Lattice softening and the martensitic transition in indium thallium alloys

Abstract

Indium-thallium alloys in the composition range l6 to 31 at.% T1 undergo a martensitic phase transition from the higher temperature fee form to the lower temperature fct one. Certain lattice properties of these alloys have been investigated in the region of the phase boundary. The alloys studied were in single crystal form and comprised of two (11.5 and 15 at.% T1) which always had a tetragonal structure, and two (25 and 27 at.% T1) which on cooling transformed from fee to fet at 195 and 125K, respectively. Ultrasonic wave velocities and attenuation have been measured by pulse methods in the temperature range 4 to 423K (melting point 427K). In addition, thermal expansion data for the alloys have been obtained. The temperature variation of the elastic constants, determined from the sound velocity results, is discussed with reference to effects associated with the martensitic transition. One elastic constant combination - 1/2(c(_11)-C(_12)) has been shown to approach zero at the transition; theoretical calculations of elastic constants from phonon dispersion curves computed from the optimised model potential theory show a similar result, and the mechanism of the transition is interpreted as the collapse of the slow transverse acoustic phonon mode propagating in the [110] direction. Crystal stability conditions have enabled the limiting values of Poisson's ratio to be derived, and the variation of that ratio and its limits has been investigated as the phase boundary is approached. The order of the phase transition is discussed; a second order process is suggested by the results obtained.