Progress in Understanding of Phase Transformations in NiTi Shape Memory Alloys

NiTi shape memory alloys were first discovered in 1961 by W.J. Buehler and F. Wang - researchers in the United Sate Naval Ordnance Laboratory. Their unique properties immediately drew attention of scientists and engineers, however commercial applications were postponed by more than decade due to tremendous difficulties in melting, processing and machining of the material. There exist other alloys in which shape memory effects occur but NiTi shows the best values of these properties coupled with very good mechanical properties and is the only one shape memory alloy that is highly biocompatible. The shape memory effects i.e. one-way, two-way shape memory effect and superelasticity are caused by thermoelastic reversible martensitic transformation that takes place in the material during temperature change or when an external stress is applied. In NiTi alloys this is the B2 (b.c.c.) phase that transforms to the monoclinic B19' martensite. The transformation characteristics depends very strongly on the alloy chemical composition and its widely understood defect structure. Moreover, in some cases the B19' transformation is preceded by another transition: the R-phase transformation which is also of thermoelastic martensitic type. The transformation sequence, its mechanism and the structure of the R-phase have been for a long time a matter of discussions and discrepancies. Also another problem that was discovered in some NiTi alloys i.e. multistage course of the martensitic transformation was a subject of many studies. The above points will be discussed in the presentation. Additionally, recent trends in NiTi alloys development will be presented. The material can be used in medicine for e.g. stents or osteosythesis clamps. Thus, the surface modification for the medical application is one of the concerns of scientists.