Lattice Instability of Premartensitic β 1 Phase in Cu 3 Al(+Pd) Alloy under Uniaxial Pressure

The ultrasonic pulse echo method and the X-ray and neutron diffraction methods have been used to investigate the lattice instability of the premartensitic β 1 phase of a Cu-27.0Al-5.5Pd alloy under uniaxial pressure. The third order elastic constants and the [110]TA 1 mode Gruneisen parameter show that the β 1 phase is strongly anharmonic. this situation causes marked instability of the β 1 phase toward the (110)[110] shear under the [001] pressure