Examples of Martensitic Transformations

Examples of martensitic transformation. This kind of transformation has been observed in a number of substances elements, compounds, alloys, minerals, metallic and ceramic materials. A few examples of systems showing a... 

Stress induced martensitic transformation is a transformation firom one ciystallographic form to another form and associated with a displacement of chains to new positions in the new crystallographic ceU in order to acconunodate the deformation. An example of martensitic transformation firom orthorhombic to monoclinic form was found in oriented polyethylene with well defined texture subjected to uniaxial compression. Martensitic transformation was also found in other polymers in poly(L-lactic acid) [119] and in nylon 6 with the a-form transforming to the 7-form [120,121]. 

The remainder of the book treats discontinuous transformations. Nucleation, which is necessary for the production of a new phase, is treated in Chapter 19. The growth of new phases under diffusion- and interface-limited conditions is treated in Chapter 20. Concurrent nucleation and growth is treated in Chapter 21. Specific examples of discontinuous transformations are discussed in detail these include solidification (Chapter 22), precipitation from solid solution (Chapter 23), and martensite formation (Chapter 24). 

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). 

Detailed consideration of the structure of many of the advanced and complex alloys which are of considerable technological importance (high-strength titanium alloys, nickel-base superalloys, etc.) is beyond the scope of this section, other than to point out that no new principles are involved. Certain titanium alloys, for example, exhibit a martensitic transformation, while many nickel-base superalloys are age hardening. Similarly, cast irons, although by no means advanced materials, are relatively complex they are considered in Section 1.3 where graphitisation is discussed. 

Figure 5.30. Schematic drawing showing the construction of an isothermal transformation diagram from measurements of the progress of the transformation at various constant temperatures. This may be done, for instance, by metallographic examination of several specimens, quenched from the 7-field quickly enough to miss the nose of the C-curve and then isothermally annealed for various length of time. Notice that curves for the transformation of different samples may be shown on the same diagram and that more complex trends may be observed in real diagrams of specific alloys. In the example reported, Ms is the temperature at which the alloy will begin to show the martensitic transformation, Mf indicates the temperature below which no additional martensite forms.

Characteristics and implementation of the treatments depend on the expected results and on the properties of the material considered a variety of processes are employed. In ferrous alloys, in steels, a eutectoid transformation plays a prominent role, and aspects described by time-temperature-transformation diagrams and martensite formation are of relevant interest. See a short presentation of these points in 5.10.4.5. Titanium alloys are an example of the formation of structures in which two phases may be present in comparable quantities. A few remarks about a and (3 Ti alloys and the relevant heat treatments have been made in 5.6.4.1.1. More generally, for the various metals, the existence of different crystal forms, their transformation temperatures, and the extension of solid-solution ranges with other metals are preliminary points in the definition of convenient heat treatments and of their effects. In the evaluation and planning of the treatments, due consideration must be given to the heating and/or cooling rate and to the diffusion processes (in pure metals and in alloys). 

Let us regard a binary A-B system that has been quenched sufficiently fast from the / -phase field into the two phase region (a + / ) (see, for example, Fig. 6-2). If the cooling did not change the state of order by activated atomic jumps, the crystal is now supersaturated with respect to component B. When further diffusional jumping is frozen, some crystals then undergo a diffusionless first-order phase transition, / ->/ , into a different crystal structure. This is called a martensitic transformation and the product of the transformation is martensite. 

We have mentioned above the tendency of atoms to preserve their coordination in solid state processes. This suggests that the diffusionless transformation tries to preserve close-packed planes and close-packed directions in both the parent and the martensite structure. For the example of the Bain-transformation this then means that 111) -> 011). (J = martensite) and <111> -. Obviously, the main question in this context is how to conduct the transformation (= advancement of the p/P boundary) and ensure that on a macroscopic scale the growth (habit) plane is undistorted (invariant). In addition, once nucleation has occurred, the observed high transformation velocity (nearly sound velocity) has to be explained. Isothermal martensitic transformations may well need a long time before significant volume fractions of P are transformed into / . This does not contradict the high interface velocity, but merely stresses the sluggish nucleation kinetics. The interface velocity is essentially temperature-independent since no thermal activation is necessary. 

There are a number of different examples within which it is possible to describe the kinematics of structural transformation. Perhaps the simplest such example is that of the transformation between a cubic parent phase and a transformed phase of lower symmetry such as a tetragonal structure. We note that we will return to precisely such structural transformations in the context of martensitic microstructures in chap. 10. If we make the simplifying assumption that the transformed axes correspond with those of the parent phase, then the deformation mapping is of the form... 

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