Thierry Chauveau

Mechanical properties of diamond films: A comparative study of polycrystalline and smooth fine-grained diamonds by Brillouin light scattering

Brillouin light scattering, Raman light scattering and x-ray diffraction were used to investigate the elastic and microstructural properties of polycrystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-assisted chemical vapor deposition process at 600 °C. Their morphology and roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the M-line spectroscopy technique. The diamond purity of all these coatings in terms of the sp3 bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be textured with a 〈011〉 crystallographic direction normal to the film plane, leading to essentially hexagonal symmetry of the elastic tensor. By taking advantage of the detection of a number of different acoustic modes, complete elastic characterization of the films was achieved. ...

The Future of Public Pensions in the Seven Major Economies

We specify a computable general equilibrium model with overlapping generations allowing for variable life-expectancy, for studying the future of public pensions in the seven major economies. The main conclusions are: First, ageing of the population is a problem very similar from one developed country to another. Second, this problem is not so much worrying as is sometimes suggested and policies in which Pay-As-You-Go financing is maintained and the retirement age held constant are allowable. Third, there is no “one best social security policy”. Fourth, there is a trade-off between, on the one hand, lower interest rates and taxes, higher investment and output, and so on, and, on the other hand, a higher standard of living of the retired. Fifth, these results hold under the assumption of closed economies as well as under the polar assumption of small open economies.

Torsion texture development of zirconium alloys

Abstract In order to study texture formation during high temperature torsion tests of Zirconium alloys, results from experiments and simulations are presented and compared in this paper. Simulations put in evidence that the Viscoplastic Selfconsistent model (VPSC), in spite of some limitations for the description of plastic deformation at high temperatures, gives a reasonable description of the experimental textures. Predictions show that prismatic and basal 〈a〉 slip are necessary to reproduce the main texture components and that pyramidal 〈c+a〉 slip must have a low activity.

Copper coverage effect on tungsten crystallites texture development in W/Cu nanocomposite thin films

Morphological and crystallographic structures of multilayered W/Cu nanocomposite thin films elaborated by physical vapor deposition were studied by varying copper and tungsten thicknesses. Sample examinations were performed by x-ray diffraction (XRD), grazing incidence small-angle x-ray scattering and transmission electron microscopy (TEM). Samples were found to be composed of copper nanoparticles, homogeneously dispersed in planes parallel to the film-substrate interface and periodically separated by tungsten layers along the growth direction. Our observations revealed an original texture development of the tungsten matrix from a mixture of unexpected α-W⟨111⟩ and α-W⟨110⟩ components to unique α-W⟨110⟩ component as the copper coverage passes a thickness threshold of 0.6 nm. Local TEM texture stereology investigations revealed simultaneous columnar growth of both preferential orientations posterior to polycrystalline development while XRD reveals strong compressive residual stresses in both texture compon...

Systematic risk and idiosyncratic risk: a useful distinction for valuing European options

Abstract Assuming that the financial market is complete and distinguishing between market risk and idiosyncratic risk, we obtain a new pricing formula for a European call where the parameters include the volatilities of the market factor and that of the stock. Such a result is then extended to the case of an incomplete market, using a market factor replicating portfolio (MFR), which can be substituted to the market factor of risk, when the number of stocks is high enough. The price of a European call on a stock may then be expressed in terms of the volatilities of the MFR portfolio and of the underlying stock (and of its beta). Finally, we compare our formula to that of Black and Scholes and to the valuation proposed by Corrado and Su. We focus on the existence of a volatility smile and we give an explanation competing with the one proposed by stochastic volatility models.

Experimental and numerical determination of the intragranular work hardening in a cold rolled multicrystal

Abstract A pure copper multicrystal has been cold rolled up to an overall true strain of e = 0.4. The intragranular work hardening state, due to plastic deformation, has been determined by the analysis of the X-ray peak-profiles measured within each individual grain with a high resolution diffractometer. The evolution of the intragranular dislocation density during rolling has been predicted by a finite element code, which explicitly accounts for the crystallographic nature of the material and models the plastic deformation within individual grains by crystallographic slip on specific slip systems. The experimental and the simulated data were compared in order to discuss the validity of the model developed for X-ray peak-profile analysis and the simulation work hardening law.

On melting of silicon carbide under pressure

The melting of silicon carbide has been studied at pressures 5–8 GPa and temperatures up to 3300 K. It has been found that SiC melts congruently, and its melting curve has negative slope of −44 ± 4 K/GPa.

Structural, static and dynamic magnetic properties of Co2MnGe thin films on a sapphire a-plane substrate

Magnetic properties of Co2MnGe thin films of different thicknesses (13, 34, 55, 83, 100, and 200 nm), grown by rf sputtering at 400 °C on single crystal sapphire substrates, were studied using vibrating sample magnetometry and conventional or microstrip line ferromagnetic resonance. Their behavior is described assuming a magnetic energy density showing twofold and fourfold in-plane anisotropies with some misalignment between their principal directions. For all the samples, the easy axis of the fourfold anisotropy is parallel to the c-axis of the substrate while the direction of the twofold anisotropy easy axis varies from sample to sample and seems to be strongly influenced by the growth conditions. Its direction is most probably monitored by the slight unavoidable miscut angle of the Al2O3 substrate. The twofold in-plane anisotropy field Hu is almost temperature independent, in contrast with the fourfold field H4 which is a decreasing function of the temperature. Finally, we study the frequency dependenc...

The Effect of Strain Path Change on Texture Evolution at Finite Strain of Multiphase Steel: Numerical and Experimental Investigations

Two-stage sequences of simple shear and/or uniaxial tensile tests conducted on TRIP800 steel sheet and supplemented by texture measurements are reported. The purpose is a better understanding of the macroscopic work-hardening behaviour and its microstructural origin. According to the previously published work on single phase ferrite steel; the peculiar macroscopic transient effect in flow stress was mainly associated to the microstructural destabilization (e.g. reinforcement, dissolution or rearrangement of the previously formed dislocation walls). In addition, the macroscopic work-softening observed at the beginning of the second stage of cross-loading was attributed to the micro-band occurrence. Considering the actual multiphase steel, the main difference lies in the absence of the peculiar transient effect in flow stress upon cross-loading (where no macroscopic work-softening is observed) and the associated microstructural mechanisms (no formation of micro-bands). Besides, the initial texture for the actual multiphase steel is in some extent different to the previously investigated single phase steel mainly made up of the γ-fibre. Therefore, a detailed analysis of the measured deformed textures is carried out in order to investigate the contribution of the texture evolution on the macroscopic work-hardening. The computations of the orientation stability map as well as the predicted texture evolutions using the classical full constraint Taylor- Bishop-Hill (TBH) model are performed for a better understanding of the observed texture development. The influence of the texture evolution on the shape of the stress-strain curves, as well as on the remaining symmetries of the material, is also discussed. Explicitly, we show that despite the presence of a well developed texture in the as-received and deformed material, the contribution of the geometrical hardening (i.e. textural evolution) on the macroscopic behaviour remains small compared to the microstructural one.

Demographic transition and social security policies in France: a welfare analysis.

A dynamic computable general equilibrium model with overlapping generations is used to appraise the consequences in France of three social security policies: a 20% cut in the replacement rate (‘TM20’), an increase in retirement age from 60 to 65 (‘RET65’), and the creation of a transitory fund (‘FUND’). These policies are described in three scenarios built up around a baseline scenario, in which the current French public pension system, financed on a pay-as-you-go basis, is maintained. Whatever the scenario, the demographic transition is assumed to be close to that described in official projections, in which the most likely fertility rate between years 1995 and 2050 is supposed to be equal to 1.8 children per woman. The main results are the following: maintaining the current system remains a reasonable option since slump should not occur during the transition period. Nevertheless, this option may seem undesirable because of the demographic risk—the realization of a scenario less favourable than the official one cannot be ruled out—and the lack of equity of the current pensions system. If equity is appraised with respect to a simple actuarial criterion (i.e. the ratio of the present value of pensions received by a generation over the present value of its social contributions), the ‘FUND’ option appears to be the most equitable. The ‘RET65’ option is the least fair since people presently working are worse off. When macroeconomic outcomes (e.g. per capita production or consumption) or social welfare are considered, the scenario in which the legal retirement age is increased seems to dominate all other options, owing to its huge effects. Nevertheless, these results hold under restrictive assumptions, among which the most crucial is full employment being restored quickly. The ‘FUND’ option is the less desirable, owing to ambiguous and small macroeconomic and social welfare effects. Finally, a 20% cut in the replacement rate would have intermediate effects: less additional production or welfare than in the ‘RET65’ scenario, but more than in ‘FUND’; less efficient in struggling against the lack of equity than the latter scenario, but more than the former.