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The R-phase of a polycrystalline 50Ti48Ni2Fe shape memory alloy is here investigated by means of X-ray analysis. With the aim to determine, by means of X-ray powder diffraction, the R-phasc lattice parameters with high accuracy, an original calibration method specific for shape memory alloys is developed. The effects of intrinsic aberrations that in X-ray powder counting diffractometry modify the peak angular positions, are taken into account. Exploiting the crystalline structure of the B2phase of the specimen under investigation, a full indexing of the X-ray spectrum of the R-phase, not achievable by raw experimental data, is obtained. The possibility to extend the method in a most general way to crystalline materials that present a first-order crystalline phase transformation it is also pointed out.Nous avons etudit la phase rhomboedrique d'un alliage polycristallin 50Ti48Ni2Fe a memoire de forme par diffraction aux rayons X. Dans le but de determiner les parametres reticulaires de la phase R avec precision elevee, nous avons developpee une methodologie originale de calibrage pour la diffraction de poudres aux rayons X. Les effects des aberrations intrinseques dans la diffraction aux rayons X a comptage, qui modifient les positions angulaires des pics, ont etk considere. En exploitant la phase B2 de I'tchantillon sous investigation, nous avons obtenue la complete indexation du spectre aux rayons X de la phase R, qui n'etait pas realisable avec les donnes grossiers. La methodologie ici developpee peux itre appliquke en faGon generale aux materiaux crystallins qui presentent une transformation cristallographique de phase du premier ordre.') Via Celoria 16, 1-20133 Milano, Italy.' ) The circle defined by the X-ray source, the moving detector, and the specimen. 4, Equatorial divergence refers to the plane parallel to diffraction circle.

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