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

Abstract

The Al content and thestabilizers in TiAl base alloys were investigated, which have an important effect on the solidification characteristics and phase selection. TiAl base alloys with single �-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 Ti48Al2Cr2Nb (atomic fraction, %) peritectic alloy. By calculating the interfacial critical nucleation work and steady state nucleation rates ofphase (bcc) andphase (hcp), it is obtained thatphase is always prior to nucleate from the Ti48Al2Cr2Nb 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. Thestabilizers 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