Heat treatment of UDIMET 720Li: the effect of microstructure on properties

The evolution during heat treatment of the as-forged microstructure of the high strength superalloy UDIMET 720Li has been studied. Particular emphasis has been placed on the characterisation of γ′ precipitation kinetics using optical and transmission electron microscopies (TEM) and subsequent image analysis. The observations are interpreted using thermodynamic, phase transformation and precipitate hardening theories. The results have implications for the gas turbine manufacturers. Through a better understanding of the evolution of the microstructure during ageing, a heat treatment of 24 h at 700°C is proposed, which is believed to be optimal. This allows full advantage to be taken of the properties of the alloy, whilst reducing the costs and time associated with the heat treatment schedules. Moreover, the data presented allows the variation in properties across a U720Li forging to be estimated.