The influence of dislocations on R-phase transformations in a NiTi shape memory alloy

TL;DR

This study investigates how dislocation density influences the R-phase transformation in NiTi shape memory alloys, revealing that deformation and recovery processes can be used to control transformation temperatures and behaviors.

Contribution

It provides new insights into the relationship between dislocation density, transformation enthalpy, and temperature, enabling tailored phase transformations in NiTi alloys.

Findings

Dislocation density affects R-phase transformation temperature.

Recovery processes are rapid and orientation-dependent.

Processing-induced residual stresses influence transformation behavior.

Abstract

The ability to control the stress-induced phase transformation of the shape memory alloy, NiTi, is an important technological challenge that must be understood for their wide application in devices that can exploit their reversible strain properties. This study elucidates the direct relationship between dislocation density and the \textit{R}-phase transformation, including its formation temperature from interrupted annealing of rolled NiTi samples. Deformation is shown to determine the enthalpy change required for the B2$\rightarrow$\textit{R}-phase transformation, with associated transformation temperatures being modifiable via dislocation density and recovery processes. Recovery is shown to be rapid, highly heterogeneous and sensitive to crystal orientation. Grains with a $\langle100\rangle$ direction close to the macroscopic rolling direction recover more rapidly than $\langle110\rangle$ and $\langle111\rangle$ orientated grains. Considered to be governed by processing induced residual stresses and resultant crystallographic dependent annihilation/slip pathways, there are opportunities to tune B2$\rightarrow$\textit{R}-phase transformation on either a grain-averaged or an orientation dependant per-grain basis.