Characteristic Flow Behaviour of γ + γʹ Duplex and Its Significant Applications in Hot Working Process of Superalloys

The characteristic flow behaviour of the γ + γ′ duplex structure during the thermomechanical processing (TMP) of highly alloyed disc alloys was comprehensively investigated. Based on the TMP of the γ + γ′ duplex, a series of promising techniques for the billet conversion and microstructure customization can be developed. The superplasticity of γ + γ′ duplex was confirmed by tensile tests, and the optimum combination of temperature and strain rate was determined. For a certain alloy, the maximum elongation up to more than 1000% can be achieved at the temperature of equilibrium γ′ solvus minus 100 °C. Near net shape forgings can be produced within this temperature region. During conversion of the VAR ingots, a proper TMP can trigger a discontinuous precipitation instead of the dynamic strain ageing effect which usually leads to a drastic degradation of plasticity. Therefore, the processing window of ingot conversion can be greatly broadened. Producing the dual microstructure disc forging using TMP from a γ + γ′ duplex billet exhibits the inherent advantages over standard heat treatment techniques. The grain size of coarse-grained region can be continuously modulated by controlling the local plastic strain, and the location of the interface between the coarse-grained region and fine-grained region can be precisely determined by controlling the strain distribution. The multi-cycle TMP techniques can play a constructive role in the conversion of the as-HIP billets of P/M material. By producing the γ + γ′ microduplex, prior particle boundaries (PPBs) and inclusions can be efficiently broken up during TMP. As a result, the hot working ductility and the quality of the ultrasonic inspections can be greatly improved.