Anomalies in the pseudoelastic effect of a Ni₄₅Ti₅₀Cu₅ shape memory alloy

Abstract
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.