Microstructural stability and mechanical properties of a new nickel-based superalloy

By means of thermodynamic calculation, OM, SEM, TEM and X-ray diffraction, microstructural stability was investigated on the new nickel-based superalloy, INCONEL alloy 740. Some mechanical properties of the new alloy were also studied. The results indicated that the microstructure of this alloy maintained good thermal stability to at least 750 °C. The major precipitates were MC, M23C6 and γ′ during prolonged aging at temperatures below 704 °C. Small amount of η precipitates formed at grain boundaries in sample aged at 750 °C in 1000 h and large amount of intragraunlar η precipitates formed a Widmanstätten pattern in sample aged at 849 °C for 1000 h. γ′ phases grew rapidly with increasing in temperature and partially resolutioned after 849 °C aging for 1000 h. Harmful phases, such as σ phase, have not been detected in these samples. The impact toughness of alloy declined and there was a ductile/brittle transition with increasing aging temperature and time. Room temperature microhardness of alloy decreased with an increase in temperature mainly due to the growth of γ′ phases. Elevated-temperature microhardness of alloy for standard heat-treated condition was higher than room temperature microhardness. In comparison to NIMONIC alloy 263, the new alloy exhibited higher ductility and higher tensile strength.