Subsonic aircraft are usually made of aluminium. However aluminium, while being light and strong, is not able to withstand temperatures much over 127 °C; above 127 °C the aluminium gradually loses its properties that were brought about by age hardening. For aircraft that fly at Mach 3, materials such as stainless steel (XB-70 Valkyrie, MiG-25) or titanium (SR-71, Sukhoi T-4) have been used, at considerable increase in expense, as the properties of these materials make the aircraft much more difficult to manufacture.