Martensitic transformations displacive transitions

There are a number of displacive transitions mentioned in this book. The order-disorder transformation of hydrogen atoms in hydrogen bonds in ferroelectric ceramics (Section 11.3.5) is one example. Displacive transitions that involve a change from an ordered arrangement of atoms to a random arrangement are commonly found in alloys. A subgroup of such order-disorder transitions, martensitic transitions, which can be used to produce shape-memory alloys, are considered in Sections 8.3.2 and 8.3.3. 

Martensitic transformations in alloys are essentially order-disorder displacive transitions that take place very rapidly, because atomic diffusion does not occur. The discussion of the formation of martensite in the Fe-C system, in Section 8.2.5, is an example. This transition is the transformation of a cubic phase containing excess carbon in interstitial sites into a tetragonal phase. As any one of three cubic axes can be elongated, three orientations of the martensite c axis can occur. This is a general feature of martensitic transformations and the different orientations that can arise are called variants or domains of the martensitic phase. These variants are simply twins (see Section 3.4.10). 

Fig. 85. Splitting of the diamagnetic transition of 123-0, at a > 6.950, indicating the phase separation of the overdoped phase. = 71 p, remains practically unchanged with x, whereas decreases with x. The onset of the splitting coincides with the displacive martensitic transformation (sect. 6.4) at the onset of the overdoped phase. After data of Conder et al. (1994a) and Zech et al. (1995a,b).

There are more general problems of stability of materials and of phase transformations that are closely related to the tensile tests described above. Namely, the tensile test may be considered as a special case of so-called displacive phase transformation path. These paths are well known in studies of martensitic transformations. Such transformations play a major role in the theory of phase transitions. They proceed by means of cooperative displacements of atoms away from their lattice sites that alter crystal symmetry without changing the atomic order or composition. A microscopic understanding of the mechanisms of these transformations is vital since they occur prominently in many materials. 

Rapid quenching of austenite results in a dkplacive or martensitic transformation in which the atoms are instantly displaced or rearranged from fee to bet, trapping the carbon atoms in the process before they have time to diffuse away (also called a diffusionless transition). Martensitic steel is extremely hard and brittle but can the tempered by heating and ageing to increase the ductility. 

As in steels, martensitic transformation t- m is an instantaneous transformation, displacive in nature, which develops when temperature decreases. In pure Zr02, cooling transformation starts at about 950°C (point known as Ms) and reversible heating transformation occnrs beyond 1,150°C (A ). We can summarize the crystallographic aspects by saying that the stractirres t and m derive from the fluorine structure c by various distortions, the most important of which is the one associated with the t- m transition, with a shearing of = 9° parallel to the base plan of the array t to lead to an angle of the monoclinic cell P = 81°. 

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. 

The solid solution LaAg Ini- crystallizes with a CsCl type stmcture (Balster et al., 1975). This pseudobinary system undergoes a martensitic (displacive) crystal structure transition. At low temperature the X-ray powder patterns of polycrystalline samples show line splitting corresponding to a cubic-to-tetragonal transformation. The indium concentration 1 - x and thus the electron count per formula unit has a large influence on the transition temperature. This structural phase transition is revealed also in the temperature dependence of the electrical resistivity. For indium concentrations above 5% the curves show a pronounced hysteresis behavior. 

---