Influence of Crystallization Conditions on Local Atomic Structure of Austenite in Rapidly Quenched Tinicu Shape Memory Alloys

The local crystalline environment of Ni and Cu atoms in the quasi-binary TiNi-TiCu shape memory alloys containing 30 and 40 at.% Cu has been studied using EXAFS spectroscopy. Amorphous melt-spun alloy ribbons have been crystallized by isothermal annealing or by exposing to a short (10 ms) electric pulse. Analysis of the EXAFS spectra has shown that electropulse crystallization has a more noticeable effect on the local atomic structure of austenite compared to martensite. In particular, it leads to a decrease in the local disorder of atoms in the Cu coordination shells, a decrease in the lengths of Cu-Ni and Cu-Cu bonds, hinders an increase in the lattice parameter of the austenite phase and stabilizes its crystal structure. The observed changes in the local atomic structure of the austenite phase of the alloys contribute to the improvement in the shape memory properties.