Microstructure control and mechanical properties of directionally solidified large size TiAl alloy by electromagnetic confinement

Ti-48Al-2Nb-2Cr alloys with a diameter of 30 mm were prepared by electromagnetic confinement directional solidification with Ti-43Al-3Si seed under different pulling rates. The macro/microstructure evolution and mechanical properties of the directionally solidified Ti-48Al-2Nb-2Cr alloys were investigated. With increasing pulling rates, the grain sizes of the directionally solidified alloys increase to the maximum at the pulling rate of 15 mu m/s and then decrease. During the process of directional solidification, the primary phase of the alloy transforms from the alpha phase into beta phase with the increase of pulling rates. At the pulling rate of 5 mu m/s, the equiaxed grains with fully lamellar microstructure are formed, and some B-2 phases and massive gamma phases are found at the grain boundary. The well aligned alpha(2/gamma) lamellar orientation is obtained at the pulling rate of 15 mu m/s, while the inclined lamellar orientation is obtained when the pulling rate increases to 20 mu m/s. The interlamellar spacing (lambda) decreases with increasing pulling rate (V) according to the relationship lambda = 6966V(-0.563) and r(2) = 0.986. For the DS samples, the nanoindentation hardness of the lamellae and the interlamellar spacing satisfy the relationship of H-N = 53.83d(-0.309) and r(2)(2) = 0.96. The room-temperature tensile strength of directionally solidified alloy reaches the maximum value at the pulling rate of 15 mu m/s. The fracture changes from interlamellar to translamellar mode with increasing pulling rate. The synergistic effect of nanotwins and dislocations contributes to the relatively high room-temperature tensile strength and elongation during the deformation.