Laurent Peltier

In situ elaboration of a binary Ti-26Nb alloy by selective laser melting of elemental titanium and niobium mixed powders.

Ti-Nb alloys are excellent candidates for biomedical applications such as implantology and joint replacement because of their very low elastic modulus, their excellent biocompatibility and their high strength. A low elastic modulus, close to that of the cortical bone minimizes the stress shielding effect that appears subsequent to the insertion of an implant. The objective of this study is to investigate the microstructural and mechanical properties of a Ti-Nb alloy elaborated by selective laser melting on powder bed of a mixture of Ti and Nb elemental powders (26 at.%). The influence of operating parameters on porosity of manufactured samples and on efficacy of dissolving Nb particles in Ti was studied. The results obtained by optical microscopy, SEM analysis and X-ray microtomography show that the laser energy has a significant effect on the compactness and homogeneity of the manufactured parts. Homogeneous and compact samples were obtained for high energy levels. Microstructure of these samples has been further characterized. Their mechanical properties were assessed by ultrasonic measures and the Youngs modulus found is close to that of classically elaborated Ti-26 Nbingot.

Synthesis and characterization of Ti-27.5Nb alloy made by CLAD® additive manufacturing process for biomedical applications

Biocompatible beta-titanium alloys such as Ti-27.5(at.%)Nb are good candidates for implantology and arthroplasty applications as their particular mechanical properties, including low Youngs modulus, could significantly reduce the stress-shielding phenomenon usually occurring after surgery. The CLAD® process is a powder blown additive manufacturing process that allows the manufacture of patient specific (i.e. custom) implants. Thus, the use of Ti-27.5(at.%)Nb alloy formed by CLAD® process for biomedical applications as a mean to increase cytocompatibility and mechanical biocompatibility was investigated in this study. The microstructural properties of the CLAD-deposited alloy were studied with optical microscopy and electron back-scattered diffraction (EBSD) analysis. The conservation of the mechanical properties of the Ti-27.5Nb material after the transformation steps (ingot-powder atomisation-CLAD) were verified with tensile tests and appear to remain close to those of reference material. Cytocompatibility of the material and subsequent cell viability tests showed that no cytotoxic elements are released in the medium and that viable cells proliferated well.

Fatigue performance evaluation of a Nickel-free titanium-based alloy for biomedical application - Effect of thermomechanical treatments

In the present work, structural fatigue experiments were performed on a Ti-26Nb alloy subjected to different thermomechanical treatments: a severe cold rolling, a solution treatment and two aging treatments at low-temperature conducted after cold rolling in order to optimize the kinetics of precipitation. The aim is to investigate the effect of microstructural refinement obtained by these processes on fatigue performances. Preliminary tensile tests were performed on each state and analyzed in terms of the microstructure documented by using X-Ray diffraction and TEM analysis. These tests clearly promote the short-time-aged cold-rolled state with a fine α and ω phases precipitation. An interesting balance between mechanical properties such as high strength and low Youngs modulus has been obtained. Cyclic bending tests were carried out in air at 0.5%, 1%, 2% and 3% imposed strain amplitudes. At low straining amplitude, where the fatigue performances are at their best, the cold-rolled state does not break at 3×106 cycles and the long-time aged precipitation hardened state seems to be a good competitor compared to the cold-rolled state. All failure characteristics are documented by Scanning Electron Microscopy (SEM) micrographs and analyzed in term of microstructure.

Variations of the Elastic Properties of the CoCrFeMnNi High Entropy Alloy Deformed by Groove Cold Rolling

The variations of the mechanical properties of the CoCrFeMnNi high entropy alloy (HEA) during groove cold rolling process were investigated with the aim of understanding their correlation relationships with the crystallographic texture. Our study revealed divergences in the variations of the microhardness and yield strength measured from samples deformed by groove cold rolling and conventional cold rolling processes. The crystallographic texture analyzed by electron back scattered diffraction (EBSD) revealed a hybrid texture between those obtained by conventional rolling and drawing processes. Though the groove cold rolling process induced a marked strengthening effect in the CoCrFeMnNi HEA, the mechanical properties were also characterized by an unusual decrease of the Young’s modulus as the applied groove cold rolled deformation increased up to about 0.5 before reaching a stabilized value. This decrease of the Young’s modulus was attributed to the increased density of mobile dislocations induced by work hardening during groove cold rolling processing.

Ultrasonic and X-Ray tomography inspection of a woven glass reinforced composite damaged by fatigue loading

An investigation of a polyamide 6.6/6 composite reinforced with woven glass fibers is described with a focus on its response to fatigue solicitations. The main purpose is to elaborate a quantitative and a qualitative study of the induced damage as well as its detectability. To do so, several nondestructive testing methods were used. The different damage mechanisms, relative to fatigue loading, were identified using X-Ray tomography as a reference for the ultrasonic investigations. As a result, each damage mechanism is visualized and a measure of the induced void is established for comparison. An ultrasonic investigation based on C-scans and guided waves is performed under different signal analysis approaches in an attempt to extract as much information as possible. The discoveries and experiences are important in the framework of a collaboration with the automotive industry en for biomechanical applications.

Ultrasonic investigation of fatigue, tensile damage, and impact damage in woven glass-fiber fabric reinforced composites with comparison to X-ray tomography

This work was done on woven glass-fiber fabric reinforced composite samples. Those materials exhibit a complex anisotropic evolution of defects induced by several damage mechanisms. In order to non-destructively evaluate the damage accumulation within this material, a methodology based on the measurement of the complete stiffness tensor is considered. After validation of the detectability of increasing damage state with this method, a new damage indicator is proposed to thoroughly quantify it. Samples were damaged by tensile tests (quasi-static and fatigue) at increasing stress levels along and out-of fibers axis. Afterwards, drop-weight impact is performed to consider several damage situations. Finally, an X-ray tomography is conducted to identify the damage mechanisms as well as the evolution of the void volume fraction. It is shown that this evolution has the same tendency with the ultrasonic damage indicator.

Characterization of direct laser deposited low modulus Ti-26Nb alloy on Ti-6Al-4V for biomedical applications

After a joint replacement, a stress-shielding phenomenon often appears resulting from Young’s modulus mismatch between the implant and the cortical bone. In accordance with Wolff’s law, it will result in bone resorption and could lead to loosening of the implant.Ti-Nb beta metastable alloys are excellent candidates for biomedical applications because of their very low elastic modulus close to that of cortical bone and their high strength. This material, associated with the direct laser deposition or CLAD® process allows the fabrication of biomimetic implants. Nevertheless, Ti-Nb powders are still rather uncommon, thus the alloy could be utilized as a replacement material for Ti-6Al-4V on the implant surface only, this configuration creating an elasticity gradient.Microstructure and phase analysis of a direct laser deposited Ti-26(at%)Nb wall revealed a fully beta microstructure. Micro-hardness tests refuted the anisotropy of the deposited alloy and tensile tests showed that the elastic properties of the CLAD® material are not far from those of the cast material. EDS analysis of a CLAD® build of Ti-26Nb on a Ti-6Al-4V substrate highlighted the diffusion of the elements of the substrate towards the deposit. Grain growth was studied with EBSD. Moreover, nanoindentation tests highlight an evolution of the elastic modulus from the substrate to the deposit.After a joint replacement, a stress-shielding phenomenon often appears resulting from Young’s modulus mismatch between the implant and the cortical bone. In accordance with Wolff’s law, it will result in bone resorption and could lead to loosening of the implant.Ti-Nb beta metastable alloys are excellent candidates for biomedical applications because of their very low elastic modulus close to that of cortical bone and their high strength. This material, associated with the direct laser deposition or CLAD® process allows the fabrication of biomimetic implants. Nevertheless, Ti-Nb powders are still rather uncommon, thus the alloy could be utilized as a replacement material for Ti-6Al-4V on the implant surface only, this configuration creating an elasticity gradient.Microstructure and phase analysis of a direct laser deposited Ti-26(at%)Nb wall revealed a fully beta microstructure. Micro-hardness tests refuted the anisotropy of the deposited alloy and tensile tests showed that the elastic properties of the C...