PHASE SELECTION AND THE SOLIDIFICATION CHARACTERISTICS OF TiAl BASE ALLOYS IN THE NONEQUILIBRIUM SOLIDIFICATION

The Al content and the β stabilizers in TiAl base alloys were investigated,which have an important effect on the solidification characteristics and phase selection.TiAl base alloys with single β-solidifying have the characteristics of low Al content and narrow crystallization temperature range.It is attributed to forming a homogeneous fine-grained microstructure.Dendrite morphology in TiAl base alloys is investigated by SEM-BSE in the nonequilibrium solidification conditions.The relationships between undercooling and nucleation of two phases are investigated systematically by using the classical nucleation theory in the undercooled Ti48A12Cr2Nb(atomic fraction,%) peritectic alloy.By calculating the interfacial critical nucleation work and steady state nucleation rates of β phase(bcc) and α phase(hcp),it is obtained that β phase is always prior to nucleate from the Ti48A12Cr2Nb undercooled melts at the cooling rate about 15 K/s in the achieved undercooling range.Due to narrow crystallization temperature range,microstructures of TiAl base alloys with high Al content as the hyper-peritectic solidification path take on the gradual solidification style and finer dendrite morphology.It is conducive to reducing and eliminating shrinkage porosity and hot cracking caused by mushy solidification.The β stabilizers have an important effect on solidification path depending on its Al equivalent and will change the solidification path of TiAl alloy from hyper-peritectic solidification to hypo-peritectic solidification.The overmuch Al content in TiAl base alloys leads to the formation of large number of massive 7 phase impacting on strength and ductility of alloys.