Ni-base superalloys are compositionally complex single crystal materials which exhibit outstanding high temperature properties and are used for the most demanding applications in modern jet engines as turbine blades. Their superior properties arise due to an unique two phase microstructure composed of γ’ cubes (ordered L12 crystal structure), coherently embedded in a solid solution fcc γ matrix. Through an appropriate heat treatment and a precise selection of alloying elements, the channel width and the γ’ volume fraction can be tuned accordingly . To obtain the best creep properties, γ’ volume fraction of roughly 70% and a cube length of 400nm is beneficial . To provide strengthening by precipitation hardening at temperatures relevant to jet engine turbine applications (700-1000°C), the stability of the γ/γ’ phases is of fundamental importance.
In this contribution we introduce a chip based “µ-bulk” setup (Fig. 1), which enables unprecedented heat treatment studies of structurally and compositionally complex materials. By using the chips from an in situ TEM heating device ultra-fast quenching and heating  can be realized owing to the small volume of both the heating spiral and the µ-bulk sample associated with a small heat capacity. Thus complex temperature-time profiles can be realized, even at high temperature regime (900-1300°C). In contrast to conventional in situ heating experiments in TEM, µ-bulk setup avoids the disadvantages associated with the thin foil effects during high temperature exposure. It thus comprises the advantages of microchip technology being able to investigate the real 3D material bulk behavior, making a big step forward to application relevant material conditions, while still using a TEM lab environment.
To demonstrate the feasibility of the µ-bulk set up two experimental approaches are pursued:
(1) Single crystal Ni-base superalloy µ-bulk samples (ERBO1) are exposed to high temperatures, then a slow cooling rate is applied. From complementary ex situ experiments we know that precipitation of secondary γ՚ nanoparticles in the γ channels should be observed (Fig. 2).
(2) ERBO1 µ-bulk samples are exposed to different temperatures in the high-temperature regime followed by ultra-fast quenching. Upon quenching the high temperature state can be frozen in . Following this approach, the evolution of the γ՚ volume fraction with increasing temperature can be investigated and compared with complementary experiments.
|Category||Short file description||File description||File Size|
|Presentation||Figure 1||FIB-prepared single crystal Ni-base superalloy µ-bulk specimen attached to a MEMS-based DENSsolutions Wildfire D3 in-situ TEM chip||296 KB||Download|
|Presentation||Figure 2||Fig 2. Nano precipitates in γ channels: (a) Dark Field TEM image, (b) EDX analysis||423 KB||Download|
|Presentation||Manuscript||This is a short manuscript to the abstract||597 KB||Download|