Microstructure evolution of an Inconel 718 alloy prepared by electron beam smelting

A new electron beam smelting technology was employed to prepare the Inconel 718 alloy, and the microstructure evolution and nucleation and coarsening mechanisms of γ', γ″ and δ phase of this alloy was investigated. The microstructures can be refined by means of electron beam smelting, and good microstructure stability is observed at 700 °C, which is at least 20 °C beyond the limiting temperature that Inconel 718 alloy can sustain for prolonged periods. The γ' and γ″ precipitates could be formed in less than 1 h when aged at 700 °C and 800 °C, in which the coarsening of γ' follows the conventional Lifshitz-Wagner theory, while the γ″ follows the Lifshitz-Slyozov encounter Modified theory. When thermal aged at 900 °C, both γ' and γ″ precipitates are absent. The nucleation of δ phase is arised from the stacking faults in γ″ precipitates induced by formation of a/6 〈211〉 Shockley partial dislocations or the lattice distortion when thermal aged at lower temperature. While at higher temperature, the arrangement of a/2 〈110〉 matrix dislocations is responsible for the nucleation of δ phase. The coarsening of δ phase is discontinuous during prolonged exposure. The growth rate of the δ lath is controlled by the coarsening in lengthwise direction at first, and then by the intermittent growth in both length and width direction at lower temperature. While for the δ phase nucleated from the matrix, the growth of δ lath is determined by its thickening firstly, and then the coherent growth occurs along the length direction with aging proceeding