Influence of Fine Grains on the Bending Fatigue Behavior of Two Implant Titanium Alloys

Abstract

By means of the ultrasonic surface impact (amplitude of 30 μm, strike number of 48,000 times/mm2), nanograins have been achieved in the surfaces of both Ti6Al4V(TC4) and Ti3Zr2Sn3Mo25Nb(TLM) titanium alloys, mainly because of the dislocation motion. Many mechanical properties are improved, such as hardness, residual stress, and roughness. The rotating–bending fatigue limits of TC4 and TLM subjected to ultrasonic impact are improved by 13.1% and 23.7%, separately. Because of the bending fatigue behavior, which is sensitive to the surface condition, cracks usually initiate from the surface defects under high stress amplitude. By means of an ultrasonic impact tip with the size of 8 mm, most of the inner cracks present at the zone with a depth range of 100~250 μm in the high life region. The inner crack core to TC4 usually appears as a deformed long and narrow α-phase, while the cracks in TLM specimens prefer to initiate at the triple grain boundary junctions. This zone crosses the grain refined layer and the deformed coarse grain layer. With the gradient change of elastic parameters, the model shows an increase of normal stress at this zone. Combined with the loss of plasticity and toughness, it is easy to understand these fatigue behaviors.

A Novel Ultrasonic Fatigue Test and Application in Bending Fatigue of TC4 Titanium Alloy

“…Although the fatigue data of axial loading can reflect the fatigue performance of materials and structures to a certain extent, the method of predicting bending life based on axial fatigue data may not be applicable due to the heterogeneity of materials and the difference in the stress of structures [16][17][18]. When comparing the typical high-frequency axial loading and rotary bending loading tests, it can be seen that rotary bending loading will make the fatigue data Materials 2023, 16, 5 2 of 14 more discrete, which has been proved to be related to the heterogeneity of the materials (microstructure, holes, and inclusions) and non-uniform loading [19,20]. In addition, the rotary bending load can usually obtain a better fatigue performance at higher stress amplitudes, but with the decrease in stress amplitude, the difference in the fatigue life gradually decreases [21,22].…”

“…Due to the immature testing scheme, the ideal results have not been obtained because there are many obvious disadvantages of the existing test scheme. Firstly, it takes a long time to get the experimental data due to the low loading frequency [19,20]. Secondly, due to the influence of complex stresses in the constrained position, the existing methods for studying very high cyclic bending fatigue cannot effectively solve the problem of cantilever bending fracture location [27].…”