Thermoelastic Equilibrium and Specimen Size Effects in Thermoelastic Martensitic Transformations

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  • Sakamoto Hidekazu
    Department of Materials Science and Engineering, School of Science and Engineering, Teikyo University
  • Sugimoto Shigeaki
    Kokan Keisoku Co. Ltd.
  • Hara Toru
    Department of Materials Science and Engineering, School of Science and Engineering, Teikyo University

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  • Thermoelastic Equilibrium and Specimen

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New thermodynamical concepts are introduced in thermoelastic martensitic transformations, since the conventional thermodynamical scheme cannot explain the upward shifts in Ms and in experimentally determined equilibrium temperature To\star=(Ms+Af)⁄2 with increasing specimen size which have been experimentally observed in Ti–Ni and Cu–Al–Ni alloys. The first concept is that the total free energy of phases Gtot, which is the sum of chemical free energy Gchem and nonchemical free energy Gnonc (such as elastic strain energy Eelas, which is generated through the shape change attending martensitic transformations, and interfacial energy Eface), equals at To\star. Increasing specimen size increases Eelas more in parent than in martensite. The larger Eelas in the parent phase than in the martensite destabilize parent phase, resulting in the upward shift in To\star. The second concept is the total free energy of a system, G#=fGtotM+(1−f)GtotP, where the symbol f indicates the transformation fraction, the superscript M martensite, and P is the parent phase. On the basis of the concept, thermoelastic equilibrium in thermoelastic martensitic transformations is formulated as −fΔGchemP→M(T)=fGnoncM(f)+(1−f)GnoncP(f). Further, the evolution of G# on cooling is found to be expressed as GchemP(T) even below Ms up to Mf under the thermoelastic equilibrium. The third is that the supercooling below To\star to Ms for the onset of the transfomations is not because necessary Gnonc has to be provided through chemical driving force but because of the presence of a potential energy barrier against pseudoshear movement of the atom cluster in the parent phase to martensite. The concept enables us to elucidate the experimentally observed upward shift of Ms with increasing specimen size.

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