Structures And Properties Of Ti46Al0.5Si0.5W Billets Directionally Solidified With Cold Crucible

In this paper, based on the research of Ti and TiAl alloys directional solidification with cold crucible and a process for Ti alloys with 2.5:1 near-rectangular cold crucible, a composition of Ti46Al0.5Si0.5W was prepared with directional and fine grains in slab shape. The effects of technical parameters on the grain modalities are studied with orthogonal experiments. The results show that with increasing power grains trends to grow up while the direction of grains is improved. Grains trend to be finer but the direction is more deviated from the growth direction with increasing withdrawal velocity. When the preheating time is prolonged, the size and the direction of grains both trend to be disbennifit. Therefore, it should be that preheating time is cut as short as possible with the guarantee of enough superheat.The macro- and microstructures and phase components of Ti46Al0.5Si0.5W slab ingots are observed and analyzed with digital camera, optical microscopy (OM), scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The region near the solidification interface is partitioned off to four distinct areas by three interfaces, which are areas of last solidified liquid, triangle with bend side, bright strip and steady solidified solid, respectively. It is ascertained that the areas of last solidified liquid and triangle with bend side are both the original liquidus zone, and the bright strip are the original mushy zone. Furthermore, with the composition variation of TiAl alloys, the width of bright strip is changed also. Inside the zone of steady directional solidification, the microstructure is mainly fully alternative lamellar structures formed withγstrips andα2 lamellae. Additives in Ti46Al0.5Si0.5W alloy exist as solid solutions in crystal and they do not participate as compounds. The forming process of lamellar structure of Ti46Al0.5Si0.5W alloy is analyzed and determined with Al equivalence. Theγ+α2 lamellae that form just during solid-solid phase transformation display many directions that deviate from the growth direction of the crystal. This is because of the segregation and the competition of the leading phase during solidification. Also, a variety of structures could exist in terms of the factors according to the steady ability within the system. The fracture toughness of lamellar structures of directionally solidified Ti46Al0.5Si0.5W is measured by the method of direct crack measurement (DCM). The anisotropy of solidified structure results in the anisotropy of fracture toughness. The value of fracture toughness is at a relatively low level because of the big grains, the large lamellar spacing and the mis-oriented lamellar directions of TiAl structures prepared accordingly. At room temperature, the average tensile strength and the elongation of Ti46Al0.5Si0.5W slab ingots are 343.1MPa and 0.202%, respectively. At high temperature, the values are 344.8MPa and 0.77% respectively at 800℃and it has shown the"R"phenomenon existence. The fracture surfaces of the specimens which have been tensiled at room and elevated temperatures respectively are observed by the SEM. The results show that the fracture of directionally solidified lamellar structures is mainly cleavage along lamellae, and they can also rupture across lamellar structure and rupture along the grain boundaries as well. The fracture of the tensile specimens at elevated temperature shows a mode of slightly plastic deformation.