Subsurface facets-induced crack nucleation behavior and microstructure-based strength evaluation of titanium alloys in ultra-long life regime

Abstract Ultra-long life fatigue tests of 10 6 -10 9 cycles under asymmetrical loading were conducted to elucidate the subsurface facets-induced crack nucleation behavior of two alpha-beta titanium alloys, and a theoretical method of describing the microstructure-based strength relationship was proposed. As a result, two titanium alloys all present the continuously descending S – N (stress - number of cycles) curve characteristics with subsurface non-defect induced failure. A bright convergence zone with facets becomes the remarkable feature of failure. By means of the milling of focused ion beam system, the coalescence and growth of subsurface micro-cracks crossing several grains along the 45° angle, controlled by the maximum shear stress, can be observable. Based on this, a subsurface facets-induced crack nucleation and growth mechanisms for titanium alloys was proposed in combination with the discussion of small crack fracture mechanics. In view of the good consistency between the evaluated nucleation life of crack at the size of convergence zone with facets and the total life, the microstructure-based modelling method for evaluating fatigue strength of titanium alloys with subsurface failure in ultra-long life regime can be acceptable.