Influence of brazing thermal cycling on microstructure and mechanical properties of GH4169 alloy

To delve into the impacts of multiple brazing thermal cycles at varying peak temperatures on the microstructure and properties of GH4169 alloy， this study conducted a comprehensive examination of how brazing thermal cycle processes influence the precipitates， grain size， tensile properties， and stress-rupture properties of the alloy.The findings reveal that as the thermal cycle temperature rises， the quantity of δ-phase precipitation diminishes， and its morphology undergoes a transformation from needle-like to rod-like and eventually to granular. Within the temperature range of 970-1010 ℃， the grain size experiences minimal alteration. However， when the temperature surpasses 1020 ℃， significant grain growth occurs. Both tensile strength and hardness initially ascend and then descend with an increase in the thermal cycle temperature， reaching their peak values at 1010 ℃. This phenomenon is mainly attributed to the dissolution of an appropriate amount of δ phase and the complete precipitation of γ″ and γ′ strengthening phases at this temperature， while the grain size does not show significant coarsening.The room-temperature impact toughness demonstrates distinct trends across different thermal cycle ranges. In the 970-990 ℃ range， it decreases with rising temperature due to the partial transformation of the δ-phase morphology from needle-like to rod-like. In the 990-1010 ℃ range， it increases with temperature as the δ phase dissolves and the strengthening-phase-free zone vanishes. Nevertheless， a further increase in the thermal cycle temperature leads to a reduction in toughness because of grain growth.The stress-rupture life initially declines and then rises with an increase in the thermal cycle temperature， hitting its lowest point in the 990-1000 ℃ range. This is caused by the partial transformation of the δ-phase morphology from needle-like to rod-like， which promotes microvoid nucleation and reduces the alloy's creep resistance. When the temperature further rises above 1020 ℃， the extensive precipitation of γ″ strengthening phases， along with significant grain growth， substantially enhances the alloy's creep performance. However， the substantial decrease in the needle-like δ-phase content results in increased notch sensitivity.Taking into account both mechanical properties and notch sensitivity， it is recommended to employ brazing thermal cycles around 1010 ℃ to achieve a well-balanced combination of strength and stress-rupture performance. For service environments with higher notch sensitivity requirements， a thermal cycle temperature in the range of 970-980 ℃ can be selected to minimize the risks of creep failure.