Dynamics of the Fcc-Hcp HELIUM-4 Transformation.

This thesis is an investigation into the dynamics of the fcc-hcp transformation in ^4He. The dynamics was investigated by studying the morphology of the transformation, the kinetics of the transformation, transverse sound velocity changes preceding the hcp to fcc transformation, and attenuation of transverse sound near the transformation. The transformation morphology was visually observed. Observed interactions between the interface and defects in the solid provided the most direct evidence that the transformation is martensitic. Observations of the transformation occurring by the migration of a single planar interface across the entire sample provided strong evidence that the transformation is a thermoelastic martensitic transformation. This also indicated that the samples were likely single crystals. The single planar interfaces had a preferred orientation. Some transformations occurred in which an interface rotated around a pivot. A model of the interface structure is presented and used to explain the above results. In this model, the interface consists of an array of coupled Shockley partial dislocations. Bands of different variants of the hcp phase were produced during rapid heating or cooling. In a study of the fcc-hcp ^4He transformation kinetics, it was demonstrated that small temperature oscillations could produce small oscillations of the fcc-hcp interface. This, along with the observation that little elastic force accumulated to oppose the transformation, indicated that the transformation was a Class I thermoelastic transformation. The transformation kinetics was dominated by the driving force that was required to activate an interface. The velocity of transverse sound was measured as a function of temperature in ^4He at 1.1 and 1.5 kbar. As predicted by the quasi-harmonic approximation, the elastic constant decreased linearly with the thermal component of the internal energy. The rate of decrease was similar to that in other hcp materials. The transverse sound velocity did not soften sufficiently for a soft mode to play a role in the dynamics of the transformation. The attenuation of transverse sound was measured at 1.5 kbar. The attenuation was generally higher in the fcc phase than in the hcp phase.