New-generation biocompatible Ti-based metallic glass ribbons for flexible implants

We introduce five new biocompatible Ti-based metallic glass (MG) compositions with different metalloid and soft metal content for a synergistic improvement in corrosion properties. Without any potentially harmful elements such as Cu, Ni or Be, these novel alloys can eliminate the risk of inflammatory reaction when utilized for permanent medical implants. Excluding Cu, Ni or Be, which are essential for Ti-based bulk MG production, on the other hand, confines the glass-forming ability of novel alloys to a moderate level. In this study, toxic-element free MG alloys with significant metalloid (Si–Ge–B, 15–18 at.%) and minor soft element (Sn, 2–5 at.%) additions are produced in ribbon form using conventional single-roller melt spinning technique. Their glass-forming abilities and their structural and thermal properties are comparatively investigated using X-ray diffraction (XRD), synchrotron XRD and differential scanning calorimetry. Their corrosion resistance is ascertained in a biological solution to analyze their biocorrosion properties and compare them with other Ti-based bulk MGs along with energy dispersive X-ray. Ti60Zr20Si8Ge7B3Sn2 and Ti50Zr30Si8Ge7B3Sn2 MG ribbons present a higher pitting potential and passivation domain compared with other Ti-based MG alloys tested in similar conditions. Human mesenchymal stem cell metabolic activity and cytocompatibility tests confirm their outstanding cytocompatibility, outperforming Ti-Al6-V4

A Re-examination of the Portevin-Le Chatelier Effect in Alloy 718 in Connection with Oxidation-Assisted Intergranular Cracking

In Alloy 718, a sharp transition exists in the fracture path changing from an intergranular brittle mode to a transgranular ductile mode which is associated with a transition of flow behavior from smooth in the dynamic strain aging regime to a serrated one in the Portevin-Le Chatelier (PLC) regime. In order to better understand both deformation and rupture behavior, PLC phenomenon in a precipitation-hardened nickel-base superalloy was carefully investigated in a wide range of temperatures [573 K to 973 K (300°C to 700°C)] and strain rates (109^-5 to 3.2910^-2 s^-1 ). Distinction was made between two PLC domains characterized by different evolutions of the critical strain to the onset of the first serration namely normal and inverse behavior. The apparent activation energies associated with both domains were determined using different methods. Results showed that normal and inverse behavior domains are related to dynamic interaction of dislocations with, respectively, interstitial and substitutional solutes atoms. This analysis confirms that normal PLC regime may be associated to the diffusion of carbon atoms, whereas the substitutional species involves in the inverse regime is discussed with an emphasis on the role of Nb and Mo

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin--Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P718 (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, Ea(P718){\thinspace}={\thinspace}2.68{\thinspace}{\textpm}{\thinspace}0.05 eV, $\tau$0{\thinspace}={\thinspace}2{\textperiodcentered}10-15 {\textpm} 1 s as well as its broadening factor $\beta${\thinspace}={\thinspace}1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superallo

Relationship between the Resistivity Profiles Obtained from the Power Law Model and the Physico-Chemical Properties of Passive Films

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Unveiling the transpassive film failure of 3D printing transition alloys

The failure of transpassive films on transition metal alloys was investigated. The results show that the failure of the transpassive film on 3D printing nickel-based superalloy in flowing NaNO3 solution is dominated by self-induced cracking of the film on niobium-segregated phases and the erosion-induced degradation on the film assisted by cation vacancy condensation at the metal/barrier layer interface. Secondary passivation induces the formation of a transpassive film, the nature of which has been debated for decades. The findings are consistent with a bi-layer structure comprising a point-defective barrier layer that grows into the substrate alloy and a precipitated outer layer

Tribocorrosion of Polyethylene/Cobalt Contact Combined with Real-Time Fluorescence Assays on Living Macrophages: Development of A Multidisciplinary Biotribocorrosion Device

International audiencehe test conditions currently used in biotribocorrosion devices often differ greatly from the physiological conditions of joint replacements, contributing to discrepancies between the simulated and actual life span of joint replacements. In this study, a multidisciplinary biotribocorrosion device was developed based on the limitations of existing tribocorrosimeters. The set-up enables corrosion measurements to be simultaneously performed with real-time visualization of living cells using fluorescence microscopy under dynamic loads and movements. The device was configured to simulate the joint contact of ankle prostheses, and the wear of ultra-high-molecular-weight polyethylene/cobalt alloy (CoCrMo) implants surrounded by murine macrophages was tested. Various characterization techniques (non-contact optical profilometry, scanning and fluorescence electron microscopy and quantitative analyses of metal ions and pro-inflammatory cytokines) were combined in-depth multidisciplinary study. Two experimental conditions were used to promote the production of either polyethylene wear particles or metal ions. The first results indicated two distinct tribocorrosion mechanisms: 1) adhesive wear coupled with slow ionic depassivation of the cobalt alloy. The main degradation products were micrometric spherical polyethylene particles that seem to have little impact effect on the metabolic activity of the macrophages. 2) Ionic wear with the production of small, fibrillar polyethylene particles was observed. The production of metal ions, mainly chromium, was the predominant degradation process. The cytotoxicity of the chromium ions was evaluated based on the secretion of pro-inflammatory cytokines (prostaglandin E2). Our findings indicate that simulated conditions that result in low mechanical wear but high ions release appear to be more harmful to cells

Influence of lubricant formulation on rolling contact fatigue of gears – interaction of lubricant additives with fatigue cracks

cited By 0International audienceThe influence of lubricant additives on rolling contact fatigue crack propagation and the mechanisms responsible for the resulting micro-scale damage, was studied via experiments conducted on complete transmissions in a test cell. Bench-scale tribological tests and the exposure of steel surfaces to two different formulated lubricants were also carried out. Scanning and Transmission Electronic Microscopy observations, Electron Dispersive Energy and X-ray Photoelectron Spectroscopy analyses indicated that the sulphur present in the extreme pressure (EP) additives has a positive impact on limiting damage propagation. Thanks to TEM observations of cracks, it was demonstrated that a sulphur rich film is formed at the crack tip. This film can act as both a barrier film towards hydrogen permeation within the metal and / or as an inhibitor of oil decomposition. The latter is associated with the nascent surface's ability to limit hydrogen generation. Without such hydrogen embrittlement, crack propagation is slowed down. © 201