Sub-solidus and super-solidus heat treatments of a single crystal superalloy

Cast Ni-base superalloys are highly heterogeneous due to coring during solidification. To increase high temperature capabilities, modern single crystal (SX) superalloys are heavily alloyed with the refractory elements, W, Ta, and Re. As a consequence, the microsegregation of these alloys is more pronounced leading to higher eutectic fractions and chemical instability. Hence, heat treatment of modern SX alloys is becoming increasingly challenging, time consuming and expensive. This thesis investigates the homogenization-solutionization behaviour of an experimental Ni-base single crystal superalloy over a wide temperature range including sub-solidus and super-solidus temperatures.;The first part of the thesis studies conventional stepwise homogenization-solution heat treatments. After characterizing the specimens by metallography, phase fraction analysis, and electron microprobe analysis (EPMA), a continuously heating procedure is proposed for heavily alloyed SX superalloys to reduce homogenization-solution heat treatment time without causing incipient melting. Though the solidus temperature is the upper limit for conventional stepwise homogenization-solution heat treatments, the second part of this research identifies that a controlled super-solidus exposure gives rise to significant microstructural refinement without affecting the SX structure. A modification to Bridgman SX processing involving a super-solidus step is proposed in the third part of the thesis. The effect of super-solidus temperature and mould withdrawal rate on the microstructure, microporosity and eutectic fraction of super-solidus processed specimens are presented and compared with conventionally cast single crystal specimens. The potential advantage of this modified method is illustrated by performing solution heat treatments on both super-solidus and normally processed specimens followed by eutectic phase fraction analysis.