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Millions worldwide suffer from arthritis of the hips, and total hip replacement is a clinically successful treatment for end‐stage arthritis patients. Typical hip implants incorporate a cobalt alloy (Co–Cr–Mo) femoral head fixed on a titanium alloy (Ti‐6Al‐4V) femoral stem via a Morse taper junction. However, fretting and corrosion at this junction can cause release of wear particles and metal ions from the metallic implant, leading to local and systemic toxicity in patients. This study is a multiscale structural‐chemical investigation, ranging from the micrometer down to the atomic scale, of the underlying mechanisms leading to metal ion release from such taper junctions. Correlative transmission electron microscopy and atom probe tomography reveals microstructural and compositional alterations in the subsurface of the titanium alloy subjected to in vitro gross‐slip fretting against the cobalt alloy. Even though the cobalt alloy is comparatively more wear‐resistant, changes in the titanium alloy promote tribocorrosion and subsequent degradation of the cobalt alloy. These observations regarding the concurrent occurrence of electrochemical and tribological phenomena are vital to further improve the design and performance of taper junctions in similar environments., Corrosion and wear at taper junctions in metallic implants can release toxic metal ions into the body. Although biomedical cobalt alloys have a higher hardness than titanium alloys, this study finds that changes on the titanium alloy during fretting can promote tribocorrosion in the cobalt alloy. Such insights can be used to improve material design in taper junctions and implants.

Titanium alloy Ti-5Al-5Mo-5V-3Cr (Ti-5553) is a near beta alloy used in structural aircraft components because of its excellent mechanical properties. Simulating the mechanical response of this material using constitutive models is an important step in understanding the relationship between its microstructure and properties. Uniaxial compression tests were conducted at temperatures both below and above the beta transus and at different loading rates. A viscoplastic self-consistent (VPSC) model was used to match the stress-strain response of the Ti-5553 alloy based on uniaxial compression tests across a range of temperatures and strain rates. Sets of parameter values were determined for two different hardening models, namely the modified Voce model, which is empirical, and the Mechanical Threshold Stress (MTS) model, which is based on dislocation theory. No consistent trends in the Voce parameter values were found as a function of temperature or strain rate. By contrast, the physically-based MTS model, which is explicitly designed to cover wide ranges of deformation conditions, was able to fit a wide range of temperatures and strain rates. It was further validated by comparison with experimental measurements with other b titanium alloys that had similar compositions. (C) 2017 Elsevier Ltd. All rights reserved.

The lifetime of moving metallic components is often limited by accelerated oxidation. Yet, the mechanisms and pathways for oxidation during tribological loading are not well understood. Using copper as a model system, tribologically-induced oxidation is systematically investigated by varying the sliding speed and test duration under mild tribological loading. We demonstrate that tribo-oxidation is controlled by test duration rather than the number of cycles or the sliding speed. Plastic deformation from tribological loading creates dislocations, grain and phase boundaries that act as high diffusivity pathways. A combination of electron microscopy and atom probe tomography revealed significantly enhanced atomic concentration of the diffusing species around dislocations. Oxygen diffusion into the bulk as well as of copper towards the free surface along these defects control the oxide formation kinetics. Our work paves the way for formulating a physics-based understanding for tribo-oxidation, which is crucial to develop strategies to slow or decrease oxidation and to strategically tailor surfaces to increase the lifetime of engineering systems.