Martensitic Transformation

a polymorphic transformation in which a change in the relative arrangement of atoms or molecules making up a crystal takes place by their ordered displacement, with the position changes of neighboring atoms being small relative to their interatomic distance. The rearrangement of a crystal lattice in microdomains usually results in a deformation of the unit cell, and the final phase of martensitic transformation may be considered a uniformly deformed initial phase.

The amount of deformation is small (̃1-10 percent), and thus the energy barrier hindering the homogeneous transition of the initial phase into the final phase is small in comparison with the bond energy in the crystal. A necessary condition for martensitic transformation that develops by the formation and growth of areas of the more stable phase within the metastable phase is the retention of ordered contact between the phases. The ordered structure of the interphase boundaries together with the low barrier for homogeneous phase transition ensures a low energy and high mobility of these boundaries. As a result, the excess energy required for the nucleation of crystals of the new phase (known as martensite crystals) is low, and at some value of the deviation from phase equilibrium it becomes comparable to the energy of defects in the initial phase. Thus, the nucleation of martensite crystals takes place more rapidly and may not require thermal fluctuations. As a consequence of the interaction of the newly formed phase with the original phase, the energy barrier for displacement of the phase boundary is significantly less than for a homogeneous transition. With small deviations from equilibrium, the energy barrier disappears and the crystal grows at a speed comparable to the speed of sound even without thermal activation. Thus, transformation is possible at temperatures close to absolute zero.

A considerable role in martensitic transformations is played by internal strain arising as a result of the elastic accommodation of crystal lattices that are in contact at the phase boundaries. The elastic strain fields lead to displacement of the equilibrium point of the interacting phases relative to the position of the true thermodynamic equilibrium of the isolated, undeformed phases. Hence, the temperature for the onset of martensitic transformation may differ significantly from the temperature of the true equilibrium. The tendency to minimize elastic strain energy determines the morphology, internal structure, and relative arrangement of the martensite crystals. The new phase is formed as thin plates with a definite orientation relative to the crystallographic axes. The plates, as a rule, are not single crystals but rather packets of plane-parallel domains of the new phase differing in orientation of the crystal lattice (twinning). Interference of the stress fields from the various domains leads to their partial extinction. A further decrease in the elastic strain fields is attained as a result of the formation of ensembles of regularly arranged plates. Thus, as a result of martensitic transformation, a polycrystalline phase with a unique hierarchical order, namely, ensembles-plates-domains, forms in the arrangement of the structural components.

The increase in internal strain during martensitic transformation under certain conditions leads to the establishment of two-phase thermoelastic equilibrium that is reversibly displaced upon changing the external conditions: the dimensions of individual crystals and the number of crystals change upon the action of a mechanical load or by change in the temperature.

The representation given, which rather closely corresponds to martensitic transformation in alloys of nonferrous metals, is usually distorted to some extent by plastic relaxation processes such as the nucleation and displacement of dislocations. The relaxation of internal strains leads to substantial irreversibility of martensitic transformation and significant hysteresis develops between the forward and reverse transformations. The “pinning” of dislocations at the interphase boundaries decreases their mobility and increases their energy such that the barrier to nucleation increases. With an increasing degree of relaxation, smaller deviations from the true equilibrium point are required for the transformation but its rate is reduced and the regular nature of the transformation products is less clearly evident. In the same material, depending on the degree of deviation from the true phase equilibrium point and relaxation rate, kinetically and structurally different types of transformations such as rapid athermal, isothermal, and normal martensitic transformations (the last has kinetics similar to crystallization) are observed.

Martensitic transformations have been discovered in many crystalline materials including pure metals, many alloys, and ionic, covalent, and molecular crystals. The martensitic transformations of iron alloys have been studied most extensively, especially in relation to the hardening of steel.

There is great promise for the practical application of the possibility of large reversible changes in shape accompanying martensitic transformation (for example, the production of “superelastic” alloys and specimens capable of returning to their original shape upon heating after plastic deformation, that is, a “memory effect”). There are also prospects for the practical application of the relationship between martensitic transformation and the appearance of superconduction properties in some metals. Martensitic transformations (often together with diffusional redistribution of components and change in atomic arrangement) are the basis of many structural transformations that account for the desired change of the properties of crystalline materials achieved by heat or mechanical treatment. A significant contribution to the study of martensitic transformation has been made by Soviet scientists, including G. V. Kurdiumov and his school.