Isothermal martensitic transformations

Abstract

¡ÐÂÎÑÅËÚÐÑÇ ÔÑÑÕÐÑÛÇÐËÇ ÐÇÒÑÔÓÇAEÔÕÄÇÐÐÑ ÔÎÇAEÖÇÕ Ë ËÊ ÎËÐÇÌÐÑÌ ÊÂÄËÔËÏÑÔÕË àÐÇÓÅËË ÂÍÕËÄÂÙËË ª®± Ä ÔÒÎÂÄÂØ Fe ë Ni ë Mn, ÒÑÎÖÚÇÐÐÑÌ Ä [103] (ÓËÔ. 13): Martensite transformations have long been considered most characteristically athermal ì in the sense that they develop, with changing temperature, at a tremendous temperature-independent rate and cease to occur at isothermal conditions. Over the past decades, however, isothermal martensite transformations (IMTs) have been discovered and studied, which take long to develop and occur at low temperatures. This review covers basic experimental data on IMT kinetics, discusses the way IMTs are affected by various inêuences, examines the current theoretical understanding of their nature, and looks at how they relate to kinetically different phase transformations and other thermally activated low-temperature processes in solids.

Reversibility of the Martensitic Transformations and Shape-Memory Effects

“…For example, it was observed that in the Fe-Ni alloys with concentrations of 20-28 wt.% of Ni martensite with a high dislocation density arises upon cooling, with the content 28-32 wt.% of Ni dislocations, and thin twins are observed in the martensite and, at a concentration of more than 32 wt.% Ni, martensite contains mainly twins [5]. It was shown that degree of the recovery in these alloys during change of substructure of the martensitic crystals from dislocations to twins increases from 10% (for Fe-20 wt.% Ni) to 35% (for Fe-32 wt.% Ni) (see Fig.…”

“…However, in the case of isothermal martensitic transition, deformation caused by MT may occur at constant temperature without change of the applied stresses. For example, using alloys of Cu-Sn-Ga, in which isothermal kinetics of MT occurs, it was shown that accumulation of fully reversible deformation may occur at constant temperature in the temperature range [196C; 20C] and under constant applied stress during 20- 30 minutes [5,6]. When removing the applied stresses and heating above A f , there is complete recovery of the original state and size.…”

“…Experimentally isothermal MT was first discovered in FeNiMn alloys and later in other ferrous alloys [3][4][5][6]. In non-ferrous alloys, isothermal MT was first observed in the CuSn alloys alloyed with 1-2 wt.% of Ga [6].…”

“…The assumption about two shifts during MT was used in the working out of the mechanisms of reconstruction in steels [28], zirconium [29], copper alloys [2] and other metals, as well as in the development of the phenomenological theories of the crystallography of the martensitic transformations [5].…”

Martensite transformations in the surface layer at grinding of parts of hardened steels

“…контактная температура, скорость нагрева, отпуск мартенсита, скорость диффузии, тем-пература аустенитизации into an austenitic structure, that is, an inverse martensitic transformation takes place. This transformation is all the more facilitated by the fact that as a result of the high specific pressures exerted by the abrasive grains on the surface of the metal, the Ac 1 point is shifted to a region of lower temperatures [2,4,5].…”