3RD-ORDER ELASTIC-CONSTANTS OF BCC CU-AL-NI

We have measured the changes in the ultrasonic wave velocity, induced by the application of uniaxial stresses in a Cu-Al-Ni single crystal. From these measurements, the complete set of third-order elastic const ants has been obtained. The comparison of results for Cu-Al-Ni with av ailable data for other Cu-based alloys has shown that all these alloys exhibit similar anharmonic behavior. By using the measured elastic co nstants in a Landau expansion for elastic phase transitions, we have b een able to give an estimation of the value of a fourth-order elastic constants combination. The experiments have also shown that the applic ation of a stress in the [001] direction, reduces the material resista nce to a (110)[1(1) over bar0$] shear and thus favors the martensitic transition.