First cycle shape memory effect in the ternary NiTiNb system

Abstract
The usage of shape memory alloys (SMAs) in coupling devices depend considerably on the phase transformation temperatures (PTTS) of the material. It is important that SMAs in such devices feature austenite start temperatures (A -temperatures) above room temperature for storage purposes as well as low martensite start temperatures (M -temperatures) in order to guarantee a high level of mechanical resistance service. It is well known that the addition of Niobium to the NiTi-System results in an increase in the width of the PTT-hysteresis, in particularly for the first phase transformation cycle. The present work studies the influence of different amounts of Nb both as a comparison between binary and temary alloys and as a comparison of diferent levels of Nb (9 and 21 at. %) on. the first cycle shape memory effect. The constrained shape memory effect is simulated using a tensile test machine and the dependence of the recovery stress on the predeformation is determined. The dependence of this functional property on cold working and subsequent annealing is considered; special emphasis is placed on the recovery stress and how it is affected by the thermomechanical treatments. In the present work, solutionannealed material and thermo-mechanically treated material is characterized on the basis of calorimetrie (differential scanning calorimetry - DSC) and mechanical experiments.