Theoretical Predictions and Experimental Confirmations of Phase Selection and Microstructure Evolution Inside Rapidly Solidifying Ti₆₂Ni₃₈ Hyperperitectic Alloy

The phase selection and microstructure evolution inside rapidly solidified Ti₆₂Ni₃₈ alloy have been theoretically predicted by a typical nucleation theory and experimentally confirmed by both the arc melting and melt spinning techniques. For the arc melted samples with a diameter greater than 3.3 mm, direct nucleation phenomenon of metastable peritectic Ti₂Ni phase occurred due to the stronger nucleation ability of Ti₂Ni phase in the sample bottom region with a relatively high undercooling, whereas peritectic solidification microstructures with directional growth characteristics formed in the other regions of the sample with a slight undercooling. Once the diameter of the arc melted sample reduced below 3.3 mm, the sample bottom region with a substantially undercooled state was still occupied by the metastable peritectic Ti₂Ni phase owing to its stronger nucleation ability, the middle region was mainly characterized by the metastable coupled growth between primary TiNi phase and peritectic Ti₂Ni phase because of the comparable nucleation abilities for constituted phases at moderate undercoolings, whereas the other regions with a relatively small undercoolings appeared as peritectic solidification microstructures. Moreover, the phase transitions in different regions of the arc melted sample have been demonstrated by a high-speed CCD. For the atomized alloy droplets under the melt spinning condition, with the reduction in droplet size, peritectic solidification microstructures evolved into metastable coupled growth microstructures. Based on the calculations of the temperature field for arc melted sample and atomized droplets, it was found that both the temperature distinction and the cooling rate difference between the droplet center and the droplet surface were much smaller in contrast with those arc melted samples, which leaded to the appearance of only sole solidification mode in one atomized droplet. The theoretical predictions of phase selection and microstructure evolution were in good agreement with the experimental observations.