Microstructural Evolution of Wrought and AM Haynes 188 Under Long Term Thermal Exposure

Haynes 188 is a cobalt-based superalloy used in hot section gas turbine components due to its excellent oxidation resistance and elevated temperature stability. Parts made from Haynes 188 are traditionally manufactured from wrought or forged material. Additive manufacturing (AM) may be a less time consuming and more cost-effective alternative, particularly for replacement parts. Additionally, necessary design changes can be quickly introduced into service using additive manufacturing. It is important to understand the effect of long-term thermal exposure on additively manufactured parts before they are put into demanding high temperature applications. Data on the effect of long-term thermal exposure on wrought Haynes 188 is readily available in the open literature. However, such data for additively manufactured Haynes 188 is scarce. This paper discusses an experimental study that investigated the microstructural evolution in wrought and AM Haynes 188 under long-term thermal exposure. Optical metallography and scanning electron microscopy (SEM) were performed on wrought and AM Haynes 188 in the virgin and long-term thermally exposed conditions. Mechanical test samples from the coupons were then extracted and tested in both conditions. Lastly, the fracture surfaces of the mechanical test specimens were evaluated by SEM fractography and metallography of sections through the fracture surfaces. The findings help to understand how long-term thermal exposure affects the microstructure and mechanical properties of wrought and AM Haynes 188. Most importantly, it illustrates a significant difference in microstructural response of the two materials to long term thermal exposure and its effect on mechanical properties.