Dominique Poquillon

Cold compaction of iron powders—relations between powder morphology and mechanical properties: Part I: Powder preparation and compaction

The effect of morphology of iron powders on their compaction behaviour has been studied in the case of compacts with a final relative density below 0.8. Two powders, one with spherical grains and the other with spongy grains, have been synthetized in order to prepare green compacts under pressures ranging from 100 to 350 MPa. The compaction behaviour of both powders has been experimentally described by the relation between the applied compaction pressure and the relative density of the material. Different stages have been identified at increasing compaction pressure: (i) fully elastic behaviour (Stage I), (ii) particle sliding (Stage II), (iii) particle irreversible deformation (Stage III). Spongy powder-based compacts achieved greater density at the same compaction pressure than spherical powder-based compacts. Models are proposed to describe the compaction behaviour of both powders.

Application of a simple statistical spalling model for the analysis of high-temperature, cyclic-oxidation kinetics data

A statistical cyclic-oxidation model is presented. This model gives analytical formulas to assess stabilized metal consumption in cyclic-oxidation experiments. The model is first detailed, then applied to se eral Ni-base superalloys, which form an α-alumina scale during oxidation above 1000C. A new map is introduced in order to compare the cyclic-oxidation beha ior of these alloys.

Cold compaction of iron powders: relations between powder morphology and mechanical properties. Part II. Bending tests: results and analysis

The purpose of this study is to determine the green strength of compacts made from iron powders and to understand how it correlates with pressure compaction and powder morphology. Two different types of powders have been tested by means of three-point bending (TPB) tests performed until fracture. In a classic approach, the obtained maximum loads are used to calculate a transverse green strength. In the compaction pressure range considered (from 100 to 290 MPa), relative densities between 48% and 71% are obtained and increased linearly together with compaction pressure. The compacts made of spongy-like particles give better green strengths than the compacts made of spherical particles. However, the global load/displacement response of the TPB tests cannot be explained by a symmetric behaviour in tension and in compression of the tested materials.

Continuous thermogravimetry under cyclic conditions

Thermogravimetry during cyclic oxidation of metallic alloys is described. A methodology is given in order to determine the Net Mass Gain, the Gross Mass Gain, the total mass of spalled oxide, the rate of metal consumption and the average oxide scale thickness as a function of the number of cycles. The fraction of oxide scale which spalls at each cycle can be also calculated, and the parabolic constant can be estimated at each cycle. Two examples are given: the cyclic oxidation of a NiAl single crystal in flowing oxygen at 1150°C, and the cyclic oxidation of alloy P91 at 800°C in laboratory air. Advantages and disadvantages of this technique are discussed in regards to classical interrupted tests in crucibles. Thermogravimetry during cyclic oxidation appears to be a powerful tool in order to model and quantify the cyclic oxidation test which is of great interest in order to qualify the resistance of materials to oxidation in conditions close to their actual use, but a specific aspparatus need to be developed in order to obtain data in an efficient and economical manner. A new apparatus designed for this purpose is described briefly.

Low Temperature Oxidation of Pure Iron: Growth Kinetics and Scale Morphologies

Isothermal oxidation of pure iron has been performed in air at atmospheric pressure between 260°C and 500°C. Growth kinetics are accurately analysed and scale morphologies are investigated by SEM and TEM observations. The calculation of the variations of the parabolic rate constant kp with scale thickness allows a better understanding of scale growth mechanisms involved at this intermediate temperature range, which have been poorly investigated up to now. These results are discussed with the objective of long term behaviour for long term interim storage of some nuclear waste containers.

Numerical Model for Oxide Scale Growth with Explicit Treatment of Vacancy Fluxes

In the framework of research on behaviour of nuclear waste containers, to evaluate the effects of possible evolution of experimental conditions, as well as evolution of parameters controlling oxidation rate during long-term interim storage, a numerical model has been developed in order to take into account non-stationary states. To anticipate effects like cold working of the metal on the scale growth kinetics and risks of scale detachment by over saturation of vacancies at the metal/oxide interface in the course of scale growth, the model is based on the calculation of chemical species, but also vacancies profiles evolution in the oxide and the metal following a simple time integration. An original numerical treatment is proposed to easily describe elimination of vacancies by introducing sink strength in the metal. The first calculations are presented and discussed.

Cyclic Oxidation Kinetics Modeling of NiAl Single Crystal

For alumina forming nickel-based alloys, sulfur content is a key element as it modifies the onset and extent of spalling of the oxide scale. To get a better understanding of that effect, isothermal and cyclic oxidation of two NiAl single crystals (with two S contents) have been carried out at 1050 and 1150°C on (100) and (110) orientated surfaces. Continuous thermogravimetry in cyclic conditions allows to follow isothermal oxidation kinetics and spalling at each cycle. Oxidation kinetics are compared between the two compositions and the two crystallographic orientations. Low sulfur NiAl oxidizes with slightly lower kinetics for both (100) and (110) surface orientations, and, in both isothermal as well as in cyclic conditions at 1150°C in O2 . Experimental results of cyclic oxidation are compared to those obtained with simulations using a previously published simple statistical model and using a more complex Monte-Carlo code. Spalling is found to be an increasing function of the average oxide thickness and seems to depend on the square of the average oxide thickness.

On the Understanding of TGO Growth and Spallation in Nickel Aluminides

This paper describes various parameters influencing oxidation kinetics, oxide’s morphology and spallation phenomena encountered while studying nickel aluminides in high temperature oxidation. Questions are raised about the possibility of making precise lifetime or failure predictions of coatings and TBCs systems. Changes in growth mechanism due to the presence of elements other than Ni and Al in nickel-aluminide base coatings, the effect of atmosphere, particularly water vapor, and the effect of surface preparation and of crystallographic orientation are the main parameters discussed in this work.

Multiscale Analysis of Viscoplastic Behavior of Recrystallized Zircaloy-4 at 400 ° C

Zirconium alloys are used in the nuclear industry as cladding tubes to prevent the fissile material from leaking into the coolant as the first safety wall of nuclear fuel. More and more requirements on fuel performance lead to stronger mechanical solicitations and integrity of cladding tubes has to be guaranteed. In this framework, the polycrystalline models, which are based on plasticity mechanisms, have interesting advantages compared to phenomenological ones. Some previous studies have shown that a polycrystalline approach could be very useful to describe the mechanical behavior of zirconium alloys. This modelling strategy has been successfully applied to fresh material and also, more recently, to irradiated material. The micromechanical approach has been developed in the light of transmission electron microscopy (TEM) observations. These experiments have been achieved to identify the main deformation mechanisms, which occur in several grains of relaxed and crept Zircaloy-4 samples. The main purpose of this paper is to describe an improved micromechanical model able to reproduce both the anisotropic creep behavior and the elasto-plastic behavior of unirradiated recrystallized Zircaloy-4 at 400 ° C . Finally, the quantitative analyses, which have been carried out with TEM correspond well with the results provided by the micromechanical approach.

Freeze-casting for PLGA/carbonated apatite composite scaffolds: Structure and properties

This paper focuses on the fabrication of three-dimensional porous PLGA-biomimetic carbonated apatite composite scaffolds by freeze-casting and using dimethyl carbonate as a solvent. Several charge/polymer ratios were tested in order to finely understand the influence of the filler rate on the scaffold porosity and mechanical and degradation properties using complementary characterization techniques (SEM, mercury porosimetry and X-ray microtomography). It was demonstrated that the apatite ratio within the composite scaffold has a strong influence in terms of architecture, material cohesion, mechanical properties and in vitro degradation properties. An optimum biomimetic apatite ratio was reached to combine good mechanical properties (higher rigidity) and material cohesion. In vitro degradation studies showed that higher apatite filler rates limited PLGA degradation and enhanced the hydrophilicity of the scaffolds which is expected to improve the biological properties of the scaffolds in addition to the bioactivity related to the presence of the apatite analogous to bone mineral.