抄録
I have been fascinated by martensitic transformations for over fifty years in view of their unique features and accessibility to quantitative experimentation in the solid state. This type of structural change first came into prominence in metallurgy because of its central role in the quench-hardening of steel; but more generally, a martensitic transformation is now defined as being diffusionless, displacive, and displaying a large component of lattice-distortive shear such that the reaction kinetics and product morphology are dominated by strain energy. The resulting crystallographic change is accompanied by a shape deformation in the transforming region, which has to be accommodated elastically or plastically within the system. Martensitic transformations, characterized by these features, are observed in nonmetallic as well as in metallic materials, and even in some biomaterials. A structural change of this kind can be regarded as a spontaneous deformation of the parent phase in response to operative thermodynamic driving forces, and so it becomes feasible to describe and quantify key aspects of martensitic transformations in terms of well-established dislocation theory.
The shape change attending a martensitic transformation imposes an unusually high energy barrier for homogeneous nucleation even under conditions where nucleation actually occurs. The latter is found to take place heterogeneously at pre-existing defects in the parent phase—a process which can now be described in the quantitative language of dislocation arrays, stress fields, and mobilities. The structure and motion of the interface which couples the parent and product phases can likewise be modeled via dislocation concepts, and it is then concluded that the measured nucleation and growth rates are both controlled by the dynamics of glissile interfacial motion. Moreover, this motion is reversible (with some hysteresis) if the transformational shape deformation can be accommodated elastically. At the same time, the transformation product is constrained to adopt plate-like morphologies and habits that tend to minimize the attendant strain energy. No evidence has yet shown that martensitic transformations are governed by impending lattice instabilities, although special cases of this sort are still being sought.
The intriguing properties exhibited by martensitic transformations are, nevertheless, entirely consistent with their diffusionless, displacive, and strain-energy-dominated nature. They can take place at temperatures approaching absolute zero, with interfaces that propagate at dislocation velocities; they can form metastable phases having exactly the same composition and degree of atomic order as does the parent phase, a state that may be impossible to achieve in any other way; and they can function as a deformation mode under the action of elastic stress or plastic strain. All these manifestations furnish the basis, not only for the quench-hardening of steel, but also for such phenomena as thermoelasticity and pseudoelasticity, transformation plasticity and toughening, and shape-memory effects.
Because of the distinctive features of martensitic transformations together with their technological importance, this class of structural change has provided a focal point for the rare convergence of solid-state thermodynamics, reaction kinetics, crystallographic relationships, dislocation theory, and mechanical behavior. The coming together of such diverse streams of thought and interpretation in order to enhance society’s understanding and utilization of nature’s processes, as in the case of martensitic transformations, embodies a powerful message in the field of materials science and engineering.
