Développement de la technologie de laminage à tiède pour la mise en forme d'alliage à mémoire de forme Ti-Ni

Abstract

Shape memory alloys (SMA) are a class of material able to generate force and/or displacement under heating after large deformation. Those alloys can also exhibit superelastic (SE) properties. SMA are used as actuators (shape memory effect) or as biomedical devices (SE). SMA’s behaviours are based on the reversible martensitic transformation and their properties are strongly influenced by the final steps of fabrication. It has been proven that severe cold working plus post deformation annealing (PDA) allows an improvement of the SMA functional properties at the expense of a shorter fatigue life. In this study, warm rolling is used for the first time on Ti-Ni SMAs to improve the material’s fatigue life while keeping excellent functional properties. Thermal and mechanical testing (differential scanning calorimetry and tensile tests) and functional experiments (recovery strain and stress) were performed on Ti-50.26at.%Ni. Six different thermomechanical treatments combining cold, warm and PDA were used to reach a true strain of e=1.2. DSC and tensile tests showed that the different thermomechanical treatments were not significantly affecting the material properties. Transformation temperatures were the same for the six tested samples, while stress-strain curves showed that warm rolled samples were less hardened than cold rolled samples. Functional experiments showed that warm rolling combined with intermediate annealing were increasing fatigue life by four times for the constrained recovery testing and 1.5 times for the stress free recovery testing. However, warm rolled samples exhibit a recovery stress 20 % lower than cold rolled samples. For comparable recovery stress, warm rolled samples still show a longer fatigue life. In order to identify the reasons for the longer fatigue life, microstructural characterization using X ray diffraction, scanning electron microscopy, transmission electron microscopy, etc will be necessary.

Date	14 Dec 2011
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Vladimir Brailovski (Supervisor)