SCIENTIFIC AND APPLIED ASPECTS OF FERROMAGNETIC SHAPE MEMORY ALLOYS

The underlying mechanisms responsible for the giant magnetic or mechanical field-induced-strains in the Ni- Mn-Ga ferromagnetic shape memory alloys are briefly discussed. The fundamental aspect is illustrated by experimental data related to the lattice instability and composition dependence of magnetization alongside literature results. An implementation of Ni-Mn-Ga single crystal as a strain sensor is described

NiTiCu shape memory alloy : Superplastic elongation during thermal cycling

We report on experimental results obtained in the characterization of a Ni45Ti50Cu5 shape memory alloy tested in order to verify its properties at the light of the use in actuators. In this field long term stability of the recovery properties and low hysteresis values are the main requests. Tests were performed in an "ad-hoc" developed device with a maximum number of cycles equal to 5000 and an applied load of 180MPa. Various thermal treatments and cold-working conditions were tested on several specimens in order to identify the combinations that can ensure the best functional properties at the light of the use in actuators. Calorimetric measurements were systematically performed on specimen taken at the beginning and at the end of the cycling procedure. Results confirm that low temperatures and short time thermal treatments account for higher stability in the cycling properties. Surprisingly it has been noted that specimens submitted to complete anneal thermal treatment, in spite of a large plastic deformation during thermomechanical cycling, exhibit only small changes in the recovery properties. A NiTiCu full annealed wire cycled up to rupture can account for a stored plastic deformation up to 60% of the initial length, at the same time demonstrating a complete shape memory effect of 5%. A detailed investigation of the material submitted to this treatment was done in order to clarify its behavior

Anomalies in the pseudoelastic effect of a Ni45Ti50Cu5 shape memory alloy

An extended investigation of the pseudoelastic properties of Ni45Ti50Cu5 alloy suggested the presence of a two-stage transformation behavior. This behavior was brought to evidence performing stress strain measurement on ternary alloy specimens submitted to different thermal treatments and pulled slightly overbehind the maximum pseudoelastic transformation strain. The concurrent measurement of Electric Resistance(ER) and of the temperature of the specimen proved clear modifications of these physical properties at the end of the pseudoelastic plateau both during the loading and unloading process. Investigation of the stress strain curve at different temperatures above the Af (austenite finish) values together with a systematic comparison of ER measurements on differently thermally treated specimens suggest the presence of an another transformation in competition with the usually accepted cubic to monoclinic. Results obtained are here presented and discussed at the light of this working hypothesis. Analysis of the ER(ε) curves allows to relate directly this physical property to the transformation strain of the pseudoelastic curve. Both at the end of the loading and unloading plateau there is evidence of a marked change of the ER(ε) , only slightly modified by the thermal treatment procedure. In more detail measurements were performed at different temperatures taking into account the modification in Af temperature induced by the thermal treatment

Smart Behaviour in a CuZnAl Single Crystal Alloy

The Step-wise Martensite to Austenite Reversible Transformation (SMART) has been widely investigated in polycrystalline shape memory alloys and its key features are, on the whole, well established. Though some working hypothesis have been put in advance, the full understanding of this “micromemory” phenomenon is still open. Specifically, the most probable origin of the SMART, relies on a local relaxation of the elastic energy. In order to clarify the advanced hypothesis, a CuZnAl single crystal alloy, where the elastic energy contribution to the thermoelastic balance is typically smaller than in the case of polycrystalline specimens, has been examined. All the investigated specimens have shown, though at different extent the SMART: results have shown that the larger the elastic energy involved, the more evident the SMART phenomenology is. Thus, the present findings further support the role of the elastic energy in the SMART

Electric Transport Properties Modified by Incomplete Cycling on Heating (ICH) in TiNi Based Alloys

The investigations till now performed on the Step-wise Martensite to Austenite Reversible Transformation (SMART) have identified the relevant key features both of the transformation itself and of the Incomplete Cycling on Heating (ICH), procedure required to induce the SMART. The SMART phenomenology, proved in several shape memory alloys, appears quite general, despite of the differences both in the kinetic and in the crystallographic characters of the thermoelastic martensitic transformations involved. The microstructural mechanism underlying SMART has however not yet been clarified. A working hypothesis, already advanced, is based on the idea that the ICH procedure is effective in inducing kinetic barriers, related to local assemblies of defects localized in the martensite phase ; that, however, contrasts, at least in TiNi, with X-ray diffraction (XRD) and Electrical Resistance (ER) measurements which support an increasing structural order as a consequence of the ICH procedure. With the aim to gain further insight into the microstructural modifications in martensite, induced by the ICH procedure, ER measurements have been here undertaken both on a Ti49.8Ni50.2 (at%) and on a Ti50Ni45Cu5 (at%) alloys. The experimental findings are discussed and contrasted with the ones already obtained on TiNi alloys

Giant two-way shape memory effect in high-temperature Ni–Mn–Ga single crystal

AbstractA perfect two-way shape memory effect (TWSME) with reversible strain of about 9% has been found in the high-temperature Ni57.5Mn22.5Ga20.0 single crystal transforming into 2M non-modulated martensitic phase. Two thermal/mechanical treatment routes were utilized in the experimental procedures. The outstanding high-temperature TWSME is observed as a result of tensile stress-strain cycling along 〈100〉 axis involving huge superelastic strains due to stress-induced martensitic transformation. It is argued that the TWSME is related to the anisotropic internal stresses produced by a network of lattice defects. The defects are generated in the course of oriented growth/shrinkage of the dominating martensitic variant in the tensile sample