Ratchetting deformation of super-elastic and shape-memory NiTi alloys

The ratchetting deformation of super-elastic NiTi alloy was first observed by uniaxial stress-controlled cyclic tests, and the dependence of ratchetting upon the applied stress and loading type was discussed. The evolutions of responded peak/valley strain, nominal elastic modulus and transformation stress, as well as dissipation energy of the alloy during the stress-controlled cyclic loading were investigated. It is shown that the super-elastic NiTi alloy presents significant “transformation ratchetting” which is mainly caused by the cyclic accumulation of remained martensite due to the incomplete reverse transformation from the stress-induced martensite to original austenite, and the transformation ratchetting and its evolution depend greatly upon the applied stress amplitude, mean stress and loading chart. For comparison, the ratchetting deformation of shape-memory NiTi alloy and its dependence upon the loading condition were also observed. It is seen that the ratchetting deformation of shape-memory NiTi alloy under the stress-controlled cyclic loading differs greatly from that of super-elastic NiTi alloy, since no reversible transformation from the austenite to the stress-induced martensite occurs in the shape-memory NiTi alloy during the stress-controlled cyclic loading at room temperature. It means that no transformation ratchetting occurs in the shape-memory NiTi alloy, and the ratchetting deformation of the alloy occurred during the asymmetrical stress-controlled cyclic loading is mainly caused by the cyclic accumulation of visco-plastic deformation of re-oriented martensite, which is similar to the ratchetting deformation of ordinary metals. For both the super-elastic and shape-memory NiTi alloys, a nearly stable stress–strain response with small dissipation energy occurs after certain cycles. Some significant conclusions are obtained, which are useful to establish a constitutive model describing the ratchetting deformation of the NiTi alloys.