Phase field simulation of grain size effects in nanograined Ti-Nb shape memory alloys

TL;DR

This study uses phase field modeling to explore how grain size influences martensitic transformation and mechanical properties in nanograined Ti-Nb shape memory alloys, revealing size-dependent phase transition behaviors and validating the Hall-Petch relation.

Contribution

It introduces a polycrystalline phase field model to analyze grain size effects on martensitic transformation in nanograined Ti-Nb alloys, highlighting new size-dependent transformation behaviors.

Findings

Smaller grains suppress martensitic transformation and increase transformation stress.

Hall-Petch relation holds across studied grain sizes.

Ultrasmall grains exhibit continuous second-order phase transition behavior.

Abstract

Titanium-based shape memory alloys, such as Ti2448, have attracted enormous attention owing to their unique thermomechanical properties and potential biomedical applications. In this study, we develop a polycrystalline phase field to investigate the grain size dependence of the martensitic transformation and associated mechanical properties of nanograined Ti-Nb alloys. It is shown that a reduction of the average grain size strengthens the suppression of the martensitic transformation (MT), leading to an increase of the transformation stress, shrinkage of the stress hysteresis, and elimination of residual strain. The time-temperature-transformation curves of nano-grained Ti-Nb alloys with different average grain sizes are obtained and the validity of Hall-Petch relation is also confirmed in all studied grain sizes. Furthermore, when the average grain size becomes ultrasmall, both the temperature- and stress-induced MTs show the continuous second-order phase transition behavior. These superior transformation characteristics are attributed to the high density of grain boundaries and the related dominant role of the gradient energy at the nanoscale. Our results have profound implications for the design and control of the properties in nano-grained shape memory alloys.