Some Observations on the Columnar-to-Equiaxed Transition in Nickel-base Superalloys

摘要

Several workers have studied the columnar/equiax transition experimentally and/or by modelling techniques, usually employing one of the highly-segregating low-melting alloys such as Al-Cu as the test material. It is clear from industrial practice that the high-temperature alloys exhibit this transition both in conventional casting and in directional solidification methods. These alloys also show the transition under appropriate thermal circumstances in the remelting methods including vacuum arc and electroslag remelting. The objective of the present study is to demonstrate the relation between results obtained for directionally solidified cast alloys, single crystal castings and remelt ingots in respect of the thermal and nucleation conditions for the transition. The results indicate that the critical nucleation density is in good agreement with previously reported models in both cases and that in the nickel-base superalloys containing Ti the main nucleant is most probably TiN.