Mechanism of α-Cr Formation in Nickel-Base Superalloy INCONE1718 after Long Time Thermal Exposure

摘要

It is commonly accepted that the drop in mechanical properties of INCONE1718(IN718) after long time exposure at high temperature is a result of coalescence of γ″, γ′ and the transformation of γ″ to δ-Ni_3Nb phase. Besides the γ″, γ′ and δ, the precipitation of B.C.C. α-Cr is another factor that affects the mechanical properties of IN718. In this paper, the Thermal-Calc software is used to reveal the thermodynamic condition of α-Cr formation. The calculated sections of the pseudo ternary diagrams of Cr-Nb-IN718 and Cr-Fe-IN718 explain why α-Cr particles do not precipitate dispersedly in γ matrix but in the vicinity of δ phase. The selected area diffraction (SAD) of TEM confirms the crystallography relationship of α-Cr with δ is (010) _δ// (-110) _(α-Cr), [100] _δ//[111] _(α-Cr°) In combination of TTT diagram with the Thermal-Calc and TEM results, the mechanism of α-Cr formation in IN718 is suggested.