•The high temperature torsion behavior of high Nb-TiAl alloy was investigated.•The hardness and the density of twins increased from the center to rim of sample.•Forming sub-grain was the transition stage between “recrystallized” and “deformed”.•γ grain recrystallized at the twin-twin junctions.•Dislocation-twin interactions formed the secondary twins.The high temperature torsion deformation of a duplex microstructure high Nb-TiAl alloy was investigated at 850 °C. The experimental results showed that the hardness and the density of twins increased with radial distance from the center to rim of 180° torsion sample. Besides, the “recrystallized”, “substructured” and “deformed” microstructures were also presented gradient and forming sub-grains was the transition stage between recrystallization and deformation. Moreover, γ grain recrystallization at twin-twin junctions occurred and the observation of dislocation-twin interactions revealed the formation of secondary twins by dissociating of the dislocation at twin-twin boundary. The dissociation of such dislocations interacting with twin boundaries and γ grain recrystallizations at twin-twin boundaries avoided excessive stress concentration, which allowed the alloy to continue plastic deformation.Download high-res image (97KB)Download full-size image

Fe-6.5 wt.%Si microwires with 40 ∼ 100 μm diameter and 2 ∼ 30 cm length were successfully fabricated by the melt extraction technique. The microwires possess a D-shaped cross-section and exhibit high ductility at room temperature due to rapid solidification inhibiting the formation of ordered phases. The fan-like microstructure, which forms as a result of a fan-like temperature gradient, consists of equiaxed crystals in the centre and columnar crystals along the radial direction. Significant dependence of magnetic moment on the sample spatial orientation with respect to the magnetic field has been observed from M-θ curves, which exhibit a “W” shape from 50 Oe to 7000 Oe due to shape anisotropy effects and a “Double-W” shape from 8000 Oe to 10000 Oe due to a compromise between the demagnetizing field energy (Ed) and external field (Zeeman) energy (Eh).

The microstructural degradation of the high Nb-TiAl alloy during the fabrication process by electron beam melting (EBM) is reported. The lamellar structure of as-EBM samples in the bottom part of the build shows significant microstructure degradation, resulting in deterioration of tensile properties at both ambient and high temperatures. Microstructural analysis has been conducted by electron backscattered diffraction and transmission Kikuchi diffraction microscopy. The results show that the degradation of the lamellar structure is not only caused by the coarsening under the high-frequency thermal cycling during the fabrication of following layers but also attributed by the discontinuous dynamic recrystallization of the unstable initial lamellar structure resulting from the rapid solidification.

Reaction behaviors occurring in Ti/Al foil metallurgy were systematically investigated. Particular emphasis was focused on the reaction between solid Al and Ti as well as subsequent reaction between TiAl3 and Ti layer. In the solid reaction between Al and Ti, the presence of residual Al is mainly caused by inhomogeneous growth of TiAl3 layer and micro-voids existing at the interface. However, through reaction between molten Al and Ti, TiAl3/Ti multilayer can be achieved with complete consumption of Al. During subsequent high-temperature heat treatment, TiAl3/Ti multilayer will eventually turn into Ti3Al/TiAl multilayer accompanying with simultaneous formation and successive disappearance of intermediate phases, such as TiAl2 and Ti2Al5. Moreover, it is found that the growth direction of TiAl layer changes as a function of annealing time between different couples in multi-intermetallics system.

•The high Nb–TiAl sheet is directly fabricated from as-cast PAM ingot without the HIP and forging.•The coarse lamella colonies of ingot can be completely broken by hot pack rolling and homogenous microstructure is obtained.•There is weak cubic texture in the high Nb–TiAl sheet, which is caused by recrystallization.•The sheet shows superplasticity above 950 °C at relatively high strain rate.High-Nb containing (6–10 at.%) TiAl alloys exhibit excellent high-temperature strength and oxidation resistance. However, they are difficult to be fabricated into sheet in comparison with the conventional TiAl alloys. In the present work, the hot-deformation behavior of a high Nb–TiAl alloy (Ti–45Al-8.5Nb-0.2W-0.2B-0.03Y) was investigated. Hot-rolling process was optimized and carried out directly from the PAM (Plasma Arc Melting) ingot without the hot isostatic pressing (HIP) and hot forging. The hot-rolled sheets were successfully manufactured with dimensions up to 360 mm × 100 mm × 3.5 mm. The microstructure of as-rolled sheet is a typical “near gamma” characteristic with an average grain size about 15 μm. In the view of breakdown the lamellar colonies of high Nb–TiAl alloy ingot, the direct hot-rolling process has advantage over hot can forging and extrusion. Moreover, mechanical properties at room and high temperatures were also tested. Noteworthily, the as-rolled high Nb–TiAl alloy shows superplasticity above 950 °C at relatively high strain rate of 5 × 10−4.