The Ni-Fe based superalloy INCONEL 706 (IN706) has a complex microstructure and many A 3 B type phases, e.g., Ni 3 Al ␥Ј, Ni 3 Nb ␥Љ, Ni 3 Nb ␦, or Ni 3 Ti precipitates can form. The various precipitates can also have different morphologies, i.e., plate, needle, cube, or disc shape, and some of them exist as co-precipitates-␥Ј/␥Љ. In the present study, in-situ measurements by small-angle neutron scattering (SANS) were performed to monitor the microstructural evolution at elevated temperatures. The SANS measurements were complemented by microstructural observations at room temperature by scanning electron microscopy and transmission electron microscopy (SEM and TEM). It is demonstrated that plates and ␥Љ needles form directly at high temperatures. Cooling from high temperature produces fine dispersions of ␥Ј/␥Љ precipitates, which coarsen on reheating to lower stabilization temperatures (893 and 993 K). The final morphology of the ␥Ј/␥Љ co-precipitates, i.e., compact/noncompact type very much depends on the stabilization temperature and the cooling rates from prior higher-temperature stabilization steps.
The new Ni-Fe-based superalloy DT706, derived from INCONEL 706, is the object of studies for potential uses in turbine-disk applications at temperatures above 973 K. This alloy aims at improving the microstructural stability while preserving the excellent machinability and good mechanical properties of the base material. This article is the first of a two-part study concentrating on the characterization of the microstructure of the DT706 precipitates, depending on the heat-treatment conditions. Analyses were performed by means of ex-situ small-angle neutron scattering (SANS) measurements, together with conventional scanning electron microscopy (SEM) and transmission electron microscopy (TEM) microscopy, on experimentally-heat-treated samples. The results, when compared to a similar analysis previously made on INCONEL 706, showed that the precipitation characteristics of DT706 reflect compositional changes, but are still remarkably dependent on the cooling stages between the different heat-treatment steps.
The development of a new steam turbine generation for use in advanced coal fired power plants with prospective operating temperatures beyond 700 °C and a projected thermodynamic efficiency of about 55 % requires, amongst other innovations, the partial substitution of ferritic steels by wrought Ni‐base superalloys. Although Ni‐base alloys are already widely used in the aerospace industry, they are faced with demands regarding component size and operation temperature, which by far exceed current aero‐engine requirements. In this article, the potential of selected alloys for 700 °C steam turbine applications is discussed with respect to their manufacturability and mechanical performance. Hereby, the focus is on the steam turbine rotor, which probably is the most critical component. It is concluded that material solutions are available for operation conditions around 600 °C but not for temperatures of 700 °C and above. Based on these results, alloy development strategies are suggested in order to close this gap and two new alloys, DT 706 and DT 750, are introduced.
The microstructural evolution at high temperatures was studied in an experimental
Ni–Fe-based superalloy DT706 by in situ small-angle neutron scattering (SANS).
Modifying the alloy heat treatment to improve the microstructural stability and
mechanical properties in Inconel 706 type superalloys has been a significant research
goal for many years. The earlier studies of phase transformation relied mostly on
optical and electron microscopy investigations of samples that were cooled down to
room temperature after the high-temperature exposure. However, the kinetics of
γ′
and γ′′
precipitation is very fast and new precipitation does occur during the cooling phase. So it is
unlikely that the high-temperature microstructure is retained at room temperature.
In this presentation, we followed the phase transformation in DT706 alloy by
studying the microstructural changes occurring at elevated temperatures. The results
of in situ SANS measurements are presented. Microstructure examination on
differently heat-treated samples by electron microscopy gives complementary
information on the phase transformation sequences. The formation and evolution of
precipitates are strongly influenced by the cooling rate from the solution to the
η
stabilization temperature. The microstructure of DT706 alloy can be tuned using the in
situ SANS results.
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