Phase precipitation behavior and its influence on mechanical properties of a solid-solution strengthened Ni-based alloy

Abstract

Phase precipitation behavior and its influence on mechanical properties of a solid-solution strengthened Ni-based alloy, used to manufacture large casting components in next generation of advanced ultra-supercritical coal fired power plant, were studied during exposure at 700 ℃ for up to 30,000 h. Detailed analysis reveals that only MC carbide exists in solution annealed alloy. During thermal exposure, γ′′, δ and M23C6 carbide precipitate, but γ′′ and MC carbide are metastable phases. γ′′ precipitation precedes δ phase, but it disappears gradually with the precipitation of δ phase. MC carbide gradually degenerates via reaction MC + γ → M23C6 + δ. δ phase first precipitates at grain boundary due to the segregation of Nb at grain boundary and M23C6/γ interface, then tends to form in the vicinity of MC carbide and then appears in the interdendritic region. Its growth expends γ′′ phase or MC carbide. Finally, the precipitation-free zone forms in the dendrite core region. The variation trend of yield strength can be divided into three stages during thermal exposure. It first increases drastically, then increases gradually, and then keeps almost constant. γ′′ phase has a major contribution in first stage, but δ phase gradually becomes the primarily strengthened phase. The elongation decreases steeply at early stage and then has no significant change. After thermal exposure, the impact toughness at room temperature also decreases drastically. The major influence factors of elongation and impact toughness are the increase of grain interior strength and the film-like M23C6 carbide.