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 memor y alloys permits to determine experimentally differences in enthalpy a nd entropy to a high degree of precision. Due to the diffusionless nat ure of the martensitic transformation the configurational entropy rema ins invariant and the entropy difference results mainly from differenc es in the phonon spectra, which are accessible to experiment, especial ly the soft branches. From measured enthalpy differences in the long r ange ordered alloys, those corresponding to the disordered state can b e deduced which, together with the vibrational entropy differences, pr ovide also valuable information on the equilibrium phases. Close packe d materials with different stacking sequences are commonly observed, a nd 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.