Interface Structure in Ferritic/Austenitic Stainless Steel Bicrystals
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Interface Structure in Ferritic/Austenitic Stainless Steel Bicrystals Aude Taisne1, Brigitte Décamps1 and Louisette Priester2 Laboratoire de Chimie Métallurgique des Terres Rares, UPR 209 du CNRS, Groupe des Laboratoires de Vitry-Thiais, 2-8 rue Henri Dunant, 94320 Thiais cedex, France. 2 Laboratoire d’Etudes des Matériaux Hors Equilibre, UMR 8647, Université Paris-Sud, 91405 Orsay cedex, France. 1
ABSTRACT Elementary mechanisms of deformation by fatigue in duplex stainless steels bicrystals are studied by transmission electron microscopy (TEM). An attempt is made to correlate the bicrystal macroscopic behaviour with the interphase interface crystallography.
INTRODUCTION Internal interfaces are significant microstructural features that control the mechanical properties in single- and two phases materials. These interfaces are sites at which the stress concentrations, due to the plastic incompatibilities between neighbouring crystals, may build-up and possibly relax during the deformation. It is thus essential to investigate the interaction mechanisms of lattice dislocations with the interphase boundary and the stress accommodation processes which allow the deformation to go further. The final aim is to understand and possibly control the material behaviour under mechanical solicitation. The main technique used in this study is the transmission electron microscopy (TEM) Jeol 2000FX. The material is a duplex stainless steels bicrystal. Such material is used in several industrial sectors (nuclear, aeronautical, petrol, metallurgic …) and has to resist to all kinds of solicitations.
EXPERIMENTAL The ferrite-austenite bicrystal has been formed by diffusion bonding, starting from two singles crystals α (Fe30Cr) and γ (Fe15Cr15Ni) grown by the Bridgman technique. Tensile specimens were spark-cut in the bicrystal and notched in the α-crystal in order to study the effects of crack propagation during fatigue tests. The interphase boundary (IB) is parallel to the stress axis and is located in the middle of each specimen (Figure 1). Fatigue crack propagation tests were performed at room temperature in air at a frequency of 10Hz. The applied maximum load was 3.8x102N and the minimum to maximum load ratio R was 0.1. Thin foils containing the interface boundaries were cut from tensile specimens closed to the fracture (Figure 1) and thinned down to be observed by TEM.
Y8.14.1
Tensile axis
BI-4
BI-7
Figure 1. Stereographic projections of the two phases and schematic illustration of the primary slip systems in the two bicrystals [1].
Two types of bicrystals not far from the Kurdjumov-Sachs crystallographic orientation relationship ({1 1 0}α // {1 1 1}γ, {1 1 1}α // {1 1 0}γ, {1 1 2}α // {1 1 2}γ ) were studied. The first bicrystal (BI-4) is deviated by 6.2° from the exact orientation relationship and the other bicrystal (BI-7) is deviated by 11.1°. The misorientation between the two crystals was precisely determined from the Kikuchi lines patterns in each bicrystal. The main differences between the two bicrystals are the IB
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