Phonon precursors to the high temperature martensitic transformation in Ti₅₀Pd₄₂Cr₈

Inelastic neutron scattering measurements were carried out on the Ti₅₀Pd_50-xCr_x alloy, which is a high temperature shape memory material. For x=0, the transformation temperature is ∼800K and for the composition studied (x=8 at.%) M_s ∼ 400K. The majority of the measurements were performed in the parent, B2-phase, up to 873K. Most of the phonons propagating along the three symmetry directions [ζ00], [ζζζ], and [ζζ0] were well defined with the exception of the [ζζ0] transverse acoustic mode with displacements along the [-ζζ0] corresponding to the C'=1/2(C₁₁-C₁₂) elastic constant in the ζ → 0 limit. These phonons are well defined for small ζ, but for ζ > 0.10 they are strongly overdamped near the transition temperature and become better defined at higher temperatures. An elastic peak develops in the cubic phase at ζ=0.22 and increases in intensity as M_s is approached. However, the dispersion curves show no anomaly at this particular wavevector, in marked contrast to the lattice dynamic studies of other systems exhibiting Martensitic transformations.