A method of manufacturing a composite rotor disc (2) in which, following diffusion bonding of a ceramic core section (4) of the rotor disc (2) to a metallic outer section (6) of the rotor disc (2) at an elevated temperature, the rotor disc (2) is allowed to cool to a predetermined temperature and is held at this predetermined temperature for a predetermined period of time. While the rotor disc (2) is held at this predetermined temperature it is loaded in a radially outward direction. The rotor disc (2) is then allowed to cool to a temperature below the predetermined temperature.