Disorder trapping in Ni3(Al, Ti) by solidification from the undercooled melt

Modelling of rapid solidification predicts disorder trapping in the superlattice structure of Ni3Al. However, experimental investigations on this compound suffer from ambiguities concerning the solidification path. There is a phase selection competition between the ordered fcc γ′-phase (Ni3Al), the ordered bcc β-phase (NiAl), the disordered fcc γ-phase (Ni), the stable γ′/β eutectic and the metastable γ/β eutectic, and there are subsequent solid state transformations. A replacement of several at.% Al by Ti leads to a stabilization of the gg′-phase and to an avoidance of most of the problems encountered on Ni3Al. The experiments on Ni3(Al, Ti) presented here clearly show the expected disordered crystallization from the undercooled melt. This was proven by measuring the dendrite growth velocity of electromagnetically levitated droplets and by analysing the data in the framework of dendrite and kinetic growth models. Complementary microstructural investigations were performed on the as-solidified samples.