T. Bretheau

Experimental characterization of the local strain field in a heterogeneous elastoplastic material

A new technique, which allows to characterize the local strain field over a domain representative of the microstructure of a heterogeneous material, is described. It is based on scanning electron microscopy, microelectrolithography, image analysis and in situ tensile tests. The in-plane components of the local strain field are characterized by their averages per phase and their distribution functions. The results are accurate for global strains between 5 and 15%. It is also possible to get contour plots of these components of the local strain field over the considered domain. The obtained strain maps give a powerful qualitative information on the strain localization modes during the deformation. This technique has basically been developed for two-phase elastoplastic materials, namely iron/silver and iron/copper blends, submitted to uniaxial tensile tests; it could also be used for polycrystals or other composite materials and for other mechanical tests.

Plastic deformation mechanisms of β treated zirconium

This study aims to determine the different deformation mechanisms of grade 702 zirconium under uniaxial tension and at room temperature. The grade 702 zirconium tested had undergone an {alpha} {yields} {beta} {yields} {alpha} cycle at a slow cooling rate ({approximately} 15{degree} s{sup {minus}1}). Three deformation mechanisms were identified: prismatic slip, (10{bar 1}2) twinning and (11{bar 2}1) twinning. The critical resolved shear stress for prismatic slip, (10{bar 1}2) twinning and (11{bar 2}1) twinning was also determined. The effect of the non-uniform redistribution of the hardening elements on location through the grain of the various mechanisms and on the tendency for localized deformation to develop is also discussed.

Cavity growth and rupture of β-treated zirconium: A crystallographic model

Abstract This study aims to understand the damage mechanisms observed in β-treated zirconium. Damage voids are characterized by a tubular morphology with hexagonal cross-section; their growth kinetics is determined experimentally. From these observations, a model based on the principle of creation of free surface by crystallographic slip permits one to explain the stability of the hexagonal shape and to predict a growth rate closer to the experimental value than traditional models. This improvement is due to the sensitivity of the free surface creation mechanism to the stress concentration factor k , which can not be accounted for in models based on continuum mechanics.

Low cycle fatigue behaviour of β treated zirconium : Partial irreversibility of twinning and consequences for damage

Abstract The aim of this study is to understand the low cycle fatigue behaviour of β treated zirconium. It focuses especially on the contribution of twinning mechanisms. In situ fatigue tests performed inside a scanning electron microscope allow the observation of the activation of twins and their partial reversibility, depending on the applied stress. Moreover, the acoustic signature (shape factor) of the different twinning systems has been characterized allowing the following of inception and evolution of twins during the fatigue tests. Finally, the consequence of such a partial irreversibility on damage and crack localization is shown.

Microstructural study of β treated grade 702 zirconium. Scanning electron microscopy and transmission electron microscopy, complementarity of two observational scales

In this study we focus on the precipitation of betagenic elements (Fe, Cr, Ni) that occurs during the β → α allotropic transition in zirconium. In particular its structural characteristics, morphology and their consequences for the mechanical behaviour of the material are probed. The observations are made by means of a scanning electron microscope and a transmission electron microscope. The results reveal the occurrence of a quasi-continuous single crystalline phase, produced by the precipitation of the betagenic additional elements which constitute the so-called lath boundaries. This phase forms a layer surrounding the laths in the bulk of the material. Its morphology could be assimilated to a net, created by precipitates with a size of several micrometres and linked by narrow bridges with a size of several hundred nanometres. The chemical composition of the precipitates are of the Zrx(Cr, Fe2)1 - x type with x ≤ 40%.