Diffusive Phenomena and the Austenite/Martensite Relative Stability in Cu-Based Shape-Memory Alloys

Abstract

The main characteristic of martensitic phase transitions is the coordinate movement of the atoms which takes place athermally, without the contribution of diffusion during its occurrence. However, the impacts of diffusive phenomena on the relative stability between the phases involved and, consequently, on the associated transformation temperatures and functional properties can be significant. This is particularly evident in the case of Cubased shape-memory alloys where atomic diffusion in both austenite and martensite metastable phases might occur even at room-temperature levels, giving rise to a variety of intensively studied phenomena. In the present study, the progresses made in the understanding of three selected diffusion-related effects of importance in Cu-Zn-Al and Cu-Al-Be alloys are reviewed. They are the after-quench retained disorder in the austenitic structure and its subsequent reordering, the stabilization of the martensite, and the effect of applied stress on the austenitic order. It is shown how the experimental results obtained from tests performed on single crystal material can be rationalized under the shed of a model developed to evaluate the variation of the relative stability between the phases in terms of atom pairs interchanges.

Sequence and Characteristics of Atomic Ordering in Ni2Mn1−xCuxGa Ferromagnetic Shape Memory Alloys

“…This is an unprecedented observation, since it has always been assumed that diffusion-related post-quench atomic reordering only takes place in austenite (being usual to observe a related exothermic peak [ 7 , 8 ]). On the other hand, atomic diffusion in martensite, and the atomic rearrangement that it entails, have always been related to the phenomenon of martensite stabilization [ 24 ] (no thermal effect associated with having been reported to date). However, as can be seen in Figure 10 , there is no stabilization of martensite, since the MT transformation temperatures are the same in the first and second heating runs.…”

A review on Nitinol shape memory alloy heat engines

“…is tetragonal, austenite-martensite diffusionless phase transformation could be performed via sheer lattice distortion when the temperature is changed [48]. The phase transformation can produce multiple martensite variants, which are martensitic crystals with different orientation directions.…”