Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles

TL;DR

This study uses molecular dynamics simulations to analyze the size-dependent thermally induced austenitic phase transition in NiTi nanoparticles, revealing surface nucleation and propagation mechanisms affecting transformation temperatures.

Contribution

It introduces a new order parameter and visualizes the heterogeneous nucleation process in NiTi nanoparticles, linking surface effects to phase transition behavior.

Findings

Austenite nucleates at the surface and propagates inward.

Transformation temperatures depend on nanoparticle size.

Surface energy influences the free energy difference between phases.

Abstract

The reverse martensitic ("austenitic") transformation upon heating of equiatomic nickel-titanium nanoparticles with diameters between 4 and 17 nm is analyzed by means of molecular-dynamics simulations with a semi-empirical model potential. After constructing an appropriate order parameter to distinguish locally between the monoclinic B19' at low and the cubic B2 structure at high temperatures, the process of the phase transition is visualized. This shows a heterogeneous nucleation of austenite at the surface of the particles, which propagates to the interior by plane sliding, explaining a difference in austenite start and end temperatures. Their absolute values and dependence on particle diameter are obtained and related to calculations of the surface induced size dependence of the difference in free energy between austenite and martensite.