ON THE USEFULNESS OF THE MARTENSITIC-TRANSFORMATION FOR THE EVALUATION OF PHASE STABILITIES AND DISLOCATION PROPERTIES IN LONG-RANGE ORDERED ALLOYS

The small hysteresis in the martensitic transformations of shape memory alloys permits to determine experimentally differences in enthalpy and entropy to a high degree of precision. Due to the diffusionless nature of the martensitic transformation the configurational entropy remains invariant and the entropy difference results mainly from differences in the phonon spectra, which are accessible to experiment, especially the soft branches. From measured enthalpy differences in the long range ordered alloys, those corresponding to the disordered state can be deduced which, together with the vibrational entropy differences, provide also valuable information on the equilibrium phases. Close packed materials with different stacking sequences are commonly observed, and from the critical transformation stresses their relative stability can be assessed. Split dislocations in long range ordered alloys with a stacking fault between them are similar to a thin martensite region, and therefore a comparison between both is of interest.