Thermodynamics of thermoelastic martensitic transformations
Abstract
A fundamental thermodynamic equation is derived for thermoelastic martensitic transformations. It describes the mutual equilibrium between the two phases at every temperature and applied stress, taking into account internal interaction between domains, the existence of interfaces and dissipative effects associated with motion of boundaries. The equation is used to study the role of these contributions on the shape of the hysteresis cycle in both thermally-induced transformations at a constant external stress and stress-induced transformations at constant temperature. It is shown that reversible (including chemical enthalpy and entropy changes) and irreversible contributions can be sorted out experimentally combining simultaneous tensile and calorimetric measurements.
Résumé
Nous formulons une équation thermodynamique fondamentale pour l'étude des transformations martensitiques thérmoélastiques. Cette équation décrit l'équilibre entre les deux phases à chaque température et à chaque valeur de la contrainte externe appliquée. L'équation tient compte des interactions internes entre domaines, de l'existence d'interfaces et des effets dissipatifs associés aux mouvements des interfaces. Nous l'avons utilisée pour étudier le rôle de ces contributions sur la forme des cycles d'hysthéresis obtenus lors de transformations induites par variation de la température à contrainte externe fixe, et des transformations induites par variation de la contrainte externe à température constante. Nous démontrons que les contributions réversibles (qui incluent le changement d'enthalpie chimique et d'entropie) et irréversibles peuvent être obtenues expérimentalement à partir d'expériencies mécaniques et calorimétriques simultanées.
Zusammenfassung
Es wird eine thermodynamische Grundgleichung für thermoelastiche martensitische Umwandlungen hergeleitet. Diese Gleichung beschreibt das Gleichgewicht zwischen den beiden Phasen bei jeder Temperatur und äuβeren Spannung, wobei die interne Wechselwirkung zwischen den Domänen, die Existenz von Grenzflächen und die dissipativen, mit der Bewegung der Grenzflächen zusammenhängenden Effekte berücksichtigt werden. Mit dieser Gleichung wird der Einfluβ dieser Beiträge auf die Form der Hysterese sowohl bei thermisch induzierten Umwandlungen bei einer konstanten äuβeren Spannung als auch bei spannungsinduzierten Umwandlungen bei konstanter Temperatur untersucht. Es wird gezeigt, daβ reversible Beiträge (einschlieβlich Änderungen der chemischen Enthalpie und Entropie) und irreversible Beiträge experimentell getrennt werden können, wenn Zugverformung und kalorimetrische Messungen kombiniert werden.
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