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Intermetallic Ti-45Al-8Nb-(W, B, Y) (at.%) and Ti-46Al-5Nb alloys are directionally solidified at a constant growth rate of 30 µm·s-1 using a Bridgman type apparatus. The quenched microstructures and lengths of different phase regions were observed and measured after various growing times of 0-30 min. Results show that the phase transformations in different phase regions are mainly depending on the high temperature microstructure and the supercooling degree during quenching process. After isothermal holding, the primary phase grows into the liquid phase, the dendrites change from equiaxed to columnar grains, and the length of the L+ß phase region, L+ß+a phase region and mushy zone varies, indicating that the entire directional solidification process can be described by a static equilibrium - nonequilibrium - dynamic equilibrium evolution process. In addition, the gap between the original growth interface and front interface shows that the actual crystal growth rate is not equal to the drawing velocity during directional solidification. [ABSTRACT FROM AUTHOR]

To improve the microstructure and microhardness, Ti-48Al (at.%) alloy was rapidly solidified by melt spinning under different cooling rates. The microstructure and microhardness of rapidly solidified Ti-48Al alloy were systematically investigated. Results show that the average lamellar colony size of the alloy reduces from 60.6 µm to 11 µm as the cooling rate increases from 2.3×105 to 5.1×105 K·s-1, caused by the increase of nucleation rate at a higher cooling rate. At the high cooling rate of (4.3- 5.1)×105 K·s-1, the a phase is the primary phase, and a few metastable a phases are reserved, which then transform into a2 phase and subsequently lead to the formation of a2 equiaxed grain. The lamellar spacing also decreases with the increase of cooling rate. The relationship between lamellar spacing (d) and cooling rate (v) is d=33.6v -1.34. The microhardness increases with the increase of cooling rate because the refined lamellar spacing and grain size can improve the microhardness. [ABSTRACT FROM AUTHOR]

Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density. In this regard, the Al-Cu-Li alloy castings with high Li content from 1.5wt.% to 4.5wt.% were prepared by near-rapid solidification, followed by two-stage homogenization treatment (490 °C/16 h and 530 °C/16 h). The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software. The results indicate that with the increase of Li content, the grain sizes decrease, the solution ability of Cu in the matrix a-Al phase increases, while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification. Additionally, by the coupling of near-rapid solidification and two-stage homogenization, the metastable precipitated phases (Al7Cu4Li and AlCu3) can be dissolved effectively in the alloys with Li content of 1.5wt.%-2.5wt.%; moreover, the stable precipitated phases (Al6CuLi3 and Al2CuLi) uniformly distribute at the grain boundaries in the alloys with Li content of 3.5wt.%-4.5wt.%. As a result, the refined and homogenized microstructure can be obtained. [ABSTRACT FROM AUTHOR]

The effect of excitation current intensity on the mechanical properties of ZL205A castings solidified under a traveling magnetic field was studied. The results of the experiment indicate that the excitation current intensity of the traveling magnetic field has a great influence on the mechanical properties of the ZL205A castings. When the excitation current intensity is 15 A, the tensile strength and elongation of ZL205A alloy castings increase 27.2% and 67.7%, respectively, compared with those of the same alloy solidified under gravity. The improvement of mechanical properties is attributed to the decrease of micro-porosity in the alloy. Under the traveling magnetic field, the feeding pressure in the alloy melt before solidification can be enhanced due to the electromagnetic force. Moreover, the melt flow induced by the traveling magnetic field can decrease the temperature gradient. The feeding resistance will be increased because the temperature gradient decrease. So traveling magnetic field has an optimum effect on feeding. [ABSTRACT FROM AUTHOR]