Lattice Instability in Cubic Cu_69.2Al_25.4Ni_5.4 Related to Martensitic Phase Transition

Abstract

The dispersion relation of [110] TA₁ phonon (e⁄⁄[1\bar10]) of β₁-phase Cu_69.2Al_25.4Ni_5.4 was measured extensively at 261, 295 and 372 K by neutron scattering technique. It was found that the dispersion surface of the transverse acoustic mode has a pronounced valley along the [110] axis. The temperature dependence of the phonon energy shows a partial softening of the phonon mode. An anomalous dip of the dispersion curve was observed around q=(2⁄3)q_max on the [110] axis. Interplaner force-constant analysis revealed that the lattice dynamics of the crystal indicates dual instabilities both to the 2H and 18R type martensites. Quasielastic scattering was investigated on and in the vicinity of the [110] axis. Peaks attributable to the 2H and 18R martensites were observed as functions of temperature, which also indicates that 2H and 18R are competing.