NONLINEAR ACOUSTIC PROPERTIES AND ACOUSTIC-MODE VIBRATIONAL ANHARMONICITY OF 18R MARTENSITE CU-ZN-AL SHAPE-MEMORY ALLOY

To shed further light on the dynamics of the martensitic transformatio n in shape-memory alloys, measurements have been made of the effects o f hydrostatic pressure on the velocities of 15 different pure ultrason ic modes propagated at room temperature in a Cu0.696Zn0.128Al0.176 sin gle-crystal alloy in its 18R martensitic phase. Although the point sym metry group of the 18R martensite is monoclinic, the elastic behaviour can be reasonably described in terms of an orthorhombic structure; th e further monoclinicity beyond such an orthorhombic representation is very small and can be neglected to a first approximation. The results obtained for the nine independent second-order elastic stiffness tenso r components C-IJ in the 18R structure and their hydrostatic-pressure derivatives (partial derivative C-IJ/partial derivative P)(P=0) have b een used to calculate the mode Gruneisen parameters gamma(p)(N), which quantify the vibrational anharmonicity of long-wavelength acoustic ph onons. Results are compared with those computed from TOEC data measure d on a B-phase Cu-Zn-Al single crystal with a similar composition. At room temperature, about 28 K below the martensitic transition temperat ure (=323 K), the longitudinal mode yp(N) have normal, positive values ; however, those yp(N) associated with some shear modes have negative values. A pronounced minimum has been found in the Gruneisen parameter of the shear mode propagated along the [111] direction with [121] pol arization vector. It is concluded that the lattice stability against t his particular shear mode decreases as the sample approaches the marte nsitic transition.