Les modèles développés afin de prédire le comportement des Alliages à Mémoire de Forme (AMF) sont généralement basés sur une description phénoménologique simplifiée de l’activation des variantes de martensite sous chargement thermomécanique. Cette étude a pour objectif de modéliser et de caractériser par nanoindentation la formation discrète des plaquettes de martensite à l’échelle nanométrique. Un nouveau critère, nommé critère de Patel-Cohen d’indentation, est proposé afin de décrire l’activation de la première variante de martensite sous l’indent et sa transformation inverse. L’évidence de transformation martensitique est observée sur les courbes d’indentation par l’apparition successive d’évènements de type « pop in » et « pop out » lors, respectivement, de la charge et de la décharge. Cela met en évidence la discontinuité spatio-temporelle de l’activation et de la propagation de la transformation martensitique à l’échelle nanométrique. L’émission de dislocations dans le nickel pur a été étudiée en tout premier lieu afin de valider et la procédure de nanoindentation utilisant un indent Berkovich et le calcul des facteurs de Schmid d’indentation décrivant l’activation de « pop ins » correspondant à l’activation et à la propagation de dislocations. Un bon accord est trouvé entre les essais réalisés sur un AMF CuAlBe superélastique et la dépendance théorique à l’orientation cristallographique des charges de « pop-ins » et de « pop outs » prédite par le critère de Patel-Cohen d’indentation introduit dans cette étude / Constitutive models developed to predict Shape Memory Alloys (SMA) behavior are often based on a simplified phenomenological description of martensite variant activation under thermomechanical loading at the micro scale. This study aims at modeling and characterizing by nanoindentation the discrete variant activation events at the nano scale. A new criterion is proposed to describe the first martensite variant activation beneath the indenter. Evidence of discrete martensitic transformation is observed during nanoindentation by the successive occurrences of pop-in and pop-out load events on the force versus displacement curve during respectively loading and unloading. Thus, the spatial-temporal discontinuity of phase transformation activation and propagation is highlighted at the nano scale with the introduction of an indentation Patel-Cohen factor for both forward austenite-martensite and reverse phase transformations. Dislocation emission in pure nickel is first studied to validate both the nanoindentation testing procedure using a Berkovich indenter and the calculations of indentation Schmid factors to describe excursion bursts corresponding to dislocation activation and propagation. Good agreement is found between nanoindentation tests performed on a superelastic CuAlBe SMA and theoretical crystallographic dependence of pop-in and pop-out loads predicted by the new introduced indentation Patel and Cohen factor

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Neste trabalho, um estudo da influencia da seleÃÃo de variantes sobre a cristalografia apÃs a transformaÃÃo martensÃtica no Maraging foi feito. O estudo cobriu a transformaÃÃo sob deformaÃÃo elÃstica e tambÃm sob deformaÃÃo plÃstica. No aÃo Maraging, austenita se transforma em martensita eu uma temperatura em torno de 200ÂC, nÃo importando a velocidade de resfriamento. Para simular a transformaÃÃo durante a deformaÃÃo elÃstica, ensaios de traÃÃo foram feitos em um forno acoplado a uma mÃquina de ensaios universais com uma tensÃo aplicada abaixo do limite de escoamento do material. A amostra foi aquecida atà 850ÂC, o forno foi aberto e a amostra resfriada ao ar sob tensÃo constante. Para estudar a influencia da deformaÃÃo plÃstica antes da deformaÃÃo, amostras foram deformadas plasticamente na temperatura acima da Ms (temperatura de inÃcio da martensita), a forÃa externa atuando sobre a amostra foi removida e o material sofreu a transformaÃÃo martensÃtica por resfriamento ao ar. Figuras de pÃlos foram medidas por EBSD (Electron Back-Scatter Diffraction) em ambas as condiÃÃes e comparadas com as figuras de pÃlos calculadas assumindo os modelos de Patel-Cohen e Taylor-Bishop-Hill. A orientaÃÃo da austenita mÃe foi obtida por envelhecimento na temperatura de 650ÂC e pelo uso de grÃos austenÃticos reconstruÃdos matematicamente. Os resultados mostraram que o modelo de Patel-Cohen foi mais adequado para a deformaÃÃo elÃstica enquanto que Taylor-Bishop-Hill foi melhor para a deformaÃÃo plÃstica. / In this work a study of the influence of variant selection on the crystallography after martensitic transformation in Maraging was studied. The study covered both the transformation under elastic deformation and also during plastic deformation. In Maraging steel, austenite becomes martensite at a temperature around 200oC regardless of the cooling speed. To simulate the transformation during elastic deformation, a tensile test was performed in a furnace attached to a universal testing machine with an applied stress below the yield strength of the material. The specimen was heated to 850o C, the furnace was opened and the sample cooled in air under a constant stress. To study the influence of plastic deformation before transformation, samples were plastically deformed in a temperature above Ms (martensite start temperature), the external force acting on the sample was removed and the material was allowed to transform into martensite by cooling in air. Pole figures were measured by EBSD (Electron Back-Scatter Diffraction) in both conditions and compared with calculated pole figures assuming Patel-Cohen model and Taylor-Bishop-Hill model. The orientation of the parent austenite was obtained either by reversing the austenite by heating at 650  C and by using the mathematically reconstructed austenite grains. Results showed that Patel-Cohen model were more suitable to elastic deformation while Taylor-Bishop-Hill model was more appropriated to plastic deformation.