Influence of a weak static magnetic field on the primary dendrite arm spacing of a directionally solidified Ni-based superalloy

Magnetic field applied in the solidification can influence the primary dendrite arm spacing (PDAS) of alloys. The effect of a weak static magnetic field on the PDAS of Ni-based superalloy DZ417G with different dimension is studied in this paper. The results show that the PDAS for directionally solidified superalloy DZ417G under the weak static magnetic field is significantly decreased. With increasing the intensity of magnetic field from 0 to 0.05 T, the PDAS is decreased from 0.3 mm to 0.23 mm and from 0.61 mm to 0.48 mm for the ingots with 4 mm and 10 mm in the diameter, respectively. When the intensity of magnetic field is larger than 0.05 T, the PDAS is slightly increased. The influence of the interactive competition between the magnetic damping and thermoelectromagnetic convection caused by magnetic field on the PDAS for superalloy DZ417G is discussed.     

静磁场对定向凝固镍基高温合金组织影响的研究进展

综述了国内外静磁场对镍基高温合金组织影响的研究现状,重点分析了施加不同方式、强度的静磁场对定向凝固镍基高温合金枝晶组织、元素偏析、凝固缺陷及高温力学性能的影响规律,并从变截面处杂晶的控制、晶体取向偏离的控制以及对凝固特性的影响机制等方面提出了静磁场在定向凝固镍基高温合金研究中潜在的发展方向。     

Recrystallization in a directionally solidified cobalt-base superalloy

The recrystallization behavior in a range of annealing temperature from 1020 to 1280 °C in a directionally solidified cobalt-base superalloy was studied. Local recrystallization first appeared at 1040 °C. The recrystallized volume increased rapidly as increasing the annealing temperature. Pinning effect of all carbides (M₂₃C₆, M₇C₃ and MC) was observed and large amount of twin formed at low annealing temperature. The size of the recrystallized grains increased significantly at high annealing temperatures accompanied with the sharp decrease of twin. The effect of annealing temperature and the role of carbide and twin on the development of the recrystallization were discussed.     

低Re含量第二代柱晶高温合金DZ59的蠕变断裂性能

采用液态金属冷却(LMC)工艺和成分优化设计,制备出一种高温性能优异的低Re含量第二代柱晶高温合金DZ59,通过扫描电镜(SEM),透射电镜(TEM)等方法对其组织特征及蠕变断裂进行了表征,研究了合金的蠕变断裂性能.结果表明,DZ59合金的高温蠕变断裂性能超过一代单晶合金,并接近二代单晶合金的水平.发现柱晶高温合金的二次晶界反应(SGRZ)现象,表明SGRZ受温度和应力的控制,由于增加了合金横向晶界的受力界面,在高负荷下可能成为蠕变空洞萌生和扩展的位置.     

On the fatigue small crack behaviors of directionally solidified superalloy DZ4 by in situ SEM observations

Small fatigue crack behaviors in a nickel-based directionally solidified superalloy DZ4 were studied by in situ Scanning Electron Microscopy. The crack initiation and propagation manners were identified under different temperatures, i.e. 25 °C, 350 °C, 700 °C. Fatigue crack growth occurred preferentially along slip bands at 25 °C and 350 °C but by Mode-I type at 700 °C. The crack growth rate generally increased with temperature, especially between room temperature and 350 °C. The anomalous small crack growth was analyzed by in situ examining the effect of microstructure. The small cracks were found to be primarily microstructurally small and secondly mechanically small.     

Processing of non-ferromagnetic materials in strong static magnetic field

Static magnetic field processing of non-ferromagnetic materials has been of broad interest and been applied in such fields as drug delivery, colloid chemistry and engineering of materials containing particles. A ‘strong’ magnetic field refers to a ‘strong’ response from the manipulated material and can vary in definitions. The response is corresponding to a local interaction between the material and the local magnetic field, being influenced by the magnetic susceptibilities of the material and the surrounding/coated medium. By carefully designing the medium, a significantly ‘strong’ response from a weakly magnetic material can even be generated by a traditional magnet, i.e. magnetic flux density ∼0.01 T. Therefore, the ability to manipulate materials by using a magnetic field depends critically on the understanding of the principles of the magnetic properties of materials and their magnetic responses. This paper provides a critical discussion on the principles including magnetic field effect thermodynamics, magnetic energy, magnetic anisotropy and different magnetic forces during ’strong’ magnetic field processing of weakly magnetic materials (focusing on metallic materials). A series of case studies and the related magnetic field effect are subsequently integrated and discussed. Overall this review aims to provide a better understanding and efficient overview on the phenomenon principles in the field of magnetic field processing.     

Formation and stability of nano-scaled M23C6 carbide in a directionally solidified Ni-base superalloy

The precipitation of nano-scaled M₂₃C₆ carbide at intragranular regions (especially in dendritic core regions) has been systematically investigated in a directionally solidified Ni-base superalloy. The crystallographic orientation relationship between the M₂₃C₆ phase and the γ matrix phase was found to be (100)_M23C6//(100)_γ, [100]_M23C6//[100]_γ. Additionally, M₂₃C₆ carbide was very stable during creep and long-term thermal exposure, which makes the fine M₂₃C₆ carbide an effective strengthening phase in γ matrix.     

In-situ scanning electron microscopy studies of small fatigue crack growth in recrystallized layer of a directionally solidified superalloy

The small fatigue crack behavior in the surface recrystallized layer of a nickel-based directionally solidified superalloy DZ4 was studied by in-situ scanning electron microscopy. Transgranular cracking manner was identified at 350 °C, distinct from the intergranular fracture of recrystallized grains reported formerly. The transition of fracture mode is attributed to the effect of temperature, which should be particularly concerned in the safety of turbine components with recrystallization. The discontinuous crack growth in recrystallized layer is associated with the local microstructure. Carbide can act as crack source, and barrier to the crack growth as well. Effect of twin boundary on impeding the fatigue crack is evident.     

Forward electric field calculation using BEM for time-varying magnetic field gradients and motion in strong static fields

A boundary element method for evaluating the electric fields induced in conducting bodies exposed to magnetic fields varying at low frequency has been developed and applied to sources of magnetic field variation that are of relevance in magnetic resonance imaging. An integral formulation based on constant boundary elements which can be used to study the effects of both temporally varying magnetic field gradients and rigid body movement in a static magnetic field is presented. The validity of this approach has been demonstrated for simple geometries with known analytical solutions and it has also been applied to the evaluation of the induced fields in more realistic models of the human head.     

Superconducting cylinder in a static longitudinal magnetic field

A study of the calculation of the magnetic field forces acting on a superconducting cylinder in a longitudinal magnetic field is reported. A computation algorithm and the results of field calculation by means of the boundary integral method, taking account of the cylinder edge singularity, are presented. The method makes it possible to compute the forces pressing the cylinder with a relative error below 1%     

A simple diagnostic method for plasma jet in strong magnetic field

An experimental study was carried out to examine the effect of strong magnetic field on the characteristics of argon plasma jet. An approximate expression was introduced to the distribution of emission line intensity and the color intensity. The distribution of the emission intensity was compared to that of the color intensity. The modified profiles of the spectral intensity agree well with those of the color intensity. The two-point analysis, that is, a calculation method with only two selected wavelengths in Boltzmann plots, was selected in the evaluation of excitation temperature because the calculation becomes significantly simple. The lateral distribution of excitation temperature obtained from the modified intensity becomes more accurate than that without the approximation. The radial distributions of excitation temperature based on the Abel inversion can be determined by the present approximation. It is found that the simple diagnostic method can utilize for understanding the characteristics on plasma jet.     

Electrocodeposition of magnetic nickel matrix nanocomposites in a static magnetic field

The effect of an external magnetic field on the electrocodeposition of composites consisting of either Co or magnetite nanoparticles in a Ni matrix has been studied. An alkaline Ni pyrophosphate bath containing citrate was used. The magnetic particles were prepared by thermal decomposition (Co) or chemical precipitation (magnetite) and characterized by transmission electron microscopy, X-ray diffraction, dynamic light scattering, zeta potential and vibrating sample magnetometry measurements. The particle incorporation showed a distinct dependency on the orientation of an externally applied magnetic field. While the particle incorporation increased in a perpendicular field (perpendicular with regard to the electrode surface), it decreased in a parallel orientation. This result is explained with the dominating action of the magnetophoretic force. The structure and the properties of the Ni layers were significantly affected by the particle codeposition. A refinement of the Ni grains was found with increasing plating current density and as a result of the nanoparticle incorporation. The magnetic hardness and the Vickers microhardness of the films increased significantly due to the incorporation of the nanoparticles.     

Two-dimensional gas of electrons in a strong magnetic field

Translated from Izmeritel'naya Tekhnika, No. 2, pp. 4–7, February, 1990.     

A transit time instrument for measurements on beam ions in a strong axial magnetic field

An instrument for measurements on beam ions in a strong axial magnetic field is presented. Information on the types of ions and an estimate of their perpendicular energy is obtained. The experiments were carried out with a hydrogen ion beam extracted from an ECR source.     

Dendritic coarsening of γ′ phase in a directionally solidified superalloy during 24,000 h of exposure at 1173 K

Dendritic coarsening of γ′ was investigated in a directionally solidified Ni-base superalloy during exposure at 1173 K for 24,000 h. Chemical homogeneity along different directions and residual internal strain in the experimental superalloy were measured by electronic probe microanalysis (EPMA) and electron back-scattered diffraction (EBSD) technique. It was indicated that the gradient of element distribution was anisotropic and the inner strain between dendrite core and interdendritic regions was different even after 24,000 h of exposure at 1173 K, which influenced the kinetics for the dendrite coarsening of γ′ phase.     

Self-assembly of fractal magnetite-silica aggregates in a static magnetic field

This paper examines the mechanism underlying the formation of linear and fractal aggregates of high-conductivity magnetic nanoparticles in a static magnetic field. A transition between dendritic and fractal structures in response to a change in magnetic field is investigated by scanning electron microscopy and atomic force microscopy.     

Low-temperature specific heat of a semiconductor in a strong magnetic field

We calculate the electronic specific heat of a semiconductor (GaAs) at very low temperatures when subjected to a magnetic field strong enough to make the electron distribution nondegenerate. The transition to nondegeneracy is characterized by a large and rapid increase in the specific heat. For large fields the value approaches that of a one-dimensional nondegenerate gas, after first exceeding this value. Zeeman splitting, even with a very smallg factor (0.32), almost doubles the maximum in the specific heat as a function of field.On leave from Centre de Recherches sur les Très Basses Températures, Grenoble, France.