooling rates between 0.05 and 1000 K s^–1. The microstructural length scales of Mg–Al alloys, such as secondary dendrite arm spacing and primary dendrite arm spacing, were determined experimentally and compared with published models. In addition, the solidification parameters of morphological transitions such as cellular to columnar dendrite and columnar to equiaxed dendrite were also determined. Based on all the experimental data and the solidification model, a solidification map was built in order to provide guidelines for the as-cast microstructural features of Mg–Al alloys.     

Influence of withdrawal rate on the microstructure of Ni-base single-crystal superalloys containing Re and Ru

The influence of elevated withdrawal rate on the microstructure and segregation behavior of Ni-base single-crystal superalloys containing Re and Ru is investigated. The experimental superalloys are processed under a high thermal gradient of approximately 250 K/cm and withdrawal rates between 10 and 500 μm/s. With increasing withdrawal rate, the dendritic structures and γ′ precipitates in as-cast microstructures are apparently refined. Electron-probe microanalyzer (EPMA) results indicate that the degree of segregation for the constituent elements (e.g. Al, Ta, W, Re etc.) increases initially and then decreases with increasing withdrawal rate. In addition, the Re and Ru additions obviously increase the amounts of γ-γ′ eutectic and the tendency of segregation for Al and Ta.     

抽拉速度对Ti-45Al合金定向凝固组织演化影响的数值模拟

采用基于溶质扩散控制模型结合CA方法对Ti-45Al合金定向凝固过程中抽拉速度对显微组织演化影响进行数值模拟。结果表明,随抽拉速度增加,凝固形态经历了平面→胞晶→胞/枝混合结构→树枝晶的转变,如果温度梯度高于20K/mm,仅经历平面→胞晶转变。此外,将模拟结果与实验结果和理论分析进行对比,证明了模型的有效性。