PhD Defense of Henrique MARTINNI RAMOS DE OLIVEIRA from BioMMat team on october the fifth at 9:30am :

 " Study of thermomechanical couplings in nanostructured superelastic nickel-titanium wires" 

Place : Salle 109, Bâtiment Boucherle, Faculté de Pharmacie Grenoble

Jury et thesis supervision :

    Pr Denis FAVIER, Professeur UGA, TIMC-IMAG, Director
    Pr Hervé LOUCHE, Professeur de l’Université Montpellier, Co-Director

    Pr Shabnam ARBAB CHIRANI, Professeur de l’École Nationale d’Ingénieurs de Brest, Reporter
    Pr Xavier BALANDRAUD, Professeur de l’Institut Pascal, Clermont-Ferrand, Reporter
    Pr Jean-Benoît LECAM, Professeur de l’Université de Rennes 1, Examiner

Abstract:

This PhD thesis is an experimental study of the thermomechanical superelastic behaviour of a Ti-50.9Ni at.% Ni Shape Memory Alloy (SMA) nanocrystalline thin wire (diameter 0.5 mm), in a Cold Worked (CW) state. SMAs are capable of inducing important temperature change when they are mechanically loaded. This phenomenon is due to an important thermomechanical coupling present in this solid phase transformation between Austenite (A) and Martensite (M) phases. The latent heat per unit of mass (∆H) throughout the phase transformation is the energy responsible of this temperature variation. The determination of ∆H is generally performed by differential scanning calorimetry (DSC). However, for nanocrystalline SMAs, the obtained DSC results are non conclusive on the determination of this property.

In this work, a method using digital image correlation (DIC) and thermal field measurements (TFM) was used to analyse the thermomechanical couplings during a stress induced phase transformation (SIPT). Kinematics and thermal full fields were acquired during superelastic tensile tests performed on the CW NiTi wire submitted to different heat treatments temperatures (HTT) ranging from 523 to 598 K during 30 min. Such a heat treatment at low temperature promoted a fully superelastic loop without stress plateau and no Lüders-like deformation. Assuming a uniform thermal model, the heat sources involved during the cyclic loading were estimated. This thermal power per unit of mass was compared to the mechanical one and integrated over the time to get energy balance. Further, through a thermodynamic analysis based on the Gibbs free energy, the values of ∆H, as well as the martensite fraction, were estimated during the forward A-M and reverse M-A phase transformations. The analysis of the results led to the following conclusions: (1) Thermal and mechanical powers and energies presented a significant dependence on the HTT. (2) Despite the strong effect of the values of the HTT on mechanical and thermal responses, the obtained ∆H were very close for all HTT and in the same range of values founded in the literature for a fully annealed Ti-50.9Ni at.% Ni alloy tested via DSC technique. (3) For a given strain, martensite fraction increases with increasing HTT.  (4) For an imposed strain of 4.5%, the martensite fraction increases from 30% to 40% when increasing HTT from 523K to 598K.

Keywords:

Full-field kinematics and thermal measurements, nanostructured NiTi wires, thermal and mechanical powers, heat balance, specific latent heat, martensite fraction.