LATTICE INSTABILITY OF PREMARTENSITIC BETA-1 PHASE IN CU3AL(+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 beta-1 phase of a Cu-27.0Al-5.5Pd alloy under uniaxial pressure. The third order elastic constants and the [110]TA1 mode Grueneisen parameter show that the beta-1 phase is strongly anharmonic. This situation causes marked instability of the beta-1 phase toward the (110)[110BAR] shear under the [001] pressure. Such an instability also appears as a gradient change of the [110]TA1 phonon dispersion relation at the GAMMA-point (q = 0). Interplanar force constant analysis of the [110]TA1 phonon dispersion relations suggests that the martensite induced by the [001] pressure would have the 18R (or 9R) structure. The 2/3[110]q(max) diffraction anomaly is found only by neutron diffraction and it does not show any noticeable pressure dependence; the anomaly is not the precursor of the stress-induced martensitic transformation.