Effect of γ' phase and microtwins on the microstructural evolution and mechanical properties of a novel Ni–Co base superalloy

Abstract

A new Ni–Co base precipitation-hardened superalloy designed using the low stacking fault energy (SFE) was used primarily in the hot components of aeroengines designed with low SFE exposed to high temperatures. Different microstructures of the new alloys were obtained by controlling the solution treatment temperature. The corresponding mechanical properties were tested at 25 °C and 750 °C. The primary γ' precipitates dissolved with increasing solution temperature, causing grain growth and an increasing microtwin fraction. Fine-grained, transient, and coarse-grained zones were formed in sequence because of the interaction between the γ′ phase and microtwins. The tensile strength decreased as the grain grew when tested at room temperature, and the strengthening mechanism was dislocation strengthening. However, when the tensile experiment temperature was increased to 750 °C, the tensile properties increased from the fine-grained to the transient zone and then decreased to the coarse-grained zone. Moreover, the fracture mechanism changed from ductile fracture to a mixed ductile and brittle fracture. The size of the secondary γ′ precipitates increased by 32 nm because the dissolved primary γ′ precipitated when the solution temperature was increased from 1090 °C to 1120 °C, which improved the tensile properties. The electron channeling contrast imaging and transmission electron microscopy results confirmed that microtwins were formed during the tensile process at 750 °C. The microtwins hindered the dislocation movement. Therefore, the transient zone of the low-SFE alloy demonstrated the best mechanical properties, which guided the optimization direction of the alloy design and service.