Fig. 13. Surface of the specimen cyclically strained under OPD-TMF cycling with mechanical strain amplitude 6×10⁻³ to fracture.

FIB cross section of two secondary cracks starting in the thick oxide layer is presented in Fig. 14. The fatigue crack initiated from the thick uniform oxide layer propagated through the oxide to the matrix. Fig. 14a shows the thick oxide layer which is cracked. The crack facilitates the access of the oxidizing atmosphere to the metal surface around the crack tip which facilitates further oxidation and following crack growth. The crack in Fig. 14b also started in the thick oxide layer and by alternating oxidation close to the crack tip and cracking of the oxide propagated deep in the material.