Philippe Vermaut

Interface mobility in Ni49.8Ti42.2Hf8 shape memory alloy

Abstract The ternary Ni49.8Ti42.2Hf8 shape memory alloy shows mechanical peculiarities compared with NiTi alloy. These are linked to (i) the distortion of the habit plane of the ternary alloy and (ii) the observed (001) mechanical twinning, which helps to accommodate plastic deformation in Ni49.8Ti42.2Hf8 and not in NiTi alloy. The distortion of the habit plane limits its mobility, evidence of which being brought here by internal friction. The (001) mechanical twinning causes the appearance of a very thin microstructure in martensite laths deformed by tension. A model is proposed as a possible mechanism giving birth to this supplementary mechanical twinning: repeated slips by a/2 on (001) planes would create translative (001) micro-twins coherent with the observed microstructure. These micro-twins may be produced by the dissociation of dislocations with the Burgers vector a.

Up-Conversion in Yb,Er-Doped Y2O3 Nanoparticles for Cell Imaging

Y2O3:19%Yb,1%Er nanoparticles were synthesized with the aim of imaging the luminescent cell. Coprecipitation and combustion synthesis were used to obtain particulate sizes ranging from 25 up to 140 nm. The powders showed predominant red upconversion and the emission efficiency is controlled by the particle size. A colloidal route was also followed and 2-5 nm-large agglomerated nanoparticles were obtained. In that case, the luminescence of Er3+ was only observed by direct excitation and no upconversion light has been detected on these very small particles.

Unexpected Constrained Twin Hierarchy in Equiatomic Ru-Based High Temperature Shape Memory Alloy Martensite

Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. Equiatomic compounds undergo two successive martensitic transformations, β (B2) → β’ (tetragonal) → β’’ (monoclinic), whereas out of stoechiometry alloys exhibit a single transition from cubic to tetragonal. In the case of two successive martensitic transformations, we expect to have a finer microstructure of the second martensite because it is supposed to develop inside the smallest twin elements of the former one. In equiatomic Ru-based alloys, if the first martensitic transformation is “normal”, the second one gives different unexpected microstructures with, for instance, twins with a thickness which is larger than the smallest spacing between twin variants of the first martensite. In fact, the reason for this unexpected hierarchy of the twins size is that the second martensitic transformation takes place in special conditions: geometrically, elastically and crystallographically constrained.

High Performance Beta Titanium Alloys as a New Material Perspective for Cardiovascular Applications

During the last few decades, titanium alloys are more and more popular and developed as biomedical devices because of their excellent biocompatibility, very good combination of mechanical properties and prominent corrosion resistance [1-3]. Recently, a new generation of beta titanium alloys dedicated to biomedical applications has been developed. Based on biocompatible alloying elements such as Ta, Nb, Zr and Mo, these alloys were designed as low modulus alloys [4] or nickel-free superelastic materials [5, 6] mainly for orthopedic or dental applications as osseointegrated implants. Beta type titanium alloys take great advantages from their capacity to display several deformation mechanisms as a function of beta phase stability. Therefore, from low to high beta stability, stress assisted martensitic phase transformation (β-α’’), mechanical twinning or simple dislocation slip can alternatively be observed [7]. As a consequence, a very large range of mechanical properties can be reached, including low apparent modulus, large reversible elastic deformation or high yield stress. Although titanium alloys display now a long history of successful applications in orthopedic and dental devices, none of them have been commercially exploited in the area of coronary stents, despite their superior long term haemocopatibility compared to the 316L stainless steel. However, according to previous researches on the biocompatibility of various metals, the corrosion behavior of stainless steel is dominated by its nickel and chromium components, which may induce redox reaction, hydrolysis and complex metal ion–organic molecule binding reactions, whereas none are observed with titanium [8, 9].

Structural and Crystallization Study of a Simplified Aluminoborosilicate Nuclear Glass Containing Rare-earths: Effect of ZrO 2 Concentration

Zirconium is an abundant element in nuclear wastes. In this paper, we present structural and crystallization results for a simplified glass composition belonging to the SiO 2-Al 2O3-B2O3Na 2O-CaO-ZrO 2-RE 2O3 system (RE = Nd or La) developed to immobilize highly concentrated waste solutions. The effect of varying ZrO 2 content on the structure and the crystallization tendency of this glass was studied using a multi-spectroscopic approach. Zr was shown to be located in six-fold coordinated sites with preferential charge compensation by Na + cations. Whereas a significant decrease of the proportion of BO 4 units was observed with ZrO 2 content, no effect was detected on the environment of AlO 4 units. However, a significant structural evolution of the silicate network occurred due to the formation of Si-O-Zr bonds. Whatever ZrO 2 concentration, the crystallization of only a rare earth silicate apatite phase was observed during either slow cooling from the melt or isothermal heat treatment. Whereas nucleation mainly occurred from the surface of the glass without ZrO 2, the introduction of zirconium induced apatite crystallization in the bulk. It is proposed that this nucleating effect of ZrO 2 is mainly due to changes induced in the neighborhood of Nd 3+ cations in glass structure.

Deformation Microstructure and Mechanisms in a Metastable β Titanium Alloy Exhibiting TWIP and TRIP Effects

Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.

Beta titanium alloys with very high ductility induced by complex deformation mechanisms: A new material perspective for coronary stent applications

The increased use of metallic biomaterials in contact with blood e.g. for application as coronary stents is steadily resulting in the development of new biomaterials. Conventional bare-metal stents made by stainless steel were reported on adverse reactions against human body and are gradually replaced by coated stainless steel. The new generation of stent requires fundamental improvements at the materials point of view. Although titanium and classical Ti-alloys display superior biocompatibility compared to other metallic materials (stainless steels, Co-Cr), the major drawback of their relatively low ductility (typically 15%-25% of elongation) seriously limits their applications as cardiovascular stents, where large ductility is basically required during the stent deployment procedure and long-term service. In this paper, new titanium alloys with high ductility, a binary Ti-12Mo (wt%) and a ternary Ti-9Mo-6W (wt%) were designed by using a chemical formulation strategy based on the electronic design method called “the d-electron alloy design method”. Both alloys were synthesized and thermo-mechanically treated into beta-metastable state. In tensile tests, both alloys exhibited outstanding ductility of 43% and 46% in total elongation at room temperature, which is almost two times greater than the normal value shown with classical titanium alloys. Optical microscopy and detailed TEM observations on the deformed specimens revealed a complex deformation mechanism, involving {332}<113> mechanical twinning, stress induced plate shaped omega phase and stress induced martensitic (SIM) transformation β-α’’.

Preparation by Twin Roll Casting and Characterization of TiNi Shape Memory Alloys Strips

The thermoelastic martensitic transformation which gives remarkable functional properties to Shape Memory Alloys (SMA) is very sensitive to the chemistry of the alloys and to its microstructure. In many cases, especially for alloys with high transformation temperatures, applications are limited by the poor ductility. To overcome the brittleness of SMA, one approach can be effective : the development of non conventional production technologies which enable to obtain materials in forms close to requirements: strips of SMA have been prepared by Twin Roll Casting (TRC). The formation of a solid sheet with plane surface is only possible for a limited combination of the different machine parameters and they will be discussed. For comprehension of the process parameters effect, the attention will be focused on TiNi. Microstructures and functional properties will be examined in relation with the microstructures induced by the technique or after specific thermal treatment.

Optimization of Superelastic Properties in Titanium-Niobium Alloys Using Short-Time Thermal Treatments

The short-time thermal treatment strategy has been proved to be very efficient in improving the mechanical properties of various titanium based alloys. The mechanical properties of alloys such as Ti-Nb, Ti-Nb-Zr and Ti-Nb-Zr-Sn based alloys, are extremely sensitive to the β phase stability, microstructure and phase constitution. The concept of the short-time treatment is designed to control precisely the material structure (phase precipitation, etc…) without extensive modification of the distribution of alloying elements. This results in reliable optimizations regarding the balance between elastic modulus, pseudo- (super-) elasticity and strength. Currently, the structural evolution mechanisms involved in the STAT are under systematic investigations in the aim of achieving accurate control of the microstructures and optimized balance of mechanical properties.

The Shape Memory Effect in Melt Spun Fe-15Mn-5Si-9Cr-5Ni Alloys

At room temperature, Fe-15Mn-5Si-9Cr-5Ni alloys are usually austenitic and the application of a stress induces a reversible martensitic transformation leading to a shape memory effect (SME). However, when a ribbon of this material is obtained by melt-spinning, the rapid solidification stabilizes a high-temperature ferritic phase. The goals of this work were to find the appropriate heat treatment in order to recover the equilibrium austenitic phase, characterize the ribbon form of this material and evaluate its shape memory behaviour. We found that annealing at 1050°C for 60 min, under a protective argon atmosphere, followed by a water quenching stabilizes the austenite to room temperature. The yield stress, measured by tensile tests, is 250 MPa. Shape-memory tests show that a strain recovery of 55% can be obtained, which is enough for certain applications.