Papers by Author: De Chang Zeng

Abstract: Isotropic and anisotropic NdFeB permanent magnets were prepared by Spark Plasma Sintering (SPS) and SPS followed hot deformation (HD), respectively, using melt spun NdFeB ribbons with various compositions as starting materials. It is found that, based on RE-rich composition, SPSed magnets sintered at low temperatures (<700 C) almost maintained the uniform fine grain structure inherited from rapid quenching. At higher temperatures, a distinct two-zone (coarse grain and fine grain zones) structure was formed in the SPSed magnets. The SPS temperature and pressure have important effects on the grain structure, which led to the variations in the magnetic properties. By employing low SPS temperature and high pressure, high-density magnets with negligible coarse grain zone and an excellent combination of magnetic properties can be obtained. For single phase NdFeB alloy, because of the deficiency of Nd-rich phases, it is relatively difficult to consolidate micro-sized melt spun powders into high density bulk magnet, but generally a larger particle size is beneficial to achieve better magnetic properties. Anisotropic magnets with a maximum energy product of ~38 MGOe were produced by the SPS+HD process. HD did not lead to obvious grain growth and the two-zone structure still existed in the hot deformed magnets. The results indicated that nanocrystalline NdFeB magnets without significant grain growth and with excellent properties could be obtained by SPS and HD processes.

Abstract: The concurrent nucleation and growth in a diffusion-controlled phase transformation is studied using the quantitative phase field method, and the transformation kinetics is obtained for a model alloy. The simulation results show that the simultaneous nucleation and growth of new phase can be described very well in the phase field model, and that the phase transformation is governed by the diffusion of solute atoms. The competition between nucleation and coarsening is also observed. The phase transformation kinetics is found to obey the JMAK equation.

Abstract: Evaluation of glass-forming ability (GFA) is important in the development of amorphous alloys. Based on phase field theory, the kinetic model of liquid-to-solid phase transition is build, and the time-temperature-transformation (TTT) diagram is plotted according to the phase field simulations of isothermal phase transformation kinetics for a model system. Furthermore, the critical cooling rate for glass formation is calculated on the basis of the TTT curve and is taken as the intrinsic criteria of reflecting the GFA for metallic melts.

Abstract: Core-shell structured nanoparticles with ferromagnetic core (Fe) and shell (Co) were prepared by a chemical reduction method. By adjusting the deposition parameters, the core-shell particles with various Fe:Co molar ratios were obtained. The saturation magnetization decreased with the increase of Cobalt content. The properties of core-shell nanoparticles synthesized under a magnetic field were compared with those prepared without a magnetic field. For the nanoparticles prepared without magnetic filed, the coercivity (H_c) increased with increasing Co content due to the large anisotropy of Co, whereas for the nanoparticles prepared under a magnetic field, the H_c was much lower. The ZFC/FC curves suggested that these particles were ferromagnetic at room temperature. The anisotropy constant K at 340K for core-shell nanoparticle is estimated to be 0.83×10⁵ erg/cm³. The second ferromagnetic phase transition may occur at the temperature lower than 25 K, which led to a drastic change of magnetization at low temperatures.

Abstract: This paper presents a detailed failure analysis on a rivet, which broke into two parts from the joint place between head and shank under a load of seven tons during service, by the combination of optical microscopy, SEM and EDS. The metallographic examination shows some micro-inclusions in the sample and many micro-cracks along the grain boundaries. SEM and EDS results show that the surface of the rivet was plated with a zinc coating and it was eroded severely. The fracture surface exhibits the characteristic of quasi-cleavage fracture, indicating an intergranular brittle fracture mode. Some micro-pores were found on the facets of intergranular cracked grains. Most importantly, the Crow-toe Pattern, one of river patterns or hairline seams, which is a typical microstructure of hydrogen embrittlement, was observed on the fracture. Based on above analysis, it is concluded that the failure of the rivet is caused by the hydrogen embrittlement. The hydrogen, most likely, comes from the zinc plating process. Due to the aggregation and diffusion of hydrogen into the matrix, the brittle cracking happens in the regions with stress concentration because of the volume expansion effect under the external force.

Abstract: The Fe–Pr binary system was thermodynamic evaluation by means of the CALPHAD method based on phase diagram experimental data from the literature and a few values of the mixing enthalpy in the liquid phase obtained by the Miedema theory technique. Each of the selected data values is given a certain weight, which is chosen and adjusted based on the thermodynamic data and diagram phase data. A consistent thermodynamic description of the Fe–Pr binary system is presented: only one intermediate compound, Fe17Pr2, is stable in the system and forms peritectically at 1371K. An eutectic reaction L↔Pr+ Fe17Pr2 occurs at 939K and the eutectic liquid contains 82 at% Pr, five solid solution phases (Fe-rich αFe, γFe and δFe, Pr-rich αPr and βPr) and the liquid solution phase were considered in the evaluation. The intermediate phase was treated as stoichiometric compound, the solid solutions as ideal and the liquid solution phase by the Redlich–Kister formalism. The calculated phase diagram and thermodynamic properties are in good agreement with available experimental data.

Abstract: The electronic structure and magnetic properties of the (Co1-xFex)Tet(CoxFe2-x)OctO4 spinels (x is defined as the degree of inversion) scenario are investigated theoretically from first-principles, using generalized gradient approximation (GGA) method for the systems with strong coulomb correlations, which gives a correct description of the electronic structure. The GGA+U method gives an improved qualitative result compared with the GGA not only for the excited-state properties such as energy gaps but also for the ground-state properties such as magnetic moments and crystal parameters. The nominal valence of the transition metal elements and the ground state structure have been established based on the study of variation of the cation distribution (x=0.0, 0.25, 0.5, 0.75 and 1.0) over the tetrahedral and octahedral sites. The site-preference calculation on bulk systems indicates that Co2+ ions strongly prefer the octahedral B sites, and the electronic structure and magnetic properties of cobalt ferrites highly depend on the cation distributions even though the chemical composition of the compound does not change. The results are in good agreement with the available experimental data and most of the other theoretical results.

Abstract: The RE4Fe72B22M2 (RE=Y, Dy or Nd; M=Nb or Ta) alloys in ribbon and rod forms were prepared by melt spinning and water cooling copper mold casting methods, respectively. Amorphous structure was observed in all ribbon samples, but the results from rod samples indicated that the composition has an important effect on the glass formability. All rod samples with RE=Y or Nd were partly amorphous, while bulk metallic glass (BMG) rod with a diameter of 2 mm was obtained for the Dy4Fe72B22Nb2 alloy. The rare earth and doping elements on the glass formability have been discussed. The effects of post-annealing on the microstructure and magnetic properties of these alloys have been investigated. The results presented here provide a viable method to get fully dense bulk hard magnets.

Abstract: Arc ion plating(AIP) has higher deposition rates, whereas macro-particles (MPs) make the film properties decreasing. In this paper, AIP with or without magnetic filter (MF or AIP) and composition of AIP followed with magnetic filter (MFAIP) were designed to deposit TiN films on silicon (Si) and high-speed steel(HSS), respectively. Scanning electron microscope (SEM), nanoindentation and microscratch tests were investigated. SEM showed that both the MF and the MFAIP films had a superfine layer among the columnar crystals grown vertically against substrate surface. However, the columnar crystals of MF films were leptosomatic and well-orientational, while MFAIP ones were coarse and short, and even ragged in size and orientation. Nanoindentation test results showed the highest hardness for MF films and the lowest one for AIP films. A new parameter Scratch Crack Propagation Resistance (CPRs) was introduced to evaluate the film adhesion properties in a scratch test. MF films had the highest adhesion. AIP films were most susceptible to failure as the CPRs was the lowest, although the Lc1 was higher than MFAIP ones. It was proposed that the MPs were effectively removed with the MF, and the MF layers were superfine to improve the properties of the films.