During vacuum carburizing, coarse reticulated carbides tend to precipitate along grain boundaries due to high-carbon-potential conditions. This phenomenon is often one of the main factors in the failure of conventional gear steels. In this paper, a novel solid-solution carburizing process was proposed to achieve nano-carbide formation in the surface of the carburizing layer, and the conventional carburizing process and material thermodynamic calculations were combined to study the carburized layer by changing the parameters of the carburizing process, and to optimize the microstructure and properties of the carburized layer. The results showed that the high carbon potential or the long-time boost carburizing process could easily cause the enrichment of many carbon atoms in the traditional carburization, thus forming a carbide network and decreasing the carburization efficiency. The minor increase in large-sized M₇C₃ carbides did not significantly improve the surface hardness and wear resistance. However, the presence of small and dispersed M₂C carbides was the main factor in improving the microhardness and mechanical properties. The novel solid-solution carburizing process could improve the carburizing efficiency and transform reticulated carbides into nano-dispersed M₂C carbides. The surface carbon content and microhardness of 1.07% and 875 HV, respectively, increased 17.7 and 2.4% compared to conventional carburizing processes at 1100 °C. On the other hand, the surface’s ultimate tensile strength was found to be 1900 MPa by mini-tensile testing, and the core had a good match of strength and toughness. It was concluded that the novel solid-solution carburizing process could dissolve the carbon network and thus effectively increase the surface carbon content, achieving fully nanosized carbide on the surface. Modifying the size, morphology, and distribution of the nano-M₂C carbides dispersed within the lath-martensite after tempering the test steel was found to be the main factor in improving the mechanical properties.

Flow pattern and inclusion removal in a novel-designed crucible in the planar flow casting process were studied in the present paper. A color dye water experiment was used to show the flow field and validate the mathematical modeling. It was shown that the flow pattern predicted by the Large Eddy Simulation model is maximally consistent with that in the physical modeling. The validated mathematical model was used to predict the flow pattern and inclusion behavior in the crucible with various prototypes using combinations of multiple facilities. The results show that the flow field, by using the stopper, enhances compared to that using none of the facilities, which causes the inclusion removal to become even worse when the stopper is individually used. Furthermore, as the magnetic field is added, the electromagnetic brake weakens the flow field in the magnetic zone. The function of the electromagnetic brake to weaken the flow pattern is most efficient for inclusion removal.

The extruded Mg-6.0wt.%Zn-0.5wt.%Zr (ZK60) alloys with 0.5wt.% and 1.0wt.% Y additions were prepared. The microstructures and tensile properties of the alloys aged in different conditions were investigated. The results show that the specimen with 1.0wt.% Y additions and aged at 453K for 15 h had the best tensile strength. The ultimate tensile strength and yield strength reached 365MPa and 352MPa, respectively. The grain refinement, the formation of the fine dispersed Mg-Zn particles and the W-Mg₃Zn₃Y₂ ternary phase may be the main reasons for the improvement. The average grain size reached to ~ 3μm. The size of the fine and dispersed particles was only several nanometers and the particle number density (Nv) was very high. The crystallographic characteristics of the ternary W-Mg₃Zn₃Y₂ phase were studied by the transmission electron microscope and selected area electron diffraction techniques. The crystallographic orientation relationship between the W phase and the α-Mg matrix is identified as: [2 0]_α//[ ]_w, (0002)_α plane near parallel to the (220)_w plane with a 3.5 degree angle difference. This orientation relationship has rarely been studied and reported.