Evolution of microstructure and properties of IN738 alloy under thermal exposure with different post-processing states

The microstructure and properties of nickel-based superalloys under thermal exposure are the key basis for evaluating the high-temperature stability and proposing process optimization strategies. This study investigated the microstructural evolution and the corresponding tensile properties of IN738 alloy with different post-processing states under thermal exposure at 700 °C. The results show that the microstructures of IN738 alloys with different post-processing states (i.e., hot isostatic pressing (HIP) and standard heat treatment (SHT)) are significantly different. For the HIPed sample, large cuboidal γ′ and a small number of ultra-fine spheroidal γ′ precipitates are present in the γ matrix. Under thermal exposure, the size and volume fraction of ultra-fine spheroidal γ′ increase with prolonged thermal exposure time. In comparison, the SHTed sample is characterized as a bimodal distribution of γ′, with large primary cuboidal γ′ and small secondary spherical γ′. The size and volume fraction of primary γ′ increase first and then remain steady under thermal exposure. In particular, the tensile properties under exposure depend highly on the post-processing states. The increase of tensile strength after the thermal exposure in the HIPed sample is attributed to the rising fine spherical γ′ volume fraction. The rising size and volume fraction of primary γ′ in the SHTed sample improve the tensile strength during thermal exposure.