Ni3Al＋Ni7Hf2共晶合金的微观结构及凝固行为研究

利用金相、扫描电镜、透射电镜观察了Ｎｉ３Ａｌ＋Ｎｉ７Ｈｆ２共晶合金的微观组织结构,并利用高梯度定向凝固技术研究了该合金的凝固行为：Ｎｉ－５．８Ａｌ－３２Ｈｆ Ｎｉ３Ａｌ＋Ｎｉ７Ｈｆ２共晶组成,是Ｎｉ３Ａｌ／Ｎｉ７Ｈｆ２共晶复合材料的合适成分。由于不平衡凝固,有害相β－ＮｉＡｌ与金属间化合物Ｎｉ７Ｈｆ２以共晶体的形式存共晶胞间,在一定的温度梯度下,可以通过降低生长速度的方式予以消除。Ｎｉ－５．８Ａｌ     

Al—0.53Zn合金近快速定向凝固条件下的胞晶间距选择

利用Ｂｒｉｄｇｍａｎ装置考察了Ａｌ－０．５３Ｚｎ单相合金在近快速定向凝固条件下的胞晶间距选择规律。结果表明：在给定的温度梯度下,随着生长速度的增大,定向凝固组织均为胞状晶、胞晶间距λ的分布存在一个较宽范围,其最大,最小及平均间距随生长速度ｖ变化的实验规律可为：λｍａｘ＝９４８．５１ｖ＾－０．４９６１,λｍｉｎ＝６６１．１６ｖ＾－０．５０１５,λ＾－＝４１２．４１ｖ＾－０．５０４９。ＫＧＴ理论模型较     

Al－Si合金的共晶共生区及组织形成规律

本文利用定向凝固、金相、电子显微技术及Ｘ射线择优取向分析等手段对Ａｌ－Ｓｉ合金的组织形态、相结晶特性等与凝固条件的依赖关系进行了研究并定量获得了低温度梯度下（Ｇ＝４５℃／ｃｍ）Ａｌ－Ｓｉ合金的共晶共生区．实验表明：Ａｌ－Ｓｉ合金的共晶共生区形态十分复杂，随凝固速度的提高，共生区变得狭窄并向富Ｓｉ方向倾斜．共生区的复杂性相关于合金组织对凝固条件的敏感性，而形貌的多样性取决于共晶相本身的结晶学特点．当存在内部扰动或外部条件不稳定时，合金中可能出现几种异常组织和缺陷．     

两种含硼Ni3Al合金的微观组织与力学性能

本文研究了两种不同硼含量Ｎｉ３Ａｌ合金的微观组织、室温拉伸及高周疲劳性能，Ｎｉ３Ａｌ（０．６ａｔ．－％Ｂ）合金为单相组织，Ｎｉ３Ａｌ（１．０ａｔ．－％Ｂ）合金中除γ相外，还有微量共晶体，虽然Ｎｉ３Ａｌ（１．０Ｂ）的拉伸强度较Ｎｉ３Ａｌ（０．６Ｂ）的高，但在相同的循环应力水平下，后者的疲劳寿命显著高于前者的，疲劳断口观察表明两种合金的疲劳裂纹萌生和扩展行为各不相同。     

电磁流体流动对Al－Al_2Cu共晶形态的影响

研究了Ａｌ－３３％Ｃｕ共晶合金在电磁流体流动下的凝固行为．发现增加流速使Ａｌ－Ａｌ_２Ｃｕ共晶由规则层片状向非规则层片状、非规则棒状、直至规则棒状共晶形态转化，同时层片间距λ略有增加．流体流动造成的溶质重新分配是产生非规则层片共晶及其它形态共晶的主要原因．     

硼在金属间化合物Ni3Al晶界的平衡偏聚

应用扫描俄歇探针研究了硼在金属间化合物Ｎｉ３Ａｌ晶界的平衡偏聚特性。研究结果表明，硼在Ｎｉ３Ａｌ晶界的偏聚符合Ｌａｎｇｒｎｕｉｒ一ＭｃＬｅａｎ平衡偏聚理论；实验得到硼在Ｎｉ－２４ａｔ．－％Ａｌ合金中晶界偏聚自由能△Ｇ＝－２２１５９－２４．７４Ｔ（Ｊ／ｍｏｌ）：硼在Ｎｉ３Ａｌ晶界的偏聚受晶界结构和晶界化学环境的影响。不同结构的晶界存在着硼偏聚的不均匀性，硼在富镍晶界的偏聚比在富Ａｌ晶界更为显著，合金元素对硼在Ｎｉ３Ａｌ晶界偏聚的影响与其在Ｎｉ３Ａｌ中占据的亚阵位置有关。     

Mg_3MNi_2(M=Ti,Al)的晶体结构

Ａｌ和Ｔｉ对Ｍｇ２Ｎｉ结构中部分Ｍｇ的取代,得到与Ｍｇ２Ｎｉ晶体结构不同的新型合金．多晶Ｘ射线结构分析表明,其化学式为Ｍｇ３ＭＮｉ２（Ｍ＝Ｔｉ,Ａｌ）,立方晶系,空间群Ｆｄ３ｍ, Ｚ＝１６,４８个Ｍｇ坐落在４８（ｆ）,１６个 Ｍ（Ｍ＝Ａｌ, Ｔｉ）坐落在１６（ｄ）位,３２个Ｎｉ坐落在３２（ｅ）位,Ｍｇ３ＡｌＮｉ２的晶胞参数ａ＝１．１５４７４（２）ｎｍ, Ｍｇ３ＴｉＮｉ２的ａ＝１．１６１７８（２）ｎｍ．与Ｍｇ２Ｎｉ相比,Ｍｇ３ＭＮｉ２合金的晶体密度更大,Ｍｇ－Ｎｉ键长更长,吸放氢温度降低,循环寿命延长．     

用机械合金化方法制备Ni—Al系金属间化合物

用球磨机分别对Ｎｉ－５０ａｔ．－％Ａｌ和Ｎｉ－２５ａｔ．－％Ａｌ混合粉末进行机械合金化，并对Ｎｉ３Ａｌ预合金粉末进行高能球磨，观察了粉末的金相组织，测定了粉末的硬度、平均直径和晶粒尺寸，并作了ＸＲＤ物相分析。结果表明，经３ｈ研磨，Ｎｉ－５０ａｔ．－％Ａｌ混合粉末变成ＮｉＡｌ金属间化合物，其晶粒直径约５ｎｍ；经５ｈ机械球磨，Ｎｉ－２５ａｔ．－％Ａｌ混合粉末成为无序的亚稳定Ｎｉ固溶体，而Ｎｉ３Ａｌ预合     

Pd韧化Ni3Al的微观机制

在对含Ｐｄ的Ｎｉ３Ａｌ合金进行成键特征分析基础上，用已建立的理论模型解释了Ｐｂ韧化Ｎｉ３Ａｌ的微观机制。Ｐｄ在Ｎｉ３Ａｌ中占据Ｎｉ原子位置，其成键电子的离域程度远大于Ｎｉ原子，可有效地削弱共价键的方向性而强化晶界结合力，改善多晶Ｎｉ３Ａｌ的韧性。偏离化学计量比对Ｐｄ韧化Ｎｉ３Ａｌ效果的影响，是因为Ｐｄ－Ｎｉ键较Ｐｄ－Ｎｉ键强。降低Ａｌ含量可增加离域的成键电子数，有利于Ｎｉ３Ａｌ的韧化。     

Zr对Ni3Al晶界及力学性能的影响

对不同Ｚｒ含量Ｎｉ３Ａｌ的力学性能及晶界的研究结果表明：随Ｚｒ含量的增加，合金的屈服强度和抗张强度不断提高；当Ｚｒ≤０．７ａｔ．－％时，塑性随Ｚｒ含量的增加而提高；但Ｚｒ含量达１．２ａｔ．－％时，Ｚｒ的增塑效果下降．Ａｕｇｅｒ能谱分析表明：Ｚｒ在晶界有偏聚，偏聚因子约为３．断口观察显示：不含Ｚｒ或Ｚｒ含量为１．２ａｔ．－％的Ｎｉ－２４．０Ａｌ合金均为沿晶断裂，其余含Ｚｒ合金的断裂方式为穿晶、沿晶混合断裂．这表明：含Ｚｒ的Ｎｉ３Ａｌ塑性和Ｚｒ在晶界的偏聚有密切关系．     

A model of solidification microstructures in nickel-based superalloys: predicting primary dendrite spacing selection

A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate solute diffusion controlled solidification of binary alloys. Constitutional and curvature undercooling were both solved to determine the growth velocity of the solid/liquid interface. A modified decentered square/octahedron (in two or three dimensions) growth technique was implemented in the cellular automaton to account for the effect of crystallographic anisotropy. The resulting model is capable of simulating the growth of equiaxed and columnar dendritic grains in 2D and 3D, with the <100> directions either aligned or inclined with the grid. The algorithm used can also be used on coarser grids, with a concomitant loss in resolution, allowing simulation of sufficiently large numbers of dendrites in 3D to investigate the distribution of spacings, as well as average behavior.Simulations were performed for directional solidification with a range of withdrawal velocities and nucleation conditions, but a constant thermal gradient. The simulations capture the full microstructural development and primary spacing selection by both branching and overgrowth mechanisms. The model illustrates that there is a range of possible stable spacings, and that the final spacing is history dependent. It was also found that a minimum deviation from the steady state dendrite spacing is required before the spacing adjustment mechanisms are activated. The influence of perturbing the withdrawal velocity upon the stability of the spacing was also investigated. It was found that perturbations significantly reduce the range of stable primary dendrite spacing.     

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.     

Microstructure selection in the interdendritic region during directional solidification of a Ni-23at.%Al alloy

Directional solidification experiments of a binary Ni-23at.%Al alloy were carried out to examine the effects of growth velocity on the microstructure selection in the interdendritic region. Only the growth velocity was changed from 5 μm/s to 60 μm/s under a given thermal gradient. As a result, the noticeable change in the microstructure during solidification occurred between the γ dendrites. The γ interdendritic microstructure was varied as a function of growth velocity from rod γ-γ′ coupled peritectic structure to planar γ′ structure and then to eutectic structures consisting of stable γ′-β eutectic and metastable γ-β, eutectic structures. The microstructure selected preferentially among the γ dendrites was considered by calculating the interface temperature of a phase growing into its parent melt. It is shown that the microstructure selection in the γ interdendritic region is determined by a phase or a structure kinetically leading at the highest interface temperature under a given growth condition.     

Simulation of microstructure evolution in directional solidification of Ti-45at.%Al alloy using cellular automaton method

The microstructural evolution of Ti-45 at.%Al alloy during directional solidification was simulated by applying a solute diffusion controlled solidification model. The obtained results have shown that under high thermal gradients the stable primary spacing can be adjusted via branching or competitive growth. For dendritic structures formed under a high thermal gradient, the secondary dendrite arms are developed not very well in many cases due to the branching mechanism under a constrained dendritic growth condition. Furthermore, it has been observed that, with increasing pulling velocity, there exists a cell/dendrite transition region consisting of cells and dendrites,which varies with the thermal gradient in a contradicting way, i.e. increase of the thermal gradient leading to the decrease of the range of the transition region. The simulations agree reasonably well with experiment results.     

History-dependent selection of primary cellular/dendritic spacing during unidirectional solidification in aluminum alloys

History-dependent selection of primary cellular/dendritic spacing is investigated systematically during unidirectional solidification of a series of aluminum alloys. A single crystal is formed in the sample before each experimental run, so that the influence of grain boundary on the primary spacing is avoided. The experimental results are compared with those of the two-dimensional crystal growth in the same alloy system and transparent model alloys. It is found that the primary cellular/dendritic spacing is remarkably history dependent. The average primary spacing is dependent not only on the current growth conditions, but also remarkably on the way those conditions were achieved. There exists a wide allowable range of primary spacings for a given growth condition. Experimental results are also compared with the Hunt–Lu model, which shows excellent fit between them, especially on the selection of cellular spacing. By comparing the three-dimensional experiments with the two-dimensional ones, it is also found that the allowable range of the primary spacing for the three-dimensional growth is wider than that for the two-dimensional growth.     

Effect of sample diameter on primary dendrite spacing of directionally solidified Al-4%Cu alloy

The relationship between primary dendrite arm spacing and sample diameter was studied during directional solidification for Al-4%Cu (mass fraction) alloy. It is shown that primary dendrite spacing is decreased with the decrease of the sample diameter at given growth rate. By regressing the relationship between primary dendrite arm spacing and the growth rate, the primary dendrite arm spacing complies with 461.76v-0.53, 417.92v-0.28 and 415.83v-0.25 for the sample diameter of 1.8, 3.5 and 7.2 mm, respectively. The primary dendrite spacing, growth rate and thermal gradient for different sample diameters comply with 28.77v-0.35G-0.70, 23.17v-0.35G-0.70 and 23.84v-0.35G-0.70, respectively. They are all consistent with the theoretical model , and b1/a1=2. By analyzing the experimental results with classical models, it is shown that KURZ-FISHER model fits for the primary dendrite spacing in smaller sample diameters with weaker thermosolute convection. Whereas TRIVEDI model is suitable for describing primary dendrite arm spacing with a larger diameter (d＞2 mm) where convection should be considered.     

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.     

CELLULAR SPACING SELECTION OF Al-0.53wt%Zn ALLOY UNDER NEAR-RAPID DIRECTIONAL SOLIDIFICATION CONDITION

1.IntroductionCellular/dendriticspacing,asoneofthemostimportantsolidificationmicrostructuralscales,isofcriticalsignificancetothestudyofsolidificationtheoryandcontrolofmicrostructures.Recently,severalimportantdevelopments[1--4]havebeenmadeintheoreticalresearchesonplanarfrontgrowthandtheselectionofcellular/dendriticstructureatlowgrowthvelocityunderconstrainedcrystalgrowthcondition,andithasbeenrecognizedinexperimentalinvestigationsls--ic]thatthereexistsawideallowablerangeofprimarydendriticspacing…     

NUMERICAL SIMULATION OF CELLULAR／DENDRITIC PRIMARY SPACING

1.IntroductionHeat, mass and momentum transports during solidification determine the structures and segregation of the solidified alloys, on which modeling of structure forming in solidification can be based. There weresomeresearchesonmodelingofcellularordendriticprimaryspacingandpatterns[1-5],inwhichtheinterfaceshapesandstabilityhadbeentheoreticallyinvestigated,butconvectionwasnotconsidered.One of the interesting problems in this literature lies in the effects of convection on crystal growth;…     

NUMERICAL SIMULATION OF COLUMNAR TO EQUIAXED TRANSITION FOR DIRECTIONALLY SOLIDIFIED Ti-44Al ALLOY

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition (CET) during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt.