NEUTRON DIFFRACTION STUDY OF THE MARTENSITIC PHASE TRANSFORMATION IN In-Tl ALLOYS. H.G. Smith and J.L. Robertson, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA, T. R. Finlayson, Monash University, Clayton, Australia M. Wuttig, University of Maryland, College Park, Maryland, USA

InTl binary solid solutions with Tl concentrations between 15.5 and 31.0 atomic percent are know to undergo a martensitic phase transition from face-centered cubic, fcc, to face centered tetragonal, fct, upon cooling below the transformation temperature, Tm. Tm depends strongly on the Tl concentration dropping from 425K at 15.5 atomic percent Tl to nearly zero Kelvins at 31.0 atomic percent Tl. Appreciable phonon softening has been predicted from the theoretical calculation of the phonon dispersion relations and because the elastic stiffness modulus, c' = (c11 - c12)/2, approaches zero near Tm. A careful measurement of the phonon dispersion curves using inelastic neutron scattering, however, shows no phonon softening within the accessible Q range, but rather a slight hardening as would be expected for a metal. The discrepancy between the temperature dependence of c' and the complicated behavior of the phonon dispersion has yet to be resolved.

The intent of the present study is to examine more closely the structural changes that take place when the alloy transforms. High resolution neutron diffraction patterns were collected at several temperatures starting at 300K and decreasing to 9K on a In(23at%)Tl(77at%) polycrystalline sample. The martensitic transformation is observed to begin around 250K when the peaks associated with the face-centered cubic phase develop small shoulders. The structure continued to change continuously as the temperature was decreased even though the sample had completely transformed to the fct phase at around 200K. Upon warming the fcc phase reappeared 15K above where it vanished when cooling and the lattice parameters showed very little histeriesis. The sample had completely transformed back into the fcc phase by 260K. From these results we conclude that this martensitic transformation is weakly first order in the sense that the c/a ratio on the fct phase continues to increase as the temperature is lowered all the way down to 9K. It is important to remember, however, that this is a polycrystalline sample and any strains present could have a large affect on these observations. Further single crystal work is necessary in order to determine with more certainty the details of this transformation.