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

Abstract
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 Ti_49.8Ni_50.2 (at%) and on a Ti₅₀Ni₄₅Cu₅ (at%) alloys. The experimental findings are discussed and contrasted with the ones already obtained on TiNi alloys.