Etude phenomenologique et microstructurale de l'alliage a memoire de forme NiTi en reponse a des sollicitations thermomecaniques cycliques (French text)

This study concerns the development of an experimental procedure to characterise the fatigue behaviour of Ni-Ti shape memory alloy subjected to thermal cycling under a constant applied load. This experimental technique combines the measure of macroscopic properties with the ray analysis of the alloy microstructure. The transformation temperatures, total recoverable strain and hysteresis effect are characterised up to 10000 heating and cooling cycles. The microstructural analysis performed under the same experimental conditions helps to understand the underlying mechanisms.; First, the effects of the heat treatment on the Ni-Ti alloy functional properties are assessed by Differential Scanning Calorimetry (DSC) and microhardness measurements. These two analyses allow the determination of the optimal heat treatment temperature. Then fatigue behaviour of the heat treated Ni-Ti ribbon is studied by using, microstructural and macrostructural analysis. The X-ray analysis will reveal three microstructural mechanisms induced by thermomechanical cycling. The first of them concerns the orientation of the martensite phase during the first cycles, they imply a growth of the memory effect of the alloy. The second and the third are respectively associated with reorganization of the dislocations induced by the cold working of the alloy prior to testing and the accumulation of internal stress during cycling. The extent of each mechanism depends on the following factors: the heat treatment conditions (temperature and applied stress) and the level of the applied stress during the fatigue testing.