An in-situ grazing incidence x-ray diffraction study of the crystallisation of Ni-Ti thin films

The shape memory effect (SMA) relies on martensitic transformations which are macroscopically due to a pseudo-shearing deformation, resulting from a deformation mode similar to slip or twinning in ordinary metals and alloys under stress. However, since martensitic thermoelastic transformations are reversible, the deformation behaviour of alloys that undergo this type of transformations is remarkably different from that of ordinary metals and alloys in which the deformation, by slip or by twinning, is not recoverable [1]. The martensitic transformation starts (in cooling) at a certain temperature Ms, and is completed when a lower temperature Mf is reached. When this martensite is deformed (below Mf) it undergoes a strain, which, within certain values, is completely recoverable upon heating. The shape recovery begins at a temperature As (where the martensite → austenite transformation starts), and is completed at a higher temperature Af (where the transformation martensite → austenite is finished). The Ms, Mf, As and Af temperatures are characteristic of the alloy system, and recoverable strains typically range from 2% to 10%. The martensite in shape memory alloys (SMA's) may also be isothermally induced above the Ms temperature by the application of a stress [2]. The NiTi system is the most popular of the SMA's because of the considerable work per unit mass it can produce during recovery (work output ~1 Joule/g), because of the value of the transformation temperature (near room temperature, i.e. from –100C to +100C) and because of its good oxidation resistance.