The electron structure ofγ/α2 phase boundaries in lamellar colonies in Ti-47Al-2M(M=Nb, Cr, V) (mole fraction, %) alloys was theoretically investigated by Empirical Electron Theory of Solid and Molecules (EET) and the bond-length-difference (BLD) method. Average-Atom-Model was employed to calculate valence electron structure of TiAl intermetallics containing site substitution elements. On this basis, the boundary condition of electron movement was employed in the improved Thomas-Fermi-Dirac (TFD) theory to decide the continuity of the electron density of the lamellar colonies interface and it is found that ofγ/α2 interface is continuous(Δρ<10%). Furthermore, it is found that adding alloying elements (including Nb, Cr, and V) can improve the electron density (ρ) ofγ/α2 interfaces, and decrease the electron density difference(Δρ)ofγ/α2 interfaces. Adding V element decreasingΔρis more remarkable than other site substitution elements. According to electron structure study ofγ/α2 interfaces in Ti-47Al-2M alloys, the added elements improve mechanical properties of the alloy in the following order: V>Cr>Nb.
A TiAl alloy from pulverized rapidly solidified ribbons with the composition of Ti-46Al-2Cr-4Nb-0.3Y（mole fraction,%） was processed by spark plasma sintering（SPS）.The effects of sintering temperature on the microstructure and mechanical properties were studied.The results show that the microstructure and phase constitution vary with sintering temperature.Sintering the milled powders at 1200 ℃ produces fully dense compact.Higher sintering temperature does not improve the densification evidently.The dominant phases are γ and α2 in the bulk alloys sintered at 1200 ℃.With higher sintering temperature,the fraction of α2 phase decreases and the microstructure changes from equiaxed near γ grain to near lamellar structure,together with a slight coarsening.The bulk alloy sintered at 1260 ℃ with refined and homogeneous near lamellar structure reveals the best overall mechanical properties.The compressional fracture stress and compression ratio are 2984 MPa and 41.5%,respectively,at room temperature.The tensile fracture stress and ductility are 527.5 MPa and 5.9%,respectively,at 800 ℃.
A fine-grained TiAl alloy with a composition of Ti-47%Al(mole fraction)was prepared by double mechanical milling(DMM)and spark plasma sintering(SPS).The relationship among sintering temperature,microstructure and mechanical properties of Ti-47%Al alloy was studied by X-ray diffractometry(XRD),scanning electron microscopy(SEM)and mechanical testing.The results show that the morphology of double mechanical milling powder is regular with size of 20-40μm.The main phase TiAl and few phases Ti3Al and Ti2Al were observed in the SPS bulk samples.For samples sintered at 1 000℃,the equiaxed crystal grain was achieved with size of 100-250 nm.The samples exhibited compressive and bending properties at room temperature with compressive strength of 2 013 MPa,compression ratio of 4.6%and bending strength of 896 MPa.For samples sintered at 1 100 ℃,the size of equiaxed crystal grain was obviously increased.The SPS bulk samples exhibited uniform microstructures,with equiaxed TiAl phase and lamellar Ti3Al phase were observed.The samples exhibited compressive and bending properties at room temperature with compressive strength of 1 990 MPa,compression ratio of 6.0%and bending strength of 705 MPa.The micro-hardness of the SPS bulk samples sintered at 1 000℃is obviously higher than that of the samples sintered at 1 100℃.The compression fracture mode of the SPS TiAl alloy samples is intergranular fracture and the bending fracture mode of the SPS TiAl alloy samples is intergranular rupture and cleavage fracture.
