Zhou Lanzhang

EFFECTS OF RARE EARTH ELEMENT Gd ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF NiAl-Cr(Mo)-Hf EUTECTIC ALLOY

采用真空电弧炉熔炼方法制备了添加稀土元素Gd的Ni-33Al-28Cr-5.9Mo-0.1Hf(原子分数,%)合金,运用扫描电镜(SEM)、电子探针显微分析(EPMA)及透射电镜(TEM)分析了该合金的微观组织形貌和相组成,采用Gleeble 1500进行了室温及高温压缩性能测试.研究表明,稀土元素Gd极少部分固溶于NiAl与Cr(Mo)相中,大部分参与形成Heusler相.适量Gd可细化共晶合金的组织,即减小NiAl/Cr(Mo)共晶胞的尺寸及共晶层片的间距,因此提高了合金的室温压缩屈服强度与压缩塑性,但对1373 K时的压缩强度影响较小.当Gd含量达到0.1(质量分数,%)时,共晶胞界宽化,且胞界的Cr(Mo)相出现粗化及不规则分布现象,导致室温和高温压缩性能降低.

CHARACTERISTICS OF A NiAl EUTECTIC ALLOY JJ-3 WITH EXCELLENT PROPERTIES

A new NiAl eutectic alloy JJ-3 with excellent comprehensive properties had been successfully developed in China for the potential vanes materials. In this paper, the study results on alloy JJ-3, including the composition, microstructure characteristics, physical, mechanical and chemical properties, and process technologies have been introduced briefly. On the basis of maintaining the low density, the higher melting point and thermal conductivity, the excellent oxidation resistance being peculiar to NiAl alloy, alloy JJ-3 have good high temperature stress-rupture strength, which are almost as the same as those of the advanced directionally solidified superalloys. Meanwhile, alloy JJ-3 possess good weldability and castability, and had already been cast the vanes with the complicated shape. With the excellent comprehensive properties, alloy JJ-3 is very close to practical engineering application, only if the poor ductility at room temperature must be improved in the future.

THE RECRYSTALLIZATION OF Ni BASE DIRECTIONALLY SOLIDIFIED SUPERALLOY DZ417G

DZ417G is a directionally solidified (DS) superalloy developed for low-pressure blade applications in gas turbine engines. The crystallization microstructures of DZ417G samples caused by sand-blasting and machining were investigated. The tensile properties of the alloy after crystalliza- tion were tested at room temperature and 900 and its stress-rupture performance was examined under conditions of 980 /216 MPa and 760 /725 MPa. The results show that after solution treat- ment equiaxed recrystallization grains form on the surface of specimens machined from directionally solidified alloy bars. After aging treatment, cellular recrystallization takes place on the surface of spec- imens pretreated by sand blasting. Both the yield strength and tensile strength at room temperature decrease after recrystallization, while those at 900 are slightly affected by recrystallization. The recrystallization depth increases after stress-rupture tests, which may be attributed to migration of recrystallization boundaries driven by high temperature and stress. For samples without recrystalliza- tion microstructure, the fracture mode is transgranular, which is controlled by the propagation rate of the cracks initiated both on surface and at inner microstructure discontinuities. While for samples with recrystallization microstructure, the cracks prefer to be initiated on transverse recrystallization grain boundaries and propagate along the recrystallization boundaries into the matrix, which may accelerate the propagation rate. TRF (transverse recrystallization area fraction) is a factor to evaluate the effect of recrystallization on the stress-ruptured performance. The stress-ruptured performance is decreased with the increase of TRF between 0 and 0.5. For samples with a TRF of 0.5, second cellular recrystallization forms in the first equiaxed recrystallization grain. The cracks initated at interfaces of cellular microstructure have a high density, which impair the stress-ruptured performance of DZ417G alloy.

APPLICATION OF ARTIFICIAL NEURAL NETWORK FOR PREDICTION OF SUPERPLASTIC BEHAVIOUR IN NiAl ALLOYS

Chemical composition, grain size, and processing conditions such as temperature and strain rate have important influence on superplasticity of NiAl alloys, which would allow the optimization of these parameters in order to achieve the desired combination of properties. In this work, the optimal superplastic deformation conditions of NiAl alloys were studied by using artificial neural networks (ANN). The standard multilayer feedforward networks were trained and tested using comprehensive datasets from previous experimentally works on the as-extruded NiAl, NiAl-25Cr, NiAl-20Fe-Y(Ce), NiAl-30Fe-Y, NiAl-9Mo, NiAl-P alloys. Different effects are modeled, including the influence of the alloying elements on the superplastic, and the influence of deformation temperature, strain rate and grain size on the elongations during the superplastic tensile tests. The artificial neural network models are combined with computer programmers for optimization of the inputs in order to achieve desirable combination of outputs. Good performances of the neural networks are achieved. Results of this research propose a range of strain rate and temperature within which the NiAl alloy possesses superplasticity with larger elongations, although the deformation temperature and strain rate of superplastic alloys alternately influence each other within the range. These models are convenient and powerful tools for practical applications in superplastic prediction in NiAl alloys.