Philippe Lours

Oxidation and oxide spallation of heat resistant cast steels for superplastic forming dies

The oxidation and oxide spallation behaviour of austenitic and ferritic heat resistant cast steels used for manufacturing superplastic forming dies is investigated in isothermal and cyclic conditions. Gravimetric, microstructural and mechanical approaches are utilised to address the oxidation and spallation kinetics, to determine the nature of oxides grown at the surface of the materials and to discriminate the different routes to spallation depending on the substrate metallurgical structure and the oxidation conditions. It is shown that both types of material develop similar oxide scales with similar kinetics. The ferritic alloys appear to be more resistant to oxide spallation than the austenic alloys. The decrease of the fracture energy at the interface substrate/oxide during oxidation resulting from an aggregation of interfacial defects, causes spallation by wedging and buckling for thin and thick oxide layers, respectively.

Determination of precipitate strength in aluminium alloy 6056-T6 from transmission electron microscopy in situ straining data

Abstract The transmission electron microscopy in-situ straining technique is employed to measure the breaking angles of strengthening precipitates in aluminium alloy 6056-T6 as they are sheared by dislocations. The experimental determination of the character of bowed dislocation segments when dislocations are pinned on precipitates allows us to calculate the corresponding line tensions. From this, the maximum forces F m that precipitates can sustain before being sheared by dislocations are deduced. It is suggested that F m may be regarded as a quantitative parameter which includes the effects of the various strengthening mechanisms operative in precipitation-hardened alloys. An attempt is made to relate the maximum force calculated from in-situ straining data to the macroscopic yield strength of the material.

Straining mechanisms in aluminium alloy 6056. In-situ investigation by transmission electron microscopy

TEM in-situ straining tests, performed in precipitation hardened aluminium alloy 6056-T6, show that precipitates are sheared or by-passed by dislocations. By-passing, assisted by dislocation cross-slip, results in the formation and stress-induced spreading of non-planar loops. The relationship between those elementary mechanisms and the mechanical properties of the alloy is discussed.

Transmission electron microscopy study of precipitate morphology and precipitate overcoming processes in aluminum alloy 6056 T6

High resolution electron microscopy and in situ straining experiments are performed on aluminum alloy 6056 T6 to charaterize the morphology of precipitates and the processes of precipitate overcoming by the dislocations. Two types of precipitates (needles and laths) are identified. They are found to be sheared by the dislocations and the maximum force precipitates can sustain before being sheared is calculated.

Oxidation resistance of ODS alloy PM2000 from 880°C to 1400°C

This letter compares the performances of alloy PM2000 exposed to air at various temperatures from 880°C to 1400°C in terms of oxidation kinetics and alumina scale morphology, crystal structure and composition.

Design of SPF Dies Based on Advanced Material Behaviour Models

Numerical simulation was performed on an axi-symmetric metallic die during a whole fife cycle. It was shown that most important stresses are generated by the thermal gradients during heating up and repeated die removal from the press furnace and also by the clamping pressure during forming. The forming itself causes non dimensioning stresses. Creep relaxation during forming induces plastic deformation and distortion of mould after cooling down. To increase simulation relevance, new behaviour models are investigated for three classes of materials candidate for SPF dies: heat resistant cast steels, fibre reinforced refractory concretes and low cost monolithic ceramics.

Isothermal Oxidation Behaviour of a Hot-Work Tool Steel

Isothermal oxidation behaviour of a hot-work tool steel (X38CrMoV5) was investigated at 600degreesC and 700degreesC in dry and wet air. Growth kinetics were determined by using TGA and oxide scales were characterised by means of SEM (EDS, X-ray mapping) and XRD examinations. Moreover, as the microstructural properties of the studied hot-work steel strongly depend on the carbides precipitates formed during its heat treatment, these carbides were extracted from the X38CrMoV5 matrix and their oxidation behaviour in dry and wet air was also studied. Oxidation behaviour of X38CrMoV5 is very sensitive to the presence of water vapour : a large increase of the scale growth kinetics was observed as soon as the water vapour partial pressure exceeds a value of 9 mbar. Microstructural characterisations showed that scales grown in wet air are porous and sometimes cracked and deformed. They are composed of an external iron-rich oxides scale (hematite alpha-Fe2O3), an internal oxide scale enriched in Cr (spinel oxides (Fe,Cr)(3)O-4) and a narrow zone of internal oxidation. Whereas no significant influence of the water vapour partial pressure has been observed on the oxide scale microstructure (composition, morphology), texture of the superficial hematite scales becomes more pronounced when p(H2O) increases from 9 to 310 mbar. Preferential orientation of alpha-Fe2O3 scales is also favoured by increasing thickness of oxides. On the other hand, the oxidation behaviour of carbide precipitates is rather complex and strongly affected by the presence of water vapour in air.

In Situ Thermal Gradient Controlled Investigation of Spallation -Experimental Design and Preliminary Results

Based on a coupled numerical and experimental approach, the design and implementation of an in situ thermal-gradient-controlled cyclic oxidation test, dedicated to the investigation of the TGO and TBC spallation, is presented. The influence of the specimen through thickness thermal gradient as well as the benefits of the video real-time monitoring of the cooling phase of an oxidation test is discussed in the case of the spallation of an alumina scale grown on FeCrAl alloys.

Novel erbia-yttria co-doped zirconia fluorescent thermal history sensor

Thermochromic pigments are commonly used for off-line temperature mapping on components from systems operating at a temperature higher than 1073 K. However, their temperature resolution is often limited by the discrete number of color transitions they offer. This paper investigates the potential of erbia-yttria co-doped zirconia as a florescent thermal history sensor alternative to thermochromic pigments. Samples of yttria-stabilized zirconia powder (YSZ, 8.3 mol% YO1.5) doped with 1.5 mol% ErO1.5 and synthesized by a sol-gel route are calcined for 15 minutes under isothermal conditions between 1173 and 1423 K. The effects of temperature on their crystal structure and room temperature fluorescence properties are then studied. Results show a steady increase of the crystallinity of the powders with temperature, causing a significant and permanent increase of the emission intensity and fluorescence lifetime which could be used to determine temperature with a calculated theoretical resolution lower than 1 K for intensity. The intensity ratio obtained using a temperature insensitive YSZ:Eu3+ reference phosphor is proposed as a more robust parameter regarding experimental conditions for determining thermal history. Finally, the possibilities for integrating this fluorescent marker into sol-gel deposited coatings for future practical thermal history sensing applications is also discussed.

Processes and equipment for superplastic forming of metals

The industrial use of materials with superplastic behaviour has required the development of dedicated processes, including all the related tooling equipment. This chapter first describes the various superplastic sheet-forming processes. Then the industrial equipment is detailed along with the technological options that may be used for heating platens. Special focus is given to tools and dies, either metallic, ceramic or refractory concrete-based: their material grades and properties are discussed, as are the methods used for modelling them. Finally, emerging processes are identified.