Advanced Materials Research Vols. 476-478

Abstract: The very poor hot workability of high-alloyed GH4742 superalloy is represented in the aspects of very narrow available deformation temperature range, high deformation-resistant force and poor ductility. In this paper, a special heat treatment to change the ingot microstructure is proposed to improve the hot workability of the GH4742 superalloy. The' phase with a appropriate size and morphology and tortuous grain boundary were obtained by slow cooling in the ' two phases region after solution treatment. Modified microstructure induces that the hot deformation flow stress of GH4742 superalloy was decreased effectively and hot deformation plasticity was increased obviously.

Abstract: The influence of Nd on structure and properties of hypereutectic Al-Si alloys (Al-17.5%Si, Al-25%Si) was investigated in this paper. For Al-17.5%Si alloy, there is no obvious variation in morphology of primary Si before and after Nd modification, but the average size of primary Si is decreased from 34.73µm to 23.39µm after 0.3%Nd modification. Compared with initial sample, the tensile strength and yield strength of 0.3%Nd-modified sample are increased by 11.6% and 1.5%, and wear resistance of Al-17.5%Si alloy is enhanced to a factor of 2.1 after 0.3%Nd modification. However, for Al-25%Si alloy, the tensile strength and yield strength of 0.3%Nd-modified sample are respectively improved by 22.1% and 9.5% as compared to initial sample. Meanwhile, wear resistance of modified sample is improved to a factor of 3 relative to initial sample. The property improvement of two alloys can be attributed to the change in morphology and size of primary Si after Nd modification.

Abstract: Effect of the solid-solution treatment on the casting structures and properties of the die-casting Mg alloy was explored through the way of adding mixed rare earths Y, Gd and Nd into the commercial Re-AZ91 Mg alloy . The results show that the solid-solution treatment could improve effectively the microstructures of the alloy, and Mg17Al12 phrase on the grain boundaries could transform from network struture into dotlike or granular one. At the same time, the compounds on the grain boundaries were less than those before the solid-solution treatment, the compounds were dispersed inside the grain . Both the microstructures and the mechanics properties of the alloy could be improved remarkably, and also, the tensile strength σb could reach over 280MPa, the elongation ratio δ over 9% when the temperature of the solid-solution treatment was between 270°C and 410°C. When the temperature of the solid-solution treatment exceeded 420°C, coarse Al-RE compounds would be formed inside the alloy, which could reduce the mechanics properties of AZ91 Mg Alloy.

Abstract: Double compound modification was carried out for hypereutectic Al-24Si alloy. The experiment showed that double modification with phosphorus and rare earth can fine the microstructure of alloy. The complex modifications of phosphorus and rare earth make the coarse block primary silicon obviously refined and the large needle eutectic silicon modified to the fine fibrous or lamella ones. The alloys with the additions of 0.10% P(phosphorus) and 0.90% RE(rare earth) have the optimal microstructure and the highest mechanical properties. Compared with the unmodified alloy, the primary silicon of alloys can be refined from 93.5μm to 24.1μm. The tensile strength is improved from 248MPa to 305MPa and the elongation is improved from 0.31% to 0.47%. Mechanism of double compound modification with phosphorus and rare earth is discussed as well.

Abstract: Process of solution treatment of 6061 aluminum alloy was done by hardness test and microanalysis in this paper. The effects of different solution treatment temperature on the microstructure and mechanical properties of 6061 aluminum alloy were studied and the influence of overburning on the microstructure and mechanical properties of 6061 aluminum alloy were also analyzed. The experimental results show that overburning occurring while 6061 aluminum alloy is heated above 580°C . The hardness measurements and microstructure analysis results show that the hardness decreased, grain boundary becomes trigemanal and compounded –melting structure (burnt structure) appeared when overburning occuring for this alloy .

Abstract: Abstract：Aluminum matrx composites reinforced by in situ ZrB2 particles are fabricated from A356-AlB-K2ZrF6 system via in-situ melt reaction method, and the morphologies, sizes and distributions of the in situ particles as well as the microstructures, mechanical mechanisms of the composites are investigated by XRD,SEM,TEM and tensile tests. The results indicate that the morphologies of the in situ particles are mainly with ball-shape, the sizes are in nanometer scale and the distributions in the matrix are uniform. The interfaces between the in situ particles and the aluminum matrix are net and no interfacial outgrowth is observed.

Abstract: The effect of magnesium on the morphology and composition of the carbide in annealed bearing steel was studied. The morphology of carbide was observed and statistical analysis was carried out. The results showed that the non-uniform distribution of carbides in bearing steel was obviously improved with increasing the content of magnesium. Segregation thermodynamics indicated that the segregation of magnesium to grain boundaries prevented element such as Cr, C from enriching on grain boundary, which was helpful for the reduction of network carbide precipitation. The chemical analysis results proved this viewpoint.

Abstract: According to the ion and molecule coexistence theory (IMCT) of slag structure and corresponding phase diagrams, a thermodynamic model for calculating mass action concentrations of structural units or ion couples of CaO-MnO-FeO-SiO₂-MgO-Al₂O₃ slags was established. Equilibrium mass action concentrations of each structure unit were gained. And the calculation results of N_FeO and N_MnO were compared with measured a_FeO and a_MnO. The comparison shows that the calculated values of N_FeO and N_MnO are in good agreement with the reported measured values of a_FeO and a_MnO, so this calculating model could fairly describe the characteristics of the slag system.

Abstract: Solidus temperature is one of the important key parameters to describe the non-equilibrium solidification states. However, the study of the non-equilibrium solidus is not enough presently. In present work, the key Influence factors on solidus temperature have been discussed, and it can be calculated by a mathematical model for solute microsegregation during the solidification. The calculated solidus can be agree with the recommended values by some dynamic soft reduction model. It is shown clearly the thermal analysis with non-equilibrium solidus was consistent with the pin shooting experiments. And the effect of the non-equilibrium solidus temperature on solid fraction and the length of soft reduction zone at the solidifying front has been studied sufficiently. Dynamic soft reduction metallurgical effect with the non-equilibrium solidus has been agreed by carbon segregation index and Macroetched experiment.

Abstract: The character of structure and the micro-area elements of inner oxidation film were investigated by using metallographic microscope, electron microscopy, energy spectrum analysis and other testing methods based on samples of superheater and reheater tube cutting from three units. The results showed that the inner wall oxide layer of T91 steel superheater tube after running exhibited a double-layer structure, including inner and outer layer, and the interface between the two layers is the original metal surface. There exists a transition zone between the inner oxide film and metal matrix. A concentration gradient of alloy element in transition zone can be observed and the intergranular oxidation is present. The Fe-rich outer oxide layer does not contain Cr, which consists of Fe₃O₄, and the internal oxidation layer is composed of (Fe,Cr)₃O₄.