Thermally Activated Martensite: Its Relationship to Non-Thermally Activated (Athermal) Martensite

Abstract

The classification of martensitic displacive transformations into athermal, isothermal or anisothermal is discussed. Athermal does not mean "no temperature dependence" as is often thought, but is best considered to be short for the notion of no thermal activation. Processes with no thermal activation do not depend on time, as there is no need to wait for sufficient statistical fluctuations in some specific order parameter to overcome an activation barrier to initiate the process. Clearly, this kind of process contrasts with those that are thermally activated. In the literature, thermally activated martensites are usually termed isothermal martensites, suggesting a constant temperature. Actually such martensites also typically occur with continuous cooling. The important distinctive feature of these martensites is that they are thermally activated and hence are distinguishable in principle from athermal martensites. A third type of process, anisothermal, has been introduced to account for those transformations which are thought to be thermally activated but which occur on continuous cooling. They may occur so rapidly that they do not appear to have an incubation time, and hence could be mistakenly called an athermal transformation. These designations will be reviewed and discussed in terms of activation energies and kinetic processes of the various martensitic transformations.

Martensite Transformation Intergrain and Intragrain Autocatalysis in Fe-Ni alloys

“…Villa et al 28 observed isothermal martensite kinetics on the isochronal cooling a Fe-Cr-Ni-Cu alloy which was attributed to thermally activated growth of athermal nucleated lath martensite. Laughlin et al 29 generalized the discussion of thermally-activated stages in the martensite transformation curve by considering the relative position of the M S temperature and the nose of the thermal-activated martensite transformation, and the imposed cooling rate. Here we provide evidence that the intergrain autocatalysis in polycrystalline Fe-31wt%Ni-0.02wt%C is controlled by a thermally activated process.…”

A review on various phases and alloy design methods of β-Ti alloys for biomedical applications

“…Anisothermal martensitic transformations are thermally activated transformations on continuous cooling. 40 Martensitic phase formation depends upon the alloy composition and rate of cooling. As the solute content increases in Ti alloy, the martensitic start temperature (M s ) and martensitic finish temperature (M f ) decrease, as illustrated in Figure 7.…”

General description of martensite transformation curves – a case for bainite

“…The martensite transformation curve has been descri bed by empirical equations whose coefficients referred to the steel composition, thermodynamic properties, and the austenite grain size [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. However, the relationship between microstructure and the volume fraction of martensite transformed still deserves consideration.…”