Investigations on the Diffusion of Oxygen in Nickel at 1000°C by SIMS Analysis

High-purity polycrystalline nickel foils have been oxidized at 1000°C in laboratory air before being analyzed in secondary ion mass spectrometry to locally measure the oxygen content in solid solution. The values obtained in metallic grains are surprisingly the same before and after the oxidation treatments (between 5 and 10 atom ppm) and they are much lower than the ones predicted from the literature solubility and diffusion coefficient data at 1000°C. It is shown that this discrepancy could have its origins in the purity level of the samples but also in the exclusive oxygen diffusion in nickel grain boundaries. This last assumption is supported by the occurrence of nickel oxide particles on the walls of voids located in grain boundaries

Experimental data about mechanical behaviour during compression tests for various matted fibres

A specific experimental device has been set up to test compressive mechanical behaviour of an assembly of fibres. Simple compression, as well as cyclic loading experiments and relaxation tests were performed. The experimental set up also allows to record the evolution of the mat fibre electrical resistance while testing. Experimental results are presented for a variety of fibrous materials. Despite the very different nature of each of these individual fibres, it appears that the mats exhibit a very similar mechanical behaviour. This common behaviour has been observed during monotonic single compression tests, as well as during cyclic or relaxation experiments. These experimental results are discussed in terms of different parameters such as the intrinsic mechanical properties of individual fibres and moreover the tangle intrinsic parameters (effect of fibre length, effect of geometrical position of fibres in the sample, fibre surface modifications. . .). The influence of the contact points between fibres is discussed in regard of the electric resistivity measurement

Effect of a heterogeneous distribution of particles on the formation of banded grain structure in wrought Alloy 718

Alloy 718 is known to be sensitive to interdendritic segregation formed during ingot solidification. The occurrence of banded grain structures under heat treating conditions close to 1000 ° C related to interdendritic segregation is often reported. In order to have a better understanding of this microstructural evolution, an extensive experimental program has been carried out. Consequently, a model taking into account the selective dissolution of δ-phase (Ni3Nb) is proposed. A grain growth simulation by Monte-Carlo method is then used to illustrate the grain structure evolution in a banded particle distribution. By comparing experimental data and computer simulation, the relationship between the Monte-Carlo step and the real time is assessed and the range of parameters when heterogeneous microstructures appear is specified

First-principles study of diffusion and interactions of vacancies and hydrogen in hcp-titanium

We present a study of the stability of n-vacancies (Vn) and hydrogens in the hexagonal close-packed titanium system computed by means of first-principles calculations. In this work, performed by using the generalized gradient approximation of density functional theory, we focused on the formation energies and the processes of migration of these defects. In the first part, the calculated formation energy of the monovacancy presents a disagreement with experimental data, as already mentioned in the literature. The activation energy is underestimated by almost 20%. The stability of compact divacancies was then studied. We show that a divacancy is more stable than a monovacancy if their migration energies are of the same order of magnitude. We also predict that the migration process in the basal plane of the divacancy is controlled by an intermediate state corresponding to a body-centered triangle(BO site). The case of the trivacancies is finally considered from an energetic point of view. In the second part, the insertion of hydrogen and the processes of its migration are discussed. We obtain a satisfactory agreement with experimental measurements. The chemical nature of the interactions between hydrogen and titanium are discussed, and we show that the H-atom presents an anionic behavior in the metal. The trapping energy of hydrogen in a monovacancy as a function of the number of hydrogen atoms is finally presented

Rafting microstructure during creep of the MC2 nickel-based superalloy at very high temperature

Directional coarsening of the single-crystalline nickel-based superalloy MC2 has been investigated by means of tensile creep tests at 1100 ◦C. Two specific specimen geometries were designed in order to generate a variety of stress and strain states. Different coarsening microstructures are observed: N- and P-type classical rafting but also coarsening oriented 45◦ away from the load axis. The comparison of microstructure maps with the local mechanical state evaluated by finite element calculations shows that the 45◦ directional coarsening appears in case of very high cumulated strain values (above 10%), independent of the stress sign. Transmission electron microscopy investigations showthat the dislocation microstructure is similar in both N-type and 45◦ coarsened areas

Static and dynamic aspects of coupling between creep behavior and oxidation on MC2 single crystal superalloy at 1150 °C

Creep tests were performed on thin wall specimens made of MC2 single crystal superalloy at 1150 °C and under controlled atmosphere. The results highlight the deleterious oxidation effect on creep properties. The assumption that oxidation leads to a non-load-bearing affected zone is insufficient to explain the difference in creep rate that was noticed between tests performed under synthetic air and under hydrogenated argon, and cannot explain the decrease of the strain rate during the tests that were carried out with a change of atmosphere from synthetic air to hydrogenated argon. On the other hand, these experimental results are consistent with vacancy injection due to partial cationic oxidation, which accelerates the creep rate by promoting creep mechanisms controlled by diffusion. The anionic protective alumina scale formed under hydrogenated argon prevents this vacancy flux

Advanced burner-rig test for oxidation-corrosion resistance evaluation of MCrAlY/superalloys systems

Protective coatings are used on gas turbine components to enable them to survive in engine-operating conditions. This study presents a recently developed cyclic burner-rig test that is used to simulate helicopter engine conditions and to assess the oxidation and hot corrosion behaviour of MCrAlY coatings on nickel-base superalloys. A diluted sea-salt solution is atomised into the burner-rig to simulate hot-corrosion. Each cycle lasts 1 h with temperatures varying in the range of 900 °C to 1000 °C followed by 15 min cooling to room temperature. Specimens are tested up to 1000 such cycles. Three different NiCoCrAlYTa coating thicknesses are used to determine the influence of the Al reservoir on the lifetime of the coated MC2 superalloy. The evolving microstructural features are identified using high resolution scanning electron microscopy and energy dispersive spectroscopy and compared with isothermal testing in pure oxidising conditions. The NiCoCrAlYTa microstructure obtained after the burner-rig test has typical features of a Type 1 hot corrosion degradation, with internal oxidation and nitruration and a front of chromium and yttrium-rich sulphides. This type of advanced burner-rig test cycle is successful in reproducing the accelerated combined hot-corrosion/oxidation damage

The effect of thermal cycling on the high-temperature creep behaviour of a single crystal nickel-based superalloy

Isothermal and thermal cycling creep behaviours of a single crystal nickel-based superalloy have been studied by means of tensile tests at 1150 °C and 80 MPa. We have demonstrated that thermal cycling creep rates are faster than isothermal creep rates and that lifetimes at high temperatures are shorter for creep tests under thermal cycling conditions. Furthermore, it is shown that thermal cycling creep lifetime increases as the thermal cycle frequency decreases