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.
Highlights
It is well known that stainless steels and titanium alloys are the main materials for manufacturing medical instruments and biomedical implants
In the present work, nanostructured surface layers were prepared by means of ultrasonic impact (UI) on TC4 and TLM specimens to study the effect of nanograins induced by plastic deformation on the fatigue behaviors of titanium alloys used in artificial bone
By means of TEM detection, it has been certain depth of near-surface severe plastic deformation (SPD) layer is observed verified Athat nanocrystals can be obtained in this SPD layer [13,14,15]
Summary
It is well known that stainless steels and titanium alloys are the main materials for manufacturing medical instruments and biomedical implants. The third-generation titanium alloys with nontoxic, low elastic modulus, and good plasticity have been introduced. Shot peening is the standard finishing process, because shot peening improves the compressive residual stress and the hardness of surface [8]; the randomness and the inhomogeneity cause the rising of surface distortion and roughness They are detrimental to the durability of materials. The fatigue fracture behaviors of titanium alloys with fine grains in their surfaces are worth exploring. In the present work, nanostructured surface layers were prepared by means of ultrasonic impact (UI) on TC4 and TLM specimens to study the effect of nanograins induced by plastic deformation on the fatigue behaviors of titanium alloys used in artificial bone. Base on the theoretical constitutive relation, the mechanisms of the crack initiation of specimens subjected to UI treatment were analyzed
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