Phase transformations athermal

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

Kak] Kakeshita, T., Sato, Y, Saburi, T., Shimizu, K., Matsuoka, Y, Kindo, K., Effects of Magnetic Field on Athermal and Isothermal Martensitic Transformations in Fe-Ni-Cr Alloys , Mater. Trans., JIM, 40(2), 100-106 (1999) (Experimental, Kinetics, Phase Relations, 13)... 

The pressure-induced transformation of h-BN to c-BN is more complex. As noted previously (Corrigan and Bundy, 1975), w-BN is not a thermodynamically stable phase in the boron nitride system. Metastable w-BN can form only from (metastable) h-BN at low temperature (Figure 11.13) by an athermal martensitic... 

The first pair of terms, (a), on the right-hand side of the equation is equivalent to ax/a7 the intrinsic.temperature dependence of the total mean susceptibility. The second pair, (b), represents the change in susceptibility that takes place during reversible co o 3 allotropic transformation. The fraction of athermal co phase, fjj, is a reversible function of temperature whose value at any temperature, say Ti, is... 

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