COMPARATIVE-STUDY OF CREEP RESISTANCE AND THERMAL-STABILITY OF TITANIUM-ALLOYS 685 AND 6242 IN AIR AND IN VACUUM

After a long period at high temperature (< 550 °C) during a creep test, some titanium commercial alloys present some embrittlement in the form of the absence of thermal stability. The addition of silicon is assumed by many authors to be the cause of this instability at high temperatures. In order to verify this assumption, the alloy Ti 685 with 0.215 wt.% silicon and the alloy Ti 6242 containing no silicon were compared during creep tests between 460 and 540 °C in air and under vacuum. This comparative study confirmed the beneficial role of silicon on the high temperature resistance of the Ti 685 alloy. Moreover, in the creep tests in air both these alloys presented a superficial oxidation and probably a preferential penetration of oxygen along the former β grain boundaries. This oxygen penetration is responsible for intergranular fracture of Ti alloys after a long period at high temperature. © 1979.