GRASSI, E. N. D.; http://lattes.cnpq.br/6767760159280554; GRASSI, Estephanie Nobre Dantas.
Abstract:
Shape Memory Alloys (SMA) are an important group of metallic active
materials that respond to thermomechanical stimuli through the Shape Memory
Effect (SME) or the Superelasticity (SE) phenomena. Both these effects are capable
of retrieving large amounts of strain by simple heating, in the former case, or simple
mechanical unload, in the latest case. The SMA of the NiTi family composition exhibit
superior properties when compared to other compositions, including biocompability,
what brings this alloy to be widely used in medical and orthodontic fields in the form
of tools and accessories to specific treatments. As an example, mini coil springs of
NiTi SMA presenting superelasticity reach strain levels hundreds of times higher than
one-dimensional elements, such as wires. However, a more suitable technique to
manipulate mechanical properties of metallic finished products is the use of heat
treatments like annealing. Mainly after experiencing cold working processes,
annealing treatments are capable of partially or totally recover the atomic mobility,
witch directly affects thermomechanical response of SMA. In this context, this
dissertation work aims to study the influence of annealing heat treatments over
thermomechanical behavior of SMA NiTi mini coil springs originally presenting the
SE. A factorial design was used to evaluate the influence of temperature and time of
annealing over some of the main thermomechanical springs’ properties: spring
constant (stiffness), shear modulus, energy dissipation capacity, phase
transformation temperatures, thermal hysteresis and transformation enthalpy
availability. It was demonstrated that heat treatments between 500°C and 600°C are
capable of converting the superelastic state of the mini coil springs to an actuator
state, as a result of the shape memory effect appearance.
