Athermal transformation

But, aside from these unique properties, Nitinol has a number of commonalities with other known martensitic transition systems (1) it is an athermal transformation, (2) it is diffusionless, (3) it involves displacive or shear-like movement of atoms, (4) the activation energy for the growth of martensite (continuous atomic shear in Nitinol) is effectively zero, i.e., the propagation rate of transformation (transition in Nitinol) is fast and independent of temperature. 

Sah] Sahu, R, De, M., Kajiwara, S., Microstructural Characterization of Fe-Mn-C Martensites Athermally Transformed at Low Temperature by Rietveld Method , Mater. Sci. Eng. A, 333, 10-23 (2002) (Crys. Structure, Experimental, 36)... 

Athermal transformation n. A reaction that proceeds without thermal (not dependent on heat, enthalpy AH = 0) activation as contrasted to isothermal transformation, which occurs at constant temperature. An athermal mixture of liquids or polymer in a solvent involves no enthalpy AH = 0) change in an ideal solution and the Gibbs free energy (—AG) is always negative for the solution (polymer dissolved in solvent) to occur while the entropy (AS) increases. Barton AFM (1983) Handbook of solubility parameters and other cohesion parameters. CRC Press, Boca Raton. 

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... 

If a fiVT ensemble simulation can be turned into a ( quasi ) NPT ensemble-type simulation (e.g., a pseudo- FT ensemble), the inverse transformation (a pseudo-NPT ensemble) is also possible. The key relationship for a pseudo-NPT ensemble technique is Eq. (5.1) [78]. Such a reverse strategy can be practical only if molecular insertion and deletion moves can be performed efficiently for the system under study (e.g., by expanded ensemble moves for polymeric fluids). Replacing volume moves by particle insertions can be advantageous for polymeric and other materials that require simulation of a large system (due to the sluggishness of volume moves for mechanical equilibration of the system) such an advantage has been clearly demonstrated for a test system of dense, athermal chains [78]. 

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. 

The transformation is athermal, i.e., it is not time dependent and proceeds very rapidly. If the transformation takes place in the Zr02 particles during fabrication of ZTA ceramics, then the 3% volume change produces stresses in the alumina matrix around the transformed particle leading to microcracking. These microcracks increase the toughness of the ceramic by their ability to deflect and... 

Under the influence of the stress field at a cracktip these particles will transform athermally to the monoclinic state. This leads to transformation toughening and the toughening increment AKi that can be achieved by this mechanism is... 

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)... 

Sah] Athermal martensite transition, XRD, microhardness 5.6 to 6% Mn, transformation temperature 77 to 200 K... 

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