Fcc-hcp transformation-related internal friction in Fe-Mn alloys

The epsilon <----> gamma reversible transformation has been studied in Fe-14 wt.% Mn and Fe-21 wt.% Mn alloys which are known to exhibit a weak shape memory effect. The internal friction changes and the associated elastic modulus changes were measured using the high frequency (40 kHz) composite oscillator technique in the temperature range 20-350degreesC in which the epsilon <----> gamma transformation occurs. The strain-induced amplitude dependent internal friction was measured in order to evaluate changes in the dislocation structure that occur during the transformations and to identify the transformation mechanism involved. The study revealed that the propagation of the epsilon/gamma interface boundary during the epsilon <----> gamma transformation is a thermally activated relaxation process obeying the Debye relaxation equation. The internal friction data were used to determine the activation energy of the process responsible for the martensitic transformation. The damping peak is observed at temperatures where both phases coexist and is due to the motion of the epsilon/gamma interface. The epsilon/gamma interface consists of Shockley partial dislocations moving in groups and causing a minimal macroscopic strain. The epsilon/gamma interface-related dislocation motion is, therefore, the cause of the internal friction peak. The internal friction is associated with a step-like change in the elastic modulus which is due to the epsilon/gamma interface related relaxation and the difference in the modulus of the epsilon and gamma phases.