Microstructural evolution and mechanical properties of Inconel 718 after thermal exposure

Abstract

Inconel 718 was subjected to various heat treatments, i.e., solution heat treatment, standard ageing treatment and standard ageing plus 700°C thermal exposure. The mechanical properties of the alloys were determined using tensile tests and Charpy pendulum impact tests at 650°C and room temperature, respectively. The highest yield strength of 988MPa was attained in the standard aged specimen, whereas a maximum impact toughness of 217Jcm−2 was attained in the solution-treated specimen. After thermal exposure, the mechanical properties of the specimens degrade. Both the yield strength and impact toughness decreased monotonically with increasing thermal exposure time. Subjected to a 10000-h long-term thermal exposure, the yield strength dramatically decreased to 475MPa (almost 50% of the maximum strength), and the impact toughness reduced to only 18Jcm−2. The microstructures of the specimens were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Coarsening of γ′ and γ″ and the transformation of γ″ to δ-Ni3Nb was observed after thermal exposure. However, a complete transformation from metastable γ″ to δ-Ni3Nb was never accomplished, even after the 10000-h long-term thermal exposure. Based on the obtained experimental results, the effects of the microstructural evolution on the mechanical properties are discussed.