Thermoelectric Bi2Te3bulk alloys were directionally solidified successfully at the pulling rate ranging from 1 μm/s to 50 μm/s under a high temperature gradient of 200 K/cm. Preferred crystal orientations of (0 1 5), (1 0 10) and (1 1 0) faces appeared at the pulling rate of 50 μm/s. In the Bi2Te3alloys directionally solidified at 5 μm/s, the maximum Seebeck coefficient of -253 μV/K was obtained and the maximum electrical resistivity of 2.26 mΩ•cm was measured at 300 K. Besides, the optimum Power Factor (PF) value reached 3.83×10-3W/K2m at 1 μm/s and the measured results show that the thermoelectric Bi2Te3 bulk alloys grown at low growth rates supply the large PF value at ambient temperate, while at high temperature, the alloy grown at 50 μm/s has a better PF value.
Microstructure of the arc-melted Cr-20at%Nb alloy solidified in different positions was investigated using optical microscope (OM) and scanning electron microscope (SEM). The results show that fully coupled eutectic was developed at the bottom of the ingot and then the eutectic experienced a columnar to equiaxed transition due to the decrease of G/V ratio (G the thermal gradient and V the solidification rate) with increasing the solidification distance. A novel lamellar structure was observed in the middle side of the arc-melted ingot, which may form by means of the eutectoid reaction of C14-Cr2Nb→C15-Cr2Nb+Crss. With the increase of the solidification distance from the middle side to the top side, the primary Cr2Nb phase underwent a nonfaceted to faceted transition.
Directional solidification experiments on Pb-Bi peritectic alloys have been conducted at very low velocity (V=0.5 μm/s) and high thermal gradient (G=25 K/mm). Incomplete banded and oscillatory structures have been observed in both of hypoperitectic and hyperperitectic compositions over several millimeters of growth. These structures resulted from the repeated nucleation and competition between properitectic α- and peritectic β-phases. The banded or oscillatory structures are found to be transient and the final steady-state phase was only the peritectic β-phases. With an increase in composition, β phase formed and α phase disappeared at a lower solidified distance. Composition variations in the banded structure are measured to determine the solute distribution along the growth direction.
The experiments were simulated based on the real environment of The East China sea. It were conducted to study HAZ seawater corrosion behaviors as well as effect of welding process on the corrosion performance of 304 stainless steel at different welding processes of argon arc welding with electrochemical technology. The results indicated that when the welding current increases, the corrosion rate of stainless steel sample will also increase while the open circuit potential will decrease. And with the increasing argon flow rate , the results will completely be opposite. The largest impedance size of the parent metal is 36.175 Ω/ cm2and the minimum impedance size of B1R is 12.531 Ω/ cm2.
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