The overview of the formation mechanisms of topologically close-packed phases in Ni-based single crystal superalloys

The nucleation and growth of topologically close-packed (TCP) phases in Ni-based single crystal superalloys (SCs) are controlled by composition, temperature, time, interface structure and stress. The formation of TCP phases causes detrimental effects on the mechanical properties of SCs. Therefore, it is crucial to study the formation mechanism and control factors of TCP phases to develop new-generation Ni-based SCs. Different alloying elements influence the TCP phase nucleation by adjusting the chemical driving force. The interface structure determined by interface energy and strain energy mainly affects the growth mechanism and morphology of the TCP phase. Various temperature determines the driving force for TCP nucleation and diffusion rates of elements during TCP phase growth. Holding time is also crucial to the growth and evolution of TCP phases. Applied stress also has an impact on the precipitation of TCP phases. Evolution between different types of TCP phases depends on the temperature, composition and crystal structure of TCP phases. TCP phases damage the mechanical properties mainly by causing stress concentration and weak interface strength. The review aims to establish a link among TCP phase formation, alloy design and microstructure from the macroscopic view to the microscopic view, providing some perspectives for future research.