Abstract
The ultrasonic pulse echo method and the X-ray and neutron diffraction methods have been used to investigate the lattice instability of the premartensitic β1 phase of a Cu–27.0Al–5.5Pd alloy under uniaxial pressure. The third order elastic constants and the [110]TA1 mode Grüneisen parameter show that the β1 phase is strongly anharmonic. This situation causes marked instability of the β1 phase toward the (110)[1\bar10] shear under the [001] pressure. Such an instability also appears as a gradient change of the [110]TA1 phonon dispersion relation at the Γ point (q=0). Interplaner 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]qmax 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.
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