Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel gamma-TiAl and, fewer, alpha(2)-Ti(3)Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 mu m x 9 mu m. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the gamma-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing gamma-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd.

The high temperature compression behavior of TiAl-Ru alloys was studied at different temperatures and strain rates. Ru was found to have a strong strengthening effect on TiAl alloys. However the Ru addition amount was limited by its low solubility in gamma-TiAl and alpha 2-Ti3Al, and the detrimental effect of excessive ternary phase precipitation. Furthermore, the melting temperature decreases when Ru >= 0.6 at.% as the alloy composition approaches a ternary eutectic point. The strengthening mechanism is discussed and two separate mechanisms are proposed, viz, solid solution strengthening and refined colony strengthening. Intergranular cracks were found in the alloys with low Ru or no Ru addition, but were barely detected as Ru content increased to above 0.6 at.%. It was suggested that Ru showed a beneficial effect on both strength and ductility of TiAl alloys due to the refined colony size. Three-point bend test results showed that the Ru addition can also improve the room temperature ductility of TiAl alloys. Hot workability was increased according to the compression tests. Thermal-mechanically treated TiAl-Ru had much smaller grain size than the heat-treated samples due to dynamic recrystallization. But it did not show superior strength in the compression test compared to the heat-treated samples. The Zener-Hollomon parameter was calculated from the compression strength of heat-treated TiAl-Ru alloys. Its relationship with dynamic recrystallization and hot work is discussed. The mechanical properties of TiAl-Ru alloys are compared with TiAl-Nb samples and demonstrate a promising combination of strength and ductility. (C) 2011 Elsevier Ltd.

AbstractA series of fine duplex microstructures with different volume ratios of equiaxed γ and lath colonies (α2+γ) were achieved from an isothermally forged Ti–46.5Al–2Cr–2Nb alloy. The internal stress determination using X-ray diffraction techniques and plane-strain fracture toughness tests were carried out at room temperature, and the effects of the internal stresses on the fracture toughness were investigated. It was found that the internal stresses play an important role in determining the fracture toughness of the TiAl-based alloy. The compressive internal stresses in γ phase and the tensile internal stresses in α2 phase may be responsible for the delamination along the γ/α2 boundaries. For the microstructures with different volume fraction of lamellar grains, the fracture toughness increases with the internal stresses, which facilitate the formation of shear ligaments; and the effects of the α2 phase and equiaxed gamma grains on the internal stresses and the fracture toughness were discussed.]]>

Solidification behaviour of TiAl-based alloys with the nominal chemical composition Ti-46Al-8Ta and Ti-46Al-8Nb (at.%) was studied at steady- and unsteady-state conditions in Y(2)O(3) moulds using Bridgman solidification technique. The effect of growth parameters (growth rate V and temperature gradient G(L)) on primary solidification phase, solidification path and microstructure parameters were determined. Directional solidification at unsteady-state conditions was applied to determine columnar to equiaxed transition (CET) in the studied systems. (C) 2010 Elsevier Ltd.

The directional solidified Ti-46Al-0.5W-0.5Si alloy with lower oxygen content has been performed successfully by electromagnetic confinement of melt within a water-cooling inductive cold crucible device. By varying the growth rate from 0.5 to 1.5 mm/min, the effects of growth rate on primary phase selection and solidification path as well as structure characteristics either in macro- or micro- scale were determined. The results reveal that the alloys will display 13 phase primarily when the peritectic reaction completed at a growth rate of less than 1.0 mm/min, which implies that the brittle lamellae with orientation that is perpendicular to the crystal growth direction might be disappeared accordingly. It is also found that the average columnar grain size decreased from 312 to 126 gm with increasing of the growth rate, which is in balance with the similarity of primary dendrite arm spacing (PDAS) lambda, decreased according to a relationship of lambda proportional to V-0.41. Due to the oxygen content is reduced to a lower degree for individual DS sample, the oxygen stabilizing effect on yielding a phase to participate primarily seems to be retarded to some extent. Thus, by controlling the directional solidification processing with cold crucible, the gamma based fully lamellar dual-phase DS Ti-46Al-0.5W-0.5Si alloy exhibits good combination of tensile properties at ambient temperature, in which it yields more than 479 MPa of yield strength and as much 3.6% in maximum of fracture elongation. (C) 2012 Elsevier Ltd.

Using the thermodynamic model of TiAl binary system, part ofphase diagram of Ti-(44-50)Al around the peritectic reactionwas calculated and the solidus and liquidus of primary phase βand peritectic phase α were determined and the parametersused for interface response function were obtained. Theinterface temperatures of primary phase βand peritectic phase α were calculated for different growthconditions. According to the criterion of the maximumgrowth temperature, the phase selection andmicrostructure evolution of Ti-(44-50)Al weretheoretically studied and a microstructure-selection map ofTiAl stable state solidification as a function of the meltcomposition and the G/v ratio was constructed.

利用TiAl二元系的热力学模型，计算了Ti-(44—50)Al(原子分数，%)合金包晶反应附近的相图，确定了初生相β和包晶相α的固、液相线，得到了界面响应函数所需的部分参数. 利用这些参数及单相合金凝固界面响应函数模型，计算了初生相β和包晶相α的界面温度与生长条件变化的关系；结合最高界面生长温度判据，分析了Ti-(44—50)Al合金凝固过程中的相选择和组织演化，并确定了Ti-Al合金稳态凝固时以成分和G/v为框架的相选择图.