Influence of Zr content on the incipient melting behavior and stress-rupture life of CM247 LC nickel base superalloy

Abstract

In the present work, a mechanism was proposed for secondary γ′ growth and incipient melting associated with Zr content in CM247 LC nickel base superalloy. In order to evaluate the onset temperature of incipient melt and its effect on stress-rupture life at 200 MPa/980 °C, several solution treatments were performed at 1220 °C, 1230 °C and 1250 °C on two different 0.1 and 0.4 wt% Zr content alloys. The results showed that an increase in Zr content from 0.1 to 0.4 wt% enhances the volume fraction of γ/γ′ eutectic phase from about 6% to 14% in as-cast microstructure. This phenomenon was attributed to the enhancing of γ′ former elements microsegregations in the interdendritic regions. Several heat treatments results showed that the incipient melting temperature of CM247 LC-0.4% Zr is less than 1220 °C, due to the presence of low-melting point intermetallic especially NiZr, while for CM247 LC-0.1% Zr, this temperature is over 1250 °C. Therefore the stress-rupture life for the 0.1% Zr alloy heat treated at 1230 °C is about three-fold higher than that of 0.4% Zr alloy. Also with increasing solution treatment temperature from 1220 °C to 1230 °C, the stress-rupture life of 0.1% Zr alloy has increased due to the higher volume fraction of γ′ phase. Furthermore for the 0.4% Zr alloy, with increasing solution treatment temperature from 1220 °C to 1230 °C, stress-rupture life is reduced due to the incipient melting affected by low-melting point phases such as NiZr and Ni5Zr.