Lattice dynamics of the high temperature shape memory alloy Nb-Ru

TL;DR

This study investigates the lattice dynamics of Nb-Ru, a high-temperature shape memory alloy, revealing phonon softening and elastic constant behavior indicative of a second order martensitic transition near 900°C.

Contribution

It provides detailed phonon dispersion measurements of Nb-Ru, showing anomalous temperature dependence and softening of specific phonon branches related to the martensitic transformation.

Findings

The [110]-TA2 phonon branch softens significantly as temperature approaches TM.

Elastic constants tend toward zero near TM, suggesting a second order transition.

No additional lattice modulation observed in the cubic phase.

Abstract

Nb-Ru is a high temperature shape memory alloy that undergoes a Martensitic transformation from a parent cubic b-phase into a tetragonal b' phase at TM 900 C. Measurements of the phonon dispersion curves show that the [110]-TA2 phonon branch, corresponding in the q=0 limit to the elastic constant C'=1/2(C11-C12) has an anomalous temperature dependence. Nearly the entire branch softens with decreasing temperature as TM is approached. The temperature dependence of the low-q phonon energies suggests that the elastic constants would approach 0 as T approaches TM, indicating a second order transition. No additional lattice modulation is observed in the cubic phase.