Ankündigung eines Vortrages von
über das Thema
Abstract:
TiNi alloys show extraordinary behaviour: transformation super-plasticity, shape memory effect (SME), super-elasticity, two-way shape memory, which take place during phase transformations. These alloys are very sensitive to preliminary thermomechanical processing, usually including plastic deformation. It can also arise at freelance exploitation of SME parts. Plastic deformation causes structure changes: density of defects, internal oriented and non-oriented stresses, texture. Considerable plastic deformation always takes place at technical processes - drawing, rolling. In this case it can be used for formation of a required complex of properties. The relevance of the research is also conditioned by the necessity of maintenance of a reliable operation of sensors, regulators, actuators with SME parts which are more and more widely introduced in engineering,.
The influence of preliminary plastic deformation effect on shape memory characteristics: transformation temperatures, capacity of the materials to accumulate deformation at direct transformation, two-way shape memory effect in TiNi specimens as well as the effect of annealing on the recovery of properties of the deformed alloys were studied. The solid cylindrical specimens with diameter 4 mm and length of the working section 30 mm were tested in torsion. Preliminarily the specimens were annealed during 15 minutes at 1073 K, cooled in air, held 2 hours at 773 K and then cooled with the furnace.
A tensile plastic deformation e p results in a non-monotonously variation of the transformation plasticity strain in torsion g tp: it grows for small preliminary strain under 2-3 % and then significantly degreases. All characteristic temperatures of the thermo-mechanical hysteresis slightly decrease.
It has been established that the interval of the martensitic transformation widens with the preliminary plastic deformation by torsion g p. At the same time the transformation plasticity strain g tp experiences an exponential decay while the two-way shape memory strain grows.
The growth of transformation plasticity deformation at low values of e p may be due to the formation of internal stress fields, which at small values favour an increase of g tp. However the further growth of plastic deformation results in too large values of internal stresses, and also in "clogging" of the material by defects. These processes become dominating reduce the role of external loading as orientating factor thus leading to the decrease of the transformation temperatures. The similar behaviour is characteristic for the reverse transformation temperatures.
An experiment on an isochronous annealing was carried out. The temperature of annealing was varied from 500 to 900 K; the holding period was constant and equalled 20 minutes. As a result of the annealing the properties of the material were partially restored. Most intensively the recovery took place at 700-800 K. This is the temperature range in which the grain re-crystallisation in TiNi takes place. It is accompanied by the fall-off of the defects density and a partial elimination of plastic deformation backtraces. A further increase of the temperature of annealing results in a decrease of the transformation plasticity deformation.
The obtained results have shown that the values of the characteristic temperatures of the martensitic transformation, strain accumulated in cooling under a constant stress (transformation plasticity) and the two-way shape memory effect are determined by the mode of preliminary deformation.