On the thermodynamics of thermoelastic martensitic transformations

Abstract

Both elastic strain energy due to the transformational shape change and frictional resistance opposing interfacial motion are shown to contribute significantly to the observed thermodynamic changes which accompany thermoelastic martensitic transformations in Cu-14 Al-2.5 Ni alloys. However, for the special case of single-interface transformations in single crystals of the parent phase, elastic strain energy is not generated because the transformational shape change takes place freely without surrounding constraints. The temperature ( T 0) of zero-change in chemical free energy can then be bracketed unambiguously between the interface-advance temperature on cooling and the interface-reverse temperature on heating. Also, by means of calorimetric measurements, the chemical enthalpy, entropy and free-energy changes can be determined. Concomitantly, the frictional resistance can be estimated from the hysteresis between the interface-advance and interface-reverse temperatures; the frictional energy turns out to be approximately 5% of the chemical enthalpy change. In multiple-interface transformations, the cooling and heating transformation curves are altered in form and depressed to lower temperatures due to the storage of elastic strain energy, and there is a corresponding effect on the measured thermodynamic quantities, e.g. as a function of decreasing grain size. Such differences relative to the single-interface results permit evaluation of the stored elastic strain energy; it amounts to about 15% of the chemical enthalpy change.