To control the negative influence of large precipitation of σ phase on the mechanical properties in hot-corrosion resistant superalloy K444, two measures, rejuvenation heat treatment aiming to dissolve the precipitated σ phase and modifying K444 alloys to control the precipitation of σ phase, are taken and the thermal stability of the modified K444 alloy is also studied in detail in the present paper. Based on the completely dissolution of σ phase and γ′ phase at the high temperature, the solution temperature is ascertained as 1170℃, and the rejuvenation heat treatment scheme is dertemined as 1170℃/4 h/AC + 1050℃/4 h/AC + 850℃/16 h/AC. The rejuvenation process could effectively reverse the microstructure, like the bimodal distribution of γ′ and discontinuous grain boundary structure composed of M23C6 and M3B2 particles, and consequently, the mechanical properties of the thermally exposed K444 alloy to the “as-new” condition. The PHACOMP method is taken to devise six K444 alloys with different chemical composition on the basis of K444 alloy. By investigating their short-term mechanical properties and microstructure stability, the modified K444 alloy is established with respect to the chemical composition and the critical electron hole number ( = 2.39). The modified K444 alloy is not sensitive to the precipitation of σ phase, and has better long-term stress rupture properties. The microstructure evolution during long-term thermal exposure and its influence on the mechanical properties are studied in detail in the modified K444 alloy. The coarsening of secondary γ′ at the expense of tertiary γ′ is mainly controlled by the diffusion of Al and Ti in the γ matrix with the activation energy of 261 kJ/mol. The coarsening of secondary γ′ and the disappearing of tertiary γ′ are the main reason for the decreasing of tensile and stress rupture properties of the modified K444. During exposure, the primary MC decomposes gradually and it can be summarized into three stages. Firstly, the MC reacts with the γ matrix and produces M23C6 and γ′, which can be described as MC + γ → M23C6 + γ′. In the second stage, the un-decomposed primary MC arrests the Ni element and forms the M6C particles and η phase, which can be expressed as MC + γ → M6C + η. In the third stage, the remaining primary MC reacts with the diffused Ni element and forms the η phase with M6C and M23C6 particles inside, which can be described as MC + γ → M6C + M23C6 + η. The reaction of the second and third stages should be ascribed to the high (Ti + Nb + Hf)/Al ratio and segregated W, Mo and Cr element in the decomposed region. As thermal exposure processes, the grain boundary structure coarsens gradually from discontinuous to half-continuous, and finally to continuous structure by the thickening of γ′ film and coarsening of M23C6 and M3B2 particles.
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