Raman microprobe spectroscopy measurements of residual stress distribution along blisters in Cr 2 O 3 thin films

Raman spectroscopy has been used to characterise the buckling phenomenon of Cr 2 O 3 films obtained by oxidation in air at 900 °C of Ni33 at%Cr. The observed circular blisters are described by measuring the radius from the optical top view, the profile via an autofocus device and the residual stress in each point of the chromia film: far away from the centre of the blister, in the vicinity and across the blister. The subsequent spalls are related to the morphology of the blisters and to the stress. © 2009 Elsevier B.V. All rights reserved

Substrate grain boundary effect on residual stress distribution at micrometer scale in chromia oxide thin films

Residual stresses in chromia oxide films formed at 700 °C on Ni Cr30 have been investigated with confocal Raman spectroscopy. The high lateral resolution allows us to determine stress distribution in the oxide growing above both substrate grains and grain boundaries. It is demonstrated that the magnitude of the equibiaxial compressive stress is related to local oxide microstructure evolution: during the initial oxidation, stress is smaller in the chromia located above substrate grain boundaries as compared to that above substrate grains. Subsequently, this gap in stress magnitude progressively vanishes due to the occurrence of oxide grains forming bridges across depressions in the oxide scale above substrate grain boundaries. © 2008 American Institute of Physics

On the discrepancy between theoretical and experimental thermal stress in chromia oxide films

Growth and residual stresses have been determined, thanks to Raman spectroscopy in chromia oxide films thermally grown in air from a Ni33at %Cr alloy between 700 and 900 °C. Due to the good sensibility of the in situ experimental device, the growth stress kinetics even at short oxidation times has been recorded and has been shown to be microstructure sensitive. At different oxidation times, the corresponding residual stress at room temperature has also been investigated and the experimental thermal stress has been deduced. The comparison between the respective magnitudes of stress variations induced by creep relaxation during isothermal oxidation, and by the additional thermal stress occurring upon cooling, has led to the definition of a stress threshold finally proposed to explain the residual stress values and the difference between theoretical and experimental thermal stress components. © 2009 American Institute of Physics

High temperature oxidation of Ni70Cr30 alloy: Determination of oxidation kinetics and stress evolution in chromia layers by Raman spectroscopy

In this work, the oxidation of Ni70Cr30 alloy has been produced at a temperature of 900 °C in air. The parabolic behaviour of oxidation was established ex situ and the corresponding kinetic constant was deduced. Microscopic observations have shown that chromia grain size does not exceed 1 μm. Raman spectroscopy was used to determine stresses in chromia (Cr2O3) layers. Stresses are found to be compressive and increase with the chromia thickness before reaching a "plateau" at -2.9 GPa. The stress relaxation phenomenon discussed here takes into account theoretical modelling that can be found in literature. © 2006 Elsevier B.V. All rights reserved

Microstructure influence on residual stresses in growing chromia oxide films as determined by in situ high temperature Raman spectroscopy

In this work, growth stresses have been investigated in relation with the microstructure in the case of α-Cr2O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range [700°C-900°C]. The low acquisition time necessary to obtain a Raman spectrum allows to follow the chromia growth kinetic with sufficient accuracy. It is demonstrated that the growth stress in such oxide films can attain more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetic - subsequent establishment and relaxation - are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress to start

Growth stresses in α -Cr2 O3 thermal oxide films determined by in situ high temperature Raman spectroscopy

Growth stresses have been investigated in relation with the microstructure in the case of α -Cr2 O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range (700-900 °C). It is established that the compressive growth stress in such oxide films can reach more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetics-subsequent establishment and relaxation-are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia mean grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress level to start. © 2007 American Institute of Physics

On the relation between damage rate and stress level evolution in α-Cr2O3 thin films growing on Ni-33at%Cr

The blistering phenomenon has been studied in chromia films formed between 700 and 900 °C on Ni-33at%Cr. The experimental conditions of the blister occurrence and the damaged surface by blisters and spalls have been determined. From the comparison between the growth stress evolution in the oxide film - which has been related to the oxide microstructure development - and the damage rate the stress relaxation by creep is shown to prevent the formation of blisters and spalls. © 2009 Elsevier Ltd. All rights reserved

Influence of pre-strain and carbon content on delayed cracking phenomenon in unstable austenitic steels

In this paper, we study the delayed cracking phenomenon that occurs after deep drawing, at various microstructure scales. Parameters such as temperature, drawing ratio, pre-strain and chemical composition are studied. From a macroscopic point of view, the number of cracks increases with decreasing temperature or increasing drawing ratios. The higher the carbon content, the more sensitive to failure the steel is. A pre-strain favors delayaed cracking too. Martensite stress state was determined by XRD. For a given set of parameters, it is heterogeneous in a cup. Moreover it varies with drawing conditions

Mechanical features optimization for α - Cr2 O3 oxide films growing on alloy NiCr30

Mechanical features have been investigated in the case of α - Cr2 O3 thermal oxide films growing on NiCr30 alloy in the temperature range 700-900 °C. From previous experimental results obtained by Raman spectroscopy and giving the growth stress evolution in α - Cr2 O3 films, and based on a modelling of the system, both asymptotic forms and an optimization procedure have been developed which give values for the mechanical characteristic parameters of the system and also its evolution with the oxidation temperature. © 2009 Elsevier B.V. All rights reserved