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1.
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  相似文献   

2.
冯坚  黄卫东  林鑫  李涛  薛玉芳  周尧和 《金属学报》1998,34(12):1267-1272
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  相似文献   

3.
Al-Si合金的共晶共生区及组织形成规律   总被引:7,自引:0,他引:7  
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.  相似文献   

4.
两种含硼Ni3Al合金的微观组织与力学性能   总被引:1,自引:0,他引:1  
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。  相似文献   

5.
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.  相似文献   

6.
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。  相似文献   

7.
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.  相似文献   

8.
用机械合金化方法制备Ni—Al系金属间化合物   总被引:8,自引:1,他引:7  
李谷松  丁炳哲 《金属学报》1994,30(2):B091-B096
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  相似文献   

9.
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。  相似文献   

10.
Zr对Ni3Al晶界及力学性能的影响   总被引:4,自引:0,他引:4  
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.  相似文献   

11.
《Acta Materialia》2003,51(10):2971-2987
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.  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
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.  相似文献   

15.
《Acta Materialia》1999,47(11):3271-3280
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.  相似文献   

16.
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.  相似文献   

17.
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.  相似文献   

18.
1.IntroductionCellular/dendriticspacing,asoneofthemostimportantsolidificationmicrostructuralscales,isofcriticalsignificancetothestudyofsolidificationtheoryandcontrolofmicrostructures.Recently,severalimportantdevelopments[1--4]havebeenmadeintheoreticalresearchesonplanarfrontgrowthandtheselectionofcellular/dendriticstructureatlowgrowthvelocityunderconstrainedcrystalgrowthcondition,andithasbeenrecognizedinexperimentalinvestigationsls--ic]thatthereexistsawideallowablerangeofprimarydendriticspacing…  相似文献   

19.
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;…  相似文献   

20.
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.  相似文献   

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