THE ELECTRIC-RESISTANCE OF SHAPE-MEMORY ALLOYS IN THE PSEUDOELASTIC REGIME

The electric resistance, sensitively dependent on the electronic struc ture and currently used to detect transformation temperatures in shape memory alloys, deserves attention also in the stress transformation d omain. In fact, the electric resistance has already been investigated under increasing stress in the martensitic phase of several shape memo ry alloys: in this case a linear relationship is found between the ele ctric 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 cl ear, notably in NiTi alloys where two transformations, the Rphase-->M and the P-->M, are often present. The aim of this paper is to investig ate the electric resistance dependence in the pseudoelastic regime in some selected shape memory alloys where one single transformation is p resent. Attention has been focused here on a Ni25Ti50Cu25 alloy, obtai ned by melt-spinning, where just the cubic(B2)<->orthorhombic(B19) tra nsformation is present and on a CuAlBe alloy where the cubic(DO3)<->or thorhombic(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, w hilst the latter shows a linear superelastic behaviour. The electric r esistance variation, detected during the stress induced martensitic tr ansformation, shows a nice linear behaviour, reversible with the trans formation direction. The results are compared with the ones obtained o n NiTi alloys.