Thermodynamic and X-ray studies of fcc to fct phase transformations in In-Tl system

The In-Tl alloy system undergoes a fcc → fct martensitic transformation on cooling, in the composition range 16.0–21.0 at.-%Tl. The thermodynamic properties of both phases were directly determined in the temperature range 300–410 K, together with the enthalpy of phase transformation. From the X-ray data, the lattice parameters of both phases were determined. The volume changes accompanying the transformation were calculated. The volume changes are directly proportional to the strain-free energy which, in this case, makes the outstanding contribution to the non-chemical free energy change. These volume changes decrease with the increase in Tl content in these alloys. The chemical free energy change necessary to overcome the non-chemical free energy change to allow the transformation to take place, is directly reflected in the enthalpy of phase transformation. This means that the enthalpies of phase transformation should also decrease with the increase in Tl content in these alloys, and this is observed. Furthermore, from the thermodynamic results it appears that the phase transformation is a first order reaction, contrary to other speculations in literature.