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.
D.09 Tutoring for postgraduate students
COBISS.SI-ID: 273707520In this work we are presenting devices to simulate the different deformations of wire that occur in orthodontic praxis. On the wire from SMA NiTi we detected a stress-induced phase transformation which was caused by the different deformations. Our goals were to observe and determine the beginning of the stress-induced phase transformation at different deformations and the benefit of the transformation plateau during orthodontic treatment. This was done by measuring the micro hardness and electrical resistivity. By these analyses we detected successfully the phase transformation while the orthodontic wire was deformed in different ways.
F.18 Transfer of new know-how to direct users (seminars, fora, conferences)
COBISS.SI-ID: 17974550This work presents a study of the mechanical properties and influence of thermomechanical training conditions on the two-way shape-memory effect (TWSME) (magnitude and stability) in NiTi and NiTiCu alloys. Correlation with a stress-assisted two-way shape-memory effect (SATWSME) was also investigated. A rapid solidification technique was employed for preparation of small-dimension NiTi samples with homogeneous microstructure. Samples were fabricated by melt spinning. The melt-spun ribbons were trained under tensile deformation by thermal cycling through the phase transformation temperature range. Results showed that the tensile trainings under both constant strain and constant stress were effective in developing a TWSME. The SATWSME was comparable in magnitude to the TWSME. Addition of copper was effective in narrowing the transformation hysteresis and transformation temperature dependence on composition.
F.10 Improvements to an existing technological process or technology
COBISS.SI-ID: 18053910