Electrostatic levitation processing and microscopic hardness property of hyperperitectic Ti60Ni40 alloy

The solidification mechanism of hyperperitectic Ti60Ni40 alloy was systematically investigated by using the electrostatic levitation (ESL) technique, and the maximum undercooling achieved was up to 373 K (0.25 TL). When the undercooling is below a threshold of 281 K, typical peritectic solidification prevailed within the levitated liquid alloy, and the double recalescence processes were observed by a high-speed camera. In this case, the primary TiNi compound preferentially grew as well-defined dendrites, and the interdendritic Ti2Ni phase was subsequently produced by the peritectic reaction and transformation. TEM analyses showed that an incomplete martensitic transformation occurs in the primary TiNi phase, which results in the complicated structure of B19’ martensite and R-phase. Once the undercooling exceeded 281 K, the coupled-growth mode between TiNi and Ti2Ni compounds was induced, leading to the lamellar and anomalous eutectic-like microstructures. The enhancement of undercooling promoted the migration speed of liquid-solid interface in both typical peritectic solidification and metastable coupled-growth modes, but it exhibited an abrupt reduction in critical undercooling. The microscopic hardness of the primary TiNi phase processed by ESL was 48.1% higher than that of the Ti60Ni40 master alloy, while it increased by 29.0% for the peritectic Ti2Ni phase. These indicated that the peritectic-type superalloys with advanced performances could be designed by modulating the solidification process and solid solubility through different undercoolings.