Local Approach : Numerical Simulations of Creep and Creep-Fatigue Crack Initiation and Crack Growth in 316L SPH Austenitic Stainless Steel

This study deals with the evaluation of local approach to assess the mechanical integrity of cracked components submitted to cyclic and steady load at elevated temperature. In this approach, a fracture criterion based on calculated intergranular damage ahead of the crack tip is introduce to simulate both crack initiation and crack propagation in 316 L type austenitic stainless steel. This numerical method, based on finite element computations is firstly described. Then numerical results are compared with sets of experimental data for two specimen geometries during load controlled tests. A damage criterion in good agreement with previous studies leads to promising load-displacement response, crack initiation and crack growth curves

Focused ion beam assisted analysis of the oxidation of a NiAl coating on pure Ni

peer-reviewedDiscs of (pure nickel 15 mm diameter and 4 nun thickness) have been polished to a 1 mu m finish, aluminised using a standard pack CVD process to give a beta-NiAl coating and then the surface of this coating polished to a 1 mu m finish again. Samples have then been isothermally oxidised in a muffle furnace for 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 hours at 950 degrees C, and subjected to surface and cross-sectional microscopy and analysis. Scales were also cross-sectioned and imaged using Focused Ion Beam Milling. The oxidation rate was determined to be 2.6 x 10(-13) g(2) cm(-4) s(-1) and for times less than 512 hours the predominant alumina phase was theta alumina. Al depletion effects were observed due to both oxidation and interdiffusion between the Ni substrate and the NiAl coating. Using these effects, it has been shown that tensile strains of up to 9% can arise and these strains readily explain the formation of intrefacial elliptical pores observed by FIB cross sectioning. FIB sectioning also indicated that different coating grains exhibit different oxide-coating interfaces which arise due to different grain orientations. The faceting of the coating-oxide interface and the development of cuboidal faceted coating structures beneath spalled oxide is thought to be due to substructure development (sub-grain development or recrystallisation) arising from the plastic strains generated by Al depletion effects.Acceptedpeer-reviewe

The Influence of Specimen Thickness on the High Temperature Corrosion Behavior of CMSX-4 during Thermal-Cycling Exposure

CMSX-4 is a single-crystalline Ni-base superalloy designed to be used at very high temperatures and high mechanical loadings. Its excellent corrosion resistance is due to external alumina-scale formation, which however can become less protective under thermal-cycling conditions. The metallic substrate in combination with its superficial oxide scale has to be considered as a composite suffering high stresses. Factors like different coefficients of thermal expansion between oxide and substrate during temperature changes or growing stresses affect the integrity of the oxide scale. This must also be strongly influenced by the thickness of the oxide scale and the substrate as well as the ability to relief such stresses, e.g., by creep deformation. In order to quantify these effects, thin-walled specimens of different thickness (t = 100500 lm) were prepared. Discontinuous measurements of their mass changes were carried out under thermal-cycling conditions at a hot dwell temperature of 1100 C up to 300 thermal cycles. Thin-walled specimens revealed a much lower oxide-spallation rate compared to thick-walled specimens, while thinwalled specimens might show a premature depletion of scale-forming elements. In order to determine which of these competetive factor is more detrimental in terms of a component’s lifetime, the degradation by internal precipitation was studied using scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS). Additionally, a recently developed statistical spallation model was applied to experimental data [D. Poquillon and D. Monceau, Oxidation of Metals, 59, 409–431 (2003)]. The model describes the overall mass change by oxide scale spallation during thermal cycling exposure and is a useful simulation tool for oxide scale spallation processes accounting for variations in the specimen geometry. The evolution of the net-mass change vs. the number of thermal cycles seems to be strongly dependent on the sample thickness

Cyclic oxidation of coated and uncoated single-crystal nickel-based superalloy MC2 analyzed by continuous thermogravimetry analysis

The previously developed cyclic thermogravimetry analysis (CTGA) [Monceau D, Poquillon D. Oxid Met 2004;61:143–163] method is applied to the cyclic oxidation at 1150 C of a NiCoCrAlYTa-coated and an uncoated single-crystal nickel-based superalloy MC2. A new procedure to correct the buoyancy effect is proposed in order to evaluate the amount of oxide formed during the heating periods, which can be important for alloys forming fast-growing transient oxides or during cycling with slow heating rates. It is shown that cyclic thermogravimetry is an useful technique for uantifying the oxidation resistance of an industrial alloy. Moreover, the behaviour of the alloy is better described with CTGA than with a classic cyclic gravimetric test, because both isothermal oxidation kinetics and spalling behaviour are assessed independently. In the present case, it is shown clearly that the NiCoCrAlYTa coating greatly improves the cyclic oxidation resistance without decreasing the isothermal oxidation kinetics. Finally, the use of CTGA for performance and time-of-life evaluations is demonstrated

Iron oxidation at low temperature (260–500 C) in air and the effect of water vapor

The oxidation of iron has been studied at low temperatures (between 260 and 500 C) in dry air or air with 2 vol% H2O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. 18O tracer experiments performed at 400 C allowed to propose a growth mechanism of the scale

Oxidation and oxygen diffusion in Ti-6al-4V alloy: improving measurements during sims analysis by rotating the sample

Titanium alloys are attractive to the industrial world, as they offer the benefits of low density, great corrosion resistance, and relatively good strength, making them viable candidates for a multitude of applications. However, above 500 °C, oxidation and oxygen diffusion in titanium alloys need to be taken into account as they change their microstructure and then their mechanical properties. Oxidations were carried out between 600 and 750 °C on a specific titanium alloy: an a-b annealed Ti-6Al-4V. Oxidation kinetics and oxygen diffusion in the matrix were studied. SIMS analyses were realized on rotating specimens of this two-phase polycrystalline alloy in order to reduce roughness. Composition profiles along the sample thickness were compared to microhardness measurements. SIMS mappings were realized on the smooth slopes of the crater

Damping Properties of Random Fibre Networks Used as Sandwich Core Material

International audienceno abstrac

Damping Properties of Random Fibre Networks Used as Sandwich Core Material

International audienceno abstrac

Fiber Networks Made Of Entangled Cross-linked Fibers : Effect Of The Fiber Length And Of Mixing Different Fibers On Compression Properties

International audienceno abstrac