High-temperature fatigue damage mechanisms in near-α titanium alloy IMI 834

The high-temperature fatigue and creep behaviour of near-a titanium alloy IMI 834 was studied for temperatures up to 650 degrees C. In order to identify the relevant damage mechanisms, the test program involved continuous cycling, dwell tests with hold periods in tension and compression, and asymmetrical strain-time cycles. The dominant damage mechanism was found to change at a temperature of about 600 degrees C. At low test temperatures fatigue life is largely dependent on the maximum stress of the fatigue cycle. At high temperatures a brittle oxygen-enriched subsurface layer forms, and thus, environmental degradation governs fatigue life in high-temperature low frequency tests. Lamella boundaries present in the bimodal microstructure were seen to deflect small fatigue cracks, and thus, this microstructure has superior fatigue properties as compared to an equiaxed one. During long-term high-temperature exposure, however, significant degradation of the lamella boundaries was observed, and then both microstructures display similar fatigue properties. (C) 1999 Elsevier Science Ltd. All rights reserved.