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In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys, cold crucible directional solidification (CCDS) technology was employed. A β-type TiAl-based alloy with the nominal composition of Ti44Al6Nb1Cr2V was prepared using the optimized CCDS parameters of 45 kW input power and 0.5 mm·min-1 solidification rate. Thermo-compression testing was utilized to evaluate the hightemperature deformation resistance and creep resistance of the CCDS Ti44Al6Nb1Cr2V alloy. Results show that the CCDS Ti44Al6Nb1Cr2V alloy billets contain aligned columnar grains and a high percentage of small-angle lamellae. Thermo-compression testing results in the radial direction of the CCDS alloy show a much higher peak stress than other reported results in similar conditions. The much higher hardening exponent and deformation activation energy are obtained, corresponding to the excellent high-temperature deformation resistance and creep resistance, which are because of the hard-oriented grains, weaker stress-strain coordination capability of lamella structure and relatively more hysteretic dynamic recrystallization. Thermo-compression testing results in the longitudinal direction of the CCDS Ti44Al6Nb1Cr2V alloy show the much higher peak stress than that in the radial direction, indicating the better high-temperature deformation resistance and creep resistance attributed to the hard-oriented lamellae in this condition.

The present work focused on the Ni3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25Al alloy. Ni3Al intermetallics were prepared at different withdrawal rates by directional solidification (DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni3Al and NiAl phases. The primary dendritic spacing (λ) decreases with the increasing of withdrawal rate (V), and the volume fraction of NiAl phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.

In the present research, microstructure refinement of a high-Nb TiAl alloy (Ti-48Al-8Nb-0.15B ) was realized by means of the electromagnetic continuous casting (EMCC) technique. The microstructure of an ingot obtained by EMCC was analyzed using scanning electron microscopy (SEM). As compared with the raw as-cast alloy, the obtained EMCC alloy presented a much finer microstructure with lamellar colonies with a mean size of about 50-70 μm because the electromagnetic stirring broke initial dendrites and enhanced the heterogeneous nucleation. As the grains were refined, the properties of the TiAl alloy were improved significantly. This implies that the EMCC technique could offer the possibility of application for high-Nb TiAl alloys with a refined microstructure and excellent properties to be used as a structural material.

The intermetallic Ti-43Al-2Cr-2Nb (at.%) alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates (V) ranging from 0.2 to 1.0 mm昺in-1, at a constant temperature gradients (G=18 K昺m-1) . Macrostructures of the alloy were observed by optical microscopy. Microstructures of the alloy were characterized by scanning electron microscopy (SEM) in back-scattered electron mode and transmission electron microscopy. Results showed that morphologies of macrostructure depend greatly on the applied withdrawal rate. Continuous columnar grains can be obtained under slow withdrawal rates ranging from 0.2 to 0.6 mm昺in-1. The microstructure of the alloy was composed of ?/?lamellar structures and a small number of mixtures of B2 phases and blocky ?phases. The columnar grain size (d) and interlamellar spacing (? decrease with an increasing withdrawal rate. The effect of withdrawal rate on microhardness was also investigated. The microhardness of the directionally solidified Ti-43Al-2Cr-2Nb alloy increases with an increase in withdrawal rate. This is mainly attributed to the increase of B2 and ? phases as well as the refinement of lamellae.

In order to improve the mechanical properties of TiAl alloys, especially the ductility at room temperature, and to study the effect of boron (B) on TiAl alloys, different contents (0, 0.1, 0.3, 0.6, 0.9, 1.2, at.%) of B were added into Ti-44Al-6Nb alloys to prepare ingots. The surface quality, macrostructure, microstructure, compressive properties and fracture surface of the ingots were studied. The results show that B has little influence on the surface quality except that there are some dark spots on the surface when the content of B is 0.9%. B can refine the grains. The average grain size decrease from about 0.8 mm to 0.088 mm with increasing B content. Meanwhile, the grain morphology of these ingots changes from big equiaxed grains with lamellars to fine equiaxed grains. When the content of B is 1.2%, the primary TiB2 phase forms in the liquid phase and increases the nucleation rate, leading to further refinement of the grains. The compressive testing results show that B can increase the strength and the ductility, the compressive strength and compressibility can reach 2,037.8 MPa and 26.7% from 1,156.2 MPa and 10.2% when the boron content is 0.6%, which is resulted from grain refining and grain boundary strengthening. It is found that the compressive strength and the compressibility are relatively stable when the B content is more than 0.3%.

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]

In order to improve mechanical properties of TiAlNb alloys, different contents of silicon were added into Ti48Al6Nb alloy. The Ti48Al6NbxSi (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5, at.%) alloys were prepared by vacuum arc melting. The phase constitution, microstructure evolution and mechanical properties of the alloys were studied. Results show that the Ti48Al6NbxSi alloys consist of -TiAl phase, a2-Ti3Al phase and B2 phase, and Ti5Si3 silicide phase is formed when the addition of silicon is higher than 0.3at.%. The addition of silicon leads to the decrease in phase and increase in a2 phase. The formation of silicide decreases the amount of Nb dissolved in the TiAl matrix, and therefore decreases B2 phase. Compressive tests show that the ultimate strength of the alloys increases from 2,063 MPa to 2,281 MPa with an increase in silicon from 0 to 0.5at.%, while the fracture strain decreases from 34.7% to 30.8%. The increase of compressive strength and decrease of fracture strain can be attributed to the decrease of B2 phase and the formation of Ti5Si3 phase by the addition of silicon. The strengthening mechanism is changed from solid solution strengthening when the addition of silicon is less than 0.3at.% to combination of solid solution strengthening and secondary phase strengthening when the addition of silicon is higher than 0.3at.%. [ABSTRACT FROM AUTHOR]

The present work focused on the Ni3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25Al alloy. Ni3Al intermetallics were prepared at different withdrawal rates by directional solidification (DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni3Al and NiAl phases. The primary dendritic spacing λdecreases with the increasing of withdrawal rate (V), and the volume fraction of NiAl phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate. [ABSTRACT FROM AUTHOR]