Abstract: Thermodynamic calculation, SEM, TEM, XRD, phase extraction and chemical analysis were employed to study the structure stability of a new Ni-Cr-Co base superalloy aged at 704℃ and 760℃ for long time. The results show that the precipitates of alloy standard heat-treated and heat-treated at 704℃ and 760℃ for time up to 2000h are γ′, MC, M23C6 and M6C, and η phases are also formed at grain boundaries and in the matrix of sample heat-treated at 760℃ as well. With increase in aging time, the mass fractions of γ′(+η), MC, M23C6 and M6C in all samples have no large changes and their chemical compositions are stable at the same temperature, but the γ′ precipitates grow obviously. The γ′-to-η transformation in sample heat-treated at 760℃ takes place with prolonged aging time. Theη precipitates form a Widmansttten pattern in samples heat-treated at 760℃. The new alloy maintains a better structure stability during prolonged aging at 704℃, but a worse structure stability during prolonged aging at 760℃.
Structure stability of Ni-Cr-Co base superalloy aged at 704 ℃ and 760 ℃
Abstract:
Thermodynamic calculation, SEM, TEM, XRD, phase extraction and chemical analysis were employed to study the structure stability of a new Ni-Cr-Co base superalloy aged at 704 ℃ and 760 ℃ for long time. The results show that the precipitates of alloy standard heat-treated and heat-treated at 704 ℃ and 760 ℃ for time up to 2 000 h are γ′, MC, M 23C 6 and M 6C, and η phases are also formed at grain boundaries and in the matrix of sample heat-treated at 760 ℃ as well. With increase in aging time, the mass fractions of γ′(+η), MC, M 23C 6 and M 6C in all samples have no large changes and their chemical compositions are stable at the same temperature, but the γ′ precipitates grow obviously. The γ′-to-η transformation in sample heat-treated at 760 ℃ takes place with prolonged aging time. The η precipitates form a Widmansttten pattern in samples heat-treated at 760 ℃. The new alloy maintains a better structure stability during prolonged aging at 704 ℃, but a worse structure stability during prolonged aging at 760 ℃.
图2 合金的原始态组织 Fig.2 Microstructures of test alloy (a)—Optical graph as solution-annealed; (b)—SEM structure as standard heat-treated; (c)—TEM morphology of γ′ as standard heat-treated
图3 合金在704 ℃时效后SEM组织 Fig.3 SEM microstructures of alloy heat-treated at 704 ℃ for 500 h(a), 1 000 h(b) and 2 000 h(c)