DISORDER TRAPPING IN NI-3(AL, TI) BY SOLIDIFICATION FROM THE UNDERCOOLED MELT

Modelling of rapid solidification predicts disorder trapping in the su perlattice structure of Ni3Al. However, experimental investigations on this compound suffer from ambiguities concerning the solidification p ath. There is a phase selection competition between the ordered fcc ga mma'-phase (Ni3Al), the ordered bcc beta-phase (NiAl), the disordered fcc gamma-phase (Ni), the stable gamma/beta eutectic and the metastabl e gamma/beta eutectic, and there are subsequent solid state transforma tions. A replacement of several at.% Al by Ti leads to a stabilization of the gamma'-phase and to an avoidance of most of the problems encou ntered on Ni3Al. The experiments on Nu(3)(Al, Ti) presented here clear ly show the expected disordered crystallization from the undercooled m elt. This was proven by measuring the dendrite growth velocity of elec tromagnetically levitated droplets and by analysing the data in the fr amework of dendrite and kinetic growth models. Complementary microstru ctural investigations were performed on the as-solidified samples. (C) 1997 Elsevier Science S.A.