Effect of environment on thermomechanical fatigue life

Results of studies on the thermomechanical fatigue (TMF) behaviour of metallic engineering materials are presented showing the effect of laboratory air as compared to high vacuum. In order to illustrate the variety of possible effects, three different materials are considered. An austenitic stainless steel is an example for rather small environmental effects on TMF lifetime. At high maximum temperatures applied in the TMF cycle, creep damage prevails. Hence, in-phase (IP) testing is more detrimental than out-of-phase (OP) loading. Vacuum shifts the crack initiation site from the surface to the interior leading to similar lives in IP and OP. The hightemperature titanium alloy IMI 834 suffers from hydrogen embrittlement at moderate temperatures and oxygen uptake at high temperatures. Consequently, the environmental influence on life is pronounced. TiAl-based intermetallic alloys show high environmental susceptibility, which is promoted by the strong ductile-to-brittle transition.