Abstract
To clarify effects of microstructure on fatigue crack initiation mechanism of Ti-4Al-6Mo-2V-5Cr-2Zr alloy, high-cycle fatigue behavior of hierarchical nanostructure (HN) and lamellar microstructure (LM) was investigated. Fatigue tests results show that the high-cycle fatigue property of LM (σ0.1(107), 685 MPa) is better than that of HN (σ0.1(107), 630 MPa). EBSD analysis and TEM observation demonstrate that with cyclic loading of stress, prism < a > slip is activated in αp phase, and basal < a > slip is activated in grain boundary α phase (GB α). Plastic deformation of HN mainly occurs in short rod αp phase and GB α. Fatigue cracks preferentially initiate at weak parts of these structures and propagate into a transformed β structure (βtrans). In addition, the intrinsic reason for the random behavior of the fatigue life can be attributed to differences in the microstructure ahead of the main crack tips. Fatigue data for HN containing GB α and short rod αp shows high scattering. Dislocation slip in LM occurred mainly in β matrix and piled up at αs/β interfaces and grain boundaries (GBs). Local high stresses caused {01¯11} nano-scale twins in αs phase to harmonize local plastic deformation, which makes cracks more preferable to nucleate at β GBs.
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