Further Aspects of the Crystallographic Model

The input data for the model consist of the description of the lattice deformation and the choice of the slip system in the lattice-invariant shear. The model has successfully predicted the observed geometrical features of many martensitic transformations. The observed and calculated habit planes generally have high indices that result from the condition that they be macroscopically invariant. 

Because of the four-fold symmetry of the [001] pole figures in Figs. 24.6-24.9, additional symmetry-related invariant planes can be produced. Also, further work shows that additional invariant planes can be obtained if a lattice-invariant shear corresponding to a = 7.3° rather than a = 11.6° (see Fig. 24.8) is employed [5]. Multiple habit planes are a common feature of martensitic transformations. 

In many cases, the martensite phase is internally twinned and is composed of two types of thin twin-related lamellae, as illustrated in Fig. 24.10. In such cases, the lattice-invariant shear is accomplished by twinning rather than by slip as has been assumed until now (see Fig. 24.106). The critical amount of shear required to produce the invariant habit plane is then obtained by adjusting the relative thicknesses of the two types of twin-related lamellae shown in Fig. 24.106. 

The crystallographic model for martensite described above is primarily due to Wechsler et al. [1], A similar model, employing a different formalism but leading to essentially equivalent results, has also been published by Bowles and MacKen-zie [2-4]. In both models, a search is made for an invariant (or near-invariant) plane which is then proposed as the habit plane, since the selection of this plane 

The two types of twin-related lamellae present are labeled 1 and 2. 

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