Abstract
Cyclic heterogeneous deformation, slip characteristics and crack nucleation with different microstructures, such as bimodal microstructure (BM) and fine lamellar microstructure (FLM) in TC21 alloy (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si), were systematically investigated and analyzed during high cycle fatigue at room temperature. The results demonstrated that the FLM microstructure possesses higher high-cycle fatigue strength than those of the BM one. For BM, the heterogeneous plastic deformation existed within the different large primary α phase, such as equiaxed primary α and primary α lath. The cracks at interfaces and slip bands easily coalesce with each other to form large cracks in BM. However, the α laths with similar morphology and size (nanosize) distributed uniformly in FLM and could relatively deform homogeneously in micro-region, which delayed the initiation of the fatigue crack. Based on the electron-backscattered diffraction (EBSD) analysis, it found that the strain was nonuniformly distributed in BM, however, it is relatively homogeneous in FLM. Moreover, lots of straight cracks are parallel and along single intrusions within the β grain which delays the coalescence of cracks.
Highlights
Due to the high strength, high operating temperature, and corrosion resistance, titanium alloys have been applied widely to aerospace industries [1,2,3]
It has been considered that fatigue life is determined by crack initiation because major of the fatigue life is spent on this region during the high cycle fatigue [8,9]
Huang et al [13,14,15] found that some small-scale heterogeneous microstructure regions (SHMR) in β grain interiors and the grain boundary (GB) α layer promote the crack initiation during fatigue loading in LM than bimodal microstructure (BM) which resulted in a lower fatigue strength for former one
Summary
Due to the high strength, high operating temperature, and corrosion resistance, titanium alloys have been applied widely to aerospace industries [1,2,3]. TC21 titanium alloy is a newα+β structural material with high strength and fracture toughness and has been successfully developed and applied in aircraft structures [4,5] During those applications, the aircraft structure parts are mainly subjected to cyclic loading, especially for the high cycle fatigue loading [6]. With further loading, localized plastic deformation was observed to initiate at individual grains with several preferred orientations which may result in crack formation [21]. The slip irreversibility and accumulative irreversible strain in BM and LM were calculated to explore the cyclic heterogeneous and localized deformation
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