The electric resistance of shape memory alloys in the pseudoelastic regime

The electric resistance, sensitively dependent on the electronic structure and currently used to detect transformation temperatures in shape memory alloys, deserves attention also in the stress transformation domain. In fact, the electric resistance has already been investigated under increasing stress in the martensitic phase of several shape memory alloys: in this case a linear relationship is found between the electric resistance variation and the built-in deformation related to the variant reorientation process, at least in NiTi alloys. In the stress transformation range at constant temperature, the results are less clear, notably in NiTi alloys where two transformations, the Rphase-->M and the P-->M, are often present. The aim of this paper is to investigate the electric resistance dependence in the pseudoelastic regime in some selected shape memory alloys where one single transformation is present. Attention has been focused here on a Ni25Ti50Cu25 alloy, obtained by melt-spinning, where just the cubic(B2)<->orthorhombic(B19) transformation is present and on a CuAlBe alloy where the cubic(DO3)<->orthorhombic(18R) is expected. Both alloys show good pseudoelastic loops though with different features: the former shows a critical stress to induce the B19 phase constant all along the transformation plateau, whilst the latter shows a linear superelastic behaviour. The electric resistance variation, detected during the stress induced martensitic transformation, shows a nice linear behaviour, reversible with the transformation direction. The results are compared with the ones obtained on NiTi alloys.