Elastic constants investigation by X-ray diffraction of in situ deformed metallic multi-layers P. Goudeau * , P. Villain, T. Girardeau, P.-O. Renault, K.-F. Badawi Laboratoire de M etallurgie Physique, UMR 6630 CNRS, Universit e de Poitiers, SP2MI, Bd Marie et Pierre Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France Accepted 20 November 2003 Abstract Decreasingperiodthicknessinbinarymulti-layersdowntoafewnanometers(<3nm)leadstoasofteningofelasticpropertiesin eachindividuallayers.Sizeeffectsrelatedtograinboundariesandinterfacesmayberesponsibleofthesedramaticchanges.Coupling X-ray diffraction with in situ tensile testing gives the unique possibility of measuring selectively intra-granular elastic constants in each phase of thin film complex systems. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Elastic constants; Tensile testing; X-ray diffraction; Size effect; Microstructure 1. Introduction Only a few experimental and theoretical works are devoted till now to elastic behavior or mechanical test- ing in the very small deformation range (e < 0:3%) in nano scale metallic thin films and nano crystalline materials. One may however notice that a real experi- mentalefforthasbeenmaderecentlyformeasuringvery precisely macroscopic elastic modulus in nano scale materials [1–4]. In fact, most of the studies concerning mechanical properties in such materials are generally dedicated to plasticity behavior [5–9]. To our opinion, plasticity and elasticity are complementary fields, which could be addressed with the same common interest concerning nano sized materials. In both cases, size ef- fect and so the role of grain boundaries still remain the samepurposetoinvestigatewhenthenumberofingrain atoms becomes smaller than the number of surface atoms.Modificationsoftheinteratomicpotentialinthe vicinity of interfaces may be expected and as a conse- quence, the elastic constants should be modified for nanograinswithrespecttothecoarse-grainedsituation. Theses considerations are very important since most of analytical models developed for interpreting the plastic deformation of materials are usually based on the knowledgeoftheelasticconstants.Moreover,boundary conditions in numerical simulations are validated through elastic constants. To our knowledge, size effect studies on elastic propertiesofmetallicmaterialsstartedwiththestudyof multi-layer thin film systems at the end of the 70s [10]. For the first time, mechanical behavior anomalies was reportedinsuchlowdimensionalsystemswithrespectto attempt in the coarse-grained reference material or the so-called bulk material. Super modulus effects [11] or breakdownofthePoisson’sratio[12]havebeendeduced from the analysis of experimental results obtained with X-ray diffraction and/or specific mechanical testing apparatus.Followingtheseveryexitingfirstresults,alot of experiments have been reproduced independently for the same multi-layer systems in different research teams all over the word. Unfortunately, contradictory results have been obtained mainly because of the fact that the samples were not prepared in the same way or with the same deposition process and thus the micro structural state was different even if the system was originally identical [13]. Another reason was related to the mechanical test used that gave macroscopic and so global behavior of the multi-phase system or to the wrong interpretation of measurements in the case of X-ray diffraction [14]. The micro structural complexity of such systems led to the conclusion that a close and * Corresponding author. Tel.: +33-549496726; fax: +33-549496692. E-mail addresses: pgoudeau@univ-poitiers.fr, philippe.goudeau@ univ-poitiers.fr (P. Goudeau). 1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.11.053 Scripta Materialia 50 (2004) 723–727 www.actamat-journals.com