Y. Le Petitcorps

In situ preparation of titanium base composites reinforced by TiB single crystals using a powder metallurgy technique

The feasibility of Ti/TiB composite by in situ precipitation of the reinforcement have been investigated. The titanium monoboride can be obtained by the chemical reaction between TiB2 and Ti powders. The fabrication method requires two stages: the first stage corresponds to the compacting of the pre-blended powders and the second one to the nucleation and growth of the TiB needles. By controlling the second stage, it is possible to monitor the density and the aspect ratio of the reinforcement. Bend tests have been performed on these composites reinforced with 20% in volume of discontinuous TiB needles and it has been demonstrated that the load transfer is efficient to get a 50% improvement of the Youngs modulus of the titanium. The simplicity of this technique makes this material very attractive for numerous applications.

The fibre/matrix interfacial shear strength in titanium alloy matrix composites reinforced by silicon carbide or boron CVD filaments

Abstract The mechanical adhesion between CVD filaments (B, SiC) and titanium matrices was studied. Because a single fiber composite was chosen for this purpose, the critical length measurement and the shear strength were calculated using a statistical analysis. The study indicated the role played by the surface treatments of the fibers on the reinforcement/matrix adhesion. The conclusions obtained on model materials are in agreement with the results obtained on real composites.

Modern boron and SiC CVD filaments: A comparative study

Abstract Large diameter filaments (100–150 μm in diameter) made by chemical vapor deposition (CVD) of two ceramic materials (i.e. boron and SiC) on a heated tungsten or carbon core are compared from a mechanical and chemical standpoint. The most interesting of the filaments studied have received a rather thick surface coating (1–3 μm) which is made of boron carbide for B(W) filaments and a sequence of pyrocarbon and silicon carbide layers for SiC filaments. The mechanical behavior of the filaments in tension is explained on the basis of a Weibull statistics approach as well as a fracture analysis. Failure appears to be mainly controlled by surface defects, a feature which emphasizes the protective role played by the coating. Annealing at high temperatures (i.e. 800–950°C) in the presence of titanium shows that coated filaments have superior behavior. The coating acts in fact as a consumable sacrificial material, the strength of the filament remaining unchanged as long as the coating is not totally consumed by chemical reaction with titanium. Modern CVD filaments appear to be the most suitable ceramic reinforcements from a fundamental point of view.

Investigation of the Young's modulus of TiB needles in situ produced in titanium matrix composite

In situ Ti/TiB composites with different volume fractions of discontinuous TiB reinforcements were produced by powder metallurgy. After compacting Ti� /TiB2 powders by hot unidirectional pressure, heat treatments led to the in situ formation of distinctive needles of TiB, randomly distributed in the titanium matrix. The Young’s modulus of TiB was evaluated using the ASW computation method and experimental Vickers micro-indentation. Three point bend tests were performed on Ti/TiB composites as a function of the TiB volume fraction in order to extract the Young’s modulus of TiB from the elastic properties of the composite. The different values obtained according to these three methods were discussed and compared with the literature. # 2002 Elsevier Science B.V. All rights reserved.

Optimization of operator and physical parameters for laser welding of dental materials.

Objective Interactions between lasers and materials are very complex phenomena. The success of laser welding procedures in dental metals depends on the operators control of many parameters. The aims of this study were to evaluate factors relating to the operators dexterity and the choice of the welding parameters (power, pulse duration and therefore energy), which are recognized determinants of weld quality.Design In vitro laboratory study.Materials and methods FeNiCr dental drawn wires were chosen for these experiments because their properties are well known. Different diameters of wires were laser welded, then tested in tension and compared to the control material as extruded, in order to evaluate the quality of the welding. Scanning electron microscopy of the fractured zone and micrograph observations perpendicular and parallel to the wire axis were also conducted in order to analyse the depth penetration and the quality of the microstructure. Additionally, the micro-hardness (Vickers type) was measured both in the welded and the heat-affected zones and then compared to the non-welded alloy.Results Adequate combination of energy and pulse duration with the power set in the range between 0.8 to 1 kW appears to improve penetration depth of the laser beam and success of the welding procedure. Operator skill is also an important variable.Conclusion The variation in laser weld quality in dental FeNiCr wires attributable to operator skill can be minimized by optimization of the physical welding parameters.

Prosthodontics: The laser welding technique applied to the non precious dental alloys procedure and results

Aim The laser welding technique was chosen for its versatility in the repair of dental metal prosthesis. The aim of this research is to assess the accuracy, quality and reproducibility of this technique as applied to Ni-Cr-Mo and Cr-Co-Mo alloys often used to make prosthesisMethod The alloys ability to weld was evaluated with a pulsed Nd-Yag Laser equipment. In order to evaluate the joining, various cast wires with different diameters were used. The efficiency of the joining was measured with tensile tests. In order to understand this difference, metallographic examinations and X-Ray microprobe analysis were performed through the welded area and compared with the cast part.Results It was found that a very slight change in the chemistry of the Ni-Cr alloys had a strong influence on the quality of the joining. The Co-Cr alloy presented an excellent weldability. A very important change in the microstructure due to the effect of the laser was pointed out in the welding zone, increasing its micro-hardness.Conclusion The higher level of carbon and boron in one of the two Ni-Cr was found to be responsible for its poor welding ability. However for the others, the maximum depth of welding was found to be around 2mm which is one of the usual thicknesses of the components which have to be repaired.

A new approach in the understanding of the SiC/Ti reaction zone composition and morphology

Abstract The morphology of the SiC/Ti reaction zone has been analysed by scanning electron microscopy. The composition of the different constituents was determined by Auger and X-ray microprobes. It was found that the two main constituents through the reaction zone were Ti 5 Si 3 (C) and TiC x . The TiC x crystals were measured from the inner part of the reaction zone (against the SiC) to the outer part of the reaction zone (against the titanium). By changing the scale of space by the scale of time, it has been possible to get the age of the TiC x crystals. This new approach has been useful to determine the kinetic rate of the SiC/Ti reaction and also to understand what was the driving force for the SiC/Ti reaction zone growth.

Compatibility between SiC filaments and aluminim in the K2ZrF6 wetting process and its effect on filament strength

Abstract The effect of the various steps of the K2ZrF6 process, used to enhance the wettability of SiC (or C) fibres by liquid aluminium, on the strength of the reinforcement, has been studied on model materials. Groups of 45 uncoated CVD SiC filaments (100 μm in diameter) were submitted to three treatments representative of the process, viz. (1) coating with a K2ZrF6 layer of variable thickness, (2) annealing of the coated filaments at 650°C in sealed tubes and quenching, and (3) embedding the coated filaments in aluminium by hot pressing. After each step, the filaments were extracted chemically and tensile tested. The UTS data were treated statistically on the basis of a two-parameter Weibull function. Steps 1 and 2 do not significantly modify the filament strength. Step 3 yields a more significant strength loss owing to the formation of Al4C3, related to the chemical reaction between SiC and aluminium. The kinetics of growth of Al4C3 is faster than for untreated filaments mainly as a result of the cleaning effect of fluoride species on the SiC/Al interfaces. Thus, the latter filament strength loss is more an indirect consequence than an intrinsic feature of the process. It could be overcome by utilizing filaments coated with a soft material (e.g, pyrocarbon).

Interfacial zone design in titanium-matrix composites reinforced by SiC filaments

Abstract The required stability at high temperature and during thermal cycling of titanium-matrix composites reinforced by SiC fibers obtained by chemical vapor deposition, has motivated the development of a procedure for designing a fiber/matrix interfacial zone. The procedure combines alternately, and sometimes simultaneously, physico-chemical and thermomechanical approaches in order to take into account the occurrence of chemical interaction and new phase formation, the presence after composite processing and use of thermally and chemically induced residual stresses and the capability of interfaces to deviate crack propagation. Although the study is mainly methodological, it shows how the SiC Ti interfacial zone could be shifted from a fairly simple sequence of phases ( SiC/TiSi 2 /Ti 5 Si 3 Ti ) toward a more complex interphase combination ( SiC/C/SiC/Si/TiSi 2 /TiSi Ti 5 Si 4 /Ti 5 Si 3 /Ti(Si) ).

Segregation of magnesium in squeeze-cast aluminium matrix composites reinforced with alumina fibres

Abstract Alumina preforms (Saffil fibres) were infiltrated into two foundry aluminium alloys (Al-7Si-0.3 Mg and Al7Si2.2Mg where the compositions are in approximate weight per cent) using the squeeze-casting technique. In order to establish the segregation and solidification mechanisms, small samples were cut from different areas in the infiltrated preform. The matrix was then dissolved in an HClHNO 3 solution and the amount of free magnesium was measured by atomic absorption spectroscopy. A noticeable loss of magnesium over the whole infiltrated preform has been observed depending on the amount of magnesium in the initial alloy. Whatever the alloy, this effect also occurs during a T6 thermal treatment. The alloying element segregations must be taken into account as they influence the age-hardening response of the matrix.