Partial Remelting of Thixotropic Magnesium-Rare Earth Alloy from Near Non-Equilibrium-Liquidus Casting

After the investigation on partial remelting of thixotropic magnesium serial alloys （ZK60） by near non-equilibrium liquidus casting （NNLC）, the primary solid grains of ZK60-2Ca alloy spheroidized notably during partial remelting processing, however, coarsening and polygonization as occurred holding time prolonged. The refining and globularity of the thixotropic alloys are promoted after further alloyed by Y, RE, Nd and/or Ag, and the results vary with those addition. The remelting structure of ZK60-2Ca-1Y alloy is finer than its base alloy. And the effect of RE, especially Ag, on the refinement of microstrueture is notable, but Nd does nothing on it. There is little impact of remelting temperature fluctuation on partial remelted microstrueture as holding time in general. On the contrary, it is more sensitive at longer holding time. The quality thixotropic silver-contained alloy can be achieved by remelted partially at 600℃ for 10 min.

NON-EQUILIBRIUM SEGREGATION OF Si ON Al-0.35 wt-% Si ALLOY SURFACE

Si segregation was observed in the vacancy condensation pits,formed by supersaturated vacancies beneath the oxidation layer of Al-0.35wt-％Si alloy,by optical microscopy,X- ray photoelectron spectroscopy and microhardness tests.This phenomenon could only be explained by vacancy-Si complex inducing non-equilibrium segregation.

Microstructural characteristics of Al-20Si-2Cu-0.4Mg-1Ni alloy formed by rheo-squeeze casting after ultrasonic vibration treatment

A swash plate for air conditioning compressor of cars was formed by rheo-squeeze casting with semi-solid Al-Si alloy slurry prepared by ultrasonic vibration process, and the microstructure of this alloy was investigated. Besides the microstructures of primary Si particles and α(Al)+β-Si eutectic phases, non-equilibrium α(Al) particles or dendrites are discovered in the microstructure of the Al-20Si-2Cu-0.4Mg-1Ni alloy. Rapid cooling generated by squeeze casting process rather than the pressure is considered as the main reason for the formation of non-equilibrium α(Al) phase. The sound pressurizing effect of ultrasonic vibration also enables the non-equilibrium α(Al) phases to form above eutectic temperature and grow into non-dendritic spheroids in the process of semi-solid slurry preparation. Non-equilibrium α(Al) phases formed in the hypereutectic Al-Si alloy with ultrasonic vibration treatment, consist of round α(Al) grains formed above the eutectic temperature and a small amount of fine α(Al) dendrites formed under the eutectic temperature. The volume fraction of primary Si particles is decreased significantly by the effect of ultrasonic vibration through increasing the solid solubility of Si atoms in α(Al) matrix and decreasing the forming temperature range of primary Si particles. The average particle diameter and the volume fraction of primary Si particles in microstructure of the swash-plate by rheo-squeeze casting are 24.3 μm and 11.1%, respectively.

Deviation of congruent composition in Fe-Pd system

The first-principles calculation of the disordered Ll0 phase boundary for the Fe-Pd system is attempted by combining the FLAPW electronic structure total energy calculations and the cluster variation method via the cluster expansion method. The lattice vibration effects are taken into account based on the Debye-Gruneisen model within quasi-harmonic approximation. The transition temperature is reproduced with very high accuracy. However, the experimental congruent composition of disordered L10 phase that significantly deviates from 1.＂ 1 stoichiometry is not reproduced. Fulther calculations are attempted based on the phenomenological Lennard-Jones type pair potential, which is capable of introducing both tetragonality of the Ll0 ordered phase and additional configurational freedom because of the magnetic spins. The prelimi- nary calculations indicate that the tetragonality enhances the stability of the L10 ordered phase and the magnetic contributions also change the transition temperature. Despite these findings, the shift of the congruent composition still remains as a subject that needs further research. The electronic origin of the shifting of the congruent composition is briefly discussed.

δ/γtransformation in non-equilibrium solidified peritectic Fe-Ni alloy

Through phase transformation kinetic analysis and experimental observation,the δ/γtransformation occurring in the non-equilibrium peritectic Fe-4.33at.%Ni alloys was systematically investigated.According to JMA solid-state transformation ki-netic theory,the Time-Temperature-Transformation(TTT)curves of the δ/γtrans-formation in peritectic Fe-Ni alloy were calculated.On this basis,the physical cor-relation between the δ/γtransformation and the initial undercooling of melt(T)was elucidated.The results indicate that the change of T can alter not only the overall δ/γtransformation pathways but also the transformation fraction with re-spect to each transformation mechanism.

Fabrication of Fe-6.5wt%Si Bulk by Spark Plasma Sintering

Fe-6.5wt%Si composite compact was fabricated by spark plasma sintering(SPS).Mec hanical alloying(MA)was used to prepare Fe-Si composite powders.The composite p owders were sintered by SPS at elevsated temperature from 500℃ to 700℃.The exp erimental results indicate that the non-equilibrium state of composite Fe-Si i s preserved in the compact.The density of the bulk rises with the increasing tem perature and there is no diffusion of silicon and iron in the interface.