THE INFLUENCE OF STRESS STATE ON THE MICROSTRUCTURE OF ORTHODONTIC SHAPE MEMORY ALLOY NiTi

V doktorski disertaciji je obravnavan problem vpliva večosnega napetostnega stanja na začetek in potek superelastičnega področja pri spominski zlitini Ni-Ti. Za raziskovalno delo je bila uporabljena komercialno dostopna ortodontska žica Ni-Ti z vsebnostjo 50,6 at.%Ni. V prvem delu raziskav smo izvedli karakterizacijo izbranega materiala. Določili smo transformacijske temperature Ms, Mf, As, Af, module elastičnosti, napetosti in deformacije na začetku in koncu superlastičnega področja ter analizirali mikrostrukturo v izhodnem nedeformiranem stanju. Z in-situ merjenjem električnega upora med obremenjevanjem pri enoosnem nategu na napravi za simulacijo enoosnega napetostnega stanja smo potrdili uporabnost te metode za določitev prehoda v superelastično stanje oziroma za spremljanje napetostno inducirane martenzitne fazne transformacije. Na osnovi teh rezultatov smo v drugem delu raziskav razvili napravo za simulacijo večosnega napetostnega stanja z možnostjo in situ merjenja električne upornosti in mikrotrdote. Z upogibom, torzijo ter kombinacijo torzije in upogiba smo simulirali obremenitve, ki so prisotne pri ortodontskem zdravljenju. S sočasnim merjenjem električnega upora smo za ta večosna napetostna stanja določili prehod v superelastično področje. Z analizo mikrostruktur pred in po obremenitvah smo identificirali spremembe in postavili modele razvoja mikrostrukture za različna napetostna stanja. This doctoral thesis deals with the problem of the influence of the multi-axial stress state at the beginning and end of the superelastic plateau in the Shape Memory Alloy Ni-Ti. For our research work commercially available orthodontic wire Ni-Ti was used with a content of 50,6 at.% Ni. In the first part of the research we determined the characterization of the selected material. We determined the transformation temperatures Ms, Mf, As, Af, modulus of elasticity, stress and strain at the beginning and end of the superelastic plateau, and analysed the microstructure of the output un-deformed state. With in-situ measurement of the electrical resistance during uni-axial loading on the device we simulated a uniaxial stress state, which confirmed the usefulness of this method for determination of the transition into superelastic condition or accompanying the stress-induced martensitic phase transformations. Based on these results, in the second part of the research we developed a device to simulate the multi-axial stress state with the possibility of in-situ measurement of electrical resistance and microhardness. We simulated the bending, torsion and combined torsion and bending loads which are present in orthodontic treatment. The simultaneous measurement of electrical resistance for this multi-axial stress state was defined during the transition into the superelastic area. By analysing the microstructures before and after load changes were identified and an evolution model of microstructure was erected for different stress states.