a b s t r a c tThe effect of laser shock peening on the high temperature oxidation resistance of commercial pure titanium at high temperature (700°C) was studied in long-time (3000 h) exposure under dry air. A reduction of the gain mass by a factor 4 was found for laser-shock peened (LSP) samples compared to untreated titanium, which supports the interest of laser-shock treatment for the improvement of high temperature resistance. Short-durations (10 h and 100 h) oxidation experiments, devoted to investigate the influence of the LSP treatment on the first stages of the oxidation process, were also carried out by TGA. Several techniques as scanning electron microscopy, hardness and roughness measurements, X-ray diffraction and X-ray photoelectron spectrometry, microRaman spectroscopy, nuclear reaction analysis and electron backscattered diffraction were used to characterize the sample after laser treatment and oxidations. The formation of a continuous nitrogen-rich layer between the oxide layer and the α-case area in LSP samples appears to be the key factor to explain the reduction of oxygen diffusion, and thus the improvement of the oxidation resistance of laser shocked titanium. Moreover, the graintexture of LSP samples after oxidation can also explain the improvement of the high temperature oxidation resistance after long times exposures.

International audienceThe excellent combination of light-weight and good mechanical propertiesmakes titanium alloys attractive for compressor section components in gasturbine engines (temperature between 250 and 600 C). However, above 600 C,the formation of an unprotective oxide layer facilitates the oxygen diffusion into thealloy. In this experimental study, pure titanium was treated with mechanical surfacetreatment to promote better protection against oxidation at high temperature. Shotpeenedand laser-shock peened specimens were compared to untreated samples interms of oxidation behavior at high temperature. We used thermal gravimetricanalysis to oxidize the samples at 700 C for 100 h. Subsequently, XRD, opticalmicroscopy, SEM/EDS, NRA, micro-Raman spectroscopy, and micro-hardnesswere used to characterize the oxide scale and the alpha-case layer formed during thehigh-temperature exposure. The shot-peened samples oxidized less (-45%) than theuntreated and laser-shock peened samples. This behavior was attributed to theformation of a continuous nitride layer between oxide and meta

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a b s t r a c tCracking is observed when a UO 2 single crystal is oxidised in air. Previous studies led to the hypothesis that cracking occurs once a critical depth of U 3 O 7 oxidised layer is reached. We present some l-Laue Xray diffraction results, which evidence that the U 3 O 7 layer, grown by topotaxy on UO 2 , is made of domains with different crystalline orientations. This observation was used to perform a modelling of oxidation coupling chemical and mechanical parameters, which showed that the domain patterning induces stress localisation. This result is discussed in comparison with stress localisation observed in thin layer deposited on a substrate and used to propose an interpretation of UO 2 oxidation and cracking.

Abstract. The present study deals with the oxidation behavior under residual stress of shot-peened plates of "commercially pure" Zirconium exposed for 30 min at 650°C. The influence of the shot-peening on a preoxidizedmaterial is presented. The results have been used to determine the influences of these chemical (preoxidation) and mechanical (shot-peening) treatments on the high temperature oxidation of Zirconium. The oxygen profile was revealed using micro-hardness techniques and confirmed by SEM-EDS techniques. After pre-oxidation the samples were first resurfaced then shot-peened in order to introduce residual stress. A significant increase of the hardness of about +400 HV was observed on pre-oxidized shot-peened samples. The thermogravimetric analysis for 30 min at 650°C under 200mbar O 2 shows a significantly slower oxidation rate for shot-peened samples.A comparison with our computations points out the role of the residual stresses on the diffusion and, consequently, on the oxidation.

An X-ray diffraction procedure has been developed to measure accurately the textures of thin wires so that the texture variation across the wire radius may be obtained. Since X-rays penetrate no further than the surface of most metals, X-ray examination can be considered to be superficial for large wires but by thinning the wire it is possible to obtain the texture variation across its radius. When the diameter is very small, X-rays can penetrate a significant proportion of the wire. In these cases a new technique using the texture changes between several thinning procedures allows a precise understanding of the texture variation between the surface and the core of the wire. To illustrate the application of this procedure, some examples are given for 175 ~tm steel cord.

Improving the high temperature (HT) resistance of titanium alloys is currently a technological challenge for extending their use in aerospace engines. Ti-Beta-21S is a metastable β titanium alloy specifically designed for high temperature applications up to 593°C. We report the effect of a surface treatment by laser-shock peening (LSP) on the high temperature behavior of Ti-Beta-21S in order to increase further its maximum service temperature. The oxidation kinetics at 700°C for duration up to 3000 h showed that the LSP treatment increases the oxidation resistance of Ti-Beta-21S. The effects of the LSP treatment on the alloy microstructure, its evolution at high temperature and the diffusion of light atmospheric elements (oxygen and nitrogen) are also reported.

A model is developed in order to study the influence of mechanical aspects during high temperature oxidation of zirconium alloys. This model accounts for oxygen diffusion within the metal and across the oxide scale. Much attention is paid on the role of the Zr-O solid solution on oxidation kinetics and on the role of mechanical anisotropy. The model shows that stresses developed in the metal, due to oxygen dissolution, have a strong influence on oxidation rates. In particular, a change of crystallographic orientation of the metal leads to a change in stress gradients which is responsible for differences in oxidation rates. Such results are confirmed by experimental results.