Vibrational entropy of the γ−α martensitic transformation in Fe₇₁Ni₂₉

Abstract

The low-temperature martensitic phase transformation in Fe₇₁Ni₂₉ was studied by X-ray diffractometry, differential scanning calorimetry (DSC) and inelastic neutron scattering. From inelastic neutron scattering spectra, the phonon density of states of the high-temperature austenite and low-temperature martensite were obtained, showing a substantial stiffening in the martensite phase. The change in vibrational entropy associated with this stiffening was found to be

. Using published results for the Fe partial density of states in disordered BCC Fe–Ni of similar composition, a Ni partial density of states was obtained, which shows a significant number of modes at much lower energies than its Fe counterpart. The martensitic and reverse-martensitic transformations occur at different temperatures and the total entropies of transformation were measured by DSC for both. From these measurements and published magnetisation results for this alloy, it is found that the change in magnetic entropy is twice as large as the phonon contribution in the reverse martensitic transformation, accounting for the inequality of the total entropies of the forward and reverse transformations.

Acknowledgements

We thank Robert McQueeney for support during the Pharos experiment and Tim Kelley for help with the data analysis. This work was supported by the Department of Energy through the Basic Energy Sciences Grant DE-FG02-03ER46055. This work has benefited from the use of the Los Alamos Neutron Science Centre at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy under Contract W-7405-ENG-36.