Microstructure of Modified Inconel 706 Superalloys - 3 Dimensional Atom-Probe and HRTEM Investigations

Abstract

Inconel 706 does not fully meet the stringent requirements of the application in new steam turbines. The thermal stability of Inconel 706 is insufficient for a long term service above 700 degC, which leads to a dramatic loss of creep and tensile strength. Two methods of compositional modification were followed to optimize the microstructural stability of Inconel 706. One is by adding rhenium to the standard composition of the superalloy and second is by refining the chemistry of Inconel 706 resulting in a new alloy composition named DT 706 alloy. The main aim of this study was to investigate the complex microstructure in Inconel 706 alloy with high resolution techniques like electron microscopy (HREM) and three dimensional atom-probe (3DAP). The microchemistry around precipitates and the local structural variations involved in phase formation and transformation sequences of the fine precipitates and the co-precipitates (as small as 10 nm) were studied. The analysis was performed to understand not only the transformation sequences but also the stability of each precipitate type. Microstructures in different heat treated conditions and after long time ageing at 750degC for 750 h and 5000 h were therefore studied in Inconel 706 alloy and compared with the modified alloys. The addition of Re to Inconel 706 composition did not show the desired effect which therefore suggests that alloying with Re is not the right choice in order to stabilize the structure of Ni-Fe wrought superalloys such as Inconel 706. On the other hand, it was observed that the thermal stability of DT 706 alloy is significantly improved. Therefore, DT 706 alloy has an advantage over the Inconel 706 alloy