Influence of precipitation and dislocation substructure on phase transformation temperatures in a Ni-rich NiTi-shape memory alloy

The present work studies the influence of different thermo mechanical treatments on phase transformation and creep of a Ti-50.7at%Ni shape memory alloy. Thermo-mechanically processed, solution annealed, tempered and pre-crept material states show different phase transformation behaviour. Precipitation of Ti3Ni4 particles can rationalize a 20°C increase in phase transition temperatures. Changes of dislocation density and subgrain size also have a systematic but small effect on the martensitic transformation. There is a strong influence of small preloads on the shape of individual creep curves in the early stages of creep which is associated with the precipitation of Ti3Ni4

First cycle shape memory effect in the ternary NiTiNb system

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$_{\rm s}$-temperatures) above room temperature for storage purposes as well as low martensite start temperatures (M$_{\rm s}$-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

Direct physical evidence for the back-transformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to BIT during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack

Direct physical evidence for the back-transformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to B19' during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack

Direct physical evidence for the back-transformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to BIT during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack