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TiAl-based alloys are potentially used as high-temperature structural materials with a high specific strength in the range of ~ 900 °C. However, the mechanical properties of TiAl-based alloys are extremely anisotropic with respect to the lamellar orientation of the microstructures. A balance combination of room-temperature ductility and strength can be achieved when the lamellar orientation are aligned parallel to the tensile stress direction. Lamellar orientation control of TiAl-based alloys by directional solidification technique has been widely studied in recent years. Two different directional solidification processes can be used to modify the lamellar Orientation. One is a seeding technique and the other is adjusting the solidification path. This paper reviews the principles of the two methods and their progress. The influence of alloy composition and solidification parameters on lamellar orientation control is also discussed.

The microstructure of as-cast Ti-46Al-0.5W-0.5Si alloy exhibits significant microinhomogenetity due to the non-equilibrium solidification and low atom diffusion rate. In order to reduce the adverse effect of this microsegregation on plasticity, the microstructure evolution of the Ti-46Al-0.5W-0.5Si alloy homogenized at different temperatures from 1,200 ℃ to 1,320 ℃ was investigated, and the optimized process of homogenizing treatment, i.e., annealing treated at 1,280 ℃ and held for 8 h, was determined. Microstructures of both the as-cast and heat treated alloys were observed by means of optical microscope and scanning electron microscopes. Tensile tests at room temperature were conducted on the homogenizing treated fully lamellar Ti-46Al-0.5W-0.5Si alloy with loading axis parallel to the lamellar interface. Results show that, at higher heat treatment temperatures, the W element diffuses sufficiently, the microstructure tends to be more homogeneous, and the profile of the silicide clusters becomes smooth. Heat treating conducted in the α+γ two phase region can keep the columnar grains and the original lamellar orientation within them. The microstructure of the alloy after heat treated in α+γ two phase region exhibits the coexisting morphology of coarse lamellar and thin lamellar. The homogenization process at 1,280 ℃ for 8 h can significantly reduce the microsegregation, and the elongation at room temperature can increase from 0.48% (as-cast) to 1.34%.

The aim of this contribution was to investigate the microstructure of in situ Al-Si functionally graded materials produced by traveling magnetic field. The research shows that the composition and associated microstructural feature of Al-Si alloys processed by this method changes from the outer of samples to the inner, respectively, from Al-Si hypereutectic with particles of primary Si to Al-Si eutectic to hypoeutectic composition with a great number of primary Al dendrites. Moreover, the hardness, the wear resistance of samples and the volume fraction of primary Si particles all have obviously gradient characteristics in the samples.

The fracture behavior of fully lamellar binary γ-TiAl alloys is extremely anisotropic with respect to the lamellar orientation. For the fully lamellar Ti-46Al-0.5W-0.5Si alloy, the existence of silicide clusters plays a critical role on the fracture behavior. In the present study, tensile test and three point bending test were performed at room temperature with the loading axis parallel and perpendicular to the lamellar orientation, respectively. To investigate the influence of silicide clusters on the initiation and propagation of cracks, the fracture surface and the cracks adjacent to the fracture zone of the specimens have been analyzed. Results show that the fracture process is related to the morphology and distribution of the silicide clusters. Crack preferentially initiates at and propagates along the interface of silicide and α2/γ lamellar with the loading axis perpendicular to the length direction of silicide. While the silicide can prevent the propagation of cracks from running across with the crack growth direction perpendicular to the length direction of silicide.

The fracture behavior of fully lamellar γ-TiAl alloys depends on the angle between the lamellar orientation and loading axis, but the role of the presentation of grain boundary cannot be ignored. To investigate the influence of the grain boundary on the initiation and propagation of cracks, the tensile test of the alloy was conducted at room temperature with loading axis parallel and perpendicular to the lamellar orientation, respectively. The cracks adjacent to the fracture zone of the tensile specimens have been investigated to analyze the fracture behavior. Results show that the grain boundary has dual influences on the fracture behavior. When the loading axis is parallel to the lamellar orientation, cracks are preferentially initiated at and propagate along the grain boundaries. When the loading axis is perpendicular to the lamellar orientation, the grain boundaries can prevent the propagation of cracks from running across. Additionally, serrated-shape grain boundaries have a better inhibiting effect on the propagation of cracks than planar boundaries.

The distribution of a magnetic field over an inductor used for sheet castings has been investigated experimentally. With an increase in height from the surface, the magnetic field density decreases according to the exponential law. In the transverse direction, the magnetic field density is approximately uniform except near the edges. The magnetic field density is direct proportional to the number of turns multiplied by the current in amperes. Ferro-magnetic material in the upper mould can enhance the magnetic field density. Under the electromagnetic force, the mould-filling process of sheet casings has been studied by physical simulation method. The difference in filling capability between gravity casting and travelling magnetic field casting has been studied. The electromagnetic force can enhance the filling process, but it also brings a problem. With an increase in the magnetic field density, the surface quality of the aluminum alloy sheet castings becomes poor. The reason is discussed.

A new method to modify the solidification microstructure of titanium alloys, named melt hydrogenation, by adding TiH2 as additive into the melt of titanium alloys during induction skull melting process (ISM), is put forward and the refining effect of this method on the solidification microstructure of Ti-6Al-4V alloy was studied experimentally. After melt hydrogenation, the grain sizes of as-cast Ti-6Al-4V alloy decreased to 612 μm from 1,072 μm, lath-shaped α phase was also refined and fine α/β lamellar microstructure was formed when 1.0 wt.% TiH2 was added. δ-hydride was found in the X-ray diffraction (XRD) spectra of Ti-6Al-4V alloy that prepared with 1.0 wt.% TiH2 added and the δ-hydride distributes in α phase as acicular precipitations.

The infl uence of Hf and B on the solidifi cation structure of cast Ti-46Al alloys was investigated. The results show that the coupling effect of Hf and B changes the solidifi cation structure morphology and strongly refi nes the grain size. When the Hf+B contents were increased from 0 + 0.0 to 3 + 0.2, 5 + 0.6 and 7 + 1.0 (in at. %), the solidification structure morphology changed from coarse columnar dendrite to fine columnar dendrite, then to equiaxed dendrite, and further to fi ne near granular grain whilst the average grain size decreased to 20 μm. It is concluded that the columnar dendrite refinement is due to the effect of Hf and B on the decrease of Al diffusion coeffi cient in the melt. The fi ne near granular grain formation is attributed to the combined constitutional supercooling formed by Al and B segregation that is strengthened by Hf and B additions at the solid/liquid interface during solidifi cation, and the TiB2 precipitates acting as heterogeneous nuclei

To investigate the effect of hafnium addition on the solidifi cation structure, Ti-46Al alloys with nominal compositions of Ti-46Al-xHf (x = 0, 3, 5, 7) (at.%) were arc-melted into small ingots in an argon atmosphere. The characteristics of the macrostructures and microstructures were studied using a linear intercept method, OM, SEM (BSE), XRD and TEM. The results showed that the ingots with Hf have near lamellar microstructure in columnar and dendrite morphology. The hafnium concentration has a strong effect on the columnar spacing refi nement. Increasing Hf from 0 to 7 (at.%), the columnar spacing can be reduced from ~ 1000 to ~ 400 μm. Constitute phases of the ingots are α2, a small amount of B2 and c. Most of the B2 phases, richer in Hf and leaner in Al and Ti, exist on the node of the dendrite core in block shape and a little across the lamellar colonies in stick shape. The c phases exist on the boundaries of lamellar colonies in small cellular shape. There also exists a segregation of Hf on the columnar and dendrite core. Particularly, both the α- and β-phase form from the melt as prior phases. The possible phase sequencing during solidifi cation and solid-state transformations with Hf is given in this paper.