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Abstract:The influence of ultrasonic nanocrystal surface modification (UNSM) on the fatigue fracture characteristics of Ti6Al4V was investigated. Two groups of specimens were separated due to different heat treatment conditions. Group one was stress-relief annealed at 650 • C, and group two was then treated with solid solution-aging. UNSM with the conditions of a static load of 25 N, vibration amplitude of 30 µm, and 36,000 strikes per unit produced about 40 µm surface severe plastic deformation (SPD) layers on both groups of specimens. UNSM improved the microhardness and the compressive residual stress. UNSM also helped achieve a neat surface, almost without changing the surface roughness. The fatigue strengths of these two groups were improved by 7% and 11.7%, respectively. After UNSM, fatigue cracks mainly initiated from the surface of the specimen before the fatigue life of 10 6 cycles, while they appeared at the internal compress deformed α-phase at the zone between the SPD layer and the core after the fatigue life of 106 cycles. The cracks usually extended along the deformation overflow bands and the process traces on the surface. Through the change of micro-dimples in the fatigue final rupture region, nanocrystals were achieved in the SPD layer. The crystal slip and the surface remodeling together influenced the energy field of crack evolution.

In this paper, the influence of Cr 2 O 3 additive (y = 0~0.4 wt%) on fracture behaviors, ferroelastic deformation, and mechanical strengths of Bi 4 Ti 2.95 W 0.05 O 12.05 (BTW) Aurivillius ceramics was investigated. For these W/Cr co-doped Bi 4 Ti 3 O 12 ceramics (BTW-yC), SEM analysis on their fractured surfaces demonstrate that the transgranular fracture is the main fracture mechanism, however, the intergranular fracture also exists in the sample at y = 0.3. Impedance analyses based electrical resonance show that their Poisson's ratio and Young's modulus vary with y in a similar trend, while the frequency constant and elastic compliance exhibit a contrary varying trend with y. On the other hand, under the Vickers indentation, the crack propagation in BTW-yC ceramics takes the form of the longstraight extension, small-angle deflection, and short-distance branching at y = 0, 0.2, and 0.4, respectively. In the uniaxial compression tests, the stress-strain behavior of ceramics consist of three stages, i.e., linear elastic deformation, ferroelastic domain switching, and microcrack propagation. The ferroelastic domain switching tends to form the resistance for the microcrack propagation. Furthermore, uniaxial bending tests prove that the fracture strength is dependent on the grain size and fracture toughness, Overall, the sample at y = 0.1 gains a better mechanical strength among BTW-yC ceramics.

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