Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.