L. A. Monassevitch, NiTi Medical Technologies Ltd., Netanya, Israel
The plateau on the stress-strain curve is one of the primary and unusual properties of the mechanical behavior of NiTi alloys. There have been various attempts to explain this phenomenon in scientific literature.
This research considers the phenomenon from the crystallographic point of view. So far, the crystallographic approach has allowed the calculation of the value of the lattice deformation of the parent phase upon martensitic transformation. This corresponds to the maximum possible value of the recoverable strain, upon SME and SE, which is 10%.
The use of the Wechsler-Lieberman-Read phenomenological theory of martensitic transformation allows us to calculate the principal deformation values of the transformed volume. The calculated value of the maximal principal deformation is 6%, which corresponds to the fact that the plateau length, as a rule, has the same value. Physically, the applied stress leads to reorientation of 24 habit planes, which previously were equivalent. As a result, their axes of maximal principal deformation turn towards the applied stress direction. Since, according to the theory, the habit plane is an invariant plane, this process does not induce an essential internal stress accumulation. As a result, the accompanying external deformation augmentation does not demand external stress growth.
Summary: The plateau on the stress-strain curve is one of the primary and unusual properties of the mechanical behavior of NiTi alloys. There have been various attempts to explain this phenomenon in scientific literature.
The use of the Wechsler-Lieberman-Read phenomenological theory of martensitic transformation allows us to calculate the principal deformation values of the transformed volume. The calculated value of the maximal principal deformation is 6%, which corresponds to the fact that the plateau length, as a rule, has the same value. Physically, the applied stress leads to reorientation of 24 habit planes, which previously were equivalent. As a result, their axes of maximal principal deformation turn towards the applied stress direction. Since, according to the theory, the habit plane is an invariant plane, this process does not induce an essential internal stress accumulation. As a result, the accompanying external deformation augmentation does not demand external stress growth.
